It’s sunny, it’s Sunday, and I’m writing a blog. How sad is that? Fortunately it’s not too bad as I’ve spent a couple of weeks in the field soaking up vitamin D. In lieu of an armchair ecology blog, I thought I’d share some natural history observations from last week’s fieldwork…
Ecologists often focus on the incredibly important process of recruitment. But it’s equally important to understand the slower processes of plant senescence and mortality. What happens when dominant plants get old? Do they re-sprout continually and maintain their dominance? Or do they die and create big, open gaps? If gaps are created, do new seedlings establish, or do other species take over?
For the past two weeks, Janet Cohn, Gill Earl and I have been surveying areas in central Victoria dominated by the native shrub, Drooping Cassinia or Chinese Scrub (Cassinia arcuata). Despite its common name, Cassinia is a native plant, indigenous to the region. Apparently it’s called Chinese Scrub as it colonized piles of bare soil (‘mullock heaps’) that were dug over by Chinese miners in the gold rushes of the 1860s. This highlights a key attribute of the species; Cassinia produces many small, wind-blown seeds, and seedlings can establish at high density in bare, disturbed soils.
Cassinia has colonized 1000s of hectares in central Victoria over the past 40 years, as land use has changed from traditional grazing and cropping to ‘amenity landscapes’, dominated by hobby farms, retirement properties and bush blocks. So there are lots of good reasons to know how these shrublands will change over time. Will the shrubs stay dense or gradually thin out, or will stands undergo succession to another vegetation type?
We spent a lot of time measuring gaps and counting seedlings to try to understand what controls whether gaps rapidly fill up with new plants. It’ll take a lot of number crunching to answer that question (that’s code for – it’s really complex!) but in the meantime, let’s look at how gaps are created.
Gap dynamics (or the ‘dynamics of nothingness’) is a fascinating topic.
Gaps can exist for two reasons. They might never have been filled by plants before. This occurs in young stands that are gradually filling up. Initial colonizing plants are scattered, and new plants then fill the spaces between the initial colonists. Alternatively, gaps can be created when old plants die. The abundance of gaps is dictated by how often plants die, and how long it takes for new plants to establish and re-fill the empty spaces. The spatial arrangement of births and deaths influences whether we get lots of small gaps or fewer big ones.
The British ecologist, Alexander (A.S.) Watt instigated the field of patch dynamics over 60 years ago in his classic paper, ‘Pattern and process in the plant community’. His great insight was to view a plant community as a dynamic or shifting mosaic of patches, or ‘phases’ as he called them. Each patch is dominated by a dominant plant at a different phase in its life cycle. Over time each patch progresses through successive phases as the dominant plants grow older and change their morphology (or shape).
Watt recognized four phases: building, mature, degenerate and hollow. Later ecologists often called the ‘hollow’ phase, the ‘pioneer’ phase, as new pioneer plants can establish in the phase.
In the building phase, vigorous, young plants grow upward. As they grow older, each plant spreads laterally, and by the degenerate phase plants begin to die out in the middle. This then forms a ‘hollow’ in the center, which other species can colonize.
Among many examples across the world, Dick Williams and David Ashton used Watt’s model to interpret the dynamics of grassland and shrubland patches in the Australian alps in the 1980s.
It’s fun to look at Cassinia dynamics through Watt’s classic framework. In the building phase, young Cassinia shrubs grow erect, their vigorous crowns covered with small, bright green leaves. As they mature, they become larger (obviously), but unlike Watt’s low sprawling shrubs, they grow upright from one or more dominant stems.
Cassinia wood isn’t very strong, and in the degenerate phase, old plants bend and sag under the weight of the heavy trunk. As Thom Yorke sang in Fake Plastic Trees, ‘gravity always wins’. This sagging causes a characteristic twist at the base of the trunk.
A key factor in Watt’s cyclic theory of pattern and process is whether old plants resprout from the base. Plant ecologists differentiate between ‘resprouters’ and ‘obligate seeders’. Seeders can regenerate from seed, but not from resprouting stems or roots. If old shrubs can resprout from the base, then degenerate plants might ‘rejuvenate’, and maintain their dominance in each patch.
