Melinda D. Smith’s research while affiliated with Colorado State University and other places

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Publications (286)


Effects of experimental drought on temporal means and variability of aboveground net primary productivity (ANPP) in Chihuahuan Desert and Great Plains grasslands. Asterisks represent *p < 0.05 and ***p < 0.001. Error bars indicate standard deviation.
Effects of experimental drought on (a–f) community‐weighted means (CWMs) of specific leaf area (SLA), leaf dry matter content (LDMC), stem dry matter content (SDMC), stem tissue density (SD), root dry matter content (RDMC), root tissue density (RD), (g) species richness, (h) functional dispersion, (i) species asynchrony and (j) species stability in Chihuahuan Desert and Great Plains grasslands. Asterisks represent *p < 0.05, **p < 0.01, and ***p < 0.001. Error bars indicate standard deviation.
Piecewise structural equation models (SEMs) showing the direct and indirect effects of community‐weighted mean (CWM) traits (i.e., specific leaf area [SLA], leaf dry matter content [LDMC] and stem dry matter content [SDMC]), plant diversity (i.e., species richness and functional dispersion), species asynchrony and species stability on the temporal variability of aboveground net primary productivity (ANPP) in Chihuahuan Desert and Great Plains grasslands under (a) control (ambient) conditions or (b) extreme drought conditions. The model was fitted using non‐standardised values first, and the overall fitting statistics correspond to these values. Solid black arrows show significant effects and dashed grey arrows show non‐significant effects. Numbers next to the solid arrows represent the standardised path coefficients. Asterisks represent *p < 0.05, **p < 0.01 and ***p < 0.001.
Extreme Drought Increases the Temporal Variability of Grassland Productivity by Suppressing Dominant Grasses
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April 2025

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Extreme droughts are intensifying, yet their impact on temporal variability of grassland functioning and its drivers remains poorly understood. We imposed a 6‐year extreme drought in two semiarid grasslands to explore how drought influences the temporal variability of ANPP and identify potential stabilising mechanisms. Drought decreased ANPP while increasing its temporal variability across grasslands. In the absence of drought, ANPP variability was strongly driven by the dominant plant species (i.e., mass‐ratio effects), as captured by community‐weighted traits and species stability. However, drought decreased the dominance of perennial grasses, providing opportunities for subordinate species to alter the stability of productivity through compensatory dynamics. Specifically, under drought, species asynchrony emerged as a more important correlate of ANPP variability than community‐weighted traits or species stability. Our findings suggest that in grasslands, prolonged, extreme droughts may decrease the relative contribution of mass‐ratio effects versus compensatory dynamics to productivity stability by reducing the influence of dominant species.

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Fig. 2. Responses of the global mean biomass-MAP relationship to fertilization with single N, P, and potassium with micronutrients (Kµ). (A) The biomass-MAP relationships for treatments fertilizing with 0, 1, 2, or 3 nutrients. Inset: the percent increase in linear regression slope relative to unfertilized controls. (B) the biomass-MAP relationships for treatments fertilizing with N, P, and Kµ in factorial combinations. Nutrient treatments are color-coded as in Panel C. (Inset) The percent increase in linear regression slopes relative to unfertilized controls for N, P, and NP treatments averaged across levels of Kµ. (C) Mean ± SE of aboveground biomass across all 71 sites for the factorial N, P, and Kµ fertilization treatments. See Table 1 for linear mixed model analyses and SI Appendix, Table S2 for linear regression equations.
Fig. 3. Aboveground biomass in relation to MAP for sites classified by form of response to N and P fertilization. (A) No Limitation, (B) Limited by Single N or P, (C) Additive limitation by N and P, (D) Synergistic limitation by N and P). Response forms are defined in Fig. 1. N and P treatments are averaged across levels of Kµ fertilization. Insets depict the slopes for unfertilized control (C), N, P, and N together with P. Upper panels are kernel-smoothed MAP distributions for the sites each form of limitation. See Table 2 for linear mixed model analyses and SI Appendix, Table S2 for linear regression equations.
Fig. 5. Summary of the standardized effects of nutrient addition and MAP on aboveground biomass production from structural equation models fit (Table 4) across grassland sites in each of four forms of nutrient limitation. (A) Sankey plot depicting Total effects (direct + indirect) of number of nutrients and MAP (Left side) mapped onto each form of nutrient limitation (Right side). The widths of the links depict the magnitude of each total effect for each limitation form. (B) Direct and (C) indirect effects (±SE) of MAP and nutrient addition for each limitation form. Indirect effects represent community mediation of MAP and nutrient effects on biomass, and combine paths through effective species richness, species evenness, and beta diversity (SI Appendix, Fig. S3).
Fit statistics for structural equation models fit across grassland sites assigned to four forms of nutri- ent limitation by nitrogen and phosphorus (Fig. 5)
Interactions among nutrients govern the global grassland biomass-precipitation relationship

