The Amazon rainforest plays a key role in regional and global biogeochemical cycling, but the region may undergo an increase in the frequency and severity of drought conditions driven by global climate change, regional deforestation and fire. The effects of this drought on carbon cycling in the Amazon, particularly below-ground, are potentially large but remain poorly understood. This thesis, therefore, examines the impacts of seasonal and longer-term drought upon ecosystem carbon allocation and cycling at an Amazon rainforest site with a particular focus upon below-ground processes. Measurements are made at three one-hectare (100 × 100 meter) forest plots with contrasting soil type and vegetation structure, to observe responses across a range of Amazon primary forest types. A fourth plot is subjected to partial rainfall exclusion to permit measurement of forest responses to a wider range of soil moisture levels than currently exists naturally.
An analysis of the number of samples required to accurately quantify key ecosystem carbon stocks and fluxes is used to guide the sampling strategy at the field site. Quantifying root dynamics, in particular, presents methodological challenges. Thus, I critically review existing methods, and develop novel techniques to accurately measure root standing biomass and production. Subsequently, these techniques are used to record root responses, in terms of standing biomass, production, morphology, turnover and nutrient content, to variation in soil moisture across the four rainforest plots. There is substantial inter-plot environmental variation in root characteristics. However, several responses remain consistent across plots: root production of biomass, length, and surface area, decreases under drier conditions, while root length and surface area per unit mass show the opposite pattern.
The other major component of the below-ground carbon cycle is soil carbon dioxide efflux. I partition this efflux, on each plot, into contributions from organic ground surface litter, roots and soil organic matter, and investigate abiotic and biotic causes for observed differences within and between plots. On average, the percentage contribution of soil organic matter respiration to total soil carbon dioxide efflux declines during the dry season, while root respiration contribution displays the opposite trend. However, these seasonal patterns are not directly attributable to changes in either soil moisture or temperature. Instead, organic surface litter mass and root mass account for 44 % of observed variation in soil carbon dioxide efflux.
Finally, information on below-ground carbon cycling is combined with above-round data, of canopy dynamics and stem wood production and mortality, to provide a holistic picture of the potential effects of drought upon an Amazon forest ecosystem. Comparison of the rainfall exclusion plot with a similar, but unmodified, control plot reveals potentially important differences in tree carbon allocation, mortality, reproduction, soil respiration and root dynamics. The apparent net consequence of these changes is that, under drier conditions, the amount of CO2 moving out of the forest and into the atmosphere is diminished. This synthesis of above-ground and below-ground data is a significant advance in our understanding of carbon cycling in rainforests, and provides information which should allow more accurate modelling of the response of the Amazon region to future drought.