In fragmented landscapes, the structure of the vegetation within a remnant strongly influences the fauna that lives there. But the vegetation outside the remnant is also important. In a recent paper, David Lindenmayer and colleagues documented how the bird composition of small woodland patches changed as pine plantations (Pinus radiata)  grew up around the woodlands. At the start of the study, the woodland patches were surrounded by open paddocks with scattered trees. At the end of the study (just 7 years later), the patches were surrounded by young pine forests. These growing forests caused big changes to the birds that lived in the remnant woodlands.

The figure below is my mind-blowing graph of the day.

The chart shows how the bird composition of the woodland remnants changed as the young pine forests grew up. The black bars show how often each bird species was seen in the first census (before pines were planted), and the white bars show how often each species was seen in the final census, 7 years later. The other colours represent surveys between these two dates. The bars are arranged so that birds that increased over time are at the top, and those that declined are at the bottom.

Changes in the bird composition of woodland patches, as pine plantations grew up around the woodlands. Figure from Lindenmayer et al. (2008). Click on the image to view in full screen mode.

As you can see, common bronzewings, weebills and peaceful doves were the big winners, while brown treecreepers, dusky woodswallows and introduced starlings were the biggest losers. Additionally, some species (including speckled warblers) disappeared completely from the area, while a number of classic ‘forest birds’ colonized the area, including olive whistlers, satin bowerbirds and even superb lyrebirds. Overall, the chart shows a change from birds that like grasslands and open woodlands, to birds that like dense forests.

We’d expect changes like these if the researchers had surveyed the open paddocks as they changed into pine plantations. But the surveys weren’t conducted in the paddocks, they were in the woodlands that were surrounded at first by paddocks and later by pines. The new pine forests led to big changes in the birds in the woodlands, and these changes happened very quickly – in just 7 years in this example.

I find the magnitude of these changes quite extraordinary. I imagined that the birds would change over time, but not this much, and not this quickly. Imagine how different the birds will be in another 20 years, when the pine plantations are tall and dense.

Satin Bowerbirds eating palm-tree fruits (Source: http://www.anhs.com.au/bowerbirdf.htm)

In the long-term these changes won’t just affect birds. Birds aren’t just ‘passengers’ in ecosystems, they also change the ecosystems they live in. They pollinate flowers, disperse seeds, eat insects and lots more. As the birds change, these functions will change, with flow-on effects on other biota, perhaps on the woodlands as a whole.

Two of the notable ‘increaser’ species were satin bowerbirds and introduced blackbirds. Both species disperse the seeds of fleshy-fruited plants. Not many native woodland plants have fleshy fruits (the flax-lilies, Dianella species, come to mind), but many exotic plants do. Invasive plants like blackberries, hawthorn, briar-rose and the native Pittosporum undulatum (which has spread from wet forests into other ecosystems in many regions) all have fleshy fruits. Based on this graph alone, we can expect woodland plants will change over time, as blackbirds and bowerbirds become more abundant, and disperse more and more fleshy fruits. Many other changes will also take place.

In conservation, it’s often hard to stop important remnants from being destroyed and replaced by other human land uses. It’s even harder to stop other land uses from being built next to important remnants, even when we know that the new activities will degrade the patches we want to conserve. The chart above highlights how strongly surrounding landscapes can affect small remnants. When the neighborhood changes, owning the best house on a bad block ain’t a great investment.

References

Gosper CR, Stansbury, CD & Vivian-Smith G (2005) Seed dispersal of fleshy-fruited invasive plants by birds: contributing factors and management options. Diversity and Distributions 11(6), 549-558. [You can download a free copy of this article from this link].

Lindenmayer DB, Fischer J, Felton A, Crane M, Michael D, Macgregor C, Montague-Drake R, Manning A & Hobbs RJ (2008) Novel ecosystems resulting from landscape transformation create dilemmas for modern conservation practice. Conservation Letters 1, 129-135. [You can download a free copy of this article from this link].

Which ecological process has the biggest impact on how your ecosystem changes over time?

Presumably, the process you selected is the one that you think is most critical to retain (or avoid) in order to conserve a diverse, functioning ecosystem. If the process is altered or removed, ecosystem health will decline.

Now, think about this question: what if you’re wrong?

Box-Ironbark Forest dominated by Red Ironbark above Gold-dust Wattle.

What if some other process is actually far more important, and the process you selected is actually rather trivial in the big picture?

I’ve been thinking about this question while wandering around box-ironbark forests in central Victoria, an ecosystem I’m pretty new to. The question I’ve asked myself is: what are the dominant processes that control how box ironbark forests function? I don’t have the answer, but the question has triggered lots of interesting discussions. But before we discuss box ironbark forests, let’s widen the canvas and think about the processes that affect most Australian ecosystems.

Fire and Rain

I imagine that the two most popular answers to the question above would be fire and water. Grazing is important in agricultural landscapes, but fire and water are arguably the most important drivers of ecosystem dynamics in most Australian ecosystems.

Fire controls ecosystem processes in lots of Australian ecosystems, especially forests, heathlands and savannahs (see recent blog). Fire regimes affect plant regeneration and mortality, and can alter ecosystem structure and habitat suitability for fauna.

In semi-arid and arid regions, the key factor that affects ecosystem processes isn’t fire, it’s water; how much water arrives in the first place (in floods and droughts), and how water is retained in, and moves through, the landscape.

Intact ecosystems are very good at retaining water. Falling rain hits ground plants and litter, and these slow the flow of water through the ecosystem. The more water that is held in the landscape, and the longer it is available, the greater the biological productivity.

By contrast, degraded dryland systems are very leaky. Degrading processes such as heavy grazing remove the plants and litter that intercept surface water. Water runs off faster and soils dry out faster, and this in turn, leads to further reductions in biological productivity. This creates a vicious cycle as ‘leaky ecosystems’ become more and more degraded.

To restore degraded dryland systems, arid zone ecologists focus on measures that slow the flow of water across the landscape. Understory and ground plants, fallen timber and leaf litter all create small barriers that prevent water from leaking, or running out of the system. Drifts of leaf litter on the ground improve water infiltration and soil condition (including organic carbon levels), so soils can store more water for longer.

In degraded dryland systems, a primary management goal is to improve the system’s ability to retain soil moisture.

It’s easy to view these two world views as applying to completely different ecosystems – fire is important in forests and water in deserts. But sharp boundaries are rare in nature. Some forests grow on poor, degraded, dry soils, and don’t burn that much. Retaining or restoring scarce resources is likely to be very important in these systems.

Which brings us back (finally!) to box-ironbark forests.

Box Ironbark forest in central Victoria dominated by Red Ironbark (Eucalyptus tricarpa)

Water for Elephants Ironbarks?

To return to our original question: which ecological processes have the biggest impact on how box-ironbark ecosystems change over time? How important are fire and water? I emphasise that I don’t know (and I don’t know anyone else who does), so this blog sketches out some early thoughts, and some research questions for future projects.

