Estimation of vegetation photosynthetic capacity from space-based measurements of chlorophyll fluorescence for terrestrial biosphere models

Photosynthesis simulations by terrestrial biosphere models are usually based on the Farquhar's model, in which the maximum rate of carboxylation (V-cmax) is a key control parameter of photosynthetic capacity. Even though V-cmax is known to vary substantially in space and time in response to environmental controls, it is typically parameterized in models with tabulated values associated to plant functional types. Remote sensing can be used to produce a spatially continuous and temporally resolved view on photosynthetic efficiency, but traditional vegetation observations based on spectral reflectance lack a direct link to plant photochemical processes. Alternatively, recent space-borne measurements of sun-induced chlorophyll fluorescence (SIF) can offer an observational constraint on photosynthesis simulations. Here, we show that top-of-canopy SIF measurements from space are sensitive to V-cmax at the ecosystem level, and present an approach to invert V-cmax from SIF data. We use the Soil-Canopy Observation of Photosynthesis and Energy (SCOPE) balance model to derive empirical relationships between seasonal V-cmax and SIF which are used to solve the inverse problem. We evaluate our V-cmax estimation method at six agricultural flux tower sites in the midwestern US using spaced-based SIF retrievals. Our V-cmax estimates agree well with literature values for corn and soybean plants (average values of 37 and 101molm(-2)s(-1), respectively) and show plausible seasonal patterns. The effect of the updated seasonally varying V-cmax parameterization on simulated gross primary productivity (GPP) is tested by comparing to simulations with fixed V-cmax values. Validation against flux tower observations demonstrate that simulations of GPP and light use efficiency improve significantly when our time-resolved V-cmax estimates from SIF are used, with R-2 for GPP comparisons increasing from 0.85 to 0.93, and for light use efficiency from 0.44 to 0.83. Our results support the use of space-based SIF data as a proxy for photosynthetic capacity and suggest the potential for global, time-resolved estimates of V-cmax.