This study evaluates age-related patterns of ecosystem carbon (C) fluxes in a chronosequence (5-, 18-, 33-, and 68-year old in 2007) of planted pine (Pinus strobus L.) forests in southern Ontario, Canada, from 2003 to 2007. Carbon fluxes determined with the eddy covariance technique were normalized by differences in site index (SI; a measure of site quality based on a species-specific relationship between the height of the dominant trees and base age), to identify the effect of stand age on gross ecosystem productivity (GEP), ecosystem respiration (RE), and net ecosystem productivity (NEP). Observed GEP, RE, and NEP were highest in the 18-year-old stand, which differed from the two older stands by having a higher SI due to sustained availability of soil water in deeper soil layers (at ∼1 m depth). Typical age-related patterns in C fluxes were observed only when normalized for differences in SI. During periods without climatic constraints, SI-normalized GEP and RE increased with stand age, whereas SI-normalized NEP peaked at the 33-year-old site. In contrast, during periods with climatic constraints (e.g. drought, reduced radiation), monthly GEP and NEP at the 18-year-old site were higher compared to the 33-year-old site because deep soil water availability may have allowed this stand to maintain high productivity during these periods. Our analysis shows that differences in site quality may affect the interpretation of age-related C flux dynamics in chronosequence and synthesis studies. We therefore suggest that SI-normalization may offer a simple and efficient way to identify age-dependent processes in C fluxes in forest chronosequence studies, which may lead to improved large-scale estimates of C exchanges in forest ecosystems.
