Testing Earth System Model Assumptions of Photosynthetic Parameters with in situ Leaf Measurements from a Temperate Zone Forest.

Abstract:

Estimates of global land CO₂ uptake vary widely across Earth system models. This uncertainty around model estimates of land-atmosphere CO₂ fluxes may result from differences in how models parameterize and scale photosynthesis from the leaf-to-global level. To test model assumptions about photosynthesis, we derive rates of maximum carboxylation (V_c,max), electron transport (J), and triose phosphate utilization (TPU) from in situ leaf measurements from a forest representative of the Great Lakes region. Leaf-level gas exchange measurements were collected across a temperature range from sun and shade leaves of canopy-dominant tree species typically grouped into the same plant functional type. We evaluate the influence of short-term increases in leaf temperature, nitrogen per leaf area (N_area), species, and leaf light environment on V_c,max, J, and TPU by testing contrasting model equations that isolate the influence of these factors on these rate-limiting steps in leaf photosynthesis. Results indicate that patterns in V_c,max are best explained by a model that includes temperature and N_area. However, J varied with species and leaf light environment in addition to temperature. TPU also varied with leaf light environment and possibly with temperature. These variations in J and TPU with species or between sun and shade leaves suggest that plant traits outside of N_area are needed to explain patterns in J and TPU. This study provides in situ evidence on how V_c,max, J, and TPU vary within a forest canopy and highlight how leaf responses to changes in climate, forest species composition, and canopy structure may alter forest CO₂ uptake.