Reproductive asynchrony and the storage effect contribute to the latitudinal gradient in forest diversity
The high biodiversity of tropical forests relative to forests at higher latitudes is perhaps the most striking of biogeographical patterns. In the tropics, speciation rates are higher and extinction rates are lower, which may be explained by latitudinal differences in species interactions. For trees, the number of potential competitors that co-occur per hectare can be orders of magnitude higher in the tropics, suggesting that competitive interactions between species are weaker and hence extinction rates are lower. Yet few quantitative analyses support explanations of why local coexistence is more likely relative to higher latitudes. Here, we ask whether lower seasonality, corresponding to lower latitudes, can contribute to tree species coexistence by permitting more asynchronous recruitment among years. Empirically, we evaluate the hypothesis that aseasonality is associated with asynchrony in inter-annual recruitment. We analyze long-term seedling recruitment data from eight forests including tropical, subtropical, temperate, and boreal forests. We then use a mathematical model to show that the observed recruitment asynchrony can reduce interspecific relative to intraspecific competition through a mechanism known as the storage effect.
Our results showed that seedling recruitment was more asynchronous in less-seasonal forests at lower latitudes (seasonality was found to be almost perfectly correlated with latitude in our study). We found that synchrony in annual seedling recruitment decreased by 0.42% for each degree decrease in latitude (p=0.005). Our mathematical analysis indicated that the observed recruitment asynchrony can reduce interspecific relative to intraspecific competition via the storage effect by 0.25% for each degree decrease in latitude (p<0.001). Thus, coexistence in less-seasonal forests potentially benefits more from asynchronous recruitment, leading to a gradient in species interactions that can contribute to the maintenance of lower extinction rates and higher diversity of tropical forests.