Precipitation variability in tropical forests most strongly affecting trees with low wood density

The frequency and intensity of extreme climate events such as drought is expected to increase with climate change, particularly in the Amazon. Reduced dry season precipitation can affect forest growth, while long-term drought can increase tree mortality. Understanding the effects of changing precipitation patterns on tree growth is essential for anticipating the effects of climate change on tropical forests. To address this question we investigate the effect of precipitation variability on tropical forest tree growth using tree data from annually censused 1-ha monitoring plots from the Tropical Ecology Assessment and Monitoring (TEAM) Network, and 5-day precipitation data from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS). Significant regional patterns of precipitation variability are apparent across the monitoring sites in the TEAM network, particularly the strong Amazon drought in 2010.

Using CHIRPS data, we calculate the Standardized Precipitation Index (SPI), and model the effects of varying SPI on tree growth, while controlling for potential differences in growth between different stem diameter and wood density classes. We find that annual growth increases with declining wood density (p < .01), increasing diameter (p < .01), and increasing SPI (p < .01). We also find statistically significant interaction effects (p < .01) between SPI and wood density: the relationship between SPI and growth is strongest for lower wood densities. This suggests that trees with lower wood density are more sensitive to drought. Interaction effects between diameter class and SPI are not significant. Given the greater sensitivity of fast growing low-density trees to drought, reduced growth of these trees is likely to affect carbon uptake during drought periods disproportionately.