Phenology of a dipterocarp forest with seasonal drought: Insights into the origin of general flowering
Our understanding of the factors that shape reproductive phenology in Southeast Asian tropical forests comes almost exclusively from studies of everwet, general flowering (GF) forests. Therefore, we examined the phenology of an evergreen dipterocarp forest in a climate exhibiting seasonal drought in Southeast Asia, with the aim of evaluating what evidence it brings to bear on the hypothesis that reproductive phenology in contemporary dipterocarp forests is the product of evolutionarily conserved responses to seasonal drought.
We hypothesized that (1) dipterocarp forests that experience seasonal drought would exhibit annual reproduction and seasonality similar to that of forests with similar climates in other parts of the world, (2) within species, flowering frequency would be climate-dependent, not a fixed trait, and (3) climatic events that cue flowering in everwet forests would also cue flowering in seasonally dry forests.
From 2001 to 2009, we monitored flowering and fruiting monthly for 1,344 trees (>300 spp.) in a forest in Khao Chong (KC), Thailand that experiences an annual 2–4 month dry season, assessing frequency, duration, seasonality and synchrony of reproduction. We also examined phenological records for trees in three dipterocarp-dominated forests in everwet climates.
Reproductive phenology of the KC forest was more similar to tropical forests with similar rainfall seasonality in other parts of the world than it was to dipterocarp-dominated forests in everwet regions of Southeast Asia. The forest exhibited annual reproduction, with peak flowering occurring at the end of the dry season and peak fruiting occurring early in the wet season. Approximately half of all species and individuals reproduced annually, including several species that are known to exhibit “general flowering” in everwet climates. Short dry spells appeared to cue flowering for early flowering species, while later flowering species may have responded to either low precipitation or temperature, consistent with data from everwet forests. GF behaviour also showed significant phylogenetic signal.
Synthesis. Our results support the hypothesis that the phenological behaviour of both seasonal and everwet dipterocarp forests may have initially evolved in response to seasonal drought and that the general flowering phenomenon is a product of evolutionary conservatism.