Abstract
Abstract. The partitioning of photosynthates toward biomass compartments plays a crucial role in the carbon (C) sink function of forests. Few studies have examined how carbon is allocated toward plant compartments in drought-prone forests. We analyzed the fate of gross primary production (GPP) in relation to yearly water deficit in an old evergreen Mediterranean Quercus ilex coppice severely affected by water limitations. Carbon fluxes between the ecosystem and the atmosphere were measured with an eddy covariance flux tower running continuously since 2001. Discrete measurements of litterfall, stem growth and fAPAR allowed us to derive annual productions of leaves, wood, flowers and acorns, and an isometric relationship between stem and belowground biomass has been used to estimate perennial belowground growth. By combining eddy covariance fluxes with annual net primary productions (NPP), we managed to close a C budget and derive values of autotrophic, heterotrophic respirations and carbon-use efficiency (CUE; the ratio between NPP and GPP). Average values of yearly net ecosystem production (NEP), GPP and Reco were 282, 1259 and 977 g C m−2. The corresponding aboveground net primary production (ANPP) components were 142.5, 26.4 and 69.6 g C m−2 for leaves, reproductive effort (flowers and fruits) and stems, respectively. NEP, GPP and Reco were affected by annual water deficit. Partitioning to the different plant compartments was also impacted by drought, with a hierarchy of responses going from the most affected – the stem growth – to the least affected – the leaf production. The average CUE was 0.40, which is well in the range for Mediterranean-type forest ecosystems. CUE tended to decrease less drastically in response to drought than GPP and NPP did, probably due to drought acclimation of autotrophic respiration. Overall, our results provide a baseline for modeling the inter-annual variations of carbon fluxes and allocation in this widespread Mediterranean ecosystem, and they highlight the value of maintaining continuous experimental measurements over the long term.
Highlights
Forest ecosystems exert a strong influence on the global carbon (C) cycle (Bonan, 2008) as forests may contribute up to 60 % of the total land carbon uptake
We analyzed the fate of gross primary production (GPP) in relation to yearly water deficit in an old evergreen Mediterranean Quercus ilex coppice severely affected by water limitations
Comparative measures of ecosystem fluxes and production components across 11 yr of contrasting water limitations in a Q. ilex coppice help to better understand how Mediterraneantype forest ecosystems will respond to the ongoing climate change and to better project future C sequestration capacity
Summary
Forest ecosystems exert a strong influence on the global carbon (C) cycle (Bonan, 2008) as forests may contribute up to 60 % of the total land carbon uptake Estimations and simulations of carbon uptake by forest ecosystems have been greatly improved in recent decades, but, how this assimilated C is transferred from the atmosphere to the terrestrial biomass remains a challenging issue. A recent synthesis has demonstrated that the partitioning of gross primary production (GPP), among aboveground and belowground production and respiration, can vary greatly across biomes, according to climate and fertility (Litton and Giardina, 2008). A more detailed understanding of how environmental factors affect the distribution of C among the different tree parts at the ecosystem scale remains a crucial step to improving the accuracy of local and global vegetation models (Fatichi et al, 2013; Leuzinger and Quinn Thomas, 2011)
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