Aspects of Alpha-niche differentiation in an herbaceous community

Rachael Helen Elizabeth Lawrence Lodge

Alpha-niche differentiation arises when plant species use resources differently enough to co-exist and persist within a community (Wilson 2011). If alpha-niche differentiation is occurring at a fine scale, a community including non-random groupings of species with divergent functional traits is expected (de Bello et al. 2009). The Botany Lawn community offers a unique opportunity to investigate alpha-niche differentiation as non-random groupings of the most abundant species; the “intrinsic guilds” have already been identified by Wilson and Roxburgh (1994). In this thesis, investigations focussed on functional traits which potentially allow alpha-niche differentiation at a fine scale with regard to the light and nutrient (resource) niches. Functional traits of fourteen of the intrinsic guild species were examined and agreement sought between species’ traits and their intrinsic guild membership, i.e. divergent traits between non-random groupings of species indicating the occurrence of alpha-niche differentiation. This thesis compliments and extends previous research seeking to identify mechanisms of alpha-niche differentiation within the Botany Lawn community. Mason and Wilson (2006) suggested differences in nutrient strategy were the basis for a mechanism driving community structure while Brownstein (2011) found agreement between intrinsic guild membership and growth responses under altered light conditions. The potential partitioning of the light niche was explored in two separate experiments. In the first, species were characterised by their relative growth rates and biomass allocation under ambient (high-) versus shaded (low-) light conditions. In the second experiment, antioxidant metabolism responses to light-mediated stress were assessed using reciprocal transfers between high and low light conditions. Aspects of the nutrient niche as a means for alpha-niche differentiation were addressed in three further experiments. The first of these compared nutrient contents, relative growth rates and biomass allocation when nitrate and phosphate were omitted or supplied. Uptake of various forms of nitrogen or phosphorus, differing in their biological availabilities, were the focus of the remaining experiments. Evidence for alpha-niche differentiation was mixed. Species differed in their functional traits throughout the experiments which, in theory at least, reflect differences in resource use. Agreement between species’ functional traits and their intrinsic guild membership was limited and appeared to be driven more by inherent differences than in response to the conditions. The clear exception was the greater antioxidant metabolism responses of intrinsic guild A compared to intrinsic guild B. The relationship between the measured species’ traits and intrinsic guilds and the benefits of employing such an approach are examined. Based on these results, it was unclear whether alpha-niche differentiation of the light or nutrient niches is driving the observed intrinsic guild structure of the Botany Lawn. While this thesis focussed on one resource niche at a time, trait responses dictating the competitive ability of a species are not expected to operate in isolation. Antioxidant metabolism responds to a range of abiotic- and biotic-derived oxidative stress and appears to be involved in longer term hormonal and growth responses. The use of antioxidant metabolism traits represents a novel avenue for exploration of alpha-niche differentiation in plant community ecology and warrants further attention.