Radiative forcing in both the short and long-wave lengths reaching the Earth's surface accounted for more than 80% of the inter-annual variations in the mean yearly temperatures measured at Potsdam, Germany, during the last 120 years. Three quarters of the increase in the long-wave flux was due to changes in the water content of the lower atmosphere; the remainder was attributed to increases in CO 2 and other anthropogenic , radiatively active gases. Over the period radiative forcing in the short-wave flux slightly exceeded that in the long wave, but its effect on air temperature was much less as the climate sensitivity to atmospheric radiation, 0.187°C per W m À2, was three times greater than to short-wave global radiation. This anomalous finding, similar to that previously reported at two coastal sites, awaits explanation as does the complex interaction existing between radiative forcing and advection in determining temperature change.

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Irrigation of protected crops requires sound knowledge of evapotranspiration. Previous studies have established that the eddy-covariance (EC) technique is suitable for whole canopy evapotranspiration measurements in large agricultural screenhouses. Nevertheless, the eddy-covariance technique remains difficult to apply in the farm due to costs, operational complexity, and postprocessing of data, thereby inviting alternative techniques to be developed. The subject of this paper is the evaluation of a turbulent transport technique, the flux variance (FV), whose instrumentation needs and operational demands are not as elaborate as the EC, to estimate evapotranspiration within large agricultural structures. Measurements were carried out in three types of agricultural structures: (i) a banana plantation in a light-shading (8%) screenhouse (S1), (ii) a pepper crop in an insect-proof (50-mesh) screenhouse (S2), and (iii) a tomato crop in a naturally ventilated greenhouse with a plastic roof and 50-mesh screened sidewalls (S3). Quality control analysis of the EC data showed that turbulence development and flow stationarity conditions in the three structures were suitable for flux measurements. However, within the insect-proof screenhouse (below the screen) and the plastic-covered greenhouse, R2 of the energy balance closure was poor; hence, the alternative simple method could not be used. Results showed that the FV technique was suitable for reliable estimates of ET in shading and insect-proof screenhouses with R2 of the regressions between FV latent heat flux and latent heat flux deduced from energy balance closure of 0.99 and 0.92 during validation for S1 and S2, respectively.

Climate change is often associated with increasing vapor pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil‒atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (g ), transpiration rate (E) and net photosynthesis (A ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the g and E response to VPD. These results demonstrate that avoiding drought on the supply side (soil moisture) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.

Climate change is often associated with increasing vapor pressure deficit (VPD) and decreasing soil moisture (SM). While atmospheric and soil drying often co-occurs, their differential effects on plant functioning and productivity remain uncertain. We aimed to elaborate on the divergent effects and underlying mechanisms of soil and atmospheric drought, based on continuous, in situ measurements of branch gas exchange, with automated chambers, in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmosphere drought (low SM, high VPD) during the rainless Mediterranean summer, and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (g ), the rates of transpiration (E) and net photosynthesis (A ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and g to the massive rise in VPD (to a maximum of 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing that plant water potential is a good predictor of the g and E response to VPD. These results demonstrate that the release from drought on the supply-side, in combination with plant hydraulic regulation, eliminates the effect of atmospheric demand (VPD) as a stressor and on canopy gas exchange in mature, drought-adapted pine trees.