In 1981, we began annual tree growth measurements in Guánica Forest, a well-protected tropical dry forest in Puerto Rico. Since then, the forest has experienced two major hurricanes, the wettest year and two of the three driest years on record. Mean annual rainfall since 1931 is 823mm. Initially, we measured diameter growth quarterly to understand seasonal growth patterns of trees that were ≥2.5 cm diameter at breast height (1.4 m) and a few juvenile trees (n = 35). More recently these measurements have continued annually. Measurements were taken at five locations on each tree bracketing breast height with a diameter tape. Juveniles were measured with a caliper at 1 m. Over time, we added trees into the census to increase sample size and replication among common species (overall: n = 475 mature trees). In 1998 we also added 240 juvenile trees (> 1m height, <2.5 dbh). Some trees have been lost to mortality and windthrow. Our aim has been to understand long-term trends in forest growth and the effects of drought and hurricanes on growth and mortality patterns.
Results/Conclusions
Tree growth clearly responds to annual rainfall (p<0.01) although the wettest year (>1400 mm) resulted in below predicted growth, likely because much of the rainfall was heavy but in isolated events. Mean annual diameter increment regardless of rainfall is low - only a few mm each year. Growth rates in the year containing Hurricane Georges (1998) were 3-4-fold greater than the long-term average, but there was not a similar growth response to Hurricane Maria in 2017. Drought years were frequently accompanied by negative increments. For some individuals, negative increments continued for 2-3 years while tree crowns produced leaves, but eventually resulted in mortality. Juvenile trees had greater variability in increment growth rates than canopy trees, but many remained small-diameter, understory trees throughout the period of study. What appears to be a “sapling” can be quite old. As expected, species responded to rainfall in different ways, some species’ growth increments responded more to rainfall in the previous year rainfall than the current year. Long-term records of growth and mortality are important to understanding forest productivity and dynamics, particularly as climate changes and modeling efforts intensify. Conclusions from the initial 5 years of this study would have been different than the overall trend.