Optimality-based modelling of plant traits and primary production along the Northeast China Transect

Traits are fundamental to understanding plant function and represent key variables for ecosystem modelling. An expanding class of models based on eco-evolutionary optimality (EEO) theory shows great potential in predicting trait–trait and trait–environment relationships at both leaf and whole-plant levels, via universal formulations that apply equally to all plant functional types. According to this theory gross primary production (GPP), the basis of the terrestrial carbon cycle, is jointly determined by the ratio of intercellular to ambient CO₂ concentrations (χ), leaf-level photosynthetic capacity (V­_cmax) and absorbed light, which depends on incident solar radiation and leaf area index (LAI). 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 V_cmax), morphological traits (e.g leaf mass per area, LMA) and biomass allocation (to roots, shoots and stems) are shaped by climate and light. The amount of N per unit area of leaf (N_area) is related in part to the quantity of photosynthetic enzymes, indexed by carboxylation capacity at standard temperature (V_cmax25), and in part to LMA. 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).

This study tested quantitative trait predictions derived from EEO principles using published and unpublished data from the Northeast China Transect (NECT), which spans a precipitation gradient from moist forest to semi-desert. Predicted and observed (or inferred) values of χ, LAI, GPP and biomass decreased with dryness, while LMA and leaf nitrogen per unit area (N_area) increased. Plant δ¹⁵N increased with dryness and soil temperature. This implied fraction of N lost in gaseous forms (f_gas) increased strongly towards the dry end of the transect. By reproducing observed patterns of trait variation along the NECT, these findings provide empirical support for an emerging, optimality-based theory for the coupling of C, N and water cycles in terrestrial ecosystems.