School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemClimate and climate change effects on carbon uptake and storage in Australia’s wooded ecosystems( 2022)Forest ecosystems are central to the land carbon sector due to their capacity to store and sequester carbon. Many studies have demonstrated that forest carbon uptake and storage is strongly dependent upon climatic conditions. However, the effects of climate on forest carbon uptake and storage in different biomes are still uncertain. Climate change may alter carbon dynamics within forest ecosystems through the direct effects of increased temperature, increased CO2 concentration and changing precipitation regimes. Yet forests may also adjust to changing climate through mechanisms such as thermal acclimation. In this thesis I used three modelling approaches (machine learning, boundary-line analysis, and a land-surface model) to examine how climate of the recent past, present, and future affect carbon uptake (as Gross Primary Productivity, GPP) and storage (as above-ground biomass, AGB) in Australian forests. Furthermore, I explored how current GPP adjusted to thermal regimes and how acclimation affected carbon uptake and storage in the future. In my first quantitative chapter (Chapter 2), I explored relationships between carbon storage (as AGB) with climate and soil in Australian forests across the continent. I developed RandomForest models with climate-only, soil-only, or climate plus soil variables to examine whether climate or soils are better predictors of forest biomass at the continental scale and to identify the most important predictor variables. In this chapter I demonstrated that climate (particularly temperature and the timing of precipitation) was more important than soil for explaining variation in AGB across Australia’s forests. In Chapter 3, I used boundary-line analysis to examine the ecosystem temperature response of carbon uptake (as GPP) in 17 wooded ecosystems representing five distinct ecoregions. These responses were represented as a convex parabolic curve that was similar in shape among ecoregions – narrow in tropical forests and broader in woodlands. I then derived the thermal optima of GPP (Topt) from these curves for each ecosystem and examined the relationship between Topt and mean air temperatures across sites. My analysis revealed a strong positive linear relationship between Topt and mean air temperature that indicated GPP was optimised to the present climate. Finally, in Chapter 4, I predicted how carbon uptake and storage will be affected by climate change in these 17 ecosystems and examined the effects of thermal acclimation of photosynthesis on these predictions. I used the CABLE-POP land surface model adapted with thermal acclimation of photosynthetic functions and forced with climate projections from the extreme climate scenario RCP8.5. My simulations indicated that increased temperature, CO2 concentration and changed precipitation patterns will have a positive effect on future carbon uptake and storage in the majority of the 17 ecosystems. Furthermore, thermal acclimation of photosynthesis is likely to enhance this effect in tropical ecosystems. My results confirm that carbon uptake and storage in Australian forests are fundamentally linked to temperature and precipitation regimes, and that these forests may be capable of adjusting to climatic conditions. My research indicates that the direct effects of climate change are likely to enhance the storage and sink capacity of Australia’s forests in the future. While I did not assess the indirect effects of climate change on carbon cycles through changes to disturbance regimes, overall, my thesis suggests that carbon uptake and above-ground biomass carbon stores in Australia’s forests are likely to be resilient to climate change.
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ItemPhotosynthetic responses to light, nitrogen, phosphorus and pruning of Eucalyptus in south-eastern Australia( 2005)Eucalypts frequently grow faster after additions of fertiliser, but more slowly in the shade or following `green pruning'. The coupling of rates of growth to environmental factors is at least partly due to acclimation of photosynthetic processes. Photosynthesis rarely proceeds at maximum rates in natural environments as photosynthetic processes and the supply of basic requirements of photosynthesis (CO2, H20, light, phosphorus and nitrogen) vary at both short (minutes to hours) and longer (days to months) time scales. Currently we lack mechanistic explanations for how these variables, alone and in combination underpin changed growth rates in Eucalyptus. This study examined growth and photosynthetic characteristics in glasshouse-grown seedlings and field-grown trees of Eucalyptus species that are commonly planted for forestry and revegetation purposes in central Victoria. Acclimation to light (among seedlings and within canopies), nutrient availability (phosphorus and nitrogen) and increased sink-strength for photosynthates were the primary foci of the study. In each instance I examined distribution of leaf nutrients within a canopy and allocation of N to Rubisco and chlorophyll to assess the degree to which nutrients limit photosynthesis in Eucalyptus. A novel technique was introduced to quantify the allocation of inorganic phosphorus within cells (cytoplasm versus vacuole), followed by an assessment of inorganic phosphorus allocation in response to a long-term reduction in phosphorus supply. In all circumstances, rates of growth were responsive to environmental conditions. Growth responses were underpinned by altered patterns of biomass partitioning and changed leaf morphology more than by rates of photosynthesis per se. There was little difference in adaptive strategies implemented by seedlings and trees: both were oriented towards the accumulation of nutrients rather than increasing rates of photosynthesis. Photosynthesis was reduced by shading (among different plants and within the canopy of a tree) and reduced phosphorus supply whereas N had little effect on photosynthesis. Analysis of pools of inorganic P revealed that adequate supplies were maintained for photosynthetic processes regardless of P supply, therefore reduced photosynthesis follows, rather than leads, a more general leaf-level response to reduced P. Similarly, changed partitioning of nitrogen between Rubisco and chlorophyll was unnecessary as leaf nitrogen concentrations were consistently maintained at well above published minimum levels. Hence, photosynthesis was not up-regulated following increased nitrogen or phosphorus supply; instead excess nutrients were accumulated and used to support increased biomass. One exception was after defoliation, when up-regulation of photosynthesis was observed, presumably to ensure the demand for photosynthates could be met by a reduced leaf area. Sensitivity analyses consistently revealed variation in photosynthetic rates owed more to altered biochemical activity (e.g. Jmax and Vcmax) rather than stomatal conductance regardless of growth condition (glasshouse versus field). Hence, whilst Eucalyptus has considerable photosynthetic potential, faster rates of carbon fixation are only exhibited in the short-term. In part, this is due to the multiplicity of factors involved in `optimisation' of photosynthesis and their individual and collective responses to environmental conditions. In the long term however, increased canopy photosynthetic capacity follows only an increased photosynthetic area.
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