Temperature is a dominant driver of distinct annual seasonality of leaf litter production of equatorial tropical rain forests

Abstract

Intra-annual periodicity of canopy photosynthetic activity and leaf development has been documented in seasonal and weakly seasonal tropical forests in the Amazon and elsewhere. However, vegetative periodicity such as leaf flush and fall in apparently ‘aseasonal’ equatorial tropical forests has not been well documented. Moreover, causal drivers of the vegetative periodicity in those forests have not been identified largely because of the difficulty in performing manipulative experiments targeting whole forest ecosystem dynamics. Here we show a distinct annual seasonality in canopy dynamics using a Fourier analysis with a statistical significance test on the long-term, fortnightly monitored dataset of leaf litterfall in nine evergreen tropical rain forests on Mount Kinabalu, Borneo. Statistically significant annual periodicity occurs across elevations and soil types in all years irrespective of the year-to-year climatic variability, suggesting that fluctuations in regional climate rather than local micro-climatic, edaphic and/or biotic conditions cause the precise 1-year periodicity. We examine climatic factors that have causative effects on the distinct 1-year periodicity using the spectrum convergent cross mapping that we developed in the present study that can distinguish causal relationships from seasonality-driven synchronization. According to the analysis, we find that mean daily air temperature is most strongly, causatively related to the 1-year periodicity of leaf litterfall. However, knowledge on ecophysiolocial and molecular mechanisms underlying temperature-control of tropical tree growth is limited and further studies are required to understand the detailed mechanisms. Synthesis. We suggest that intra-annual temperature changes in association with the movement of the intertropical convergence zone cause the distinct annual vegetative periodicity. Because vegetative periodicity can be transmitted to the dynamics of higher trophic levels through a trophic cascade, interactions between vegetative periodicity and daily air temperature, not rainfall, would more strongly cause changes in the dynamics of equatorial tropical rain forests. Our results show that clear vegetative periodicity (i.e. annual seasonality) can be found in equatorial tropical rain forests under diverse local environments, and that air temperature is a more important factor than the other climate variables in the climate–forest ecosystem interactions.