Climatic controls of tropical rainforest productivity: Results from long-term forest monitoring

Background/Question/Methods Because tropical rain forests store globally-significant amounts of carbon and annually process vast quantities of CO₂, the response of this biome to climate change can strongly influence the rate of planetary warming.  Understanding the relation between forest productivity and climate variation in tropical forests has remained elusive because long-term measurements of annual productivity and local climate are virtually non-existent.  Here we present data from two parallel studies of annually-measured tropical tree performance and associated climate over periods of 10-24 years in old-growth tropical wet forest in Costa Rica.  The results challenge current hypotheses that tropical rain forest productivity increases with higher CO₂ and radiation or that it decreases with elevated rainfall. 

Results/Conclusions Tree growth decreased with increasing nighttime temperatures in forest inventory plots (10 annual censuses) as well as in focal–species studies (24 annual censuses).  In the inventory plots, annual tree mortality increased with increasing nighttime temperatures, and annual tree growth was strongly negatively correlated with dry season intensity.  These results suggest that the warming and drying climates predicted for many areas of the tropics will produce rapid and large changes in carbon cycling and in the physical and biological structure of tropical rain forests.  Given the strength of the relationships observed in this study, such changes may well be underway already. 
Long-term studies on ecologically-relevant (annual to sub-annual) times steps are urgently needed to monitor forest response to variations in current climate. 

These data on climatic sensitivity of forest performance provide critical information on how increasing temperature and CO2 are likely to affect productivity in these systems.  We illustrate with examples from these studies the twin effects of length of study interval and date of initiation on the probability of detecting climatic sensitivity in annual-scale forest performance data.  Long time series, taken at annual time steps at the same site and carried out with systematic attention to accuracy and precision issues, are key for understanding forest response to constantly changing climate conditions.  Unfortunately these types of studies are exceedingly difficult to fund in the current culture of short-term studies addressing novel hypotheses.