Distinct annual seasonality in Bornean tropical rain forests

Authors Kanehiro Kitayama, Masayuki Ushio & Shin‐Ichiro Aiba discuss their recently published Journal of Ecology article: Temperature is a dominant driver of distinct annual seasonality of leaf litter production of equatorial tropical rain forests

Find out more about this driver of annual seasonality in tropical rain forests and discover the future implications of this research.

Aseasonal equatorial tropical rain-forest climate

Tropical rainforest climate remains hot, humid and wet throughout the year, without any marked seasonal changes. Equatorial tropical rain forest climate, occurring near the equator under the influences of the intertropical convergence zone (ITCZ) away from the trade winds, is considered even more aseasonal (seasonless). Monthly air temperature does not change much and monthly rainfall is always ample during the course of a year. Tropical rain forests flourish under such a climate, unlimited by coldness or dryness. Ecologists have long believed that tropical rain forests in this region do not show annual (12-month) periodicity in vegetative growth. Equatorial tropical trees commonly lack annual rings.

Mount Kinabalu, north Borneo (4095 m), where this study was conducted. Photo: Kanehiro Kitayama

Examining the annual seasonality of vegetative growth on Mount Kinabalu, Borneo

We monitored annual periodicity of vegetative growth at various locations on Mount Kinabalu, north Borneo. Kinabalu is the highest mountain (4095 m) in Southeast Asia and located near the equator at 6°5’N. Kinabalu consists of various soil types with different nutrient availabilities, which may affect vegetative growth and its seasonality. We selected nine forests between 700 and 3100 m elevation on various soil types and collected fallen litter at 2-week intervals for ten years from 1996 until 2006 (one site abandoned later). We collected litter because litterfall was considered to reflect vegetative growth and show most sensitive responses to meteorological changes. We also monitored meteorology on the slope of Kinabalu.

Leaf-litterfall amount at 2-week intervals demonstrated a distinct and exact 12-month periodicity in nearly all forests after we applied a Fourier analysis (which is a time-series analytical tool to detect dominant periodicity). Periodicity of 12 months in leaf litterfall was consistent irrespective of elevations and soil types, suggesting that regional climate rather than local environments determines the periodicity. Our finding contradicts the traditional thoughts that equatorial tropical rain forests lack annual seasonality.

Distinct annual seasonality with an exact 12 month periodicity in the study sites.

Next, we tried to determine causative climatic drivers for the 12-mo periodicity. Traditionally, we apply a correlation analysis on the temporal patterns between response variables and climate variables. However, such correlation analyses cannot distinguish true causative drivers from simply correlating drivers. We, therefore, invented a new statistical method (the spectrum convergent cross mapping), which could identify a causative driver even when variables are highly synchronized. According to our analyses, daily air temperature was the most consistent, powerful causative driver. Only exception is the cloud forest at 2700 m, where both air temperature and leaf litterfall did not show a 12-mo periodicity. Cloud zone is annual laden by thick cloud (ground mists) and characterized by a super-humid meteorological condition with water-logged soils. Therefore, annual periodicity in vegetative growth is divergent in the cloud forest.

Monthly air temperature at our sites (except for the cloud zone) show two slight peaks in May and November. The ITCZ crosses the latitude 6°5’N (where Kinabalu is located) in the Western Pacific in May and November as mean meridional migration patterns. ITCZ is accompanied with high sea surface temperatures, which is probably a reason for explaining the two slight peaks in monthly air temperature. The timing of the arrival of the ITCZ and its magnitude may slightly vary from year to year, being affected by such a large-scale air circulation as El-Niño Southern Oscillation. The timing and magnitude of air temperature changes during the year are probably then transmitted as a signal to leaf primordia formation as well as leaf shedding. New leaf formation is tightly linked with a litterfall event.

Cloud forest at 2700 m, where vegetative periodicity is different from the other forests. Photo: Kanehiro Kitayama.

Implications for the future

Exact and distinct 12-mo periodicity in leaf litterfall in Bornean forests is celestially determined via the ITCZ. However, the consistent occurrence of such vegetative periodicity implies evolutionary significance because trees must have adapted to such seasonal changes and trees with seasonal changes must have been selected for. A sudden increase of fallen litter on the ground may satiate soil microbes for nutrient demand and divert the competition of trees for nutrients. A sudden increase in new leaves in tree canopies also satiates leaf grazers. Vegetative periodicity must be transmitted both to grazer and decomposer food chains and affect the dynamics of the whole ecosystems. Therefore, the subtle change in annual periodicity in daily air temperature should be taken into account to predict the influences of climate change.

Kanehiro Kitayama Kyoto University, Japan
Masayuki Ushio Kyoto University, Japan
Shin‐Ichiro Aiba Hokkaido University, Japan

You can read the full paper online: Temperature is a dominant driver of distinct annual seasonality of leaf litter production of equatorial tropical rain forests

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