Tamir Klein, Uria Ramon
Forests play a major role in the global carbon cycle, assimilating large amounts of CO2, and thereby moderating the effects of its human-induced increase in the atmosphere. In turn, forest trees also respond to changes in atmospheric CO2 level, primarily by adjusting their stomata, the microscopic pores in the leaf surface that are specifically designed to admit CO2 and release water vapor. In this study, a rich dataset of tree leaf responses to increased CO2 was compiled. We used 144 records of stomatal responses to CO2, coming from dozens of independent manipulation experiments and representing 57 tree species. In all but four species, stomatal aperture was reduced under high CO2, thereby reducing leaf activity. Yet, the extent of reduction was very different among tree types, with only a minor reduction in evergreen conifer species, a significant reduction in deciduous broadleaf species, and the largest reduction in evergreen broadleaf species. The reasons for this stronger reduction include an overall higher stomatal conductance of broadleaf species (offering a wider range for reduction); their thinner leaves, in turn losing water faster; and the decreasing atmospheric CO2 concentration at the geologic time of their evolution. In addition, there was a stronger reduction response in tropical trees, compared to temperate trees. Such stronger reduction means a lower carbon uptake capacity than originally expected. Moreover, since water evaporates as part of the stomatal gas exchange process, this can have major implications for the water cycle. Specifically, their stomatal response to high CO2 might mean that coniferous forests continue to use the same amounts of water as today, whereas broadleaf forests might reduce their water-use . The new insights about species and forest type differences should be integrated into models that are applied to forecast future forest fluxes.