Leaf- and stand-level responses of a forested mesocosm to independent manipulations of temperature and vapor pressure deficit

Alterations in temperature (T) and vapor pressure deficit (VPD) strongly influence gas exchange, but because VPD is highly influenced by T, the effects of these two factors are difficult to separate. Here, the concomitant effects of T and VPD on CO₂ uptake, stomatal conductance, and transpiration at leaf- and canopy-levels were examined for a stand of trees (Populus deltoides) enclosed within large mesocosms. T and VPD were independently altered to yield a factorial combination of treatments of low (24°C) or high (30°C) T and low (0.75) or high (1.75 kPa) VPD. Traditional leaf-level gas exchange measurements were compared with whole-canopy exchange to verify typical scaling methods. Elevated T resulted in an average 40% and 14% increase in midday leaf-level and canopy-level net CO₂ uptake, respectively. Other physiological responses to elevated T and VPD were similar at both scales, but the magnitude of change was usually less pronounced at the canopy-level. Surprisingly, only minimal interactions between T and VPD were found to influence responses of CO₂ uptake and stomatal conductance at either level.