Date of Award

2009

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Bachelor of Science Honours

School

School of Natural Sciences

Faculty

Faculty of Computing, Health and Science

First Supervisor

Ray Froend

Second Supervisor

Will Stock

Abstract

Water availability (the defining feature of a 'hydrological habitat') has a major effect on plant growth, and the physiological and structural characteristics of plants often reflect the availability of this resource. In relation to development of vegetation structural characteristics as a consequence of a given hydrological habitat, it is important to understand whether self-organisation of plant cover (productivity, structure and function) interacts with water availability and if adjustment to water limitation exists ('ecohydrological optimization' or establishment of a 'hydrological equilibrium'). This study tested, through three hypotheses, whether structural and functional attributes of Banksia woodland vary with water availability, i.e. whether stand-level productivity and capacity to transpire is optimised under the prevailing hydrological conditions. The hypotheses are that Banksia woodland i) leaf area, ii) density, and iii) sapwood area (of the dominant overstorey species) are greatest where water availability is highest. The study used spatial variation in groundwater availability (defined by depth to groundwater and phreatophyte water source partitioning) in the Gnangara Groundwater System on the Swan Coastal Plain, Western Australia, to select sites from three contrasting hydrological habitats across which comparisons of Banksia woodland structural characteristics were made. To quantify differences in leaf area associated with water availability, Foliage Projective Cover (of the understorey and overstorey) and leaf age (of the dominant overstorey species B. attenuata) were measured. To quantify differences in spatial distributions of plants, the density and distances to nearest overstorey, conspecific and similar aged neighbour were calculated for the same overstorey species. B. attenuata sapwood area per basal area and sapwood area per hectare were calculated to assess the transpirational capacity of each habitat. All of these variables were statistically tested for significant difference between hydrological habitats. It was found that significant differences occur in the structural characteristics of Banksia woodland in contrasting hydrological habitats along an ecohydrological gradient. These findings suppmi the hypotheses that leaf area and sapwood area are greatest where water availability is highest, but do not support the same pattern for density. As water availability increased, growth per overstorey individual (B. attenuata, including tree size and amount of foliage) was maximised, with a trade off reflected in decreased plant densities. In more xeric hydrological habitats, increased plant densities were observed, with smaller individual tree sizes and leaf area. The more mesic habitats also have a taller overstorey stratum, with a canopy of increased leaf area and longer foliage retention times. This in-tum is correlated with increased total habitat sapwood area (despite lower plant densities) and increased individual tree sapwood area. The nature of Banksia woodland structural adjustments to water availability seen involve processes of hydrological optimisation to achieve vegetative equilibrium with the hydrologic environment. These are habitat-specific and occur over different time scales. These mechanisms reflect Banksia woodland's adaptive capacity to respond to variability in water availability and have impacts on NPP, ecosystem function, habitat and environmental water requirements. This study enables the prediction of vegetation responses to changes in water availability (climatic or anthropogenic) and can also aid in formulating better infom1ed objectives for vegetation rehabilitation.

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