Carbon isotope discrimination by Eucalyptus species in the Northern Territory, Australia
Carbon isotope discrimination (Δ) reflects the balance between the supply of C0₂ into the intercellular airspaces of a leaf and the demand of photosynthesis for C0₂ from those intercellular airspaces. The conductance of C0₂ into a leaf depends upon the plant being able to sustain the co-occurring transpiration, which requires the acquisition and transport of water at rates equal to the evaporative demand. The photosynthetic demand for C0₂ depends upon the investment of nutrients, primarily...[Show more] nitrogen, in photosynthetic enzymes. Plants with high conductance, relative to photosynthetic capacity, will have little reduction in intercellular C0₂ concentrations relative to the ambient source C0₂ concentration outside the leaf, and therefore high Δ values. Plants with either low conductances or high photosynthetic capacities will have low intercellular C0₂ concentrations and low Δ values, reflecting a high degree of stomatal limitation on photosynthesis. Carbon isotope discrimination therefore reflects the balance between the water and nutrient economies of plants, and when combined with information on the distribution of a species may yield insight into the success of the genetically determined growth strategy for the uptake and utilization of these resources. From Darwin, on the north coast of Australia, to the southern border of the Northern Territory there is an eight-fold decrease in total annual rainfall with little topographic complexity. A transect across this gradient includes plant communities in the wetter northern areas that are dominated by Eucalyptus species, while the arid southern areas are dominated by potentially N-fixing Acacia species. This broad-scale replacement of a possibly high water-using growth strategy by a high nitrogen-using strategy along a gradient of decreasing water availability would seem ideal for study using carbon isotope discrimination. This is especially true since carbon isotope discrimination records a production-weighted measure of the ability of conductance to supply C0₂ for photosynthesis over time periods approximating the duration of the time required for the growth of the sampled tissue. The carbon isotope discrimination recorded in wood tissue would presumably record conditions over a full year in an annual growth increment, with conditions over many years being integrated by combining the growth during multiple years. Although carbon isotope discrimination is a leaf-level phenomenon, it has been used as a successful surrogate measure of the water-use efficiency of co-occurring whole plants. Where the evaporative demands are the same, i.e. plants have similar leaf temperatures and therefore similar leaf-to-air vapor pressure deficits, and are not competing for the same limited water supply, slow rates of water-use (and necessarily limited rates of carbon gain) are more efficient, and will result in greater final biomass accumulation, than high rates of water use, assuming similar allocation patterns for reinvesting photosynthate towards resource acquisition. Low rates of water-use, and a high degree of stomatal limitation on C0₂ supply for photosynthesis will result in low Δ values correlating with this high biomass production. When any of those pre-conditions is not completely satisfied, arguments can be made that the plant with lower stomatal limitation on photosynthesis should be a better performer. The ambiguity of the potential response of Δ to decreasing water availability in naturally occurring plant populations is work. Carbon isotope discrimination was measured in leaf and wood tissue across the distributions of a series of co-occurring and replacement species along a rainfall gradient. With decreasing rainfall, the whole-tissue leaf Δ of five of 13 species decreased, seven exhibited no trend, and one increased. The whole-tissue wood Δ of eight species decreased, showed no trend in four species, and increased in one species. Species replacements were marked by a shift in Δ reflecting greater stomatal limitation on assimilation. There was a non-linear response of the multi-species average leaf and wood Δ to decreasing total annual rainfall. This response reflected the spatial pattern of the sensitivities of Δ to decreasing rainfall of the individual species and was not the result of a proposed emergent behavior where the mean would differ from the responses of the individual species. Because the patterns in Δ in response to decreasing water availability were not common to all species it was not possible to determine the extent to which Δ is a measure of the stomatal limitation on photosynthesis, i.e. is a measure of the reaction to environmental stress. The patterns of Δ across the distributions of the species did not provide a simple measure of the physiological limits of the distribution of eucalypts in northwestern Australia. As Δ measures the ratio of leaf conductance to assimilation capacity, movement in either factor could generate a signal, while combined movement could result in no signal. Although the opportunity to directly measure the magnitude of leaf conductance and assimilation capacity in a subset of the sampling locations would have greatly assisted in the understanding of their dynamics across the transect and the patterns in Δ, assimilation capacity