Gold mining in the 1800s stripped the topsoil from many Box Ironbark forests. Early illustration by S.T. Gill. Source: http://oldwww.ballarat.edu.au/sovhill/gold150/gf2b.gif.

Box-ironbark forests are really interesting. They occur on soils that are naturally infertile, but which were made less productive by widespread gold mining in the mid-1800s. Mining degraded soils over vast areas, and many soils now have no A horizon at all. The loss of the original topsoil must have reduced soil water holding capacity and, by inference, the biological productivity of the forests as a whole.

Box-ironbark forests in Victoria don’t burn very often. They don’t produce a lot of ground fuels, so they don’t carry frequent fires. Many of the dominant understory shrubs seem to regenerate well without fires (although fire undoubtedly promotes regeneration of many species). Unlike most forests in south-east Australia, there is little evidence that fire is an important driver of ecosystem dynamics. (Who knows what role fire may have played before European settlement, but that’s a matter for conjecture).

By contrast, box ironbark forests are undoubtedly subject to seasonal water limitation. Annual rainfall is moderate (450-600 mm on average) but summers are hot and dry. Annual rates of biomass accumulation are low compared to ecosystems on more fertile, well watered soils. Prolonged droughts (as seen during the past decade) killed many understory shrubs which, in turn, is thought to have contributed to declining numbers of birds during the drought.

Shallow soils with little leaf litter are easily eroded.

Box ironbark forests also seem to be pretty ‘leaky’. Soils are often bare and crusted, and rates of water infiltration are (presumably) very slow. The paucity of humus and organic matter means that their water holding capacity is also low. Because the ground surface is relatively open, there often aren’t a lot of barriers to impede the flow of water during heavy rains. Following heavy summer rains, water rushes down shallow drainage lines, washing leaf litter off the slopes and down to the gullies. This process must further reduce soil productivity. Indeed, this is the vicious cycle that arid zone ecologists try to avoid.

Piecing together these casual observations, one can make a good case that, to maintain biological productivity in box-ironbark forests, it’s useful to view these forests through the lens of arid zone ecology. This approach becomes even more compelling given the perils of climate change. As temperatures increase, box ironbark forests will come under increasing water stress.

As in arid ecosystems, a valuable management strategy to minimize the effects of climate change will be to increase the system’s ability to retain soil moisture.

I’m starting to wonder – is water availability a far more important issue than fire in box ironbark forests? Instead of this simple either/or question, a better question might be: how do fires (where they do occur) affect soil moisture retention in box ironbark forests? Do they make the system more or less leaky?

Litter for water, water for ecosystems

Small barriers trap and prevent leaf litter from being washed from the system.

How do we retain soil moisture in water-limited ecosystems? David Tongway, John Ludwig and other arid zone ecologists have studied this for decades now.

The goal isn’t to create ‘dams’ of deep water (this is a dryland system after all). Instead, the goal is to increase the capacity of topsoil and litter layers to hold and slow the flow of surface water. Anything that promotes ‘surface roughness’, slows the flow of water. Branches, leaves and twigs on the soil surface trap fine litter and create small barriers that hold water for longer periods. Soil accumulates, soil micro-organisms become more abundant, and system productivity increases.

In the longer term, as trees grow older and drop more larger limbs, fallen coarse woody debris (‘brown gold’) helps to capture even more litter and water. By contrast, processes that reduce leaf litter accumulation are likely to reduce soil water holding capacity, biological productivity and resilience to climate change.

Fallen timber and deep leaf litter beneath an old grey box. Old trees drop more branches than young trees.

A key goal for conservation management is to increase ecosystem resilience to changing climate. The most likely impact of climate change in south-eastern Australia is a decline in soil water due to higher evaporation as temperatures increase. So any process that retains soil moisture and reduces system ‘leakage’ seems like a great process to promote (or at least to investigate), yes?

Water for ironbarks may not be the goal, but water for soil development, soil productivity, and biological diversity seems like an important ecosystem process to promote. Ultimately, retaining leaf litter (fallen leaves, twigs and branches) and old trees (that produce more branches) may be the simplest way to hold soil water. Unfortunately, litter for water and resilient ecosystems doesn’t sound that sexy does it? Nevertheless, as climate change intensifies, it may be the issue that we have to focus on more and more.

Further reading

Environment Conservation Council (1997). Box – Ironbark Forests and Woodlands Investigation Resources and Issues Report. (ECC: Melbourne)

Ludwig J, Tongway D, Freudenberger D, Noble J, Hodgkinson K (1997). Landscape Ecology, Function and Management: Principles from Australia’s Rangelands. (CSIRO: Melbourne).

Mac Nally R, Bennett AF, Thomson JR, Radford JQ, Unmack G, Horrocks G, Vesk PA (2009). Collapse of an avifauna: climate change appears to exacerbate habitat loss and degradation. Diversity and Distributions 15(4), 720-730.

Taylor SG (2008) Leaf litter invertebrate assemblages in box-ironbark forest: composition, size and seasonal variation in biomass. Victorian Naturalist 125(1), 19-27.

Watson DM (2011) A productivity-based explanation for woodland bird declines: poorer soils yield less food. Emu 111(1), 10-18.

Related blogs

• Interact, said the tortoise to the hare
• Growing old in a shrubland: gravity always wins
• Fire and rain: what makes a woodland?

I read of a scheme to manipulate aerosols to reduce positive feedbacks between anthropogenic CO2 levels and positive temporal patterns in climate change.

We scientists know that our wordy sentences get translated in lots of ways. But sometimes there’s an enormous difference between what we think we said and what others believe we said. Believe it or not (according to a new study), the sentence above could easily be interpreted to mean…

I read of a devious plot to illicitly tamper with spray cans to reduce the great response between CO2 levels, attractive foreheads and climate change.

Wow, where did that mess come from? Taken as a whole, the interpretation is incomprehensible. But word by word, the translation could be impeccable. What went wrong? What got lost in translation?

In a great new paper, Richard Somerville and Susan Hassol describe why it’s so difficult to communicate the science of climate change to the general public. It’s an interesting, and highly readable, article. Many of the issues they describe are relevant to many conservation issues, not just to climate change.

Misinformation campaigns and poor reporting certainly make it hard to get climate change messages across. But the authors also place the breakdown in communications at the feet of scientists like me. And since many of my colleagues in environmental management talk the same way I do, this problem must extend beyond research to the broader field of environmental management.

Scientists typically fail to craft simple, clear messages and repeat them often. They commonly overdo the level of detail, and people can have difficulty sorting out what is important. In short, the more you say, the less they hear. And scientists tend to speak in code. We encourage them to speak in plain language and choose their words with care (Somerville & Hassol 2011).

They point out the obvious, of course.

Many words that seem perfectly normal to scientists are incomprehensible jargon to the wider world. And there are usually simpler substitutes. Rather than “anthropogenic,” scientists can say “human-caused.” Instead of “spatial” and “temporal,” they could say “space” and “time.”…. And they shouldn’t expect a lay audience to do mental arithmetic (Somerville & Hassol 2011).

