Long‐term C, N and P allocation to reproduction in Bornean tropical rain forests

Abstract

Summary Tropical rain forests in SE Asia are well known for the occurrence of supra‐annual synchronous reproductive events, masting. Answering the question how trees allocate carbon (C), nitrogen (N) and phosphorus (P) to such irregular but gregarious reproduction requires a long‐term observation. We conducted a 10‐year continuous monitoring of litterfall in eight tropical rain forests, which differ significantly in P (and N) availability on Mount Kinabalu, Borneo. Mean P concentration in reproductive organ litter decreased significantly with increasing P‐use efficiency of net primary production (PUE), an index of P deficiency. Therefore, P in reproductive organ litter became diluted as the magnitude of P deficiency increased. Mean annual litterfall (kg ha−1 yr−1) of reproductive organs over the 10 years ranged from 128.5 to 730.9 across the eight forests. Long‐term C allocation ratio to reproductive organs (i.e. C in reproductive organs per C in total litterfall) varied from 2.0% to 7.8% across the eight forests and did not relate with PUE, indicating that long‐term C allocation ratio to reproduction was not controlled by the availability of the most critical soil nutrient. Long‐term N allocation ratio to reproduction varied from 2.7% to 9.9% and significantly positively related with C allocation ratio. The quotient of N allocation ratio to C allocation ratio ranged from 1.1 to 1.4. Long‐term P allocation ratio to reproduction varied from 9.8% to 16.4%. The quotient of P allocation ratio to C allocation ratio ranged from 1.6 to 5.0. Therefore, tropical trees allocated much greater proportion of P to reproduction than C and N over the 10 years. Moreover, trees disproportionately increased P allocation to reproduction with decreasing C allocation to reproduction. Trees adjusted P allocation relative to C allocation and maintained a narrow range of P allocation ratio to reproduction in the long run in each site. Synthesis. Reproduction in Bornean tropical rain forests costs more P than C and N. Our results suggest that reproductive events in these forests are regulated by P at the level of overall long‐term mean. Understanding patterns and processes of reproductive events requires a long‐term monitoring of nutrient dynamics.