April 2025

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618 Reads

Proceedings of the National Academy of Sciences

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass–MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass–MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass–MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass–MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass–MAP relationship.


C4 photosynthesis, trait spectra, and the fast‐efficient phenotype

March 2025

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355 Reads

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1 Citation

It has been 60 years since the discovery of C4 photosynthesis, an event that rewrote our understanding of plant adaptation, ecosystem responses to global change, and global food security. Despite six decades of research, one aspect of C4 photosynthesis that remains poorly understood is how the pathway fits into the broader context of adaptive trait spectra, which form our modern view of functional trait ecology. The C4 CO2‐concentrating mechanism supports a general C4 plant phenotype capable of fast growth and high resource‐use efficiencies. The fast‐efficient C4 phenotype has the potential to operate at high productivity rates, while allowing for less biomass allocation to root production and nutrient acquisition, thereby providing opportunities for the evolution of novel trait covariances and the exploitation of new ecological niches. We propose the placement of the C4 fast‐efficient phenotype near the acquisitive pole of the world‐wide leaf economic spectrum, but with a pathway‐specific span of trait space, wherein selection shapes both acquisitive and conservative adaptive strategies. A trait‐based perspective of C4 photosynthesis will open new paths to crop improvement, global biogeochemical modeling, the management of invasive species, and the restoration of disturbed ecosystems, particularly in grasslands.


The EDGE
a, The geographical distribution of experimental sites in Eurasia and North America (additional information is provided in Supplementary Table 1). The data of distribution of grassland types were sourced from a previous study⁵². CHY, Cheyenne; ERG, Erguna; HYS, Hays Agricultural Research Center; KNZ, Konza Prairie; MUR, Sheila MuRen; SGS, Shortgrass Steppe; TAR, Sher Tara; URA, Urat; XLL, Xilingol-Leymus chinensis; XLS, Xilingol-Stipa grandis. b, Photograph of experimental treatments at the Xilingol-Stipa grandis site in Eurasian grasslands. c, Example of the effect of the extreme drought treatment at the Sheila MuRen site.
Effects of four years of extreme drought on plant productivity and richness
a,b, The response ratios (Methods) of ANPP (a) and plant species richness (b) over the four-year drought period. Data are mean ± s.e.m. of the response ratios of ANPP and species richness (Methods) in six Eurasian grasslands (n = 36) and six North American grasslands (n = 60). Different uppercase letters and lowercase letters indicate significant differences in response ratios among treatment years in Eurasian and North American grasslands, respectively, assessed using two-sided Tukey’s honest significant difference (HSD) tests.
The relationships between plant species richness response to extreme drought and ANPP response to extreme drought
a, The relationship between the response ratio of total species richness and the response ratio of ANPP. b, The relationship between the response ratio of subordinate species richness and the response ratio of ANPP. Fitted lines are from linear mixed-effects models. The purple, yellow and grey lines indicate the trends averaged across all Eurasian grassland sites (n = 24), averaged across all North American grassland sites (n = 24) and the trends for each site (n = 4), respectively (the detailed model specification and summary statistics are provided in Supplementary Table 4).
Contrasting drought sensitivity of Eurasian and North American grasslands

