During 1984 -2000, canopy tree growth in old-growth tropical rain forest at La Selva, Costa Rica, varied >2-fold among years. The trees' annual diameter increments in this 16-yr period were negatively correlated with annual means of daily minimum temperatures. The tree growth variations also negatively covaried with the net carbon exchange of the terrestrial tropics as a whole, as inferred from nearly pole-to-pole measurements of atmospheric carbon dioxide (CO 2) interpreted by an inverse tracer-transport model. Strong reductions in tree growth and large inferred tropical releases of CO 2 to the atmosphere occurred during the record-hot 1997-1998 El Niñ o. These and other recent findings are consistent with decreased net primary production in tropical forests in the warmer years of the last two decades. As has been projected by recent process model studies, such a sensitivity of tropical forest productivity to on-going climate change would accelerate the rate of atmospheric CO 2 accumulation. A lthough human activities are rapidly increasing atmospheric levels of the greenhouse gas CO 2 (1), understanding of the global carbon budget and how it is affected by climatic change remains approximate and evolving (2). Current knowledge of plant function, however, raises the likelihood that continued warming will alter the net carbon balance of global vegetation (3,4). Plant respiration increases exponentially with increasing temperature, whereas photosynthetic rates increase to a temperature optimum and then decline (5). At the ecosystem level, the balance between these two processes determines net primary production (NPP). Decreasing terrestrial NPP with rising temperatures would constitute a biotic positive feedback to the increase in atmospheric CO 2 (3, 4).Tropical rain forests, among the warmest terrestrial ecosystems, might be expected to be among the first to show negative temperature responses (6). Because these forests account for a third of global terrestrial NPP (7), any such responses could strongly affect atmospheric CO 2 levels. Studies at the leaf (8, 9) and stand level (10, 11) in this biome already suggest reduced carbon uptake with even small temperature increases. Measured respiration rates of tree boles in one tropical rain forest show an 8% increase with a 1°C temperature rise (12); compared with that of boles, the respiration of other plant parts tends to be even more sensitive to temperature changes (13). In addition, carbon losses by tropical trees in the form of volatile organic compounds increase exponentially over current temperature ranges (8,9). Quantifying responses of these processes at the ecosystem level, however, remains elusive because of limitations of both data and methods (14,15). Estimating forest carbon exchange with eddy covariance techniques has been found to be problematic for tropical forests, because of the prevalent still-air conditions at night (Ͼ80% of nights; ref. 11). In addition, short-term data may be poor indicators of longer-term trends, given the possibilities of c...
