Trends in Global Plant Water Use Efficiency diagnosed from Fluxnet and Tree Ring observations

Plant Water Use Efficiency (WUE), which is the ratio of the uptake of carbon dioxide through photosynthesis to the loss of water through transpiration, is a powerful metric of the functioning of the land biosphere. WUE is expected to increase with atmospheric CO₂, but to decline with increasing atmospheric evaporative demand – which can arise from increases in near-surface temperature or decreases in relative humidity. We have used eddy covariance measurements from the Fluxnet sites, along with d¹³C measurements from tree rings, to estimate the sensitivities of WUE to changes in CO₂ (C_a) and atmospheric humidity deficit (D). This enables us to reconstruct fractional changes in WUE, based on changes in atmospheric climate and CO₂, for the entire period of the global climate record.

The shorter-term Fluxnet observations span a small range in CO₂, but have the temporal resolution to enable the sensitivity of WUE to D to be constrained. By contrast, the tree rings give only annual resolution but are multi-decade timescale records that enable the CO₂-sensitivity of WUE to be estimated. In order to derive a robust sensitivity to both Ca and D, we combined all Fluxnet and tree ring sites using a weighted-mean (maximum-likelihood) approach which gives greater emphasis to the sites where each parameter is more tightly defined. We found that WUE most likely increases even more quickly than C_a (fractional sensitivity = 1.36±0.35) and decreases with D (fractional sensitivity = -0.50±0.18). These numbers were used to produce complete spatial maps of WUE changes for every year since 1900. Overall we estimate that WUE increased by 36±9% over the 20^th century. This long-term trend is largely driven by increases in CO₂, but significant inter-annual variability and regional differences are evident due to variations in temperature and relative humidity, especially in highly-populated regions of Western Europe and China.