The authors present a neat table in which they compare how scientists and the public interpret many words that we use in our papers and reports.

Table 1. Terms that have different meanings for scientists and the public

(From Somerville & Hassol 2011; http://dx.doi.org/10.1063/PT.3.1296).

A quick glance through the titles of my papers (let alone their contents), reveals a litany of abuses. I’ve only been a short step from crafting titles like, Spatial and temporal impacts of enhanced anthropogenic disturbances. No wonder no one understands me.

The list raises some interesting questions. Should we use these words in our papers, but different words when we speak to a wider audience? Or are some of the words just as useless in our papers? Do I write enhance instead of increase just cause it sounds more posh? Is there any other good reason to do so?

When I did my PhD many years ago, whenever one of my relatives asked – “what are you studying?” – I realized that their eyes glazed over when I described my real project. I worked out that it was better to invent a parallel project that meant something to them. So I’d say I was “working out how to save endangered species”. And they’d say, “that’s wonderful dear!” Delighted by the brevity of my reply as much as its content, no doubt.

My ‘parallel project’ description may have stretched the boundaries of factuality, but at least it communicated something. Somerville and Hassol emphasize the value of simple metaphors and analogies in communicating science.

Consider what your audience cares about…. Try to craft messages that are not only simple but memorable, and repeat them often. Make more effective use of imagery, metaphor, and narrative. In short, be a better storyteller, lead with what you know, and let your passion show (Somerville & Hassol 2011).

It’s a big challenge to write good papers AND to communicate research findings to a wide readership. New researchers face lots of pressure to adopt the latest concepts, frameworks, buzzwords and jargon. This is awesome. This is how science progresses. But our system gives few rewards for translating research findings into ‘simple, memorable narratives’ that don’t rely on the buzzwords. When we strip out the concepts and jargon, what have we got to deliver? It’s not simply a matter of running an ‘edit replace’ through our wordy text. To get our messages across we have to repackage things completely.

Opposite ways of communicating. Somerville & Hassol recommend that we, “start with the bottom line and tell people why they should care.”

Somerville and Haslon provide a fun picture that compares these two ways of communicating. It highlights the need to turn our language inside out, so we can get our messages across better.

There’s an old story (possibly apocryphal), about a survey of what the public thought the word biodiversity meant. One common response was, a laundry detergent. It would be interesting to find out how the ‘general public’ [and I know that’s a condescending word – ‘which public’ I hear you ask?] interprets lots of terms that we use in conservation management. Probably a lot less than we think – not because people aren’t interested, but because we often fall into a fanzine geek-speak that can be impenetrable except to the converted.

I’m sure everyone has experienced this challenge to communicate. How do you describe your work, research or passion to Uncle Jack and Auntie Mabel? Most importantly, how do you get them to say, “That’s wonderful dear, tell me more”?

Further reading

Somerville RCJ & Hassol SJ (2011). Communicating the science of climate change. Physics Today 64(10), p. 48. http://dx.doi.org/10.1063/PT.3.1296. [This link should let you read the whole article on line for free].

Related blogs

• Local and/or general
  
• Interact, said the tortoise to the hare.

To celebrate the festive season and my first year of blogging, I’ve abstained from writing another technical post. Instead, here are some links to some great blogs on ecology and natural history. My searches have been pretty random,  so if you know of other really good sites, please leave a comment below. My blogs range from vaguely academic to casual field observations, so these links follow the same thread…

My favorite ecological blog site is the Oikos Blog – a refreshingly informal blog from a  very formal academic journal. Posts range from really technical to really funny, and often trigger great discussions in the comments section. This is a must read site for ecology post-grad students.

Anyone who is terrorized by the notion of giving a seminar at an academic conference will have their fears put to rest (or perhaps heightened!) by this link from the Oikos Blog on bad conference questions. After reading it, everyone should take time off to elect the biggest “wonk hipster” in their faculty.

The company that hosts my blog, WordPress, boasts that, “403,739 bloggers, 616,918 new posts, 417,719 comments, & 119,701,074 words were posted today on WordPress.com”. No wonder the world’s energy needs are rising. Given this surfeit of self expression, I’m still bemused by how few good blogs there are that present ecological concepts in an engaging way for a broad audience.

From John Morgan's blog on "Ecological divides"

My gong for the best Australian ecology blog goes, of course, to John Morgan for his great site on vegetation ecology. John has too many great posts to select just one, so scroll through them all. Why don’t more ecologists write like this?

Similarly, my 2011 award for the best blog on “how to describe your research project in a really interesting way”, goes to one of John’s students, Brad Farmilo. Lots of ecology students and research groups set up blog sites this year, but Brad’s site makes a great effort at communicating science in a really engaging way. If we want to save biodiversity, we ecologists need to improve our ability to communicate our research findings. John and Brad’s sites are great examples of fantastic science communication.

Moving out of academia, Chris Helzer’s site on prairie restoration is awesome. He mixes great photographs with interesting reflections on the nuts and bolts of prairie management and restoration. I shouldn’t be, but I’m constantly surprised at how similar Chris’s issues are to those faced by grassland and woodland managers in Australia. As an example, Oz grassland aficionados will enjoy the post, A skeptical look at mob grazing.

As an aside, Australia’s premier restoration journal, Ecological Management and Restoration has just launched a new site devoted to restoration and rehabilitation projects across the country. Each project is described in about 500 words, and projects can be searched by topic or ecosystem. This is a great resource for restorationists.

I began the year intending to set up a standard (i.e. boring) research web site, with no thoughts of posting a new blog every month, but the blogs somehow took over. My foray into blogging was triggered by two friends, Bert and Manu, who write really different blogs, about natural history and environmental issues. Check out Bert’s post on discovering Aboriginal stone tools in the Strathbogies, and Manu’s post on the state of science education in Australia.

Through Bert’s site, I discovered the wide world of Nature Blogs. To see how beautiful a blog can be, take a look at Farmhouse Stories. WordPress makes great templates for blogging, but this gorgeous site takes things to another plane. (To see it at its best, view it in a web browser rather than on a RSS reader or an iPad). Where do people find the time to visit beautiful places, take great photos, design awesome web sites, write regular blogs, plus get out and enjoy nature? Thanks to a regular commenter on my posts, Watching Seasons, for the link to this site.

If you follow a few of the links above, you’ll find they don’t have much in common stylistically. Some are really technical, some very chatty, some look pretty ordinary while others look awesome. Apart from a common theme of ecology, natural history and conservation, the one thing all of these sites have in common is a burning desire to communicate.

The only reason that bloggers crouch over a laptop late at night is that nerdish compulsion to have people read their stuff. So a big thank you to all of the subscribers and ‘followers’ who regularly read my posts, and to everyone who took the time to post a comment, I’m most grateful. I’d encourage everyone to subscribe to blogs that you really like, as there’s nothing like knowing that lots of people (or even just a few) want to read your next post, to encourage authors to keep posting good content.

