Ecosystem water use efficiency in a forest and a peatland in northern Patagonia

During 8 years (2015-2022), we measured net ecosystem exchange of CO₂ (NEE) and evapotranspiration (ET) using eddy covariance systems in a temperate rainforest and an anthropogenic peatland in northern Patagonia (southern Chile). NEE was partitioned into gross primary production (GPP) and ecosystem respiration (Reco), while ET was partitioned into evaporation (E) and transpiration (T), which in turn were used to calculate different formulas of water use efficiency (WUE). We identified the main environmental drivers of WUE, GPP, ET, E and T. Results showed that while the forest was a consistent carbon sink (-17.82 Mg CO₂ ha^-1 year^-1), the peatland was in average a small source (1.21 Mg CO₂ ha^-1 year^-1). Only the expressions of WUE that included atmospheric water demand showed seasonal variation. Variations in WUE were more related to changes in ET than with changes in GPP. For both ecosystems, E increased with higher global radiation and higher surface conductance and when water table depth was closer to the soil surface. Also, higher values of E were related to higher wind speed in the forest and higher air temperature in the peatland. The absence of a close relation between ET and GPP was likely related to the dominance of plant species with low or no stomatal control. The observed increase in potential ET suggests that WUE could increase in the future in these ecosystems.