Science Inventory

Warmer Temperatures Reduce Net Carbon Uptake, but Do Not Affect Water Use, in a Mature Southern Appalachian Forest

Citation:

Oishi, C., C. Miniat, K. Novick, S. Brantley, J. Vose, AND Johnt Walker. Warmer Temperatures Reduce Net Carbon Uptake, but Do Not Affect Water Use, in a Mature Southern Appalachian Forest. AGRICULTURAL AND FOREST METEOROLOGY. Elsevier Science Ltd, New York, NY, 252:269-282, (2018).

Impact/Purpose:

Mature forests in the mesic, southern Appalachian mountain region have a strong potential for carbon uptake and storage and provide an important source of water. Warming in the region has likely extended the growing season, but these changes have been less dramatic than in some comparable forests and appear to have only a minimal effect on water and carbon dynamics. The increasing frequency of warm days is expected to increase respiratory carbon losses, likely reducing the potential for carbon sequestration, even in the absence of drought. Forest evapotranspiration (ET) shows low interannual variability, meaning that the increasing variability in precipitation will result in greater fluctuations in water yield from these forests. Seasonally, understory ET may compensate for phenology-driven changes in canopy ET. Uncertainty remains about how severe growing season droughts may affect these ecosystem processes.

Description:

Increasing air temperature is expected to extend growing season length in temperate, broadleaf forests, leading to potential increases in evapotranspiration and net carbon uptake. However, other key processes affecting water and carbon cycles are also highly temperature-dependent. Warmer temperatures may result in higher ecosystem carbon loss through respiration and higher potential evapotranspiration through increased atmospheric demand for water. Thus, the net effects of a warming planet are uncertain and highly dependent on local climate and vegetation. We analyzed five years of data from the Coweeta eddy covariance tower in the southern Appalachian Mountains of western North Carolina, USA, a highly productive region that has historically been underrepresented in flux observation networks, due to methodological difficulties in estimating fluxes in complex terrain. We examined how leaf phenology and climate affect water and carbon cycling in a mature forest in one of the wettest biomes in North America. Warm temperatures in early 2012 caused leaf-out to occur two weeks earlier than in cooler years and led to higher seasonal carbon uptake. However, these warmer temperatures also drove higher winter ecosystem respiration, offsetting much of the springtime carbon gain. Interannual variability in net carbon uptake was high (147 to 364 g C m-2 y-1), but unrelated to growing season length. Instead, years with warmer growing seasons had 10% higher respiration and sequestered ~40% less carbon than cooler years. In contrast, annual evapotranspiration was relatively consistent among years (coefficient of variation = 4%) despite large differences in precipitation (17%, range = 800 mm). Transpiration by the evergreen understory likely helped to compensate for phenologically-driven differences in canopy transpiration. Long-term temperature trends predict that growing season length is increasing by only ~1 day per decade, but that the increasing frequency of high summer temperatures will likely compromise the forest’s ability to store carbon.