Unfortunately, I have no idea what I’ll write about next year. I don’t have a dozen new papers in press that I can write new stories about, so next year’s posts will have to draw upon sources far beyond our research group, perhaps like the last post on savanna fires. It’ll be interesting to see how the stories evolve. Please leave a comment if there are topics you’d really like to read about.

In closing, I’ll leave you with a beautiful visual blog from the artist, Barbara Bash. I hope you enjoy it. Have a great break over Christmas and a fantastic year in 2012.

Merry Christmas, Ian

“Fire, grass, kangaroos, and human inhabitants, seem all dependent on each other for existence in Australia; for any one of these being wanting, the others could no longer continue. Fire is necessary to burn the grass, and form those open forests, in which we find the large forest-kangaroo…. But for this simple process, the Australian woods had probably contained as thick a jungle as those of New Zealand or America…” (Mitchell 1848).

I don’t care if he was completely right or bizarrely deluded. Mitchell’s genius lay in linking sensitive observations of indigenous people and ecosystem dynamics, at a time when such subtleties were rare. He earns a gong for putting down the first big hypothesis (not the first big fact) about the role of disturbance in Australian ecosystems.

160 years later, many ecologists still wrestle with the question, “to what extent do disturbances such as burning and grazing control vegetation patterns, compared with abiotic factors such as climate and soils?”

We can address this question in many ways. We can develop computer models, and we can run experiments to see how ecosystems respond to different fire regimes. Or, simpler still, we can record the outcomes from ‘natural experiments’, by comparing vegetation patterns between areas with different fire histories.

Abundant Prickly Bursaria in an unburnt Cumberland Plain woodland. Source: http://www.rbgsyd.nsw.gov.au/

Despite their simplicity, natural experiments are surprisingly rare in grassy woodlands in southern Australia. In a good example, Penny Watson showed that Prickly Bursaria (Bursaria spinosa) forms a dense understorey in rarely burnt woodlands on the Cumberland plain near Sydney, but is sparse in frequently burnt woodlands, where grasses dominate. Thus, in woodlands on the Cumberland plain, fire frequency controls the abundance of understorey shrubs, as Mitchell suggested.

Unfortunately, at global scales, we can’t easily run huge fire experiments, so ecologists often use correlative studies to uncover relationships between global patterns of vegetation, rainfall and fire. Correlations can’t tell us what caused the observed patterns, but they can uncover patterns that we were otherwise unaware of. In a recent issue of the journal Science, two great papers use correlative approaches to see how fire and climate interact to control global vegetation patterns.

In one study, Marina Hirota and colleagues used global maps of vegetation and rainfall to understand ecosystem dynamics in tropical Australia, Africa and South America. At the global scale, the major driver of vegetation patterns is rainfall. Before I read the paper, I’d have guessed that, if I drove from a dry region to a wet region, I’d see tree cover steadily increase as the climate got wetter and wetter.

At a general level, the authors found that this does happen. Extremely dry areas (< 300 mm annual rainfall) usually support treeless grasslands while, at the other extreme, dense forests are common in the wettest areas (> 2400 mm rainfall). Nothing surprising there. But in between, something really interesting happens.

Tree cover does increase as rainfall increases, but it doesn’t increase steadily, and some cover classes are extremely rare at the global scale. Thus, tree cover tends to ‘jump’ from some levels to others rather than increase gradually. In eco-speak, the relationship between tree cover and rainfall is not linear, and there are rapid, non-linear transitions in tree cover at certain threshold levels of tree cover.

Forests, savannas and grasslands occur in regions with different rainfall and fire patterns. Surprisingly, intermediate (unstable) cover levels between these three ecosystems are rarely found (Mayer & Khalyani 2011).

As a result, we tend to find three discrete vegetation types: treeless grasslands (with 0-1% cover), open savanna woodlands (5-50% cover) and dense closed forests (>60% cover). But very, very few areas have intermediate levels of tree cover between these three states. The most surprising finding from the study is that so few areas in tropical Africa, Australia and South America have 1-5% tree cover or 50-60% cover.

Why would this be the case?

Clearly, each of the three widespread ecosystems (grasslands, savannahs and forests) must be very stable, as they are so abundant. By contrast, the two rare, intermediate configurations (1-5% cover and 50-60% cover) must be very unstable, otherwise we’d expect to see them much more often. Whenever tree cover approaches 50-60%, some process must act to make tree cover quickly increase (to form a forest) or decrease (to form a savanna woodland).

As our historical hypothesis generator, Thomas Mitchell noted, fire is an obvious candidate. Flammable grasses dominate the ground layer in open savannah woodlands. These grasses carry frequent fires that kill tree seedlings and prevent dense forests from forming. Frequent fires in savannah woodlands maintain open savannah woodlands, and may prevent tree cover from increasing into the unstable 50-60% cover range.

By contrast, dense forests have little grass, so fire cannot easily penetrate forest stands. Gaps that open up when trees die may quickly be filled by new trees. In unburnt forests, rapid gap filling may prevent tree cover from declining into the unstable 50-60% range.

Savanna fires often kill woody plant seedlings and prevent tree encroachment. Source: http://www.abc.net.au/news/stories.

As I described in an earlier blog, this feedback between vegetation and fires drives the formation of ‘alternative stable states’. In their new paper, Hirota and colleagues demonstrate that, at a global scale, feedback mechanisms prevent the formation of unstable, intermediate states, to a degree far greater than was previously known.

The second paper, by Carla Staver and co-workers, addresses a similar question. They also used global data on vegetation, rainfall, soils and fires to tease apart the influence of fire and rainfall on vegetation patterns. At the broad scale, they showed much the same patterns as Hirota and co. They also showed major differences between Australia and the other two continents. In their words, “At intermediate rainfall, where both savanna and forest occurred on other continents, Australia was dominated by savanna.” (Staver et al. 2011, p. 231).

This raises the obvious question, why don’t dense forests occur in ‘intermediate rainfall’ areas (1000-2500 mm) in Australia? Staver found that dense forests need more than just high rainfall. Across all three continents, forests occur where there is more than 1000 mm of rain each year and where this rain falls across most of the year. Dense forests hardly ever occur where the dry season lasts for half a year or more.

Staver and colleagues concluded that a long dry season prevents forest from forming in high rainfall areas in northern Australia. The climate in northern Australia is dominated by the monsoon, which delivers torrential rainfall in a few short months, and hot, dry weather for the rest of the year. The authors suggest that the monsoonal climate, and the long dry seasons it creates, prevents the development of dense forests across northern Australia.

Staver's map of climatic biomes in South America, Africa and Australia (Staver et al. 2011).

The continental differences in rainfall and vegetation patterns can be seen in the above map from Staver’s Science paper. The ‘deterministic forest’ zone (shown in dark green) has mean annual rainfall > 2500 mm. This climate nearly always produces forests. This zone is conspicuously absent from northern Australia.

