Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Sunday, 31 December 2017

Where have all the trees gone?


In the 2nd of a series of guest posts from my Post-Grad students, MSc candidate Simon Heyes gets us to think about savanna in southern Australia.


Imagine the Victorian volcanic plains prior to the arrival of Europeans. They stretch from Melbourne to near the border with South Australia. Most of us picture the plains as covered by copper-coloured Kangaroo Grass (that amazingly widespread C4 grass that has taken over Australia since it's arrival 1.8 M yrs ago), a riot of wildflowers and maybe a fire on the horizon. Now picture this same landscape, but include a sparse cover of trees such as Drooping She-oak, Lightwood, Bursaria and Banksia. This kind of landscape wouldn’t look out of place with a pride of lions lounging under the shade of an Acacia, but this isn’t Africa, this is southern Australia.
 
Scene from a savanna in western Victoria in the 1850’s?
Painting by Duncan Elphinstone Cooper from Challicum, Western Victoria

We know trees, such as Silver Banksia, were probably much more widespread on the plains at the time of European settlement, although they have largely been forgotten over the last century or more as trees were likely quickly utilised/removed, being a valuable commodity to early settlers.  Historic anecdotes and paintings - such as the one by Duncan Elphinstone Cooper painted in the 1850’s at Challicum, Western Victoria - provide insight into the past, a time-capsule only now being opened to eager eyes. Looking at some of this work has lead me to ask: “did the pre-European plains constitute a savanna? Or is this a grassland with trees?" Have we been looking at the plains through a grassland lens when, in fact, a unique, non-eucalypt savanna (comprising Acacia, She-oak, Banksia and Sweet Bursaria) may have been the original vegetation state over much of this area? To answer this, it helps to know what a savanna is?

Savannas have been variously defined; many authors seem to settle on the definition as "a tropical continuous grassland and scattered trees with a long dry season" (Scholes & Archer 1997). Others have
defined savannas as "open (sparse to no trees) and dominated by C4 grasses". So, are savannas a tropical to subtropical thing only? Perhaps not. [Note: there is no mention of mechanism in these definitions as to why trees are spare]. The longleaf pine (Pinus palustris) savanna of southeastern USA, for example, is an ecosystem dominated by Wiregrasses (Aristida spp.), a group of C4 grasses.  The climate (for SE USA in places like Louisiana & Mississippi) is humid subtropical, with rainfall between 3-6" in each month of the year. A long dry season it does not have.

Another interesting way to define and classify a savanna was proposed by Ratnam et al. (2011). In their paper, they provide a key based upon a trait approach to distinguish between savanna and forest. So, how does this approach work for “savannas” in western Victoria?
 

1.      Well our savanna certainly doesn’t have a closed canopy ()
2.      Kangaroo Grass is the monodominant C4 grass species that accumulates combustable biomass quickly ()
3.      Ok, so here at number 3 we find ourselves falling down because all of our tree species on the plains - She-oak, Banksia and Bursaria - are also found in forests, heathlands and coastal woodlands. This is because they have not evolved as savanna species but, rather, occupy a savanna habitat ( X )

However, if we take Banksia as an example, we hit some uncertainty.

This species is not just found in grasslands of western Victoria but is also under the canopy of various forest types and also occurs in coastal heathland where is recruits after fire (i.e. in a high light environment). Plants on the plains are small, non-obligate seeding trees and their saplings appear to be able to resprout and persist under frequent fire; sapling regeneration does not appear to depend on fire. Many of these traits and adaptations to fire probably evolved well before the arrival of C4 grasslands (in response to climate drivers such as drought) and it goes some way to explaining why these species exist on the plains as savanna-like trees. Whether fire or herbivores (such as Diprotodon) kept the trees open, or there are inherent limits to trees imposed by soils and climate, are still only vaguely resolved. Indeed, only one Honours thesis (from back in the 1970s) has explicitly tried to answer this question - for eucalypts, a group of trees that were not the treed component of the plains. There's a great opportunity for more research to tease this out.
Under frequent low intensity fire, Silver Banksia resprouts from lignotubers such as these in a savanna fragment near Ballarat, Western Victoria. Photo S Heyes 2017

 For now, I think I will still call the treed-landscapes of western Victoria savanna-like. I think that trees are sufficiently sparse to not constitute a woodland, but trees were likely a persistent element of the landscape (and widespread enough across the plains) to be more than a local phenomena only. What is really important is that we recognize the important contribution that trees did make to the fabric of the original vegetation. In some small way, the creation of the new Friends of Forgotten Woodlands is perhaps that first step to wider appreciation of these hitherto forgotten ecosystems.

References

Ratnam J, Bond WJ, Fensham RJ, Hoffmann WA, Archibald S, Lehmann CER, Anderson MT, Higgins SI & Sankaran M. (2011) When is a ‘forest’ a savanna, and why does it matter?. Global Ecology and Biogeography, 20: 653–660

Scholes RJ, & Archer SR. (1997) Tree-grass interaction in savannas. Annual Review of Ecology and Systematics 1997 28:1517-544 

1 comment:

Stanislaw Wawrzyczek said...

Very well written, Simon. I in order to defend the "savanna hypothesis" would be good to demonstrate that the sparse trees are not just stray individuals, but actually have an ecological function. This may be difficult to test for obvious reasons, but what would happen to a savanna if you tool out the trees? Would this affect other plant species or animals? Or would it just look a little different and we would call it grassland? Btw, does Ratnam elaborate on why the savanna trees must be different to forest species? Do you think s/he added this extra to narrow down the meaning more than was necessary?