Age-related variations in 13C of ecosystem respiration across a coniferous forest chronosequence in the Pacific Northwest

Open Access
Publisher Website
Google Scholar
Abstract
We tested the hypothesis that forest age influences the carbon isotope ratio (δ13C) of carbon reservoirs and CO2 at local and regional levels. Carbon isotope ratios of ecosystem respiration (δ13CR), soil respiration (δ13CR-soil), bulk needle tissue (δ13CP) and soil organic carbon (δ13CSOC) were measured in > 450-, 40- and 20-year-old temperate, mixed coniferous forests in southern Washington, USA. Values of δ13CR, δ13CR-soil, δ13CP and δ13CSOC showed consistent enrichment with increasing stand age. Between the youngest and oldest forests there was a ∼1‰ enrichment in δ13CP (at similar canopy levels), δ13CSOC (throughout the soil column), δ13CR-soil (during the wet season) and δ13CR (during the dry season). Mean values of δ13CR were –25.9, –26.5 and –27.0‰ for the 450-, 40- and 20-year-old forests, respectively. Both δ13CR-soil and the difference between δ13CR and δ13CR-soil were more 13C enriched in older forests than in young forest: δ13CR – δ13CR-soil = 2.3, 1.1 and 0.5‰ for the 450-, 40- and 20-year-old forests, respectively. Values of δ13CP were proportionally more depleted relative to δ13CR: δ13CR – δ13CP = 0.5, 2.2 and 2.5‰ for the 450-, 40- and 20-year-old forests, respectively. Values of δ13CP were most 13C-enriched at the top of the canopy and in the oldest forest regardless of season (overall values were –26.9, –28.7 and –29.4‰ for the 450-, 40- and 20-year-old forests, respectively). Values of δ13CSOC from shallow soil depths were similar to δ13CP values of upper- and mid-canopy needles. All δ13C data are consistent with the hypothesis that a decrease in stomatal conductance associated with decreased hydraulic conductance leads to increased CO2 diffusional limitations in older coniferous trees. The strong associations between δ13CP in needles with δ13CR and δ13CR-soil at the forest level suggest that 13C observations scale between leaf and ecosystem levels.

This publication has 0 references indexed in Scilit: