Flooding and Profuse Flowering Result in High Litterfall in Novel Spathodea campanulata Forests in Northern Puerto Rico

The African tulip tree, Spathodea campanulata, has been introduced to and dominates many post-agricultural secondary forests in the moist tropics, particularly in islands. Some consider these novel forests have null to negative ecological value, yet they appear to restore ecosystem processes on degraded sites. This study describes the litterfall mass and seasonality, canopy phenology, and microclimate of S. campanulata forests on alluvial and karst substrates in northern Puerto Rico. These substrates have different water drainage properties and I hypothesized that (1) annual leaf fall mass and seasonality would differ between substrate types; because (2) leaf fall would be related to water availability and seasonality. I used analysis of variance to compare annual and biweekly litterfall mass across three sites on each substrate type, and multiple linear regression analysis to relate biweekly litterfall to environmental variables. Litterfall mass was high (13.8 Mg/ha/yr, n = 6, SE = 0.60) yet its components did not differ by substrate type except for reproductive part mass which was higher on karst due to more S. campanulata flowers. Leaf fall had a bimodal seasonality and was negatively related to the number of dry days indicating it occurs when water is readily available or in excess as during floods. Observations show systematic leaf senescence in this deciduous species can be caused by water and nutrient demand from flowering. Litterfall mass and seasonality of novel S. campanulata forests is similar to that of native forests in Puerto Rico, yet flower fall appears to be higher than that of tropical forests worldwide. The environmental variables that affect litterfall seasonality and canopy phenology are similar to those in tropical forests in Puerto Rico and elsewhere. Litterfall seasonality and canopy phenology regulate understory microclimate, and influence the establishment and growth of juvenile trees and other organisms within S. campanulata forests. Thus, the forest ecosystem processes and properties restored by novel S. campanulata forests facilitate tree species establishment, growth, and turnover in deforested, abandoned, and degraded agricultural lands in Puerto Rico. This study illustrates how anthropogenic land use change and species transport interact to modify the phenology of current forest cover, and suggests that anthropogenic climate change that modifies seasonal patterns of tempreature and precipitation will have an influence on the litterfall and phenology of novel S. campanulata forests.