Patterns of Leaf, Flower, and Fruit Phenology and Environmental Relationships in a Seasonal Tropical Forest in the Indian Eastern Himalaya

Abstract

Tree phenology plays an important role in determining the structure and function of tropical forest communities. However, there are few long-term studies on tree phenology from South Asia. We monitored 716 trees of 54 species monthly from 2011 to 2023 for leaf flush, flowering, and fruiting in the Pakke Tiger Reserve, Arunachal Pradesh, and North-east India. We examined monthly patterns in the percentage of species and trees in leaf flush, flower, and fruit and characterized phenological seasonality using circular statistics. We hypothesized that phenological patterns would show significant seasonality, with phenological activity being triggered by increasing solar irradiance and daylength. Flowering periodicity was classified using Fourier analysis, and we examined the relationships between phenological activity and temperature, rainfall, solar radiation, and daylength using GAMLSS. Leaf flush and flowering were moderately seasonal, peaking in the warm dry season. Fruiting patterns and their seasonality differed among dispersal modes. Most species (78.13%) and trees (51.17%) flowered annually. In accordance with our hypothesis, daylength and solar radiation served as proximate cues for phenological activity through nonlinear effects. This indicated the existence of narrow ranges of optimal climatic conditions for phenology. Rainfall was not an important proximate cue as water availability was not a constraint for tree phenology in the site. Our study is the first long-term phenological monitoring effort that establishes the proximate climatic cues for the observed patterns of leaf flush, flowering, and fruiting in the seasonal tropical forests of the Eastern Himalaya.