At the other extreme, the ‘deterministic low tree cover’ zone (pale orange) has low rainfall (< 1000 mm a year) or strongly seasonal rainfall (dry season > 7 months long). Tree cover is nearly always low in this zone. This zone is common on all three continents, but occupies nearly all of northern and central Australia.

The two ‘bistable zones’ have the same rainfall (mildly seasonal, 1000-2500 mm) and currently support either forest (light green) or open savanna (dark orange). The two ‘bistable zones’ are where ‘alternative stable states’ can form. Where fires are frequent, open savannahs occur, and where they are rare, closed forests form.

So what do these two studies tell us about Mitchell’s hypothesis, about the relative importance of climate and fire in controlling vegetation patterns?

At one level, the papers appear to address Mitchell’s hypothesis really well. In some parts of the globe, fire plays a big role in controlling tree cover. In other regions, it’s not at all important. But – and this is a big but – there’s a limit to how far we can push this argument.

Lightning from monsoonal storms lights many fires in northern Australia. Source: http://www.northauschasers.com/forum

Both studies suggest that climate, not fire, prevents the development of closed forests in northern Australia. But they don’t tell us how (or why) climate prevents dense forests from forming in high rainfall, monsoonal areas of northern Australia.

The monsoon delivers many things. It delivers a heap of water in a few, short months. It delivers a long dry period. And it also delivers a lot of lightning. This lightning starts lots of fires, which burn vast areas. Staver and colleagues note that, ‘on all three continents, fire was ubiquitously present in savanna’.

Unfortunately, these two great papers can’t tell us whether there are few dense forests in monsoonal Australia because the monsoon delivers a long dry season or because the monsoon delivers a lot of fires or both factors. And the monsoon does deliver a lot of fires. We often hear that south-east Australia and California are among ‘the most fire prone’ areas in the world. Both areas have very severe fires, but the extent and frequency of fires in these areas are trivial compared to fires in northern Australia. The video below from NASA demonstrates the ubiquity of fires in monsoonal northern Australia. It’s an extraordinary show.

Over 160 years ago, Thomas Mitchell raised a big question that continues to tax the brains of some of the world’s best ecologists – how important are disturbances such as fire and grazing in controlling global vegetation patterns? Nowadays, satellite images and videos provide a global view of vegetation, climate and fire. But we still haven’t nailed the issue. Will ecologists still try to answer it in the future? Who knows. And who knows what amazing technology they’ll have to help them in another 160 years time.

Related blogs

• Dancing with flames: Callitris, fire and Patrick Swayze
• Fire and rain #2: water for ironbarks

References

Hirota M, Holmgren M, Van Nes EH & Scheffer M (2011). Global resilience of tropical forest and savanna to critical transitions. Science 334, 232-235.

Mayer AL & Khalyani AH (2011). Grass trumps trees with fire. Science 334, 188-189.

Mitchell TL (1848). Journal of an expedition into the interior of tropical Australia. In search of a route from Sydney to the Gulf of Carpentaria. (Link to full text ebook)

Staver AC, Archibald S & Levin SA (2011). The global extent and determinants of savanna and forest as alternative biome states. Science 334, 230-232.

Watson PJ, Bradstock RA & Morris EC (2009). Fire frequency influences composition and structure of the shrub layer in an Australian subcoastal temperate grassy woodland. Austral Ecology 34, 218–232.

Everyone at the field day will receive a handout that summarises the key findings from the study. It’ll be a long while before the scientific papers from the study get finished and published. In the meantime, we hope you enjoy the field day summary below. Please contact us if you would like more information on the project or results.

Restoring Woodland Understoreys Project Summary, November 2011

Fig 1. A healthy, forb-rich native understorey in box gum woodlands near Cowra, NSW.

Box-gum woodlands, dominated by White Box (Eucalyptus albens), Yellow Box (E. melliodora) and Red Gum (E. blakelyi) are one of Australia’s most en­dang­ered ecosystems. They once formed park-like landscapes with scattered trees and a wide array of native grasses and forbs in the understorey (Fig. 1). These ecosystems have been cleared for agriculture, and most remnants are small, degraded and weed-invaded.

In the past 20 years there has been a ground­swell of activity to restore these ecosystems and their landscapes. One of the biggest challenges faced by on-ground projects is to find practical ways to improve the quality of degraded understoreys.

This summary describes the key findings of studies undertaken by CSIRO Ecosystem Sciences and Charles Sturt University, that have focused on restoring the native grassy sward in Box-Gum woodlands.

Field experiments

Fig. 2. Experimental fencing for pulse grazing experiment at Bakes TSR near Albury.

We established field experiments in a number of remnants on the south-west slopes of NSW, to trial different options for encouraging native grasses and reducing the most prominent weeds, particularly exotic annuals such as Wild oats (Avena spp.) and Paterson’s Curse (Echium plantagineum)  (Fig. 2). Building on earlier research, we used pulse grazing and spring burning to prevent annual weeds from seeding, and added carbohydrate (sugar) to reduce avail­able soil nutrients and weed growth. We sowed native grasses to improve native grass cover and increase long term resistance to weed invasions.

Key findings

Success may not be immediate.

Confirming what land managers already know, we found that climatic variability can over-ride management inputs. Thus, man­age­ment activities will have better outcomes in some years than others. There is also a high level of variation in the abundance of different exotic annuals during wetter com­par­ed with drier cycles, with exotic annual grasses more common in wetter periods. Native C4 grasses such as Bothriochloa macra and Themeda triandra es­tab­lish part­icularly well following wet summers.

Exotic annuals can be suppressed by managing nutrients.

Fig. 3. Sugar application (right) dramatically reduced exotic annuals which dominated untreated areas (left) (Bakes TSR).

A very reliable method for suppressing exotic annuals is to add sugar to the soil surface, from the time the exotic annuals are establishing (Figs 3&4). Sugar feeds soil micro-organisms, which in turn use up soil nutrients and limit the growth of the nitrogen-loving exotic annuals.

Sugar only offers a short-term solution, as each application lasts about three months. It is best used to provide a window-of-oppor­tun­ity for establishing native species, possibly in small nodes across a site to min­im­ize costs. We applied about 0.5 kg/m² of sugar about 3 times per year (May, August, November). Cheaper alternatives like molass­es may also be effective. Sugar should never be applied near waterways.

Grazing and burning can suppress some (but not all) exotic annuals.

Fig. 4. Cumulative treatment effects on exotic annuals at Cumberoona TSR. All treatments controlled annuals better than doing nothing. Note there were few broadleaf exotic annuals in 2011.

Burning and grazing can be used to limit seed production of exotic annuals, alter seed-bed conditions and renew the vigour of native grasses. Exotic annual grasses such as Wild Oats (Avena spp.) can reliably be suppressed over small areas by burning in spring (prior to the weeds’ seeding) with a weed burner (Fig. 4). This method does not control broad-leaf exotic annuals such as Paterson’s Curse (Echium plantag-ineum), that have longer-lived seed banks and germinate well on bare ground.

