Photosynthesis may be light-dependent, but what about other (and co-related) climate variables such as temperature? Is it possible to describe the environmental dependencies of gross primary productivity (GPP)? Moving outside the controlled conditions of a growth chamber or greenhouse, attempting to answer such questions for natural ecosystems becomes complicated. Further complexity is generated by plant responses to changing seasons and asynchronous climate variables – often dependent on latitude.
Here we use open access eddy covariance data from over 100 sites collated over 20 years (the number of sites has grown with time). These sites, located in a wide range of biomes and climate zones, form part of the FLUXNET network. We have combined the flux data with a satellite product (FPAR, MODIS) that provides spatial estimates of the fraction of incident light absorbed by green vegetation.
Using these data (available at a range of timescales) we can derive site-based estimates of light use efficiency (LUE) defined as the amount of carbon fixed by the plants per unit absorbed light (effectively GPP normalised for light conditions). Here we show a plot of LUE as a function of air temperature; the data limited to each site’s growing season (Figure 1). Many chemical reactions respond to temperature variation – and photosynthesis is no exception. But finding a discrete response with global application may be unfeasible.
As a next step we consider additive models (incorporating multiple explanatory factors) that support non-linear responses. For a given site, could the asynchrony of light, temperature and moisture allow us to differentiate individual effects on primary productivity?