Multidecadal records of intrinsic water-use efficiency in the desert shrub Encelia farinosa reveal strong responses to climate change

By Avery W. Driscoll et al.

While tree rings have enabled interannual examination of the
influence of climate on trees, this is not possible for most shrubs.
Here, we leverage a multidecadal record of annual foliar carbon
isotope ratio collections coupled with 39 y of survey data from
two populations of the drought-deciduous desert shrub Encelia
farinosa to provide insight into water-use dynamics and climate.
This carbon isotope record provides a unique opportunity to examine
the response of desert shrubs to increasing temperature
and water stress in a region where climate is changing rapidly.
Population mean carbon isotope ratios fluctuated predictably in
response to interannual variations in temperature, vapor pressure
deficit, and precipitation, and responses were similar among individuals.
We leveraged the well-established relationships between
leaf carbon isotope ratios and the ratio of intracellular to ambient
CO2 concentrations to calculate intrinsic water-use efficiency
(iWUE) of the plants and to quantify plant responses to longterm
environmental change. The population mean iWUE value increased
by 53 to 58% over the study period, much more than the
20 to 30% increase that has been measured in forests [J. Peñuelas,
J. G. Canadell, R. Ogaya, Glob. Ecol. Biogeogr. 20, 597–608 (2011)].
Changes were associated with both increased CO2 concentration
and increased water stress. Individuals whose lifetimes spanned
the entire study period exhibited increases in iWUE that were very
similar to the population mean, suggesting that there was significant
plasticity within individuals rather than selection at the
population scale.

As the proportion of land area covered by arid land vegetation
continues to expand and water limitations for plants increase,
understanding if and how desert shrubs are responding to environmental
change has become more urgent. Among two populations
of Mojave Desert shrubs, we found that intrinsic water use
efficiency has increased substantially over the last three decades
in response to increasing aridity and CO2 concentration.
While increases in intrinsic water-use efficiency have been widely
assumed to mitigate negative effects of decreasing water availability,
precise effects on plant productivity, reproduction, and
survival remain unknown.

The full article is available on the Proceedings of the National Academy of Science website at 18161.full


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