How wrong can I be? I blithely predicted that last week’s quiz was far simpler than the previous one, but few readers guessed the correct answer. To re-cap, the question was: which species colonized the eastern side of Little Wallenjoe Swamp?
The chart shows the votes, as they stood last night. The People’s Choice was the dryland shrub, Drooping Cassinia (Cassinia arcuata). It’s a great choice but unfortunately was the wrong answer. Cassinia has colonized many areas [see blogs 1 & 2] including intermittent wetlands (at least temporarily). But it doesn’t like wet feet and quickly dies when flooded. The photo below shows a stand of drowned Cassinia in the nearby Doctor’s Swamp. Cassinia colonized parts of Doctor’s Swamp during the drought and died when that wetland re-flooded in 2010.
The runner-up was the wetland shrub, Tangled Lignum (Muehlenbeckia florulenta). This is a great choice and is actually the species that I expected to see, before I visited the site. Like many voters, I was dead wrong; the culprit wasn’t Lignum.
The next most popular choices were wetland reeds, rushes and sedges. The photographic evidence isn’t conclusive, but the patterns suggest to me that the culprit is unlikely to be a ground plant like a rush, reed or sedge.
Have another look at the air photo above. The colonizing plants have a very even texture, and there are no obvious tracks or waterways between the plants. By contrast, the photo below is from another nearby wetland (Two Tree Swamp) which is dominated by the wetland grass, Cane-grass (Eragrostis infecunda) and Common Spike-sedge (Eleocharis acuta). I’ve manipulated the colors to accentuate the patterns (you can enlarge the photo by clicking on it and can view the original air photo here).
A striking feature of the Cane-grass Swamp is the network of tracks created by grazing animals. Track networks are often obvious in old stands of shrubs and ground plants. [This photo from an earlier blog shows a similar pattern in an old, dryland stand of Drooping Cassinia]. When studying air photos, I often use animal tracks as a spotting character for old shrub stands, as new stands usually have few track networks.
The complete absence of animal tracks and waterways on our air photo suggests that the regrowth may be very recent or – more likely – may be very tall. Tall plants would obscure any tracks below the canopy. This line of evidence is suggestive rather than conclusive, but is one of many tricks of the trade in air photo interpretation. One more clue: as some of you noted, the culprit doesn’t have a very wide canopy. The regrowth has a fine-textured pattern, which suggests that each plant has a small crown.
Who Done It?
My crude profiling suggests our culprit is likely to be a moderately tall, small canopied plant, capable of regenerating very densely, and spreading over a large area.
As well as colonizing Little Wallenjoe Swamp, the same species has formed dense stands in other wetlands in the region, including Doctor’s Swamp (see earlier blog) and the nearby (Big) Wallenjoe Swamp. In both Wallenjoe wetlands, 1000s and 1000s of plants regenerated in the 1970s. They’re now about 40 years old. They’re much taller than I am, but many aren’t much wider than I am. They’re thin for a reason; they’re so crowded that they don’t have any room to spread. The dense patches look like this…
The colonizer is River Red Gum (Eucalyptus camaldulensis). Before the colonization event, both Wallenjoe wetlands contained old, wide-canopied trees. Remember the glorious old trees in the recent blog on old woodland trees? Like those below? The old red gums were photographed at Wallenjoe Swamp.
Now they don’t form a woodland. Instead, they’re surrounded by a thicket of dense regrowth. Everything on the right side of the wetland below is River Red Gum. That’s a lot of young trees, isn’t it? They’ve thinned out a bit over 40 years, but are still extremely dense in many areas.
Congratulations to those of you who guessed the right answer. You did better than I did. Like many readers, I expected to see dense Lignum when I first visited the area. I got quite a surprise when I found dense red gums instead.
It’s interesting that so few people voted for any of the tree species in the quiz. Instead shrubs, reeds and rushes were the most popular options. I guess we don’t often see trees regenerating to form uniform dense stands over such large areas.
Thanks again to everyone who voted and submitted comments.
The next question for you all to answer is – Why? What caused the change? What was the ‘motive’ – apart from ‘
world wetland domination’?
What will happen next? Will the dense stands form big, wide-canopied, woodland trees ever again? Will the dense regrowth die off or thin out rapidly? What do you think?
Please continue the discussion by leaving a comment below. Next installment, same bat time, same bat channel…