© 1995 Heron Publishing—Victoria, Canada
Carbon isotope variation in Douglas-fir foliage: improving the δ13C–climate relationship
Jeanne A. Panek and Richard H. Waring
Department of Forest Science, Oregon State University, Corvallis, OR 97331, USA / Received January 17, 1995
Summary
The natural abundance of stable carbon isotopes in the annual rings of forest trees is used as a tracer of environmental changes
such as climate and atmospheric pollution. Although tree-ring δ13C varies by about 2‰ from year to year, variability within the foliage can be as high as 6‰. Recent studies have shown that
branch length affects stomatal response, which influences the integrated foliar δ13C signal. To improve the ability of δ13C to predict climate differences, we examined the relationship between branch length and foliar δ13C in Pseudotsuga menziesii (Mirb.) Franco from four sites across a steep climate gradient in Oregon. The transect spanned the boundary between the ranges
of the coastal variety, P. menziesii var. menziesii (three sites), and the Rocky Mountain variety, P. menziesii var. glauca (one site). At the most maritime site, branch length explained 76% of within-site variation of 5‰, whereas at the harshest
site, branch length accounted for only 15% of this variation.
We considered the possibility that cavitation in the water-conducting xylem obscures the branch length effect in the harsher
climates. Cavitation, as measured by dye perfusion, was most extensive at sites where the branch length effect in the coastal
variety was weakest. Trees at the site with the most substantial cavitation displayed seasonal xylem refilling.
Branch length standardization significantly improved the relationship between δ13C and climate. With standardization to constant length, δ13C values were significantly related to the degree that climatic variables, as modeled with a forest growth simulation model,
constrain transpiration (R2 = 0.69, P < 0.0001). Without standardization, the R2 was 0.27. We conclude that sampling standard length branches or tree rings from trees of similar shape and size is desirable
when seeking correlations between isotopic composition and climate.
Keywords:
hydraulic conductivity, recovery, transpiration, tree rings, xylem cavitation.
