Tree-ring responses to interannual climate variability within the Fluxnet-Canada network: an isotope approach

Tree-ring width provides a retrospective record of the amount of growth that has taken place during one year. Carbon isotope composition in tree-rings provides an integrated record of the balance between CO2 assimilation rate and stomatal conductance during the period in which the ring was produced. We compared the influence of climatic variations on tree-ring width and carbon isotope composition in the major tree species of three contrasting ecosystems: a coastal Douglas-fir (Pseudotsuga menziesii) forest in British Columbia, a boreal aspen (Populus tremuloides) forest and a boreal Jack pine (Pinus banksiana) forest, both in Saskatchewan. At least four cores were sampled from five dominant trees in each site. In addition, five trees from the co-dominant red-cedar species (Thuja occidentalis) were also sampled in British Columbia. After measurements of annual radial increment, the α-cellulose was extracted from the rings formed within the 1981-2001 period. A year-by-year analysis of their carbon and oxygen isotope compositions (δ13C and δ18O, respectively) was performed. Over the two decades, the average δ13C values of the four species were in the range –23.2‰ to –24.0‰. Maximum interannual variations of δ13C ranged from 1.2‰ to 2.5‰ for T. occidentalis and P. tremuloides, respectively. In all species, interannual changes in δ13C and δ18O were significantly related to variations in vapor pressure deficit averaged over the growing season period. These covariations were interpreted as reflecting reduction in stomatal conductance in response to atmospheric moisture stress. Whereas a negative correlation between tree-ring width and δ13C was expected under such conditions, no significant relationship was observed between radial growth and δ13C in any site. However, in the Jack Pine site, which is characterized by a very low soil water reserve, 80% of the variation in radial growth was explained by an empirical model that included: (i) early growing season precipitation; (ii) summer mean temperature; and (iii) previous-year ring width. Relationships between tree-ring measurements and climate variables were much less obvious in the Douglas-fir and the aspen sites. These results emphasize the complexity of the environmental controls on carbon isotope discrimination and growth in trees that dominate these northern ecosystems.