Tree leaf-and branch-trait coordination along an aridity gradient
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NO FULL TEXT AVAILABLE. Access is restricted indefinitely. ----- Functional attributes of leaves and branches have a significant influence on a plant’s establishment, survival and fitness to particular environments. A major environmental variable to which plants must adapt is annual rainfall, or soil water availability. This thesis presents an analysis of variation in leaf- and branch-traits of trees across a rainfall gradient within NSW, Australia. In particular, it quantifies relationships amongst five leaf-traits: C0₂ assimilation rate, stomatal conductance to water vapour (gs), foliar nitrogen (N) concentration, specific leaf area, and foliar carbon isotope ratio (δ¹³C) (a measure of leaf water-use-efficiency and intercellular C0₂ concentration; Cᵢ), and three branch-traits: branch hydraulic conductivity, wood density and the ratio of leaf area to sapwood area (LA:SA), across a total of 16 perennial tree species across 13 native woodland sites spanning a mean annual rainfall gradient of 455 to 1310 mm. The generality of the trends found within tree species of NSW are then compared with the ‘global’ leaf-trait relationships of data obtained through Wright et al. (2004a) (referred to as the GLOPNET dataset), including comparisons among phenological and plant functional groups. Increased gs and foliar N concentration were associated with increased C0₂ assimilation rate. Higher foliar N concentrations were also associated with reduced Cᵢ and higher δ¹³C, suggesting that increased N concentration increases water-use-efficiency. Trees growing at low rainfall sites had lower mean gs, allocated more N to leaves, and consequently had higher foliar δ¹³C, suggesting a water conservation strategy at low rainfall sites. Similarly, reduced sapwood-specific hydraulic conductivity (ks) was correlated with reduced gs, higher foliar N concentration, a lower Cᵢ, but, most importantly, a statistically similar average rate of C0₂ assimilation as branches with higher ks. Overall, these results suggest that trees or species subject to water limitation, either by reduced rainfall or through a lower water transport capacity (a lower ks) ‘substitute’ water for N to achieve a given rate of leaf-level carbon gain. This mechanism represents an optimisation strategy for the use of limited supplies of water and N. Consequently, trees growing at low rainfalls sites, with low ks, maximise water-use-efficiency and minimise any decrease in C0₂ assimilation rate. This is achieved by these low rainfall trees operating at low Cᵢ which increases the diffusional gradient of C0₂ into the leaf. Branch wood density was inversely correlated with leaf area per branch, per unit branch length and per unit sapwood area, consistent with a reduced hydraulic capacity of denser wood. However, leaves of branches with high wood density and low LA:SA exhibited higher rates of C0₂ assimilation per unit leaf area and had a lower average Cᵢ. Thus, whilst branches with low ks invested more carbon in sapwood (higher density wood and more sapwood area per unit leaf area), this investment was supported by maximising C0₂ assimilation per unit leaf area. The patterns of correlation amongst gs, foliar N concentration and C0₂ assimilation observed in NSW were replicated in an analyses of the global data contained in the GLOPNET dataset. Thus, species with low gs had higher foliar N concentration for any given C0₂ assimilation rate, demonstrating the broad generality of water and N substitution. More importantly this pattern was observed only within certain functional groups. Evergreen species, trees, shrubs and N-fixing species exhibited this pattern but functional groups with short leaf lifespan, including deciduous species and grasses, did not display resource substitution. These differences suggest that species which utilise limited quantities of water and N over a long leaf lifespan use water and N sparingly and optimise resource use. In contrast, species with short leaf-lifespan use water and N in a more profligate manner and do not optimise resource use. In addition, N-fixing species were capable of substituting water for N to a greater degree than non-N-fixers, by allocating more N to foliage at low gs. The greater plasticity of N-fixers (such as Acacias) to substitute the two resources to achieve a given rate of carbon assimilation might explain their relative success in low rainfall environments.
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