Fig. 5. Sheep grazing plots at Cumberoona TSR, near Albury.

For larger areas, one to two weeks of intense grazing by livestock (Fig. 5) each year in spring (‘pulse grazing’) may achieve a similar result. In our experiment it took three years to obtain a detectable effect from pulse grazing on exotic annual grasses (Fig. 4). As for burning, pulse grazing is less effective for controlling broadleaf annuals. We hope to continue to apply grazing treatments in our experiments to evaluate the longer-term reliability of grazing strategies.

Anything is better than doing nothing for extended periods.

Fig. 6. The dominant native grass, Austrostipa bigeniculata, increased after drought and release from heavy grazing, then declined (shown in green) if no treatments were applied (Cumberoona TSR).

Notably, all of the management options des­cribed above led to better outcomes by 2011 than doing nothing (Figs. 4, 6). In particular, in sites dominated by Tall spear grass (Austrostipa bigeniculata), the native grass sward tended to die back and become swamped by weeds when untreated for several years, whereas burning, grazing and/or sugar allowed it to remain vigorous. We hope to continue monitoring the untreat­ed swards to see if they recover naturally after these die-back events.

While all three treatments had positive out­comes, the reliability and effectiveness of each treatment for controlling exotic annuals and promoting native grasses varied, as shown in the table below.

Controlling exotic annuals can improve establishment of native grasses.

Our results confirmed a common observ­ation that native grass establishment is erratic and likely to fail in dry seasons. None­the­less, establishment was often more successful in treated plots where exotic annuals were suppressed.

Fig. 7. Sown sward of Kangaroo grass at Bakes TSR

Establishment success also varies between species. For seed sown on the surface, we had greatest success with C4 grasses, particularly Kangaroo grass (Themeda australis, Fig. 7), the likely original domin­ant of these woodlands, and the disturbance-increaser, Red grass (Bothrio­chloa macra). Over the period of our studies, which were mostly dry years, we recorded minimal estab­lishment of the C3 grasses, Snow Tussock (Poa sieberiana) and Wallaby grasses (Austro­danthonia spp.), despite providing ample seed.

Native grasses are themselves a restoration tool.

Fig. 8. Reduction in exotic annuals resulting from sowing different native grass species into swards initially dominated by Tall spear grass.

Our studies have shown that native grasses help to make degraded woodlands function more naturally, and that this is ultimately the key to sustainable restoration of box-gum wood­lands. In particular, native grasses help to maintain low soil nutrient availability and can strongly suppress exotic annuals over the longer term. This in turn should benefit re-establishment of a wider diversity of wood­land plants.

Kangaroo grass (Themeda australis) is the most effective species for reducing soil available nutrients and minimizing growth of exotic annuals (Fig. 8). Other native grasses such as Tall spear grass (Austrostipa bigeniculata) and Red grass (Bothriochloa macra) provide similar but weaker benefits. These species are more tolerant of livestock grazing than Kangaroo grass, so may be more appropriate in restoration areas that undergo more prolonged grazing. Figure 6 also emphasizes that it is important to maintain the vigour of the native swards. This can be achieved through pulse disturb­ances such as fire or grazing every few years.

Three key approaches to restoring weed-invaded woodland understoreys are to:

• Control nutrients to discourage weeds.
• Use pulse grazing or burning in spring every few years to maintain a healthy native sward and suppress exotic annual grasses.
• Re-introduce other native grasses, particularly Kangaroo grass, to suppress nutrients and weeds.

By Suzanne Prober, Ian Lunt & Ian Cole, November 2011

Acknowledgements

We gratefully acknowledge support for our research from the NSW Government through its Environment Trust, the Murray CMA, CRC for Future Farm Industries, Sugar Australia, Hume LPHA and local graziers. Neil Padbury and many helpers provided invaluable field assistance.

If you would like more information about this project, please contact:

• Dr Suzanne Prober, Suzanne.Prober [at] csiro.au
• Assoc. Professor Ian Lunt, ilunt [at] csu.edu.au
• Mr Ian Cole, Ian.Cole [at] environment.nsw.gov.au

Related blogs

• Woodlands on TV!
• From Weeds to Wildfire: Restoring Woodland Understoreys #3
• Sugar Stunts Weeds: Restoring Woodland Understoreys #2
• Restoring Woodland Understoreys #1

You can also download a pdf file of the information in this post.

But she did more than that. She left a legacy that allowed others to answer, not just her old questions, but new questions, by building on her data.

She banged a stake into the ground. In fact she banged lots of them in the ground, one picket in the corner of each of her plots.

Forty years later, the student no longer does research, her papers aren’t read that often, and her questions aren’t that popular any more. But the stakes are still there. And they show us things we couldn’t otherwise see. That’s an amazing legacy for a student who abandoned research at the end of her PhD.

The Withers Pickets. The fence was erected in an open gap 40 years ago. Now its covered in dense vegetation.

The student’s name was Jennifer Withers. Dr Withers did a series of fantastic experiments in the laboratory, glasshouse and field, to try to understand why some species were declining and others becoming dominant in an unburnt woodland. In the language of today’s ecology, she attempted to link plant traits and ecosystem assembly. In the language of her era, she investigated how the ecophysiology of dominant trees influenced vegetation succession. And fortunately, she left some stakes, fences and plastic tags that marked the seedlings she planted, many of which still survive today.

Dense closed forest dominated by Black She-oak at the Ocean Grove Nature Reserve. This area was much more open when Jennifer Withers surveyed it forty years ago.

When Dr Withers set up her plots in the early 1970s, the Ocean Grove Nature Reserve contained an open grassy woodland that was beginning to be overtaken by dense shrubs and small trees, especially Golden Wattle (Acacia pycnantha) and two Allocasuarina species, Black She-oak (Allocasuarina littoralis) and Drooping She-oak (A. verticillata). At that time, many open gaps remained, but it seemed inevitable that the woodland would continue to thicken up if it remained unburnt.

Plastic tag at the base of a Drooping She-oak that was planted over 40 years ago. The tree is still less than 3 m tall.

Jennifer and her supervisor, David Ashton, did lots of experiments to work out why some species were regenerating while others didn’t. In the glasshouse, they measured how well seedlings grew in pots, some of which they left shaded or unshaded, while others were watered and left dry. In the field, they put seeds in trays and counted how quickly ants ate the seeds of different species. They sowed seeds and seedlings into plots that they burnt, to see if some species needed fire to promote their establishment. Plastic plant tags must have been a lot tougher in the 1970s than they are nowadays, because many of the tags they used still survive today.

