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

Abstract:

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.