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.

Thursday, 19 September 2013

Restoring ecosystem stability

There are many aims of ecological restoration. Some might include: to enhance native diversity. To stop soil erosion. To connect fragmented ecosystems. To recover disturbances caused by D9 bulldozers.

In bushfires, firebreaks often get bulldozed along tracks. These areas need to be restored afterwards. Here, in the mountains of Victoria, restoration usually is about getting high vegetation cover back (as soon as possible) to reduce soil loss.
(Photo: Sera Cutler, somewhere near Mt Hotham, 2003)

Given these aims, it is often fairly clear what needs to be reported upon to determine the success of restoration. So, for the above examples, you'd probably want to report on the following:
- changes in native species richness or native:exotic species richness ratio
- reduce bare ground (%)
- movements of habitat specialists between patches via corridors
- increased native species cover and low exotic species cover.

As an ecologist, I'm interested in longer-term outcomes of restoration. I'm really curious to know whether initial recovery predicts longer-term recovery. And, importantly, having restored vegetation, how 'stable' is this vegetation in the decades after it was first introduced.

I think one of the under-reported outcomes of restoration is stability. We know that ecosystems fluctuate from year to year in response to climate and biotic factors - hence, the term "non-equilibrium dynamics" is often applied when considering vegetation dynamics. Yet, as a restoration practitioner, I'm pretty sure that you'd hope your restoration outcomes, while fluctuating across the years, was introducing a vegetation that was rather resilient to big changes. Big change might normally be considered negative change. For example, change could involve (a) invasion by exotic species overwhelming the re-introduced natives or, (b) recruitment failure leading to restoration collapse once the lifespan of the plants has been reached.

Tilman's experimental plots in Minnesota
test ideas about the role of species diversity
on ecosystem function.
Theory suggests that introducing more native species gives the system greater stability. Dave Tilman's experimental grassland plots in Minnesota are often cited as the best example of this - severe drought had less impacts on rich assemblages compared to species-poor plots. Diverse plots also had effects on invasion by exotics (rich plots resisted invasion) and nutrient capture/drawdown (rich plots use more resources, reducing leakage of resources).

However, in the native grasslands that I work in, very few species contribute most of the biomass and occupy most of the basal area, i.e. the dominant tussock grasses. So, is stability driven by many species, or few abundant ones?

This is not something we have much data on in Australia, so I'm not going to be able to answer that question very well at this stage. But, we can use one of the longest running grassland restoration projects in Australia to get insight into the capacity of restored grasslands to persist over two decades and to resist invasion by exotic species.

In the mid 1980s, a group of ambitious (and enthusiastic) volunteers from the Friends of the Organ Pipes National Park teamed up with a state government scientist to re-introduce native grassland plants to an area that had been transformed to exotic species by agriculture.  This was a mighty task, for in those days there was little seed available of the now threatened grassland species, there was little known about the autecology and cultivation of the native species, nor had anyone tried to reintroduce them back into the 'wild' to create the 'wild'!

I dug up some photos from about 1985/86 when I first got interested in native grasslands. Keith McDougall, a fabulous grassland ecologist, took me on as a Student Research Cadet for 6 months, and I helped harvest seed of native grasses, sow the seed in experimental plots testing different establishment treatments,  and then monitored the resulting seedlings and recovery. I've been doing it ever since!

You can see in this photo the plots we initially created.
The experimental plots at Organ Pipes National Park, around 1985. Each 10 x 10m plot is examining
how Kangaroo Grass establishment is effected by treatments such as burning.
About 2-2.5 ha was sown in this way over a 5 year period.
(Photo: Keith McDougall)
Here you can see Kangaroo Grass thatch being spread onto plots that have been unburnt & burnt.
(Photo: Keith McDougall)

Each treatment plot was about 10 x 10 m. The aim here was to create the matrix first - the native tussock grasses - so as to build the grassland structure before introducing diversity later. Exotic species were variously removed by fire, herbicides and their interaction, but only for the initial establishment phase of the grassland. In this ecosystem restoration model, we explicitly assumed that native diversity would increase with time as we first occupied the site with native grasses, then added inter-tussock native species later.

I decided to go back to Organ Pipes recently. I was curious whether the grasses that we had sown (almost exclusively Kangaroo Grass) had (a) persisted after 25 years, (b) had resisted invasion by exotic species. While the plot markers are gone, it is very obvious where we sowed native grasses because the plot outlines are almost still visible. Inside the plots was Kangaroo Grass - at high density, with tussocks of varying sizes hinting at self-replacement,, and seemingly acting as the mono-dominant species we see in nature. And, surprisingly, there were very few exotic grasses present. These were restricted to areas outside our sowings where native grasses had no yet colonised.

Restored native grasses creating a matrix for future re-introduction of native species.
Note the tall exotic grass Avena fatua in the general vicinity of our initial walkways.
Organ Pipes National Park
(Photo: John Morgan)
Kangaroo Grasses dominate in areas first sown in 1985.
Organ Pipes National Park
(Photo: John Morgan)

So, from a field of exotic annual grasses (comprising Avena spp, Bromus spp. and Vulpia spp.) a native-dominated grassland has arisen. This hints at a great (perhaps increasing) stability of the native grassland - one that maintains itself yet buffers against invasion. While native species diversity remains low (reintroducing herbs to the matrix has proven a challenge), one could argue that an unstable annual exotic system has been replaced by a more stable native system. And, importantly, the native system still provides an opportunity to return the ever-diminishing native grassland plants to a secure habitat.

I hope you've found this insight inspiring. And maybe it makes us think about restoration in a slightly different way. We need to create systems for the long-term, ones that can buffer environmental change, and ones that provide us with opportunities to continue to enhance their biological value over the decades.