Application of stable isotope analyses to examine patterns of water uptake, water use strategies and water-use-efficiency of contrasting ecosystems in Australia

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Spatial differences and temporal variation in the availability of water accessible by plants have significant influences on a range of plant functions, including photosynthesis, transpiration, growth and yield. Coexisting plant species often adopt contrasting water use strategies to reduce inter-species competition for water and to cope with seasonal and/or geographical differences in water availability. The work described in this thesis aimed to analyse the variations in sources of water uptake by vegetation as well as contrasting water use strategies across spatial and temporal scales in a range of sites across Australia. In particular, it employs stable hydrogen, oxygen and carbon isotopes to investigate variations in sources of water uptake by vegetation and discrimination against carbon-13 and water-use-efficiency at a range of spatial scales (from 10s of metres to thousands of kilometres) across a range of sites located along gradients of water availability. The generality of the trends found within smaller spatial scales in contrasting mesic and semi-arid ecosystems across water availability gradients are compared with trends obtained from different sites at a continental scale across a climate gradient (with mean annual precipitation ranging from 255 mm to 2140 mm). At smaller spatial scales, analyses were performed along two naturally occurring depth-to-groundwater gradients in contrasting mesic and arid ecosystems. Results from stable deuterium and ¹⁸O analysis revealed that, even in the mesic site that received >1000 mm rainfall for two consecutive years prior to sampling, increased water availability (via access to groundwater) still exerted some influence on sources of water uptake by dominant vegetation. However, stable isotope analysis more consistently and reliably identified access to groundwater by deep-rooted vegetation in the semi-arid sites that receives rainfall < 350 mm year⁻¹. A key result identified in this research was that discrimination against stable ¹³C isotopes (Δ¹³C) and leaf intrinsic water-use-efficiency (WUEᵢ) calculated from ¹³C at the leaf-level accurately identified water availability, specifically, groundwater use and spatial and/or seasonal patterns of groundwater use especially in water-limited regions. Consistent with the results of (branch water) deuterium and ¹⁸O analysis, discrimination against ¹³C and WUEᵢ (leaf-level) did not vary significantly with depth-to-groundwater in shallower rooted (Acacia) species but showed significant trends up to a threshold value of depth-to-groundwater of 13.9 m in the deep-rooted (Eucalyptus and Corymbia) species, indicating possible access to groundwater. A strong positive correlation of bulk-leaf Δ¹³C (strong negative correlation with WUEᵢ) of the dominant overstorey species with MAP and moisture index was observed during both dry- and wet-season across an eight-fold increase in mean annual precipitation (MAP) along a continental-scale climate gradient. Up to 3.7‰ and 4.5‰ differences among biomes during the dry- and wet-season respectively were observed in Δ¹³C. Among 19 other climate parameters in addition to MAP that were considered in the multiple regression analyses, isothermality as a secondary predictor during the wet-season was the only model with notable explanatory power. Among the gas-exchange traits, Δ¹³C and WUEᵢ was tightly associated with Cᵢ/Cₐ ratio and stomatal conductance and this explained the strong response of Δ¹³C to MAP during both seasons. During the dry-season, Δ¹³C and WUEᵢ were more strongly regulated by stomatal conductance than photosynthetic capacity. In contrast, during the wet-season when canopy photosynthesis is the most active under favourable conditions of water availability, photosynthetic capacity was found to have a stronger relationship with Δ¹³C than stomatal conductance at a continental scale. Leaf mass per unit area (LMA) among the leaf structural traits, showed a strong negative correlation with Δ¹³C suggesting that these two structural (LMA) and functional (Δ¹³C) leaf-traits are possibly linked through the effects of low (soil) water availability on decreasing Cᵢ. In 55 dominant overstorey species distributed widely across the country and experiencing distinctly different climates and ecosystem structures and compositions, a pattern of similarity was identified in terms of species mean values of Δ¹³C among closely related species. Phylogenetic analysis suggests that more closely related species were found under a given set of climate conditions across the gradient at a large spatial scale in Australia. Cross-species relationships of Δ¹³C/WUEᵢ and climate variables/leaf-traits were generally quite robust and well supported after taking their genealogical relationships into account. Combining phylogenetic and trait-based information supported the hypothesis that climate variables, in particular variables related to precipitation, influenced the evolution of discriminatory traits of leaves of the dominant species at the scale of this present analysis by reducing discrimination against ¹³C with decreasing water availability and with increasing temperature.
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