Relationships Between Tree Height and Carbon Isotope Discrimination

Abstract Understanding how tree size impacts leaf- and crown-level gas exchange is essential to predicting forest yields and carbon and water budgets. The stable carbon isotope ratio (d13C) of organic matter has been used to examine the relation-ship of gas exchange to tree size for a host of species because it carries a temporally integrated signature of foliar photosynthesis and stomatal conductance. The carbon isotope composition of leaves reflects discrimination against 13C relative to 12C during photosynthesis and is the net result of the balance of change in CO2 supply and demand at the sites of photosynthesis within the leaf mesophyll. Interpreting the patterns of changes in d13C with tree size are not always clear, however, because multiple factors that regulate gas exchange and carbon isotope discrimination (D) co-vary with height, such as solar irradiance and hydraulic conductance. Here we review 36 carbon isotope datasets from 38 tree species and conclude that there is a consistent, linear decline of D with height. The most parsimonious explanation o