January 2025

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965 Reads

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6 Citations

Nature

Extreme droughts generally decrease productivity in grassland ecosystems1, 2–3 with negative consequences for nature’s contribution to people4, 5, 6–7. The extent to which this negative effect varies among grassland types and over time in response to multi-year extreme drought remains unclear. Here, using a coordinated distributed experiment that simulated four years of growing-season drought (around 66% rainfall reduction), we compared drought sensitivity within and among six representative grasslands spanning broad precipitation gradients in each of Eurasia and North America—two of the Northern Hemisphere’s largest grass-dominated regions. Aboveground plant production declined substantially with drought in the Eurasian grasslands and the effects accumulated over time, while the declines were less severe and more muted over time in the North American grasslands. Drought effects on species richness shifted from positive to negative in Eurasia, but from negative to positive in North America over time. The differing responses of plant production in these grasslands were accompanied by less common (subordinate) plant species declining in Eurasian grasslands but increasing in North American grasslands. Our findings demonstrate the high production sensitivity of Eurasian compared with North American grasslands to extreme drought (43.6% versus 25.2% reduction), and the key role of subordinate species in determining impacts of extreme drought on grassland productivity.


Fig. 1. A globally distributed network of experiments to investigate the impacts of drought on soil carbon. (A) experimental sites are displayed as black dots over a global Ai map. the more arid an ecosystem is, the lower its Ai. (B) Range of mean annual temperature and MAP of the 19 study sites, which are color-coded according to ecosystem type. examples of drought shelters at representative sites are shown in the photos.
Fig. 2. The responses of SOC, POC, and MAOC concentrations to experimental drought. (A) total changes in SOc, POc, and MAOc concentrations across sites assembled by levels of Ai. Water availability increases with higher Ai. (B) Responses of SOc, POc, and MAOc concentrations to drought across all sites and as moderated by (C) Ai and (D) ecosystem type. Points indicate mean treatment effects across all sites, error bars indicate 95% confidence intervals, and n indicates number of observations, *P < 0.05. Site codes are listed in table S1.
Fig. 3. AI-moderated responses of soil carbon fractions to drought. the responses of (A) SOc, (B) POc, and (C) MAOc concentrations and (D) ratio between MAOc and SOc to drought in drylands (Ai < 0.65) and mesic (Ai > 0.65) ecosystems. Red dot shows the mean. *P < 0.05; ns, P > 0.05.
Fig. 4. Environmental factors influencing SOC concentrations responses to drought. Relationships between (A) total changes in SOc concentrations and the Ai, (B) total changes in POc concentrations and the Ai, (C) total changes in SOc concentrations and the cv in precipitation, (D) standing SOc concentration, (E) total change in priming effect, and (F) water holding capacity. Solid lines indicate significant correlations, while dashed lines indicate nonsignificant correlations. Shaded areas reflect the 95% confidence interval for the line of fit.
Aridity drives the response of soil total and particulate organic carbon to drought in temperate grasslands and shrublands

October 2024

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1,033 Reads

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8 Citations

Science Advances

The increasing prevalence of drought events in grasslands and shrublands worldwide potentially has impacts on soil organic carbon (SOC). We leveraged the International Drought Experiment to study how SOC, including particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) concentrations, responds to extreme drought treatments (1-in-100-year) for 1 to 5 years at 19 sites worldwide. In more mesic areas (aridity index > 0.65), SOC and POC concentrations decreased by 7.9% (±3.9) and 15.9% (±6.2) with drought, respectively, but there were no impacts on MAOC concentrations. However, drought had no impact on SOC, POC, or MAOC concentrations in drylands (aridity index < 0.65). The response of SOC to drought varied along an aridity gradient, concomitant with interannual precipitation variability and standing SOC concentration gradients. These findings highlight the differing response magnitudes of POC and MAOC concentrations to drought and the key regulating role of aridity.


