¹³C content of ecosystem respiration is linked to precipitation and vapor pressure deficit

Abstract.

Variation in the carbon isotopic composition of ecosystem respiration (δ¹³C_R) was studied for 3  years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipitation across these sites ranged from 227 to 2,760 mm. Overall δ¹³C_R varied from –23.1 to –33.1‰, and within a single forest, it varied in magnitude by 3.5–8.5‰. Mean annual δ¹³C_R differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between δ¹³C_R and the vapor saturation deficit of air (vpd) 5–10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the δ¹³C_R versus vpd relationship, resulting in higher than expected δ¹³C_R values.