I first visited the plots in the mid-1990s, 25 years after they were first surveyed. It was fascinating to see how the vegetation had changed in the interim. The density of trees had almost doubled and the large open gaps had almost completely disappeared. In 1996, there were over 5,600 trees and saplings per hectare, which is extremely dense! Surprisingly, many of the eucalypt seedlings that Dr Withers planted in her burnt plots were still alive 25 years later, even though they were less than knee-high and spindly, suppressed by tall she-oaks and wattles. Suppressed eucalypt seedlings just seem to hang in there forever.

Many old trees died during the recent drought. This dead She-oak created a large open gap when it died.

Last weekend I re-visited the plots for the first time since 1996 – the first time in 15 years. It inspires a certain sense of awe to sit in a small fenced plot where someone counted plants 40 years ago, sowing seeds that would stimulate even more ecological inquiries 40 years later.

In the last decade we’ve had a long, severe drought, and there are many more dead trees now than there were in the 1990s. In the 1990s, hardly any She-oaks were dead (only about 1%), but many have since died. There don’t seem to be as many young saplings now either, whereas saplings were really common 15 years ago. These are impressionistic observations from a short visit, and I wouldn’t put much stock by them at this stage.

Fortunately, courtesy of the Withers Pickets, we can easily re-measure the plots, and see how one of the worst droughts of the last century has influenced the long-term process of vegetation encroachment (or ‘thickening’). In other regions, severe droughts have completely reversed the process of thickening that occurred during wet decades. The changes at Ocean Grove are not that dramatic, but it shall be interesting to see how many trees died during the drought , and how the relative abundance of different species has changed. Was tree mortality density dependent, with more deaths in areas that had more plants initially?

Pick your technology. The GPS is cool, but the picket will outlive it.

Quantitative ecology is a young profession. In Australia we have precious few data sets that extend longer than 40 years. The Ocean Grove plots are an important scientific resource for documenting how Australian ecosystems have changed over many decades. This legacy is solely due to a student who vanished from the research world soon after she finished her PhD.

Our new ways of storing information aren’t built to last. To view the data that I collected 15 years ago, I now need to find a computer that will read my old floppy discs! By contrast, a stake in the ground is immutable.

The Withers Pickets gave me an opportunity to act as a link in a longer chain, updating Withers and Ashton’s work for another generation of ecologists. The stakes I leave will let others answer new questions in the future.

To the class of 2011: Leave a legacy. If you seek immortality, hammer a stake through your plots. It’ll probably outlive everything else that seems important right now.

Related blogs

• Growing old in a shrubland: gravity always wins

Further reading

Lunt ID (1998a) Two hundred years of land use and vegetation change in a remnant coastal woodland in southern Australia. Australian Journal of Botany, 46, 629-647.

Lunt ID (1998b) Allocasuarina (Casuarinaceae) invasion of an unburnt coastal woodland at Ocean Grove, Victoria: structural changes 1971-1996. Australian Journal of Botany, 46, 649-656.

Withers JR & Ashton DH (1977) Studies on the status of unburnt Eucalyptus woodland at Ocean Grove, Victoria. I. The structure and regeneration. Australian Journal of Botany, 25, 623-637.

Another curious aspect of this ‘waves of migration’ view is that it implies that species will respond to climate change in isolation. Thus, ten different species might all move uphill as the world gets warmer, without interacting with each other or with other species. But in nature, interactions rule. All populations are influenced by many factors other than climate, including other species and ecological disturbances.

We get a different perspective on how global warming will affect natural ecosystems if we change our view point from the regional to the local.

Picture a reserve that you know well, and imagine how the vegetation might change as global warming progresses.

At this scale, we’re more likely to perceive changes as being much more chaotic. New species may appear and others may disappear for short or long periods. As one species declines another will become more abundant and take its place.

At this scale, we’re unlikely to think about species moving independently in waves. Instead, we’re might view populations as being buffeted around like balls in a pinball machine, with numbers increasing and decreasing as each species responds to changes in the abundance of other species and changing disturbances. For example, a relatively rare species might increase in abundance – even if it is less suited to the new climate – if a dominant species declines and more resources become available for the rare species.

As the vegetation and climate continue to change, disturbances such as fire regimes and insect herbivory may also change in unpredictable ways. Days of extreme fire danger are expected to become more common in south-east Australia. But this doesn’t necessarily mean that there will be more severe fires in the distant future. Fires don’t just need hot windy days, they also need dry fine fuels. As the climate changes, vegetation will change, which will alter fuel levels, which in turn will create new fire regimes. In some places, we may get more extreme fire weather, but less fuels to burn, as Ross Bradstock describes in a great review.

Interactions among species, and between species and disturbance regimes, mean that we can’t just focus on one species at a time when we try to predict how ecosystems will change. We also need to understand how species interact, and how they affect each other.

The Name’s Bond – William Bond

Many years ago, a prominent South African ecologist, William Bond, developed a theory to explain one of the biggest plant distribution patterns across the globe. Why are gymnosperms (conifers and cycads) abundant in some regions, and angiosperms (flowering plants) more abundant in others? Bond’s theory was based on differences in plant physiology between angiosperms and gymnosperms, and on interactions: interactions between species (gymnosperms versus angiosperms), and interactions between plants and disturbances, such as grazing and burning.

The other Mr Bond enjoying a spot of field work. The ecologist is much less photogenic.

Bond noted that seedlings of most gymnosperm species grow more slowly than angiosperm seedlings. He argued that gymnosperms were most abundant in stressed, unproductive ecosystems, such as cold and dry regions, where they didn’t have to deal with competition from vigorously growing angiosperms. In productive ecosystems, Bond argued, competition from vigorous angiosperms would either kill gymnosperm seedlings directly, or would cause gymnosperm seedlings to be suppressed. Small, suppressed gymnosperm seedlings would then be vulnerable to disturbances such as grazing and burning. In some cases, gymnosperms would face further challenges if angiosperms promoted disturbances that were fatal to gymnosperm seedlings. For example, dense grasses promote frequent fires that will kill fire-sensitive gymnosperm seedlings.

Bond’s model is known as the ‘slow seedling’ or the ‘tortoise and hare’ hypothesis. It has spawned papers with more imaginative titles, such as The Hare, The Tortoise and The Crocodile – in which tall ferns (aka the ‘crocodiles’) influence whether fast-growing angiosperms (‘hares’) win the race against slow-growing, tortoise-like, gymnosperms. Who said that scientists lose their child-like sense of imagination?

Bond’s theory illustrates why we need to consider species interactions if we want to understand how climate change will affect plant distributions. According to this theory, the distribution and abundance of gymnosperms is influenced not just by climate, but also by soils, competition from angiosperms, and by disturbance regimes which themselves are influenced by angiosperms. This is the type of complex interaction between species and disturbance regimes that makes it so difficult to predict how global change will affect natural ecosystems.

