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 (δ13C) of organic matter has been used to examine the relationship 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 δ13C with tree size are not always clear, however, because multiple factors that regulate gas exchange and carbon isotope discrimination (Δ) 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 Δ with height. The most parsimonious explanation of this result is that gravitational constraints on maximum leaf water potential set an ultimate boundary on the shape and sign of the relationship. These hydraulic constraints are manifest both over the long term through impacts on leaf structure, and over diel periods via impacts on stomatal conductance, photosynthesis and leaf hydraulic conductance. Shading induces a positive offset to the linear decline, consistent with light limitations reducing carbon fixation and increasing partial pressures of CO2 inside of the leaf, p c at a given height. Biome differences between tropical and temperate forests were more important in predicting Δ and its relationship to height than wood type associated with being an angiosperm or gymnosperm. It is not yet clear how leaf internal conductance varies with leaf mass area, but some data in particularly tall, temperate conifers suggest that photosynthetic capacity may not vary dramatically with height when compared between tree-tops, while stomatal and leaf internal conductances do decline in unison with height within canopy gradients. It is also clear that light is a critical variable low in the canopy, whereas hydrostatic constraints dominate the relationship between Δ and height in the upper canopy. The trend of increasing maximum height with decreasing minimum Δ suggests that trees that become particularly tall may be adapted to tolerate particularly low values of p c.