had to be estimated from leaf nitrogen contents. The increase in area-based leaf nitrogen contents is explored in Chapter 4. The uptake of C0₂ can be viewed as a two-step process, the first involving C0₂ entering the leaf and stomatal conductance and the second involving the actual _photosynthetic uptake from the intercellular airspace depending on leaf nitrogen contents and investment in photosynthetic enzymes. The two processes are in series such that a decrease in either could be offset by an increase in the other to maintain the overall photosynthetic rate. This principle of resource substitution is a basic idea of economics and there is a well developed mathematical treatment of the issue coming from the fields of consumer behavior and the production of firms. Such concepts as the intrinsic antagonism of water-use efficiency (the rate of carbon gain per unit water lost in transpiration) and nitrogen-use efficiency (the rate of carbon gain per unit nitrogen lost in leaf senescence) are obvious outcomes of resource substitution. The mathematics of resource substitution and related concepts as applicable to a simple model of photosynthesis, transpiration, and root investment for acquiring water and nitrogen is developed in Chapter 4. The measured increases in leaf nitrogen contents were estimated to account for between 50 and 75% of the projected change in water-use as water availability decreased across the transect. The increasing leaf nitrogen contents with decreasing rainfall across the transect were predicted to result in increased photosynthetic capacities. As water availability decreased there was also an increase in the leaf-mass per unit area and the leaf-carbon content per unit area. This increased investment of carbon in leaves was presumed to be a mechanism to withstand low water potentials and would result in longer periods to recover the costs of the leaves. From the calculated gas exchange rates (based on leaf nitrogen contents and leaf Δ) the patterns in the leaf payback period are predicted in Chapter 5. The predicted increases in photosynthetic capacities as a result of increases in leaf nitrogen contents were not large enough to balance the increases in leaf carbon contents, and all but one species was predicted to have increasing leaf payback periods with decreasing rainfall across their distribution. This increase in the time required to recover the cost of the leaves, combined with a predicted decrease in the maximum supportable leaf area index could indicate the physiological limits to the distribution of each species. The environmental influences of decreasing water supply and increasing evaporative demand on leaf conductance are relatively well understood from experiments in controlled conditions (see Chapter 6). The link between leaf nitrogen contents and photosynthetic capacity, and then the theory relating Δ to leaf conductance and assimilation capacity are also well understood. The extent to which these links could be found in the 1300 measurements of leaf or wood Δ is explored in Chapter 6. Long-term predicted mean meteorological conditions did not explain the patterns or variability in leaf or wood Δ in a satisfying manner. The rainfall in the Northern Territory is strongly seasonal, so that the gradient in annual rainfall masks the fact that all along the transect there is a dry season with little or no rainfall for around three months or longer. The mean interval between rain events, or drought length, was often as important as rainfall in explaining patterns of b.. This could indicate that selection has tuned the rate of water-use to match the time period between supply events. The degree to which the trees of the Northern Territory are able to tap a reliable ground-water supply by increasing their allocation to root growth, and the degree to which they operate with partially open stomata (which would generate a low D. signal) rather than merely operating in an open or shut mode (with would avoid recording a low D. signal as there would be no concurrent assimilation) were beyond the scope of Chapter 7 which addressed the ability of the species to change their physiological and morphological characteristics across their distributions. Plasticity would allow a species to adjust to varying conditions, but is thought to be a risky behavior under the most extreme conditions when genetically controlled conservative behavior is often observed. Variability within populations may also decrease with increasing stress levels where only a limited sub-population of growth strategies remain viable. Physiological parameters, specifically wood and leaf Δ, had lower elasticities than the measured morphological parameters, emphasizing that subtle changes in Δ are of potentially great importance as indicators of plant performance. Although carbon isotope discrimination should have allowed a quick and efficient method for broadly establishing the potential ecophysiological controls on the distributions of Eucalyptus species in the Northern Territory, to be followed upon by direct measures of gas exchange parameters, the realities of the method preclude its viability in the limited duration of a PhD thesis. No single measure conducted in a laboratory thousands of kilometers from the field can compensate for a lack of direct field experience.
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