Carbon assimilation, water consumption and water use efficiency under different land use types in subtropical ecosystems: from native forests to pine plantations

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The effects of selective logging in Argentinean native subtropical forests or its replacement by pine plantations on carbon and hydrological cycles is of importance for developing strategies for a sustainable use of water resources and climate mitigation in subtropical humid regions. Ecosystem-level gross primary productivity (GPP), evapotranspiration (ET) and water use efficiency (WUE) were assessed using multi-temporal MODIS remote sensing data and its response to climatic variables were analyzed in well-preserved native forests, degraded native forests by selective harvesting and pine plantations. Field measurements of leaf area index (LAI), tree density, basal area, tree high, diameter at breast height and understory cover was implemented. The GPP was high throughout the year across all three-land use types. Selective harvesting of commercial trees resulted in a significant decrease in tree density and in basal area compared to well-preserved native forests but did not have a negative impact on GPP. Mean annual ET was similar between both native forests because although the degraded forest had a lower tree density they had a greater maximun DBH per tree than the well-preserved forests and therefore high transpiration. The WUE was similar across the ecosystem types. Removal of 5.5 m2 of canopy trees produced a compensatory change in ecosystem structure and function resulting in the maintenance of GPP and ET. Replacement of native forests by pine plantations for wood production did not change annual GPP compared to native forests, but the final carbon assimilation should be lower due to the CO2 released before canopy closure and after harvesting. All climatic variables were linearly and significantly related to monthly GPP, ET and WUE with the exception of precipitation with monthly GPP. Finding forest management techniques for increasing carbon assimilation is of importance for limiting the rise of global CO2 emissions and for regulation of the water cycle.