Daniel C. Laughlin

Summary: Belowground plant trait research has predominantly focused on trade‐offs in fine‐root traits via the root economics space (RES). Yet, this fine‐root framework captures only a fraction of the functional strategies plants employ beneath the soil surface. Here, we broaden the perspective on belowground plant functioning by integrating traits related to root system extent, clonality, and bud banks, using data from the new UNDERPLOT dataset. This integration links measurable traits to key belowground functions: resource acquisition, spatial exploration, and persistence. Our analysis shows that the fine‐root economics space explains less than 5% of the variation in traits related to root system extent, clonality, and bud banks. Instead, an expanded trait analysis reveals three significant dimensions, explaining 62% of total trait variation. The third dimension represents an independent, persistence‐related gradient, not captured by existing root economics frameworks. We propose that understanding belowground plant strategies requires embracing additional functional gradients. The strategy of persistence, in particular, varies significantly across growth forms and is a critical dimension of plant response to resource limitation and stress, becoming increasingly important as global change shifts disturbance regimes.

Predicting which non-native plant species will become established and where is critical for conserving and managing biodiversity. Theory suggests that the mycorrhizal strategy of non-native plants may predict their establishment success. Here we combine a global dataset of 440,788 vegetation plots with data on plant native status and mycorrhizal type to assess mycorrhizal strategy of non-native plants. The mycorrhizal strategy of non-native plants varies strongly across biomes. Across grassland and desert biomes, non-native species are more frequently non-mycorrhizal than native species, whereas in other biomes non-native species are more likely to be mycorrhizal, most commonly arbuscular-mycorrhizal. Disturbance type and intensity are key predictors of mycorrhizal strategy of non-native species, as mycorrhizal species are favoured by landscape modification and non-mycorrhizal species by natural and human-caused disturbance events. Facultatively mycorrhizal species are consistently under-represented among non-native plants compared with natives, suggesting that symbiotic flexibility does not confer an advantage for non-natives as previously expected. Our study shows that non-native mycorrhizal strategy varies across biogeographical contexts and disturbance, highlighting the need for region-specific prevention and management approaches to plant species introductions.

Drought and invasive species are threats to primary productivity in grassland ecosystems worldwide. A central challenge in ecological restoration is establishing plant communities that can withstand these abiotic and biotic stressors. We tested the efficacy of a trait‐based framework for enhancing drought tolerance and invasion resistance in experimental restorations of contrasting grassland systems, a perennial mixed‐grass prairie in Wyoming and annual interior grassland in California. Each experiment included four trait‐based seeding treatments: drought tolerant, invasion resistant, functionally diverse, and a random control. All seeded communities were subjected to extreme precipitation reduction (−50% annual) and invasion by non‐native annual grasses. We asked two questions: (1) Can we establish communities from seed that meet and maintain these desired trait‐based targets? (2) Did the trait‐based seeding treatments tolerate extreme precipitation reduction or resist invasion by non‐native annual grasses? Based on analyses of compositional dissimilarities, we found trait‐based restoration targets were more difficult to maintain in an annual‐dominant grassland rather than in a perennial‐dominant one, and targets composed of multiple community‐weighted mean traits were more difficult to meet than targets that maximized functional diversity. In both grasslands, plant communities with drought‐tolerant traits maintained growth rates under reduced precipitation, but these effects diminished after multiple years of drought and during drought release. This highlights the importance of including a diversity of strategies when restoring plant communities. Our proposed invasion‐resistant traits did not consistently reduce non‐native annual grass establishment, but communities with traits that conferred drought tolerance were the most effective at resisting invasion, suggesting that similar traits enhance drought tolerance and resist invasion in these grasslands. Our findings indicate that restored plant communities with high functional diversity may be able to respond to variable conditions better than communities with traits designed to meet specific restoration objectives. The results from this restoration experiment suggest that our understanding of the traits underlying drought tolerance is better than our understanding of the traits underlying invasion resistance. However, if drought tolerance enhances competitive ability in arid and semiarid grassland ecosystems, then physiological theories of resource use can be applied to enhance invasion resistance.

