Print PageMost climate predictions agree that the southern US will be 2-4°C warmer by the end of the 21st century, whereas the average temperature increase in North Carolina over the last century was 0.7°C. It is currently unclear how global warming will affect atmospheric vapor pressure deficit (VPD) and transpiration in natural ecosystems. If VPD remains unchanged, forest production may increase, but if VPD increases, productivity of forest ecosystems may decline in the future. We measured changes in phenology, stomatal conductance, and growth of saplings of four temperate forest tree species (Acer rubrum, maple; Carya tomentosa, hickory; Quercus alba, white oak; Quercus rubra, red oak) to understand how future climate change may affect tree productivity. This work was performed in an experimental warming site in the piedmont of NC. Warming was achieved by actively heating open-top chambers from +1.5 to +5.5°C above ambient temperature, with 0.5°C steps between treatments. As a consequence of heating, mean VPD also varied among chambers, ranging from +0.3 to +1.5 kPa. Large increases in VPD are in contrast to patterns observed in recent climate change, however, and failure to account for changes in VPD in warming experiments may result in large biases in results.
Results/Conclusions
Experimental warming extended the 2010 growing season for the four tree species by advancing spring leaf expansion by 6-14 days and delaying fall leaf senescence by 3-7 days. Extended growing seasons may lead to increased production of terrestrial ecosystems, although this is contingent on the direction and extent of future changes in water availability and VPD. In this study, for instance, all species reduced stomatal conductance (P<0.0001) during a drought in September 2010, which limited carbon uptake. Species differences in the sensitivity of stomata to VPD also contributes to variation in forest productivity. In the summer of 2010, maple had significantly lower stomatal conductance than the other three species (P<0.0001), and differences among species were greatest under conditions of high water availability. In the first year of experimental warming, there was no significant effect of warming on growth (R2=0.05, P=0.48 for maple; R2=0.13, P=0.25 for hickory; R2=0.03, P=0.63 for white oak; R2=0.007, P=0.81 for red oak). These results emphasize the importance of explicitly accounting for changes in VPD in warming experiments. This work generates quantitative estimates of the uncertainty resulting from our incomplete knowledge of future changes in soil and atmospheric moisture under warming scenarios.