The value of long-term ecological research for evolutionary insights

August 2024

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432 Reads

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6 Citations

Nature Ecology & Evolution

Scientists must have an integrative understanding of ecology and evolution across spatial and temporal scales to predict how species will respond to global change. Although comprehensively investigating these processes in nature is challenging, the infrastructure and data from long-term ecological research networks can support cross-disciplinary investigations. We propose using these networks to advance our understanding of fundamental evolutionary processes and responses to global change. For ecologists, we outline how long-term ecological experiments can be expanded for evolutionary inquiry, and for evolutionary biologists, we illustrate how observed long-term ecological patterns may motivate new evolutionary questions. We advocate for collaborative, multi-site investigations and discuss barriers to conducting evolutionary work at network sites. Ultimately, these networks offer valuable information and opportunities to improve predictions of species' responses to global change.


Shifting from competition toward facilitation with abiotic stress is limited for two codominant grass species

July 2024

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27 Reads

It’s unclear why the C 4 grasses, Andropogon gerardii and Sorghastrum nutans , codominate in the tallgrass prairie despite asymmetric competition and drought tolerance that favors A. gerardii . According to the stress gradient hypothesis (SGH), net interactions between these species may shift from negative (competitive) to positive (facilitative) in harsher conditions. We tested this hypothesis in a greenhouse experiment manipulating community composition and water availability. We found no evidence of such a shift, suggesting limits to the SGH and the existence of other mechanisms driving codominance between these grasses.


Grazing herbivores reduce herbaceous biomass and fire activity across African savannas

June 2024

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437 Reads

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10 Citations

Fire and herbivory interact to alter ecosystems and carbon cycling. In savannas, herbivores can reduce fire activity by removing grass biomass, but the size of these effects and what regulates them remain uncertain. To examine grazing effects on fuels and fire regimes across African savannas, we combined data from herbivore exclosure experiments with remotely sensed data on fire activity and herbivore density. We show that, broadly across African savannas, grazing herbivores substantially reduce both herbaceous biomass and fire activity. The size of these effects was strongly associated with grazing herbivore densities, and surprisingly, was mostly consistent across different environments. A one‐zebra increase in herbivore biomass density (~100 kg/km² of metabolic biomass) resulted in a ~53 kg/ha reduction in standing herbaceous biomass and a ~0.43 percentage point reduction in burned area. Our results indicate that fire models can be improved by incorporating grazing effects on grass biomass.


Figure 1: Precipitation attributes (mean ± SE) for control conditions and the two types of extreme drought in 2015-2018. Growing season (May 1-August 31) precipitation (mm) (a); average number of rain events during the growing season (b); average size (mm) of a rain event during the growing season (c); length (days) of the average dry period experienced between rain events during the growing season (d). Capital letters indicate significant differences among the three treatments.
Figure 3: The response of grass cover (a), species richness of grass (b), forb cover (c) and species richness of forb (d) to extreme drought patterns with or without insect herbivores. The response ratio of each functional group cover and species richness to extreme drought patterns with or without insect herbivores are shown in the upper right corner, respectively. Capital letters indicate significant differences without insect herbivores. Lowercase letters indicate significant differences with insect herbivores. * indicates a significant difference between HR and HP in the same treatment, *P < 0.05, **P < 0.01 and ***P < 0.001.
Figure 5: The regression relationship between cover response ratio and leaf carbon content (a), leaf dry matter content (b), starch (c), calcium (d), magnesium (e) and manganese (f), respectively.
The relative importance of functional groups to total cover
Interaction of extreme drought and insect herbivores on grassland community is affected by drought pattern

