Warmer temperatures reduce net carbon uptake, but do not affect water use, in a mature southern Appalachian forest

Abstract

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 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. The increasing frequency of high summer temperatures is expected to have a greater effect on respiration than growing season length, reducing forest carbon storage.

Ecosystem processes at the watershed scale: Influence of flowpath patterns of canopy ecophysiology on emergent catchment water and carbon cycling

“…First, a more realistic evergreen:deciduous ratio in the SPECIES model with the addition of the evergreen understorey compared with the two NLCD models. Oishi et al () have shown that evergreen understorey at Coweeta Basin contributes significantly to the annual ET, and high density of evergreen understorey under mature deciduous forest area is quite common at the southern Appalachian Mountains (Bolstad et al, ). Second, the impact of the accumulated soil moisture deficit during the growth season which conditions the required soil water recharge required for peak flows during the dormant season.…”

Riparian canopy openings on mountain streams: Landscape controls upon temperature increases within openings and cooling downstream

“…Rainfall is evenly distributed throughout the year, and less than 5% of precipitation comes in the form of snow and ice (Swift & Messer, ). Mean annual temperature is approximately 12.7°C (Swift, Cunningham, & Douglass, ) but has been increasing (Laseter, Ford, Vose, & Swift, ; Oishi et al, ). Growing seasons are also lengthening (Hwang et al, ).…”

The functional role of temperate forest understorey vegetation in a changing world

“…The understorey contributes 10%–15% of ET in forests with a dense canopy and/or a sparse understorey vegetation, but this contribution can rise to 40% in more open forests (LAI around 3 or less; Table S4). Oshi et al () showed that the understorey contribution to total ET varies throughout the year and is particularly high just before the leafing out of the canopy (up to 76%). The results from our review seem in line with Roberts' () hypothesis.…”