School of Agriculture, Food and Ecosystem Sciences - Theses

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Start date: 2004 × End date: 2009 × Author: Simpson, Robert Randall. ×

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    Edaphic and vegetation factors controlling carbon and nitrogen pool sizes and greenhouse gas emissions in south-eastern (sub-alpine) Australia
    Simpson, Robert Randall. (University of Melbourne, 2005)
    Climate, vegetation and disturbance regimes in part determine both pools and fluxes of carbon and nitrogen in terrestrial ecosystems. These interactions have not been examined in the Australian Alps. I examined the influence of edaphic and vegetation factors on the size of the pools and fluxes of C and N on the Bogong High Plains (BHP) - a paleoplains formation located at the southern reaches of the mainland alpine formation. Vegetation distribution on the BHP is a function of microclimatic gradients that result from the topography common to the Plains formation. Differences in mean minimum temperatures along the vegetation/topographical gradient were small; usually less than 2 �C separated the different communities. These results might suggest the BHP is at risk of substantial changes in vegetation distribution should there be the predicted 2-4 �C increase in ambient air temperatures in coming decades, and particularly if there is an increase of this magnitude in the minimum temperature or in the diurnal range. Nonetheless there is insufficient data and knowledge to predict if either individual species or communities will acclimate to increasing temperatures or if there will be significant shifts in their distribution. On the other hand, considerable accumulated knowledge suggests soil processes will respond strongly to changes in temperature. Rates of decomposition did not vary amongst communities, however they were strongly correlated with initial litter type (p < 0.001). Such relationships suggest that microclimate exerts control over some elements of the C and N cycles indirectly through its effect on vegetation distribution. Gaseous fluxes of C as CH4 were consistently negative with no effect of vegetation community. Seasonal mean rates of CH4 oxidation ranged from -65 pg m-2 h-1 to -115 ?g m -2 h-1, suggesting that the BHP are a strong sink for atmospheric methane over the entire year. Rates of CO2 flux were generally high with seasonal means ranging from 100 to 700 mg m-2 h-1. Surprisingly, the alpine heathlands were stronger sources of CO2 than the seemingly more productive Alpine Ash forests (p = 0.05) that fringe the BHP. Losses of gaseous N from study plots were strongly stochastic in nature and not correlated with vegetation community. Seasonal mean N2O fluxes ranged from a net uptake of -6 ?g m-2 h-1 to a net flux of 13 ?g M-2 h-1. Fluxes of all three gases displayed general trends with both soil temperature and moisture content in both in situ and in vivo measurements. The greatest moderator of system C and N was undoubtedly fire. Fire stimulated the turnover of soil N through increases in soil inorganic N and pH that remained evident for up to one year following fire, allowing for the possibility of nitrification. Recolonisation of sites following fire further altered C and N dynamics, particularly in the Alpine Ash forest where N-fixing Acacia spp. maintained high levels of soil inorganic N and supported nitrification for up to three years (and possibly longer) after fire. Overall, cycles of C and N in the communities studied here are likely to be only moderately directly affected by changes in climate. On the other hand, there is evidence from this study and others to suggest that increased fire frequency, that may be a result of climate change, has the potential to significantly alter C and N on the BHP.