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

Variation in the carbon isotopic composition of ecosystem respiration (delta(13)C(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 delta(13)C(R) varied from -23.1 to -33.1parts per thousand, and within a single forest, it varied in magnitude by 3.5-8.5parts per thousand. Mean annual delta(13)C(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 delta(13)C(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 513 delta(13)C(R) versus vpd relationship, resulting in higher than expected delta(13)C(R) values.