Plants influence the atmosphere through fluxes of carbon, water and energy1, and can intensify drought through land–atmosphere feedback effects2,3,4. The diversity of plant functional traits in forests, especially physiological traits related to water (hydraulic) transport, may have a critical role in land–atmosphere feedback, particularly during drought. Here we combine 352 site-years of eddy covariance measurements from 40 forest sites, remote-sensing observations of plant water content and plant functional-trait data to test whether the diversity in plant traits affects the response of the ecosystem to drought. We find evidence that higher hydraulic diversity buffers variation in ecosystem flux during dry periods across temperate and boreal forests. Hydraulic traits were the predominant significant predictors of cross-site patterns in drought response. By contrast, standard leaf and wood traits, such as specific leaf area and wood density, had little explanatory power. Our results demonstrate that diversity in the hydraulic traits of trees mediates ecosystem resilience to drought and is likely to have an important role in future ecosystem–atmosphere feedback effects in a changing climate.
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Eddy flux data are available at http://fluxnet.fluxdata.org/data/fluxnet2015-dataset/; community trait data are available at http://www.anderegglab.net/data/trait-data/; detailed trait data are available in Extended Data Figs. 1–10 and at https://datadryad.org//handle/10255/dryad.235, https://datadryad.org//handle/10255/dryad.80340 and from a previous publication18.
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We thank M. Beninati, R. O’Dell and J. Gallafent for assistance with trait compilation. W.R.L.A. acknowledges funding from the University of Utah Global Change and Sustainability Center, NSF Grant 1714972 and 1802880 and the USDA National Institute of Food and Agriculture, Agricultural and Food Research Initiative Competitive Programme, Ecosystem Services and Agro-ecosystem Management, grant no. 2018-67019-27850. A.T.T. acknowledges funding from USDA National Institute of Food and Agriculture Postdoctoral Research Fellowship grant no. 2017-07164. This work used eddy covariance data acquired and shared by the FLUXNET community, including these networks: AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, CarboItaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KoFlux, LBA, NECC, OzFlux-TERN, TCOS-Siberia and USCCC. The ERA-Interim reanalysis data are provided by ECMWF and processed by LSCE. The FLUXNET eddy covariance data processing and harmonization was carried out by the European Fluxes Database Cluster, AmeriFlux Management Project and Fluxdata project of FLUXNET, with the support of CDIAC and ICOS Ecosystem Thematic Center and the OzFlux, ChinaFlux and AsiaFlux offices.
Nature thanks D. Baldocchi, S. Delzon, S. Jansen and the other anonymous reviewer(s) for their contribution to the peer review of this work.
The authors declare no competing interests.
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
Species richness is shown by different colours (data from http://ecotope.org/anthromes/biodiversity/plants/data/). Size of the circle is representative of the sample size of included days ranging from 10 (smallest circles) to 1,057 (largest circles) samples.
a, b, Variable importance (total decrease in node impurities) results from the machine-learning algorithm, random forests, for each variable for drought sensitivity (a) and drought coupling (b) metrics. Traits include SLA, (Amax), P50 and HSM. The suffix ‘m’ indicates the community-weighted mean; the suffix ‘SD’ indicates the community-weighted standard deviation. See Supplementary Table 3 for sample sizes.
a, c, Drought coupling is expressed as the percentage of explained variation (R2). b, d, Drought sensitivity is shown as the summed absolute values of standardized coefficients for drought variables that are regressed against latent energy (LE) exchange. Regression: LE ≈ VPD + SM + VPD × SM. a, b, Panels are identical to Fig. 2 but with site identifications shown. Hydraulic variation is expressed as the community-weighted standard deviation in the hydraulic safety margin of species. c, d, The hydraulic safety margin was calculated from the 50% loss of hydraulic conductivity in gymnosperms and 88% loss of hydraulic conductivity in angiosperms. Colours indicate biomes of deciduous broadleaf (green) and needleleaf (red) forests. The size of the dot indicates the number of days included for each flux site. The solid black line is the best fit of the ordinary least-squares linear regression (c, P = 0.008; d, P = 0.01) and dashed lines are the 95% confidence interval of the regression fit.
a–c, Relationship between daily total latent energy (LE) exchange measured via eddy covariance and midday canopy water content from remote-sensing of VOD for global forest sites (a; n = 4,525 grid cells), broadleaf forest sites (b; n = 1,915 grid cells) and evergreen forest sites (c; n = 2,610 grid cells). Red lines indicate best fits for ordinary least-squares regressions. Note that the canopy water content at each pixel integrates a spatial area that is two orders of magnitude greater than the eddy covariance sites.
