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Title Warmer temperatures reduce net carbon uptake, but do not affect water use, in a mature southern Appalachian forest
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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. 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 Cm-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 phenologicallydriven differences in canopy transpiration. The increasing frequency of high summer temperatures is expected to have a greater effect on respiration than growing season length, reducing forest carbon storage.

Contributors A. Christopher Oishi, Chelcy F. Miniat, Kimberly A. Novick, Steven T. Brantley, James M, Vose and John T. Walker

Oishi, A.C. & Miniat, C.F. & Novick, K.A. & Brantley, Steven & Vose, James & Walker, John. 2018. Warmer temperatures reduce net carbon uptake, but do not affect water use, in a mature southern Appalachian forest. Agricultural and Forest Meteorology. 252. 269-282. 10.1016/j.agrformet.2018.01.011.

Key Words Complex terrain, Drought, Ecosystem respiration, Gross primary productivity, Net ecosystem exchange, Net ecosystem productivity
File Date 2018
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