Tropical climates are often considered to be stable relative to higher latitudes that experience large seasonal fluctuations in temperature and rainfall. Climate stability in turn implies constancy, yet, we are finding that tropical forests are surprisingly dynamic, with high local turnover rates of individuals and species. Variation in plant reproduction and seedling performance drive much of this turnover. Some of the variation among years in reproduction or seedling performance is driven by climate cycles ranging from annual seasonality to decadal fluctuations with effects at multiple spatial scales, varying from local orographic effects to global climate cycles. At the same time, anthropogenic factors are causing unidirectional changes in atmospheric CO2and temperature, which, in turn, are expected to alter the distribution and frequency of storms and droughts. We are quantifying flower and seed production with high temporal resolution to evaluate the effects of climate variation on weekly to decadal time scales. Associated annual seedling dynamics data relate variation in seed production to seedling recruitment. All of this is done in fully enumerated forest dynamics plots in three Neotropical sites to place tree reproduction and seedling performance in the context of local species composition.
During a decade of LTREB funding to study intra- and interannual change in forest reproduction in Puerto Rico, Ecuador, and Panama, we discovered very high inherent variation in plant reproduction. Even at the most aseasonal site in Ecuador, the peak date of flowering for a species can vary by six weeks among years. Each year at our study sites, pulse reproductive events contribute to further variation, such that two or three decades are needed to capture even basic dynamics. El Niño Southern Oscillation (ENSO) was found to strongly impact patterns of reproduction in Panama, an early surprise result that now contributes to our understanding of the worldwide impact of ENSO cycles. In Puerto Rico, ENSO was also a strong driver of reproductive variation, and to a lesser degree, two Atlantic climate cycles, but these latter effects may have been overwhelmed by damage from two severe hurricanes within 25 years. Flower production has increased with time via the effects of CO2 fertilization in Panama, but this pattern was not evident in Puerto Rico or Ecuador. Thus, evidence of anthropogenic effects was not consistent among sites.