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Australian National University (2002)

Carbon isotope discrimination by Eucalyptus species in the Northern Territory, Australia

Miller, Jeffrey M. 

Titre : Carbon isotope discrimination by Eucalyptus species in the Northern Territory, Australia

Auteur : Miller, Jeffrey M. 

Université de soutenance : Australian National University

Grade : Doctor of Philosophy (PhD) 2002

Description partiel
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 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.

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