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 very useful 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 Δ13C measurements from tree rings, along with
eddy covariance measurements from Fluxnet sites, to estimate the
sensitivities of WUE to changes in CO2 and atmospheric humidity deficit.
This enables us to reconstruct fractional changes in WUE, based on changes in
atmospheric climate and CO2, for the entire period of the instrumental global climate record. We estimate that overall WUE increased from 1900 to
2010 by 48 ± 22 %, which is more than double that simulated by the
latest Earth System Models. 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. There are several highly populated regions, such as western Europe
and East Asia, where the rate of increase of WUE has declined sharply in the
last 2 decades. Our data-based analysis indicates increases in WUE that
typically exceed those simulated by Earth System Models – implying that
these models are either underestimating increases in photosynthesis or
underestimating reductions in transpiration.
Comment on "Spatial and temporal variations in plant Water Use Efficiency inferred from tree-ring, eddy covariance and atmospheric observations" by Margriet Groenendijk et al.