Comparison of dominant tree species richness from gridded data of US forests against the hydraulic diversity—the standard deviation in HSM—at six eddy covariance sites in the United States that have adequate trait data. The red line indicates the best fit of the ordinary least-squares linear regression (n = 6 sites; R2 = 0.65; P = 0.04).
Extended Data Fig. 6 Higher species diversity is associated with more buffered drought responses in US forests.
a, b, Drought coupling (a; n = 163 grid cells) and drought sensitivity (b; n = 163 grid cells) from variation in remotely sensed canopy vegetation water content compared to tree species richness in the United States. Black line is the best fit of the ordinary least-squares linear regressions.
Extended Data Fig. 7 Higher species diversity is associated with more-buffered drought responses in forests globally.
a, Drought sensitivity as the slope (β) in an ordinary least-squares linear regression of an index of the variation in aboveground plant water content at midday compared to night (regression: VODmidday = β × VODnight + ε). b, Native plant species richness (percentage of maximum; data from http://ecotope.org/anthromes/biodiversity/plants/data/). c–h, Ordinary least-squares linear regressions between these two variables for six major biomes. c, Tropical and subtropical moist broadleaf forests (n = 1,380 grid cells). d, Tropical and subtropical dry broadleaf forests (n = 241 grid cells). e, Temperate broadleaf and mixed forests (n = 1,289 grid cells). f, Temperate coniferous forests (n = 318 grid cells). g, Boreal forests (n = 1,784 grid cells). h, Mediterranean-type forests, woodlands and shrub (n = 319 grid cells). Each point represents an individual grid cell from the map and colours that are more red indicate a higher density of points. Red lines show the best fit of ordinary least-squares linear regression lines.
The importance of the variables (total decrease in node impurities) results obtained using the machine learning algorithm, random forests, for each variable of the drought coupling metric (n = 6,698 grid cells). ‘CanHeight’, lidar-derived canopy height.
Extended Data Fig. 9 Ecosystem sensitivity to drought saturates with species richness (the percentage of maximum) across forests globally.
a, Drought coupling is expressed as the explained variation (R2) of midday aboveground plant water content in forest ecosystems regressed against drought variables using ordinary least-squares linear regression. b, Drought sensitivity is expressed as the regression coefficient of midday aboveground plant water content regressed against a metric of soil water stress using ordinary least-squares linear regression. n = 6,698 grid cells. The black line shows the best fit generalized additive model and dashed lines are the 99% confidence interval.
Nine randomly selected sites of observed latent energy (LE) fluxes versus predicted fluxes from the multiple regression model based on VPD, soil moisture and their interaction. Red lines are the ordinary least-squares best-fit regression line. Sites are as follows (abbreviations can be found in Supplementary Table 1). a, US-UMB (R2 = 0.12, P < 0.0001). b, US-WCr (R2 = 0.13, P < 0.0001). c, DK-Sor (R2 = 0.11, P < 0.0001). d, IT-Ca1 (R2 = 0.44, P < 0.0001). e, IT-PT1 (R2 = 0.06, P = 0.002). f, IT-Ro1 (R2 = 0.10, P < 0.0001). g, JP-SMF (R2 = 0.40, P < 0.0001). h, NL-Loo (R2 = 0.25, P < 0.0001). i, US-NWR (R2 = 0.15, P < 0.0001). k, Performance of the drought multivariate ordinary least-squares linear regression model (LE = f(VPD, SM, VPD × SM) across eddy covariance sites shown in a histogram of site-level model P values from those regressions.
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Anderegg, W.R.L., Konings, A.G., Trugman, A.T. et al. Hydraulic diversity of forests regulates ecosystem resilience during drought. Nature 561, 538–541 (2018). https://doi.org/10.1038/s41586-018-0539-7
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