Water availability and carbon isotope discrimination in conifers.

The stable C isotope composition (delta C-13) of leaf and wood tissue has been used as an index of water availability at both the species and landscape level. However, the generality of this relationship across species has received little attention. We compiled literature data for a range of conifers and examined relationships among landscape and environmental variables (altitude, precipitation, evaporation) and delta C-13. A significant component of the variation in delta C-13 was related to altitude (discrimination decreased with altitude in stemwood, 2.53 parts per thousand km(-1) altitude, r(2)=0.49, and in foliage, 1.91 parts per thousand km(-1), r(2)=0.42), as has been noted previously. The decrease in discrimination with altitude was such that the gradient in CO2 partial pressure into the leaf (P-a-P-i) and altitude were generally unrelated. The ratio of precipitation to evaporation (PIE) explained significant variation in P-a-P-i of stemwood (r(2)=0.45) and foliage (r(2)=0.27), but only at low (