Join the Plant Ecology Lab - Honours Projects in 2017
For further info: J.Morgan@latrobe.edu.au
Re-introducing fire into long unburnt grassy ecosystems– accelerated recovery of the ecosystem, or stasis?
Many grasslands and grassy woodlands are now rarely burnt, although it is likely that patch burning once played an important role in the structure and function of these ecosystems. Fire exclusion has led to tree recruitment and loss of diversity (in some cases because biomass accumulates to outcompete poor competitors). Land managers are increasingly re-introducing fire to long unburned landscapes, but what changes occur when fire is re-introduced when it has been absent? Are trees resilient to fire (or does it depend on their size)? Do species appear that haven’t been seen for a while, presumably re-appearing from dormant soil stored seed? Do some species disappear, having initially profited from the absence of fire?
In this project, we will test ideas about re-introduction of fire to landscapes where much benefit might be derived from such activities. Grassy ecosystems in western Victoria are much restricted (due to agriculture and, increasingly, timber plantations) and need sympathetic management to maintain their natural values. Re–introducing frequent fire to long unburnt grasslands is seen as a desirable management activity – it should serve to open up opportunities for seed regeneration and species coexistence. However, there are almost no examples where this has been tested, at least in good quality vegetation.
In this study, we will burnt long unburnt grasslands and ask whether often reported reductions in species richness due to the cessation of frequent fire can be spontaneously reversed by the return of fire. Additionally, will the abundance of currently sparse species be improved? How will exotic species respond to a change in disturbance regime? What about trees that have established in the inter-fire period? The student will work closely with the CFA, who will be responsible for conducting the trial burns, to design and implement the burning experiment.
‘Extinction debt’ in grassy woodlands
In 1975, Cliff Beauglehole (an excellent amateur botanist) surveyed fragmented grassy woodlands on the Brim Brim Plateau in western Victoria, identifying all native species and their abundance. Across a range of sites (many small, linear and isolated), the surveys provides a time stamp on floristic composition of these woodlands from which to assess changes in composition. In particular, they provide a capacity to identify which plant species can persist in highly modified agricultural landscapes, and which can’t.
Many species occur at very small population sizes and face threat (such as invasion by exotic species), meaning their local extinction is likely. The timescales over which this process are unknown. In 2006, my Honours student Fiona Sutton revisited these sites and identified substantial change that likely result from habitat fragmentation and small population size (Journal of Ecology 97, 718–727). This project would build on prior work by revisiting the sites to determine just how the extinction debt is playing out a decade later. One important new aspect of the work will involve assessing seed production and dispersal capacity of species in grassy woodlands. In my Lab, we’ve been interested in trying to understand which species can persist in fragmented woodlands and grasslands, and which ones can’t. Maybe the native species that go extinct are those that have low seed production, while those that expand in the landscape are those that have lots of seed production and whose seeds are easily dispersed. After all, regeneration is a key plant trait but rarely has it been directly tested in terms of its importance for predicting persistence. We’d need to do two things. We would need to quantify seed production (per plant) across a large range of species (increasers, decreasers) to see if seed production is a trait that differs dramatically across these groups. We would also need to quantify something about how seed production varies with plant size (what is called ‘reproductive economy’). Are plants that are successful the ones that produce lots of seeds regardless of size, while decliners are the ones that have size-related thresholds for production. You can read more about the genesis of these ideas in a paper by Poschlod et al. (2013) Seed ecology and assembly rules in plant communities. pp 164-202. In: Vegetation Ecology (eds. E. van der Maarel and J. Franklin), John Wiley & Sons). This project would suit a student who wants to learn lots of plants!
Endemic plant species on restricted soils: 'early victims' or 'hardy survivors' of climate change?
One of the greatest challenges that land managers face today is anticipating how climate change will affect the diversity and composition of ecological communities to develop effective strategies for adaptation and mitigation. The direct effects of climate change on species via changes in temperature and precipitation have been the focus of many studies. Many conclude that altitudinal and latitudinal shifts in distribution will be necessary to survive the impacts of predicted climate change. Little attention, however, has been given to how plant species on 'restricted' soil (i.e. very infertile) will respond to climate change. Here, suitable habitats for such species are patchily distributed, and the dispersal distances required to move to newly suitable habitat are large, making successful migration unlikely. Are species confined to low-nutrient soils, which may reflect their tolerance of such conditions and intolerance of other biotic factors such as competition, make them particularly vulnerable to climate change? Some studies suggest that soil specialists may be at less risk than species on 'normal' soils due to their stress-tolerant functional traits, but there is also contrary evidence.
Plant communities on low-nutrient soils have two distinctive attributes that may cause them to respond uniquely to climate change. First, they are often found in discrete areas making them more spatially isolated from one another than species on ‘normal’ soils that tend to be more contiguous. This spatial isolation may make it much more difficult for species to successfully migrate under climate change. Second, because these species are on unproductive substrates, they may differ from communities on ‘normal soils’ in terms of limiting resources, functional traits, and the relative importance of disturbance, competition and other ecological processes. Plants in these special soil habitats often have traits associated with tolerance of drought and nutrient-limitation [e.g. small stature, low-specific leaf area (SLA), high allocation to roots relative to shoots] because nutrient availability is limited, water can be scarce, and soils may have additional unusual chemistries (e.g. particularly acidic pH). Special soil communities are more strongly water-limited than others; therefore, they may be especially responsive to changes in available precipitation. On the other hand, because plants on special soils already have adaptations for stress tolerance, they may be particularly well-suited to withstand climatic changes.
