Carbon use efficiency (CUE) is a difficult-to-measure ecosystem attribute that describes the fraction of gross photosynthesis (P_g) used by plants to construct new tissues, with the remainder allocated to autotrophic respiration (R_a) and other metabolic pathways.  Many temperate forest studies suggest that CUE should be a constant 0.50 across ecosystems, and few if any studies have demonstrated clear mechanistic controls over CUE variability.  Chambers et al. (2004) found that Central Amazon forests exhibit among the lowest measured CUE (0.30), and hypothesized that carbon may be relatively easy to acquire in a tropical environment, but that acquiring sufficient nutrients to build new tissues is difficult on highly nutrient deficient soils.  A Masters student at INPA in Brazil continued this line of research to demonstrate that trees surviving in a logged experiment demonstrated higher CUE, closer to the proposed temperate forest constant.  Recently, researchers at an Eastern Amazon site also quantified an increase in CUE for trees surviving logging, strengthening our results from the Central Amazon.  Studying shifts in plant carbon allocation along resources gradients will be a productive line of future research.  Tests of these hypotheses will require experiments in the field and under controlled greenhouse environments, using a combination of ecological, physiological, and isotopic measurements.  A review article covering these and other ecophysiological adn biogeochemical topics was recently published in Philosophical Transactions of the Royal Society of London (Chambers and Silver 2004).

Dr. Edgard Tribuzy at INPA is leading a project studying canopy photosynthetic response to changes in leaf temperature using a portable photosynthesis system (LiCor 6400) and an automated thermocouple method we developed.  By experimentally manipulating temperature and CO₂ concentration, key biochemical characteristics were quantified (e.g., V_cmax, J_max, temperature optimum), including how these parameters vary with other ecological characteristics important for scaling from leaf to ecosystem.  Using these manipulations, we found that when leaf temperature increased beyond ~36 ^oC, net photosynthesis fell sharply, although there was interesting response variability among individuals.  Further investigations using the automated thermocouple system distributed among trees located near a number of canopy access towers demonstrated that leaf temperatures exceeding 36 ^oC were surprisingly common, reaching a maximum near 45 ^oC.  We are currently preparing manuscripts based on this work, and some of this research was recently highlighted in the 27 Oct 2007 issue of New Scientist.