Restoration outcomes are often variable and can fall short of targets due to poorly understood factors, especially in arid ecosystems. Identifying the drivers of variation during community assembly is critical for achieving restoration outcomes. Community trajectories and levels of recovery could be contingent on conditions at the time of establishment, a point at which practitioners may have the most control, though we currently lack a predictive understanding of these factors. Furthermore, it has been proposed that functional traits will respond more predictably than compositional metrics and could improve forecasts of restoration outcomes. Here, we use NMDS ordination and mixed‐effects models to evaluate sagebrush steppe restoration trajectories across 13 sites and 11 years. We compare the relative importance of time‐since‐restoration (age) and establishment conditions (soil, seed mix, establishment‐year climate) to variation in restoration outcomes. Most compositional and trait metrics responded predictably through time, with sites converging with each other towards reference conditions despite differences at establishment. However, communities fell short of or overshot reference targets after 11 years, reaching a maximum 15% compositional similarity and 69% functional similarity compared with reference sites. Restored communities were dominated by native and exotic graminoid species, corresponding with a conservative and drought‐tolerant composition of species with high leaf dry matter content and low turgor loss point. Restoration outcomes and convergence towards reference sites were contingent upon establishment year climate (year effects), soil sand content (site effects), and, to a lesser degree, seed mix design. These establishment effects on restoration trajectories often persisted or increased in strength over 11 years. Practical implication . Full restoration may require many more decades, but practitioners could speed success and increase control over outcomes by designing soil‐specific seed mixes and scheduling seed sowing or post‐seeding management to match short and long‐term climate forecasts. When restoration efforts fall short of targets or lead to undesirable stable states, practitioners may improve recovery through actions such as supplemental targeted planting of desired shrub and forb species to redirect successional trajectories. Seed mixes with a higher proportion of forbs and shrubs to graminoids should be further tested.

Summary: Drought stress constrains the productivity of terrestrial ecosystems and the distribution of plant species. To withstand drought stress, plants have evolved diverse water‐use strategies. Despite the critical function of roots in whole‐plant water regulation, drought strategies have been primarily studied from an aboveground perspective, and so our knowledge of how fine‐root traits are related to aboveground hydraulic traits remains limited. Here, we compiled a dataset of primarily woody species comprising five aboveground plant hydraulic traits associated with water‐use strategies and four fine‐root traits from the root economics space (RES). We investigated how the global diversity in ecological strategies of the RES traits relates to aboveground hydraulic traits contributing to drought resistance. We found a slight trend towards acquisitive species with higher root nitrogen content and lower root tissue density having lower drought resistance in aboveground tissues. Species with thicker roots displayed a tendency towards slightly higher drought resistance than thin‐rooted species. The weakness of these relationships suggests that aboveground drought adaptations are largely independent of classical axes of fine root ecological strategies, pointing to either a decoupling between aboveground and belowground responses or a stronger coordinating role for other root traits such as maximum rooting depth or root cortex fraction in hydraulic adaptation.

Understanding the drivers of plant community stability is crucial for predicting ecosystem responses to extreme drought events. In grasslands, drought resistance supports the maintenance of key functions such as above‐ground primary productivity, making the identification of resistance drivers essential to guide management under climate change. Proposed factors contributing to grassland stability include multiple diversity facets, functional traits and long‐term climate, but most assessments focus on temporal invariability under historical disturbance regimes, leaving mechanisms of extreme drought resistance and their variation across climatic contexts relatively underexplored. Here, we analysed data from 54 grassland sites of the International Drought Experiment to examine the resistance of above‐ground net primary productivity to a short‐term (i.e. 1 year) extreme drought. We investigated the relative importance and joint influence of functional composition (i.e. community‐weighted means of leaf and root traits), plant diversity facets (taxonomic, functional and phylogenetic) and climate (aridity and rainfall variability) on drought resistance. We used structural equation models to disentangle direct, indirect, and moderating pathways linking these drivers to drought resistance. Long‐term aridity appeared as one of the most important drivers of grassland resistance to drought, with more arid sites showing lower resistance. Moreover, aridity impacted resistance through indirect effects by shaping functional composition and plant diversity, and by moderating the influence of plant diversity and functional composition. Functional composition related to dehydration avoidance and dehydration tolerance was also positively associated with resistance, while diversity had a weaker relationship with resistance, mostly through functional and phylogenetic facets. Interannual rainfall variability also influenced resistance, with different effects in more arid versus humid and less arid sites. Synthesis . Widely studied stability drivers such as plant diversity and functional composition have only partial explanatory power for short‐term drought resistance of above‐ground productivity in grasslands at a global scale. The abiotic context, particularly long‐term aridity, is crucial for understanding ecosystem responses to rainfall variation and can improve predictive models for advancing the study of ecosystem resistance to drought. Along with management practices that target high species diversity or specific traits, restoration and conservation practices should support vulnerable sites experiencing high aridity

Summary: Belowground plant trait research has predominantly focused on trade-offs in fine root traits via the root economics space. Yet, this fine root framework captures only a fraction of the functional strategies plants employ beneath the soil surface. Here, we broaden the perspective on belowground plant functioning by integrating traits related to root system extent, clonality and bud banks, using data from the new UNDERPLOT database. This integration links measurable traits to key belowground functions: resource acquisition, spatial exploration, and persistence. Our analysis shows that the fine root economics space explains less than 5% of the variation in traits related to root system extent, clonality, and bud banks. Instead, an expanded trait analysis reveals three significant dimensions, explaining 62% of total trait variation. The third dimension, represents an independent, persistence-related gradient, not captured by existing root economics frameworks. We propose that understanding belowground plant strategies requires embracing additional functional gradients. The strategy of persistence, in particular, varies significantly across growth forms and is a critical dimension of plant response to resource limitation and stress, becoming increasingly important as global change shifts disturbance regimes.

