The way the wind blows matters to ecosystem water use efficiency

In many regions, atmospheric conditions change frequently with shifts of wind direction, extending maritime influences far inland or continental influences to coastal ecosystems. Climate models predict changes in both wind direction and velocity; these changes could potentially impact ecosystems mass and energy exchanges with the atmosphere. Using data on climate (wind speed and direction, air temperature and humidity) and ecosystem-scale fluxes (eddy-covariance evapotranspiration and CO2 flux) from the Mediterranean island of Sardinia, we evaluated whether the frequency of certain wind characteristics, potentially affecting ecosystem CO2 uptake, have changed over five decades, and whether these characteristics are indeed linked to ecosystem gas-exchange responses of the studied ecosystem. The analyses show that days dominated by summer Mistral winds decreased on average 3% per decade, and that wind direction affects biosphere-atmosphere exchange of carbon but not water. High velocity cool Mistral winds from continental Europe undergo limited thermal equilibration with the land surface after embarking on the land, trebling vapor pressure deficit (D) as they cross the island. In contrast, arriving with a similar D, lower velocity, warmer Saharan Sirocco winds heat up, thus increasing D 5-fold only 50 km inland. Over a mixed pasture-woodland (grass-wild olive), while soil moisture was low and constant, daytime net carbon exchange (NEEd) averaged 2.3-fold higher (p < 0.001) in Mistral than Sirocco days, largely reflecting the theoretically expected response of canopy conductance (g(c)) to variation of D. Because the product of g(c) and D encodes the key ecosystem compensatory mechanism, the reciprocal g(c)-D response maintained similar ecosystem evapotranspiration (E-e). Thus, summertime ecosystem water-use efficiency (W-e = NEEd/E-e) was similar to 66% higher during Mistral than other days. The historical decrease of Mistral frequency reduced the estimated summertime NEEd > 30%. The analyses demonstrate that alteration of dominance of air masses predicted with future climate will amplify or negate the positive effect of increased atmospheric [CO2] on We, and should be considered when assessing climate change impact on NEE.