Stem CO2 efflux of Abies fabri in subalpine forests in the Gongga Mountains, Eastern Tibetan Plateau

Abstract

Aims Despite the importance of stem CO2 efflux (Es) in ecosystem carbon cycling and energy balance, little is known about temporal variation in the temperature coefficient (Q10) and sapwood nitrogen concentrations ([N]) and their intrinsic links with Es. The objectives of this study were: (i) to examine the response of Es to temperature in a subalpine region and (ii) to explore the influence of stem diameter and [N] on Es. Also, we will test the hypothesis that (i) Es in trees has thermal acclimation and (ii) Es will be well correlated with diameter at breast height (DBH) and [N]. Methods Here, a horizontally oriented soil chamber technique was applied to measure Es of Abies fabri in two subalpine forest stands in Southwestern China from May to December 2014. We also examined the variability in Es, Q10 and [N] in trees and monitored the relationship between temperature, [N], DBH and Es. Important Findings During the measurement period, Es showed an apparent seasonal trend, following the change in air temperature, increasing from May and peaking in July, then continuously decreasing until December. The mean Es for the growing and dormant seasons were 1.45 and 0.25 μmol m-2 s-1, respectively, and Es in the mature forest was significantly higher than in the immature forest. The area-based Es was positively correlated with DBH and sapwood width (SW), while volume-based Es showed negative relationship with DBH and SW. Across the five diameter classes, 69.8-89.0% of the variation in Es could be explained by air temperature. The temperature sensitivity (Q10) of Es ranged from 2.98 to 5.61 during the measurement period, with a higher Q10 appearing in the growing season than in the dormant season. There was a significant linear relationship (P < 0.01) between [N] and Es (expressed based on two different units). Additionally, exponential models of Es against [N] and air temperature were applied to estimate Es.