Quantifying responses of these processes at the ecosystem level, however, remains elusive because of limitations of both data and methods ( 14, 15). In addition, carbon losses by tropical trees in the form of volatile organic compounds increase exponentially over current temperature ranges ( 8, 9). Measured respiration rates of tree boles in one tropical rain forest show an 8% increase with a 1☌ temperature rise ( 12) compared with that of boles, the respiration of other plant parts tends to be even more sensitive to temperature changes ( 13). Studies at the leaf ( 8, 9) and stand level ( 10, 11) in this biome already suggest reduced carbon uptake with even small temperature increases. Because these forests account for a third of global terrestrial NPP ( 7), any such responses could strongly affect atmospheric CO 2 levels. Tropical rain forests, among the warmest terrestrial ecosystems, might be expected to be among the first to show negative temperature responses ( 6). 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. These and other recent findings are consistent with decreased net primary production in tropical forests in the warmer years of the last two decades. Strong reductions in tree growth and large inferred tropical releases of CO 2 to the atmosphere occurred during the record-hot 1997–1998 El Niño. 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. The trees' annual diameter increments in this 16-yr period were negatively correlated with annual means of daily minimum temperatures. During 1984–2000, canopy tree growth in old-growth tropical rain forest at La Selva, Costa Rica, varied >2-fold among years.
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