Seasonal and interannual patterns of carbon and water fluxes of a poplar plantation under peculiar eco-climatic conditions

This paper reports 3 years of eddy covariance measurements (2002–2004) on a poplar plantation (Populus × canadensis Moench, Clone I-214) in Northern Italy. We analyzed seasonal and interannual variability of the net ecosystem exchange (NEE), gross primary production (GPP), ecosystem respiration (RECO) and evapotranspiration (ET) in relation to different meteorological and environmental conditions experienced by the plantation and to natural disturbances. In particular, during 2003 climatic conditions were exceptionally severe, with the highest mean air temperatures and the lowest precipitations observed in the study area in the last 50 years. In addition, during the late summer of 2004, the plantation was attacked by fall webworm larvae (Hyphantria cunea). The cumulated NEE during the three growing season (April–September) was −752.8, −626.7 and −702.7 g C m−2 for 2002, 2003 and 2004, respectively. In the period June–August 2003, NEE was 35% and 29% higher than the NEE measured in the same period in 2002 and 2004, respectively, resulting in a lower net carbon uptake. As RECO did not show large differences over the three growing seasons, the reduction in net carbon sequestration of June–August 2003 is mainly explained by a reduction of GPP (−18% and −17% in 2002 and 2004, respectively). The cumulated ET over the three growing seasons was 388, 471, 484 kg H2O m−2 for 2002, 2003 and 2004, respectively. This behaviour was due to the fact that the position of the water table remained close to the roots, therefore supplying enough water to the plantation, even during the driest period. The analysis of light curve parameters, their residuals and canopy conductance suggests that the reduction of CO2 uptake during summer 2003 was mainly controlled by stomatal and non-stomatal limitations due to high temperatures, directly and in particular indirectly (i.e. through D) rather than by soil water stress. In conclusion, our study shows that in the presence of a heat-wave, a significant reduction of net productivity during summer may occur even in the absence of marked soil water stress. In addition to this sensitivity to high temperatures, which will likely increase in the future, the effects of natural disturbances may add further uncertainties, thus suggesting caution in the evaluation of the potential carbon sequestration of these ecosystems.