Drought, strong negative departures of rainfall and stream flows from historical trends, occur rarely in wet tropical forests but are predicted to become more frequent under modeled global climate change scenarios. 2015 was unusually dry in northeastern Puerto Rico, resulting from one of the strongest recorded El Niño events in history. The Luquillo Long-Term Ecological Research Program monitors rainfall, stream flow, litterfall, forest reproductive phenology, and tree growth in tabonuco forest, (200m – 550 m elevation) where rainfall is normally 3500 mm yr-1 without a significant dry season. We used these long-term measurements to characterize the ecosystem responses to drought focusing on vegetation responses by contrasting the observed patterns from 2015 with patterns from previous decades. Rainfall was measured at El Verde Field Station (EVFS; 350 masl); stream flow was gauged in the nearby Quebrada Sonadora (~400 m masl), and litterfall was collected in 3 replicate 0.09 ha plots located between 350 – 500 masl (~1 km from EVFS). Reproductive phenology (120 flower/seed traps) and tree diameter growth (from the 1000 largest trees) were monitored in the 16–ha Luquillo Forest Dynamics Plot (LFDP; 333–428 masl and 0.5–1 km from EVFS).
During all of 2015, rainfall was approximately 50% of normal. Departure from the 40-year average of cumulative rainfall was evident by April. Stream flows were well below 25-year average levels by early May and this departure was evident through early November. Litter fall exhibited a strong peak in mid-May followed by reduced inputs until early September, when Tropical Storm Erika brought down additional litter. The peak was 3.5-fold greater than the 12-yr average for May and was associated with large numbers of aborted fruits in seed/flower traps. Diameter increments of trees in the LFDP were 30% reduced in 2015 in contrast to the previous two years. Fall storms brought an end to meteorological drought and, eventually, the hydrological drought. The timing of the 2105 drought mimicked patterns predicted by global circulation models (GCMs), i.e., a much stronger mid-summer drought than has been normally observed (usually no more than a month in duration). The drought was clearly stressful for forest vegetation at this elevation in the Luquillo Mountains. Assuming these conditions become more common as currently predicted by GCMs, these forests would suffer significant alteration of phenology and tree growth at increasing frequency. At what point increased drought will lead to alteration of forest species composition is currently unclear.