Cassinia can’t do this. It looks like the buds in the lower trunk die as plants get old, as we never saw shrubs resprout from the lower trunk. However, Cassinia does resprout from buds higher up on the plant. As degenerate plants droop and sag, new shoots grow from these buds to form upright ‘secondary branches’. This secondary growth gives the old sagging plants a new upright crown, for a period at least.
As in Watt’s model, gaps begin to form before the dominant shrubs die completely, as old degenerate plants sag, twist and lie on the ground. The canopy of old plants may lie a meter or more from where the trunk is rooted in the ground.
It’s interesting to speculate on how gaps created by sagging degenerate plants might differ from gaps created after all shrubs die. When degenerate plants bend and sag, they create gaps in the canopy, but their roots survive below the ground, where they might compete against new seedlings. Perhaps this might help to prevent recruitment and keep these gaps open for a period?
Since gravity and death are both insurmountable, all plants ultimately die. This is where things get really interesting. Cassinia is well known as a ‘weedy’ colonizing shrub that can regenerate rapidly on bare, disturbed soil. But many gaps don’t contain bare, disturbed soil. Instead, they contain other plants, especially grasses and sedges, and when these are absent the soil surface is often covered by a thin crust of lichens and mosses – the ‘biological soil crust’.
So a key question is, how fast can Cassinia re-colonize gaps that don’t contain disturbed soil? It’ll take us a while to answer this question. What was obvious in the field though, was that some large gaps have been open for many, many years. Perhaps they’ll be colonized by young plants over time. If so, then a ‘shifting mosaic’ will form, as gaps come and go in different places at different times. Alternatively, perhaps some of these stands will gradually open out, and Cassinia may gradually decline in importance. I suspect that this will depend on whether the stands are disturbed or grazed heavily by feral, native and domestic animals. What a great topic for an experimental study using grazing exclosures.
But what about other successional changes? Do other species colonize Cassinia stands over time? Interestingly, another short-lived native tree did colonize many gaps: Golden Wattle (Acacia pycnantha). Many Acacia species have ‘hard’ seeds, which only germinate after fires, but Golden Wattle produces many ‘soft’ seeds that can germinate without fire.
In many gaps, Golden Wattle seedlings were much more common than Cassinia seedlings. Thus, its likely that Golden Wattle will dominate or co-dominate these areas in the future. This may or may not represent a ‘successional’ change. Some stands contained many dead old Golden Wattle trees that died in the recent drought. The new crop of seedlings may have been triggered by heavy, drought-breaking rains in the last year or two. The long-term dynamics of Acacia pycnantha stands would make a fascinating project. Does Golden Wattle regenerate abundantly in wet periods, and die off en masse in severe droughts? Are population dynamics driven by long-term climatic (ENSO) cycles? So many questions.
Regardless of its role in successional dynamics, Golden Wattle may play an important role in altering soils in regrowth shrublands. Like all Acacia species (and Pea species in general), Golden Wattles fix nitrogen, so soil nutrient reserves may increase when wattles are abundant.
One of the fascinations of ecology is that many questions can only be answered by using a combination of scientific approaches. We need surveys to see what’s going on and to document large-scale patterns. But surveys raise questions that surveys just can’t answer. We then do experiments to answer these questions, by intentionally manipulating one part of the system to see how others parts respond. We can also use modeling to help us better understand how the system works as a whole, and to investigate how it responds to interventions over long time periods.
Unfashionable as it may be, we always need a solid knowledge of the basic ecology of dominant species. Among other things, if we want to know how regenerating landscapes will change over time, we need to know what happens when shrubs get old.
In the end, the future’s driven by senescence and mortality.
Watt, A.S. (1947) Pattern and process in the plant community. Journal of Ecology 35, 1-22.
Watt, A.S. (1955). Bracken versus heather, a study in plant sociology. Journal of Ecology 43, 490-506.
Williams, R.J. and Ashton, D.H. (1988). Cyclical patterns of regeneration in subalpine heathland communities on the Bogong High Plains. Victoria. Australian Journal of Botany 36, 605–619.