Optimal trait theory: testing predictions on the Northeast China Transect

Recently developed ecosystem models based on eco-evolutionary optimality hypotheses can predict many aspects of the carbon, water and nutrient economy of ecosystems. These models have focused on various key plant functional traits and their environmental controls. Gross primary production (GPP) is partly determined by the ratio of intercellular to ambient CO₂ concentrations (χ), which can be inferred from leaf stable carbon isotope ratios (δ¹³C). The effect of nitrogen (N) supply on GPP is mediated by the allocation of carbon (C) to leaves, while leaf-level photosynthetic traits (e.g. χ and photosynthetic capacity) and morphological traits (e.g leaf size and leaf mass per area, LMA) are modified or constrained by climate. The amount of N in the leaf is related in part to the quantity of photosynthetic enzymes, indexed by carboxylation capacity at standard temperature (V_cmax,25), and in part to LMA – as all plant tissues, including cell walls, contain N. Plant N isotope ratios (δ¹⁵N) are sensitive to the partitioning of N loss from soil between the gaseous and leaching pathways (a balance that is strongly under climatic control), and also to plants’ N uptake strategy (mycorrhizal type or symbiotic N-fixation).

Plant and ecosystem data collected on the Northeast China Transect (NECT) are used here to test a series of quantitative trait predictions based on optimality principles. The NECT is characterized by a long continuous gradient in precipitation and community structure, ranging from moist forests in the east, via grasslands, to semi-desert in the west. We investigated the relationships among leaf traits, ecosystem properties and N loss pathways, including χ, LMA, leaf N per unit area (N_area), leaf area index (LAI, inferred from satellite data), above-ground biomass, and δ¹⁵N. The calculations involve testable predictions of intermediate quantities, including community-mean photosynthetic capacity and GPP. By reproducing observed patterns of trait variation along the NECT, this analysis has provided empirical support for an emerging, optimality-based theory for the coupling of C and N cycles in terrestrial ecosystems.