Increasing fruiting synchrony at the community level in an Afromontane tropical forest

Changes in community-level patterns of fruit production can affect resource availability for frugivores, causing cascading effects in forest ecosystems. However, few studies have assessed changes in community patterns of fruit production on timescales sufficient to capture variation in tropical forests. As different tree species respond idiosyncratically to changes in environmental cues, synchronicity or complementarity (overlap or non-overlap) in fruit production among species may shift, with important consequences for the reliability of fruit resources. Such community-level changes may be context-specific; for example, different successional guilds of trees (e.g., pioneers, non-pioneer light demanders, and shade-tolerant species) are adapted to different environmental conditions and likely have unique responses to environmental cues. We examine a 24-year dataset of fruit production in Nyungwe National Park, Rwanda, a tropical montane forest, to assess changes in complementarity and synchrony of fruit production and differences in phenological patterns among three successional guilds of trees: pioneers, non-pioneer light demanders (NPLD), and shade-tolerant (ST). We used generalized additive models to assess nonlinear temporal trends and elevational influences and Bayesian model selection to assess climate predictors of the observed phenological patterns. Pioneer species fruited more synchronously and with more variation than late-successional NPLD and ST species. All guilds increased in synchrony from 2015 to 2019 and higher synchrony coincided with greater fruit production, suggesting that species respond to similar environmental cues for higher fruit production. Synchrony increased with elevation for NPLD, while for pioneers, synchrony was highest at the mid-to-high elevations. Irradiance was positively related to complementarity and inversely related to fruit production. These findings show that differences between functional groups and species in fruiting synchrony or complementarity may decrease when species respond similarly to environmental cues, such as changing irradiance levels. Such changes in community patterns of fruit production could have consequences for frugivore resource availability, plant species coexistence, and long-term community stability and biodiversity.