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.

Tuesday, 18 October 2011

Ecological divides

Fencelines provide excellent opportunities to do comparative ecology.

Across southern Australia, livestock grazing has been so widespread and intense that it's transformed the natural grassy ecosystems, sometimes beyond recognition. Early settlers were drawn to the vast grassy plains of the lowlands, as well as the drought relief that the alpine high plains provided. For ecosystems that evolved in the absence of large, hooved, congregating animals, it's no wonder that changes in the native vegetation were recorded within five or so years of occupation by europeans. In many cases, we can only dream of what the original ecosystems might have looked like. Early paintings provide some insights, particularly about the structure of the vegetation and perhaps some of the dominant tree species. For example, the image below shows that the Yanakie Isthmus at Wilsons Promontory in the 1870s was undoubtedly a grassy woodland at the time of settlement, and extensive at that. Today, the Isthmus is covered in shrubs and hardly a blade of grass can be found, a consequence of 100 yrs of stock grazing and fire exclusion.

From Lookout Hill, towards Mt Latrobe.The trees here are probably She Oaks (Allocasuarina verticillata).
Painted by John Black Henderson, about 1870. (Image provided by Jim Whelan)
Fenceline comparisons, however, provide much more detailed information about the effects of historical and current regimes on the composition of native vegetation. In some cases, the fences went up in the very early days of settlement (around cemeteries, along railway lines, and to demarcate travelling stock routes), preventing the ecological transformation that occurred throughout the remainder of the landscape that was to be grazed. It's no wonder they have been used extensively in ecology as "natural experiments".

To illustrate the importance of fenceline comparisons, I've been just looking at the distribution of C4 grasses on the riverine plains of northern Victoria with a view to thinking about how to restore and manage these endangered ecosystems. Currently, grasslands here are dominated by C3 grasses such as Wallaby Grass and Spear Grass, with a wide variety of annual and perennial intertussock herbs. C4 grasses are very rare (despite their implied importance in the pre-european flora). The current structural and compositional values are being managed by status quo management - using sheep grazing - based on the idea that the vegetation we see today is a product of its recent grazing history and hence, this is the best way to manage it to maintain those values. But what elements might be negatively affected by such a regime?

At Terrick Terrick National Park, there's a little gem of an unused laneway that runs for perhaps 1.5 km. It's no more than 20 m wide and it adjoins large paddocks with native grasslands that have been grazed for a century, as well as cropped at some stage (as evidenced by the cultivation lines). Such a laneway offers an amazing insight into what grasslands might have looked like in the absence of grazing.

Land use on the Northern Plains of Victoria. This native grassland shows the typical
signs of having been 'used' before it was conserved. There are cropping lines, obvious effects
 of water points on sheep movements. And, at the top of the picture, a laneway - different
in colour from much of the vegetation. Is this because it supports a different flora?
(Photo: Google Earth image)

Two things strike me about the fenceline comparison here:
1) in the ungrazed area (left), C4 grasses that are incredibly uncommon in the grazed grasslands - such as Enteropogon (Spider Grass) - are super abundant (and co-dominate with other C4 grasses). Strikingly, C3 grasses are almost absent from the ungrazed area (yet are ubiquitous in the grazed area). Hence, functionally, the laneway grassland is a summer-active one, not a winter-active grassland as now dominates most of the landscape. This is a profound change.
2) unsurprisingly, in the ungrazed laneway, there are lots of sub-shrubs and herbs which are absent from the grazed areas. Maireana and Atriplex, in particular, only exist at high abundance where grazing has been minimal.

These are not new insights. Changes in species composition due to grazing have been recorded many times before in Australia. This is often due to differences in palatability. But, it does point to some key questions.

Does grazing promote C3 grasses that have a regeneration niche dependent on disturbance? It is thought that the C4 grasses regenerate better where the ground is covered in plant litter, but this is removed in grazed areas, potentially favouring Spear Grasses. Perhaps this is why there are no Spear Grasses in the laneway. This needs to be experimentally tested.

Can status quo management ever allow the system to recover poorly represented components of the original flora? The laneway is so different that it is clear that ongoing grazing by sheep maintains different vegetation states across a fenceline. It does this because it prevents the C4 components (and other subshrubs & herbs) from transitioning back into the system. If every grassland is managed in the same way on the northertn plains, then we are conserving a subset of the original flora. And a subset of the ecosystem services and functions performed by such a flora.

It would be incredibly useful to assess the plant trait distributions of species found in the laneway and adjoining grazed grasslands. Does grazing maintain species richness, but at the expense of functional diversity? Are we causing a functional homogenization of the grasslands in our pursuit of conserving exisiting values?

I've highlighted just how important fencing experiments can be to show that vegetation patterns are affected by management history. Providing a mechanistic understanding of the changes that occur, however, has been less well demonstrated. I hope to fill in these gaps soon. Till next time....

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