Leaf respiration in tropical and temperate rainforest tree species : responses to environmental gradients

Abstract

Among forest biomes, tropical and temperate rainforests often show high rates of net primary production (NPP), with rainforests playing a major role in controlling current and future atmospheric carbon dioxide concentrations, and the total amount of carbon stored in vegetation globally. Quantifying rates of leaf respiration (R) of rainforest species and how changes in environment (e.g. light, short- and long-term changes in temperature (T), gradients in nutrient availability) affect R in rainforest species, is crucial in determining how much carbon dioxide is released now and will be released in the future. In this thesis, I sought to explore the effect of a range of environmental gradients on leaf R rates of rainforest tree species, both under controlled and field conditions. Leaf R in darkness (RD) and in the light (RL) exhibited marked differences between upper and lower canopy position in a tropical rainforest. When measured at prevailing growth T at each site, no systematic differences in RD or RL were evident across thermally contrasting rainforest sites in Australia, or across an elevation gradient from Amazon to Andean sites in Peru. When averaged across all forest sites, 30% light inhibition of leaf R was found. Over the 25-45 degrees Celsius range, the T sensitivity of RD was similar in upper and lower canopy leaves; although the Q10 (i.e. proportional change in leaf RD per 10 degrees Celsius rise in temperature) exhibited a decreasing trend with increasing measuring T, there were no marked differences among the thermally contrasting rainforest sites in the measuring T response. Strong evidence was found that tropical and temperate rainforest plants are equally capable of acclimating leaf RD to different growth Ts, including growth Ts higher than that were normally experiencing in their current habitats. The results suggest that rainforest species may be capable of acclimating leaf respiratory metabolism to increases in growth T associated with future global warming; with the result that warming will not necessarily lead to exponentially higher rates of respiratory carbon dioxide release from temperate and tropical rainforest ecosystems. The field studies in Peru highlighted the fact that variations in soil P availability have no systematic impact on leaf RD of tropical lowland forests; this finding was supported by controlled environment study showing that deficiencies in leaf phosphate did not result in lower rates of leaf RD. Collectively, these findings have implications for vegetation-climate models that seek to predict carbon fluxes between rainforests and the atmosphere. Show more