Increased variability in precipitation associated with climate change creates extreme conditions of drought and deluge that can have profound effects on the abundance and composition of plant communities. Responses to these extremes likely vary across climatic gradients and depend on local plant community composition, which includes the emergent, aboveground vegetation as well as belowground seed banks. Because seed banks can both buffer the effects of environmental change and influence the future trajectories of communities, it is critical to understand seed bank responses to precipitation extremes in relation to the aboveground vegetation and how patterns vary across environmental gradients. Here we quantified the responses of aboveground and seed bank communities at five perennial grass-dominated sites across an elevational gradient to 6 years of extreme drought and deluge, by implementing experimental water exclusion and water addition treatments. Responses were stronger for drought than for deluge. Drought decreased abundance aboveground, while seed bank abundances were generally unaffected. Similarly, drought decreased richness and diversity of aboveground vegetation at intermediate elevations, without concurrent changes in seed banks. Surprisingly, the lowest and middle elevation sites showed stronger shifts in functional composition and dissimilarity in response to treatments, despite the expectation of greater buffering in seed banks in more arid environments. The relatively attenuated responses of seed bank communities to drought and deluge suggest potential for resistance and recovery, though species and functional composition may show greater responses to change particularly in more arid, lower elevation sites.

Summary: Humans are driving biodiversity change, which also alters community functional traits. However, how changes in the functional traits of the community alter ecosystem functions—especially belowground—remains an important gap in our understanding of the consequences of biodiversity change. We test hypotheses for how the root traits of the root economics space (composed of the collaboration and conservation gradients) are associated with proxies for ecosystem functioning across grassland and forest ecosystems in both observational and experimental datasets from 810 plant communities. First, we assessed whether community‐weighted means of the root economics space traits adhered to the same trade‐offs as species‐level root traits. Then, we examined the relationships between community‐weighted mean root traits and aboveground biomass production, root standing biomass, soil fauna biomass, soil microbial biomass, decomposition of standard and plot‐specific material, ammonification, nitrification, phosphatase activity, and drought resistance. We found evidence for a community collaboration gradient but not for a community conservation gradient. Yet, links between community root traits and ecosystem functions were more common than we expected, especially for aboveground biomass, microbial biomass, and decomposition. These findings suggest that changes in species composition, which alter root trait means, will in turn affect critical ecosystem functions.

Para entender la dinámica de las poblaciones de plantas se requieren datos demográficos asociados con supervivencia, crecimiento y reproducción. Mientras que la supervivencia y el crecimiento son bastante sencillos de medir, la reproducción puede ser difícil de cuantificar, especialmente en las plantas graminoides. Para incrementar nuestro entendimiento sobre el rendimiento reproductivo (producción de semillas) de 10 especies dominantes y perennes de gramíneas en los ecosistemas de bosques de pino ponderosa del suroeste, modelamos la producción de semillas y su relación con tres características de las plantas: tamaño (área basal en cm2), número de tallos florales (registro de tallos florales) y altura máxima del tallo floral (en cm). Desarrollamos modelos lineales generalizados para predecir el rendimiento reproductivo (semillas producidas) y la probabilidad de floración por especie, y su relación con el tamaño de la planta. Posteriormente, demostramos cómo estos datos de producción de semillas pueden ser utilizados para parametrizar los modelos de rendimiento reproductivo (núcleos de fecundidad) dentro de un modelo de población de plantas estructurado por tamaños (modelo de proyección integral). Para medir los núcleos de fecundidad, utilizamos 19 años (2002–2020) de datos a largo plazo de mapas permanentes de vegetación en cuadrantes, con el fin de estimar las tasas anuales y medias de reclutamiento, así como las distribuciones de tamaño de los individuos reclutados, en función del tamaño de la planta para cada especie. Tal como esperábamos, el número de semillas se relacionó positivamente con el tamaño de la planta en nueve de las diez especies perennes de plantas graminoides. Las tasas de reclutamiento para todas las especies durante el período de 19 años fueron bajas (<3%). Los núcleos de fecundidad demostraron que algunas especies de gramíneas perennes produjeron reclutas a partir de individuos a lo largo de su distribución de tamaño, mientras que otras especies, únicamente produjeron reclutas cuando eran de mayor tamaño. Nuestros resultados apoyan la idea de que el tamaño de la planta es un predictor confiable del número de tallos florales y de la producción de semillas en gramíneas perennes longevas. Estas estimaciones de fecundidad pueden ser integradas en estudios demográficos a largo plazo que rastreen el crecimiento y la supervivencia de individuos, con el objeto de generar modelos poblacionales de plantas estructurados y completamente parametrizados en el sotobosque del pino ponderosa del suroeste.