Plant Structure-Function Relationships and Woody Tissue Respiration: Upscaling to Forests from Laser-Derived Measurements

Land surface processes dominate the observed global signal of large inter-annual variability in the global carbon cycle , and this signal is itself dominated by responses of tropical forests to climatic variation and extremes. However, our understanding of the functioning of these forests is poorly constrained, not least in terms of the size and climate-sensitivity of gross ecosystem respiratory CO2 emission. Woody tissue CO2 effluxes contribute substantially to gross ecosystem CO2 emissions, thereby influencing the net ecosystem exchange of carbon. Our ability to estimate this component of the forest respiration budget has been limited by our technical capacity to measure vegetation size and structure in sufficient detail and at sufficient scale. The outcome has been to leave large uncertainties in land-surface model performance and prediction. A key challenge in estimating woody tissue CO2 efflux for the ecosystem has been the scaling of measurements made with chambers from the level of an organ to the stand. Appropriate scalars such as woody tissue mass, surface area and volume all require accurate structural information on both size and pattern. For individual trees, pattern is dominated by branching structure and this fundamentally determines how trees partition resources to address the trade-offs inherent in the simultaneous maintenance of structural integrity and metabolism. The detailed structural information needed to address this challenge has until recently been extremely scarce because of the difficulty of acquiring it, even for a single large tree. Recent developments in terrestrial light detection and ranging (LiDAR) technology have made possible a step change in our ability to quantify and describe tree form for continuous forest, for example describing hundreds of adjacent trees at the hectare scale. Connecting this new capability with tree physiology and fundamental theories of plant structure and metabolism offers to change the way we understand plant functional biology and its variation with environment, biogeography and phylogeny.