Abstract
Increased atmospheric [CO 2 ] could theoretically lead to increased forest productivity (‘CO 2 fertilization’). This mechanism was hypothesized as a possible explanation for biomass increases reported from tropical forests in the last 30+ years. We used unique long‐term records of annually measured stands (eighteen 0.5 ha plots, 10 years) and focal tree species (six species, 24 years) to assess the effects of rainfall, temperature, and atmospheric [CO 2 ] on annual wood production in a neotropical rain forest. Our study area was a meso‐scale section (600 ha) of old‐growth Tropical Wet Forest in NE Costa Rica. Using the repeated remeasurements we directly assessed the relative effects of interannual climatic variation and increasing atmospheric [CO 2 ] on wood production. A remarkably simple two‐factor model explained 91% of the interannual variance in stand‐level tree growth; the statistically independent factors were total dry season rainfall (positive effect, r 2 =0.85) and night‐time temperature (negative effect, r 2 =0.42). Stand‐level tree mortality increased significantly with night‐time temperature. After accounting for dry season rainfall and night‐time temperature, there was no effect of annual [CO 2 ] on tree growth in either the stand or focal species data. Tree growth in this Tropical Wet Forest was surprisingly sensitive to the current range of dry season conditions and to variations in mean annual night‐time temperature of 1–2°. Our results suggest that wood production in the lowland rainforests of NE Costa Rica (and by extension in other tropical regions) may be severely reduced in future climates that are only slightly drier and/or warmer.