COS 154-3 - In-synch in good times--and other long-term fruiting patterns in a montane tropical forest

Abstract: Changes in the relative timing of fruit production of different plant species in a community can affect resource availability for consumers and may have cascading effects throughout the ecosystem. However, few studies have assessed changes in community-scale fruit production on timescales sufficient to capture the variation in fruit production cycles, which, in tropical forests, can range from months to several years. Community patterns of fruit production depend on species-specific responses to environmental cues and on selective pressures influencing whether plants fruit synchronously or complementarily relative to other species or individuals. Here, we examine a 24-year dataset of fruit production in Nyungwe National Park, Rwanda to assess changes in the complementarity (temporal non-overlap) and synchrony (temporal overlap) of fruit production and differences in phenological patterns between three successional guilds of trees: pioneers, non-pioneer light demanders (NPLD), and shade-tolerant (ST) species. We used generalized additive models to assess non-linear temporal trends and the influence of elevation on patterns of fruiting complementarity and synchrony. We also used Bayesian model selection of generalized additive models to identify potential climate predictors of the observed fruiting phenology patterns. Pioneer species had more synchronous fruiting than the later-successional NPLD and ST species and varied more in complementarity and synchrony over the course of the study, though all successional guilds increased in synchrony in the last years of the study. Higher synchrony was accompanied by higher fruit production in a given year, potentially owing to mast-seeding events or different species and individuals responding similarly to favorable environmental conditions for fruiting. Synchrony varied by elevation for NPLD and pioneer species, potentially due to species turnover with increasing elevation and different sensitivities of tree species to environmental changes at different elevations. The only climate factor identified as a good predictor of fruit production was irradiance, which was positively related to complementarity and inversely related to fruit production, though it is not yet clear how irradiance is mechanistically related to the fruit production patterns in this study. These findings show that differences between functional groups in the synchrony or complementarity of fruiting may be countered when these functional groups respond similarly to changing environmental cues that promote higher fruit production. Such changes in community-level patterns of fruit production could have consequences for plant species coexistence, resource availability for frugivores, and long-term community stability and biodiversity.