Investigation into the effects of elevated carbon dioxide and temperature on nutrient cycling and understorey vegetation in a Eucalyptus woodland

Forest ecosystems contribute substantially to biogeochemical processes on the earth. Understanding their responses to climate change is essential to the prediction of future climate as they could accelerate or decelerate the rates at which atmospheric CO2 concentrations and associated global temperatures are rising, depending on their carbon (C) storage capacity. However, very little is known about how mature forests respond to climate change, particularly those growing under P limitation, or NP co-limitation. This thesis aimed to investigate the effects of elevated (e)CO2 (ambient +150 ppm) on soil nutrient dynamics and understorey plant community composition in a novel field CO2 exposure experiment in a mature, P-limited, native Eucalyptus woodland (EucFACE). It also investigated the effects of elevated temperature (eTemp; ambient +3 ℃) on soil nutrient dynamics using whole-tree chambers (WTCs) in which Eucalyptus tereticornis trees were grown. I found seasonally-dependent positive effects of eCO2 on the availability and turnover of soil N and P over an 18-month period, with concurrent higher concentrations of dissolved organic carbon (DOC) in soil solution and lower soil pH. P availability was enhanced to a greater extent than N, resulting in decreased N:P ratios. The WTC experiment showed that, whilst eTemp enhanced P turnover and availability, and increased DOC in soil solution, effects on N availability and turnover were not statistically significant. In the case of both eCO2 and eTemp, increased DOC alongside increased soil P availability and decreased soil N:P ratios, suggests that plants adjusted their C investment strategies as their demand for nutrients, particularly P, increased. The EucFACE experiment also showed that eCO2 increased C3 plant species abundance compared to C4, and decreased species diversity in the understorey community. This thesis provides novel insights into and empirical evidence of soil nutrient, particularly P, dynamics under P-limited ecosystems in a higher CO2 and warmer world.Open Acces