In this study, we ask: what are the potential responses to climate change of endemic plant species when soil factors appear to limit their current distribution? We focus on the Wellington Mint-bush (Prostanthera galbraithiae), a vulnerable species, as a model species. The species is endemic to the Gippsland region of Victoria, restricted to sandy podzol soils typically low in macronutrients (especially N, P and K) and subject to long periods of soil moisture stress. To address the role of non-spatial factors, we will compare the plasticity to water and temperature stress of the endemic Mint-bush to that of two more widespread species (Prostanthera lasianthos, P. rotundifolia) to test the hypothesis that soil specialists are already well-adapted to environmental stress and they may be particularly well-adapted to withstand climatic changes.
Quantifying the wind dispersal capacity of seeds at mountain summits
The dispersal of plants and animals is of fundamental importance as it underlies landscape-scale ecological processes such as species invasion, immigration and meta-community dynamics. Dispersal is particularly critical if plants are to keep pace with climate change, migrating into new locations within the climate niche envelope. This is particularly true in mountain ecosystems where upslope migration will be crucial for cold-adapted species to maintain their advantage over warm-adapted species.
Understanding the aerial movement of seed, however, has proven difficult to measure and I am not aware of community-wide quantification of seed dispersal in alpine areas (although models of seed dispersal have been developed by Morgan & Venn). We will use a quantitative approach to measuring the aerial movement of seed in field situations, using a newly developed seed trap design, to assess the capacity of wind-dispersed seeds for long-distance migration. Using a series of alpine peaks distributed across the Victorian Alps where long-term observations of vegetation change are being conducted, we will quantify the composition of the seed rain and contrast it to local community assemblages. Quantifying dispersal remains a critical part of determining the contribution of these processes to shaping patterns of biodiversity at a landscape-scale. Determining rates of propagule supply to different parts of the landscape will provide guidance on which areas might respond to climate warming by natural regeneration processes, and which (poorly dispersed) species could be priorities for climate mitigation strategies.
The student will work closely with DECRA fellow Dr Susanna Venn (Australian National University)
Grassland litter decomposition across climate gradients
In south-eastern Australia, fire and grazing regimes have long been recognised as influential drivers of species diversity in native grasslands. Over time, in the absence of such disturbances, senescent Kangaroo Grass (Themeda triandra) tussocks create a thatch of dead leaves over the soil surface. This layer of leaf litter decomposes very slowly in temperate climates, shading out inter-tussock species, smothering seedlings and contributing to the decline of plant diversity. As such, a common assumption is that native grasslands require frequent disturbance to remove accumulated biomass in order to optimise species diversity. However, not all grasslands accumulate litter. In sub-tropical and semi-arid ecosystems, low biomass accumulation might reflect high decomposition rates. Here, litter is unlikely to smother intertussock species, as is the case in temperate climates. Different climates may influence the rate of decomposition and accumulated biomass within tussock grasslands and hence, the disturbance requirements of grasslands to maintain species diversity. However, this contention remains untested.
To quantify how litter decomposition varies across native grasslands, and how this may be related to litter type (C4 vs C3 grasses, native perennials vs exotic annuals), climate and rates of photodegradation, we will conduct a litter decomposition experiment in the field across northern, central and southern Victoria using litterbags. The effect of UV radiation on decomposition rates will be investigated using structures to manipulate UV radiation (see Brandt et al. (2010) Ecosystems 13, 765-781). The field experiment will be complemented with a glasshouse experiment to test germination success of grassland species according to high or low biomass accumulation. A range of species typical of grasslands (e.g. daisies, lilies) will be sown under grass litter (at depths that correspond to field observations) to determine whether litter suppresses germination. This is a critical experiment as it has been assumed that litter smothers herbaceous germination, but quantitative data are lacking.
The student will have the opportunity to work with a range of grassland managers (e.g. Trust for Nature, Brimbank City Council, Parks Victoria) and scientists.
Exotic grasses as drivers of species decline in threatened native grasslands
Weedy grasses pose a significant threat to the diversity of native temperate grasslands. This is particularly true in the Victorian Volcanic Plains where soils are fertile and rainfall is moderate. Although we know weedy grasses can outcompete native species at local scales if appropriate disturbance regimes are not maintained (e.g. frequent burning), we do not know at what point weedy grasses will cause a significant, and often irreversible decline in native grassland diversity at larger spatial scales. It seems intuitive that there would be a negative relationship between the cover of weedy grasses and the number of native species (as weed cover goes up, native species diversity goes down). However, the strength and shape of this relationship remains unclear. This applied project aims to provide grassland managers with clear and unambiguous recommendations that will enable the identification of problem areas that can be treated prior to the deleterious effects of weed invasion.
This study will utilise a series of sites across the VVP to assess the broad scale effects of a high threat perennial weedy grass, Phalaris aquatica, on temperate native grasslands. In particular, this study aims to: i) establish what the relationship between Phalaris cover and native species diversity looks like (i.e. is it linear or non-linear); ii) establish whether an impact threshold is evident (i.e. is there a point at which Phalaris cover leads to dramatic declines in native species diversity?) And, iii) identify whether species losses were associated with particular life forms or whether impacts were across many life forms. The student will work in collaboration with ecologists at the Arthur Rylah Institute for Environmental Research (the Victorian Government Dept. of Environment, Land, Water and Planning biodiversity research base).