Abstract
Abstract. The fixation of carbon in tropical forests mainly occurs through the production of wood and leaves, both being the principal components of net primary production. Currently field and satellite observations are independently used to describe the forest carbon cycle, but the link between satellite-derived forest phenology and field-derived forest productivity remains opaque. We used a unique combination of a MODIS enhanced vegetation index (EVI) dataset, a wood production model based on climate data and direct litterfall observations at an intra-annual timescale in order to question the synchronism of leaf and wood production in tropical forests. Even though leaf and wood biomass fluxes had the same range (respectively 2.4 ± 1.4 and 2.2 ± 0.4 Mg C ha−1 yr−1), they occurred separately in time. EVI increased with leaf renewal at the beginning of the dry season, when solar irradiance was at its maximum. At this time, wood production stopped. At the onset of the rainy season, when new leaves were fully mature and water available again, wood production quickly increased to reach its maximum in less than a month, reflecting a change in carbon allocation from short-lived pools (leaves) to long-lived pools (wood). The time lag between peaks of EVI and wood production (109 days) revealed a substantial decoupling between the leaf renewal assumed to be driven by irradiance and the water-driven wood production. Our work is a first attempt to link EVI data, wood production and leaf phenology at a seasonal timescale in a tropical evergreen rainforest and pave the way to develop more sophisticated global carbon cycle models in tropical forests.
Highlights
Tropical forests have a primordial role in the terrestrial carbon (C) cycle
We used a unique combination of a Moderate Resolution Imaging Spectroradiometer (MODIS) enhanced vegetation index (EVI) dataset, a wood production model based on climate data and direct litterfall observations at an intra-annual timescale in order to question the synchronism of leaf and wood production in tropical forests
Inventorybased studies catch wood production through secondary growth, while satellite studies based on EVI catch only a part of net primary production (NPP) related to leaf production
Summary
Tropical forests have a primordial role in the terrestrial carbon (C) cycle. First, 55 % of the total forest C stocks are stored in live biomass, deadwood, litter and soil of tropical areas (471 ± 93 Pg C). Carbon sequestration in tropical intact forests represents about half (1.19 ± 0.41 Pg C yr−1 for the period 1990–2007; Pan et al, 2011; Baccini et al, 2012) of the total sink in global established forest. Accumulation of carbohydrates in woody tissues during secondary growth constitutes the main component of carbon sequestration in trees. Tree growth occurs in two ways: primary growth, which corresponds to the length extension of shoots from the apical meristems, where the leaves grow, and to root development, and secondary growth, which corresponds to all the biological mechanisms behind cambial activity and stem growth in thickness (Kozlowski, 1992). We will use seasonal tree diameter growth as a proxy of seasonal variations in wood production and leaf phenology to assess leaf production, extension and fall.
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