May 2024

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189 Reads

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1 Citation

Journal of Plant Ecology

Extreme drought and insect herbivores can affect plant community non-independently, and their interaction may be influenced by drought pattern. However, few studies have explored the interaction on plant community structure, and no study has investigated whether the interaction is affected by drought pattern. We explored the interaction of different extreme drought patterns and insect herbivores with a manipulated experiment in a semiarid grassland. There were three treatments for drought: control – ambient precipitation; chronic drought – 66% precipitation reduction in growing season (May – August); intense drought – completely exclusion rain events from June to July. Herbivores removal and present treatments were imposed within each drought treatments. We found that although herbivores had no significant effects on the impacts of droughts on total species richness, it alleviated the impacts of chronic drought on total cover, and the alleviating role decreased under intense drought. Comparing with intense drought, more increasing of grass cover led to more alleviation under chronic drought associated with the decrease of forb which have low leaf carbon and dry matter content, along with high starch, calcium, magnesium and manganese concentration. These results indicate that the interaction of drought and herbivores is affected by drought pattern, and the changes of leaf traits that reduce herbivores feeding in grass might alleviate the impact of drought on grassland.


The response of photosynthetic rate of dominant species to extreme drought in Inner Mongolia grasslands

April 2024

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110 Reads

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2 Citations

Journal of Plant Ecology

Estimating the effects of extreme drought on the photosynthetic rates (Pn) of dominant plant species is crucial for understanding the mechanisms driving the impacts of extreme drought on ecosystem functioning. Extreme drought may result from either reduced rainfall amounts or decreased rainfall frequency, and the impacts of different patterns of extreme drought may vary greatly. In addition, different grasslands likely appeared varied sensitivity to different extreme drought patterns. However, there have been no reports on the effects of different extreme drought patterns on dominant species Pn in different grassland types. Here, we conducted multi-year extreme drought simulation experiments (reducing each rainfall event by 66% during growing season, CHR vs. completely excluding rainfall during a shorter portion of growing season, INT) in two different grasslands (desert grassland vs. typical grassland) from 2014. The Pn of two dominant species in each grassland were measured in July and August 2017. Both CHR and INT significantly decreased dominant species Pn, with INT causing more negative impacts on Pn regardless of grassland types. The response ratios of Pn in desert grassland were generally higher than that of typical grassland, especially for Leymus chinensis in CHR. These results indicate that decreased rainfall frequency had a more negative effect on Pn compared to reduced rainfall amount, with grassland types changing the magnitude, but not the direction, of the effects of extreme drought patterns. These findings highlight the importance of considering extreme drought patterns and grassland types in ecosystem management in the face of future extreme droughts.


Citations (81)


... For the past few decades, a wide range of studies have found that drought hinders plant growth and induces plant death by increasing their susceptibility to wildfire, insect pests, and disease. Yu et al. revealed the different response mechanisms of drought in different grassland ecosystems, providing theoretical support for addressing climate change and adaptive management [28]. Ernandez et al. found that during droughts, highly correlated hydraulic constraints are often the underlying mechanism leading to an increase in forest mortality [29]. ...

Reference:

Socio-Economic Aspects of Drought Impact on Forest Industry in China
Contrasting drought sensitivity of Eurasian and North American grasslands

Nature

... productivity (Maestre et al. 2015;Delgado-Baquerizo et al. 2020;Hu et al. 2021;Berdugo et al. 2022;Shi et al. 2024). By the end of the century, rising aridity could transform one-fifth of all land, shifting ecosystems and causing widespread extinctions of plants, animals, and other life forms (Vicente-Serrano et al. 2024). ...

Aridity drives the response of soil total and particulate organic carbon to drought in temperate grasslands and shrublands

Science Advances

... In the face of unprecedented human-induced pressures, it is essential to understand how environmental change will impact the eco-evolutionary dynamics of natural populations inhabiting fragmented landscapes (Cocciardi et al. 2024;Hanski 2012). Our study highlights the importance of collecting long-term, individual-based data across large spatial scales to understand the stochastic processes determining the viability of populations through their effects on population dynamics and adaptive capacity. ...

