Timing matters: the seasonal effect of drought on tree growth

Droughts are expected to increase in frequency and severity in many regions as climate changes. The impact drought has on tree growth may depend on the timing of drought. The historical climate-growth response of trees provides insight into how a species may respond to future changes in climate and also enables us to understand how drought and the timing of drought has impacted growth. In this study, we use dendrochronology to examine the climate and drought response of Quercus myrtifolia Willd. growth on sites from three different scrub ridge systems in central Florida. Five site chronologies and a regional chronology were created from tree-ring measurements. Growth of Q. myrtifolia correlated positively with spring precipitation and the standardized precipitation index (SPI). March, April, May, and June SPI explained 21.5 to 58.3% of the variance in growth of Q. myrtifolia. The growth response of Q. myrtifolia to spring SPI was similar between sites, with the exception of Malabar West, where Q. myrtifolia occurred on poorly-drained soils. Seasonal droughts explained more of the variation in Q. myrtifolia growth than annual droughts. Spring droughts significantly decreased Q. myrtifolia growth at all sites, and summer droughts significantly reduced growth during the subsequent year at one site. Our data suggest that spring drought is the climatic limiting factor for Q. myrtifolia growth.