Bringing it all back home: Callitris vs Eucalyptus

In 2003, during a severe drought, wildfires burnt over a million hectares in south-east Australia. One high intensity crown fire burnt over 10,000 hectares at Mt Pilot in north-east Victoria. As described in an earlier blog, Mt Pilot supports large stands of the native gymnosperm, Black Cypress-pine (Callitris endlicheri). Callitris stands are usually co-dominated by eucalypts (angiosperms) including Red Stringybark (Eucalyptus macrorhyncha), Long-leaved Box (E. goniocalyx) and Red Box (E. polyanthemos). The high intensity fire killed virtually all Callitris trees. After the fire, Callitris regenerated from seedlings and the eucalypts regenerated from seedlings and basal resprouts.

Learning about vegetation ecology at Mt Pilot

We just published a paper in which we compared the regeneration patterns at Mt Pilot against Bond’s ‘tortoise and hare’ theory of interactions between gymnosperms and angiosperms. When I say ‘we’, I am not just indebted to my two co-authors, Heidi Zimmer and David Cheal, but to over 400 enthusiastic students who measured seedlings and coppice regeneration as part of a 3rd year undergraduate ecology subject from 2004 to 2011.

What did we expect to see? Based on Bond’s theory, we thought that eucalypt (angiosperm) seedlings would grow faster than Callitris (gymnosperm) seedlings. We predicted that Callitris seedlings would grow more slowly, and would die more often, if they grew with lots of eucalypt seedlings, due to competition from the eucalypts. We also expected that small, slow-growing Callitris seedlings would suffer more damage by browsing animals which would lead to even more deaths. And, taking things one step further, we expected that fast-growing Callitris seedlings that grew in areas with few eucalypt seedlings would set seed cones earlier than slow growing seedlings in areas with many eucalypt seedlings. In short, we expected eucalypt seedlings would give Callitris seedlings a really hard time. By contrast, we thought that Callitris seedlings would have very little effect on the growth and survival of eucalypt seedlings.

Dense eucalypt saplings above small Callitris seedlings, 6 years after the fire. This area was dominated by Callitris, not eucalypts, before the fire.

Perhaps surprisingly (since, in my experience, good field data always complicates great theories), the patterns that the students recorded were really similar to our predictions from Bond’s theory. Eucalyptus seedlings did grow much faster than Callitris seedlings. In 2010, when they were 7 years old, the tallest eucalypt seedlings were 8 meters tall, whereas the tallest Callitris seedlings were just 3.5 meters tall. Callitris seedlings were eaten more often than eucalypt seedlings by animals, and Callitris seedlings grew taller in fenced plots that kept out browsing animals. Furthermore, Callitris seedlings grew faster and produced more seed cones where there were fewer eucalypt seedlings. For eucalypts, the opposite occurred; eucalypt seedlings actually grew faster, not slower, in plots that had more Callitris. All of which is entirely compatible with Bond’s theory.

Tall Callitris seedlings in an open area.

As an aside, I should point out that areas with dense eucalypt seedlings weren’t innately less suitable for Callitris, as many of these areas were dominated by tall Callitris before the fire.

However, one important pattern didn’t turn out as we expected. Bond’s theory states that slow-growing gymnosperm seedlings should die off when faced with competition from vigorous angiosperms. However, hardly any Callitris seedlings died during the seven year period, whereas over half of the eucalypt seedlings died. Virtually all of the Callitris seedlings managed to survive, which is quite extraordinary, as many were browsed by wallabies and rabbits .

Seed cones on a tall Callitris sapling eight years after the fire.

This surprisingly high survival rate may be because of an amazing quirk of Callitris ecology. Recent studies have found that Callitris species are among the most drought tolerant trees on earth. One consequence of this behaviour is that dense stands of regenerating Callitris self-thin very slowly. Instead, plants can persist in dense stands for many decades.

In an unburnt area at Mt Pilot, 50 Callitris trees that are thought to be over 100 years old are packed together in an area of just ten by ten meters. This is a really high density of old trees. A single 100-year old tree growing in the open would occupy more space than this. These old trees have grown extremely slowly, and some have trunks thinner than my wrist.

The outstanding ability of suppressed Callitris to persist in dense stands for lengthy periods may be an important factor that enables small Callitris seedlings to hang on beneath dense eucalypt saplings. It shall be fascinating to see what happens to these small seedlings in the future; will they slowly die out, or will the fairy tale ‘tortoises’ slowly overtake the fast-growing eucalypts?

Suppressed old Callitris in an unburnt area. Growth rates can be very slow in dense stands.

The fact that hardly any Callitris seedlings died in 7 years is surprising, but it doesn’t invalidate Bond’s model. Bond predicted that angiosperms would triumph over gymnosperms in productive ecosystems by two mechanisms. The first is by out-competing gymnosperm seedlings and causing them to die. This certainly didn’t happen at Mt Pilot. The second mechanism is by suppressing the growth of gymnosperm seedlings so that gymnosperms remain vulnerable to other threats, such as browsing and burning. And this does appear to happen.

At Mt Pilot, the size of Callitris seedlings is influenced by how dense the eucalypt seedlings are. In areas with few eucalypt seedlings, Callitris seedlings grow relatively fast, and many of the tallest seedlings have already produced seed cones. By contrast, in areas with dense, tall eucalypt seedlings, Callitris seedlings are growing much more slowly, and none have produced seed cones yet. These small seedlings will remain highly susceptible to future fires.

If the area remains undisturbed, then the fairy tale ‘tortoises’ have a chance of winning, or at least reaching a draw, in the race. But if the area is burnt by another high intensity fire in the near future, then the odds flip to the hares. And the more hares there are, the greater the odds turn in their favor.

Back To The Future

The students’ field work at Mt Pilot illustrates the value of long-term, repeated observations. Long-term observations don’t need to be complicated. Everyone can record how their local ecosystem changes over time using simple techniques, such as recording the date that flowers first bloom each spring and by taking repeat photographs from the same place. Long term observations will be immensely valuable to help us understand how climate change affects natural ecosystems.

The study also illustrates how important it is to study interactions between species rather than focusing on single species in isolation. At Mt Pilot, long-term changes in the abundance of Callitris will be influenced by changes in climate, and by how climate change affects disturbance regimes, especially fire. But Callitris will also be influenced by other species, especially the dominant eucalypts, and by how these species also respond to climate change and future disturbances. Changes in the abundance of these two dominant trees will flow on to affect many more species of plants and animals.

I won’t be around to see what Mt Pilot looks like when the effects of climate change really kick in. But with 400 students we’ve left a small legacy that will help others to better understand how species interactions contribute to the ecosystems of the future.

Will the tortoises win? I won’t place any bets. We’ll watch and see.

Related blogs

• Reading the bush: juxtapositions in history
• Dancing with flames: Callitris, fire and Patrick Swayze

References

Bond WJ (1989). The tortoise and the hare: ecology of angiosperm dominance and gymnosperm persistence. Biological Journal of the Linnean Society 36, 227–249.

Lunt ID, Zimmer HC & Cheal DC (2001). The tortoise and the hare? Post-fire regeneration in mixed Eucalyptus–Callitris forest. Australian Journal of Botany 59, 575–581. [This link is to an Open Access paper which can be downloaded for free].