The value of long-term ecological research for evolutionary insights

Nature Ecology & Evolution

... Along with fire, the presence of large mammalian herbivores also plays a vital role shaping the savanna ecosystems (Karp et al. 2024). These herbivores, found in high densities in many African savannas, regulate plant communities by consuming vegetation and contribute to nutrient cycling and redistribution by eating biomass and dispersing nutrients across the landscape through dung (Augustine and Frank 2001;Veldhuis et al. 2018). ...

Grazing herbivores reduce herbaceous biomass and fire activity across African savannas

... Given the variable effects of precipitation on plant diversity and biomass across studies (i.e., Kardol et al. 2010;Cherwin and Knapp 2012;Cantarel et al. 2013;Zang et al. 2020;Smith et al. 2020), it suggests that the effects of precipitation on plant communities are often context dependent, and it could be that interactions with insects mediate the effect of altered precipitation on plant communities. Importantly, recent evidence suggests that insects do mediate the effect of altered precipitation on plant communities (Xu et al. 2021;Luo et al. 2024). For example, insects can facilitate a higher abundance of drought-resistant plant species and subsequently maintain total plant abundance during drought (Xu et al. 2021). ...

Interaction of extreme drought and insect herbivores on grassland community is affected by drought pattern

Journal of Plant Ecology

... Severe drought was identified in Yijinhuoluoqi Banner and the surrounding areas, which could be attributed to a low rainfall and high temperature. This high evapotranspiration influences the vertical flux of energy and soil moisture, thereby changing soil filtration and triggering drought [45]. Zhungeer Banner and northern Wushenqi Banner were unique, with typical grassland in the former and sandy land in the latter preventing conditions of low rainfall and high temperature causing drought. ...

The response of photosynthetic rate of dominant species to extreme drought in Inner Mongolia grasslands
  • Citing Article
  • April 2024

Journal of Plant Ecology

... Previous studies have found that background climatic conditions (i.e. mean annual temperature and mean annual precipitation) affect not only plant biomass production (Lin et al., 2010) but also the response of plant biomass production to GCDs (Feng et al., 2023;Huxman et al., 2004;Maurer et al., 2020;Smith et al., 2024;Zhou et al., 2020;Zhou, Zhou, et al., 2022b). Global change effects on plant biomass production are also expected to vary with the intensity of global changes. ...

Extreme drought impacts have been underestimated in grasslands and shrublands globally

Proceedings of the National Academy of Sciences

... We drew on both conservativeacquisitive and competitor-stress tolerator frameworks to disentangle our results. Recent work assessing grassland responses to drought showed a positive correlation between drought tolerance and resistance to competition-grassland species that are stresstolerators are often also good competitors (Mount et al., 2024). ...

Drought‐tolerant grassland species are generally more resistant to competition

... Despite similar climatic conditions at both sites, experimental drought had contrasting impacts on ANPP and its temporal variability. This suggests that factors beyond climate, such as plant functional traits and species composition, play key roles in modulating drought impacts across these common ecosystems (Muraina et al. 2021;Song et al. 2024). ...

Grassland sensitivity to drought is related to functional composition across East Asia and North America

... Effects of drought on more fundamental processes in the boreal biome and adjacent northern hardwood forest transition zone, that is, nutrient cycling, largely remain unknown (Houle et al. 2016). However, more widespread evidence has been steadily accumulating by using precipitation shelters (Homyak et al. 2017;Jourdan and Hättenschwiler 2021;Quer et al. 2022;Courcot et al. 2024;Knapp et al. 2024). ...

Field experiments have enhanced our understanding of drought impacts on terrestrial ecosystems—But where do we go from here?