Interannual variation in rainfall modulates temperature sensitivity of carbon allocation and flux in a tropical montane wet forest

Tropical forests exert a disproportionately large influence on terrestrial carbon (C) balance but projecting the effects of climate change on C cycling in tropical forests remains uncertain. Reducing this uncertainty requires improved quantification of the independent and interactive effects of variable and changing temperature and precipitation regimes on C inputs to, cycling within and loss from tropical forests. Here, we quantified aboveground litterfall and soil-surface CO2 efflux (soil respiration; F-S) in nine plots organized across a highly constrained 5.2 degrees C mean annual temperature (MAT) gradient in tropical montane wet forest. We used five consecutive years of these measurements, during which annual rainfall (AR) steadily increased, in order to: (a) estimate total belowground C flux (TBCF); (b) examine how interannual variation in AR alters the apparent temperature dependency (Q(10)) of above- and belowground C fluxes; and (c) quantify stand-level C allocation responses to MAT and AR. Averaged across all years, F-S, litterfall, and TBCF increased positively and linearly with MAT, which accounted for 49, 47, and 46% of flux rate variation, respectively. Rising AR lowered TBCF and F-S, but increased litterfall, with patterns representing interacting responses to declining light. The Q(10) of F-S, litterfall, and TBCF all decreased with increasing AR, with peak sensitivity to MAT in the driest year and lowest sensitivity in the wettest. These findings support the conclusion that for this tropical montane wet forest, variations in light, water, and nutrient availability interact to strongly influence productivity (litterfall+TBCF), the sensitivity of above- and belowground C fluxes to rising MAT (Q(10) of F-S, litterfall, and TBCF), and C allocation patterns (TBCF:[litterfall+TBCF]).

Automated summary

The study found that aboveground litterfall and soil respiration in a tropical montane wet forest increased positively and linearly with mean annual temperature, with total belowground carbon flux and soil respiration decreasing, but litterfall increasing, with rising annual rainfall. The sensitivity of carbon fluxes to temperature and precipitation also varied, with different responses in the driest and wettest years.