What drives long-term variations in carbon flux and balance in a tropical rainforest in French Guiana?

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A thorough understanding of how tropical forests respond to climate is important to improve ecosystem process models and to reduce uncertainties in current and future global carbon balance calculations. The Amazon rainforest, a major contributor to the global carbon cycle, is subject to strong intra- and interannual variations in climate conditions. Understanding their effect on carbon fluxes between the ecosystem and the atmosphere and on the resulting carbon balance is still incomplete. We examined the long-term (over a 12-year period; 2004–2015) variations in gross primary productivity (GPP), ecosystem respiration (RE) and net ecosystem exchange (NEE) in a tropical rainforest in French Guiana and identified key climatic drivers influencing the changes.The study period was characterized by strong differences in climatic conditions among years, particularly differences in the intensity of the dry and wet seasons, as well as differences in annual carbon fluxes and balance. Annual average GPP varied from 3384.9 g C m−2 yr‒1 (95% CI [3320.7, 3445.9]) to 4061.2 g C m−2 yr‒1 (95% CI [3980.1, 4145.0]). RE varied even more than GPP, with a difference of 933.1 C m−2 yr‒1 between the minimum (3020.6 g C m−2 yr‒1; 95% CI [2889.4, 3051.3]) and maximum (3953.7 g C m−2 yr‒1; 95% CI [3887.6, 4019.6]) values. Although NEE showed large interannual variability (nine-fold), from ‒65.6 g C m−2 yr‒1 (95% CI [‒4.4, ‒126.0]) to ‒590.5 g C m−2 yr‒1 (95% CI [‒532.3, ‒651.6]), the forest remained a carbon sink over the 12-year period.A combination of global radiation (Rg), relative extractable water (REW) and soil temperature (Ts) explained 51% of the daily variations for GPP, 30% for RE and 39% for NEE. Global radiation was always the best predictor of these variations, but soil water content and temperature did also influence carbon fluxes and balance. Seasonally, Rg was the major controlling factor for GPP, RE and NEE during the wet season. During the dry season, variations in carbon fluxes and balance were poorly explained by climate factors. Yet, REW was the key driver of variations in NEE during the dry season.This study highlights that, over the long-term, carbon fluxes and balance in such tropical rainforest ecosystems are largely controlled by both radiation and water limitation. Even though variations in Rg have a greater impact on these fluxes, water limitation during seasonal droughts is enough to reduce ecosystem productivity, respiration and carbon uptake. The reduced precipitation expected in tropical rainforest areas under future climatic conditions will therefore strongly influence carbon fluxes and carbon uptake. This study also highlights the importance for land surface or dynamic global vegetation models to consider the main drivers of carbon fluxes and balance separately for dry and wet seasons.