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Despite progress in trait‐based ecology, there is limited understanding of the plant traits that structure semi‐arid grasslands. In particular, it remains unclear how traits that enable plants to cope with water limitation are related to traits that influence other key functions such as herbivore defence and growth. The hypothesis that drought and herbivory exert convergent selection pressures is supported for morphological traits, but largely untested for structural, physiological and phenological traits. Drought and economic traits can also covary, but where and to what degree remains uncertain.Here we address these uncertainties in semi‐arid shortgrass steppe and mixedgrass prairie, the largest remaining grasslands in North America. Using a broad selection of traits for 37 of the most common plant species in each ecosystem, we ask whether traits that confer drought tolerance, avoidance and escape covary with herbivore resistance traits and economic traits.Results reveal that both drought tolerance and escape are coordinated with other functions, but in opposite fashion. Drought‐tolerant species (low leaf osmotic potential and high leaf dry matter content, LDMC) were also herbivore resistant (high leaf toughness and cellulose) and at the 'slow' end of the economic spectrum (low leaf nitrogen, leaf phosphorus and high stem density). Conversely, drought escape via early senescence was associated with lower drought tolerance, lower herbivore resistance and 'fast' economic traits. Drought avoidance, as indicated by thick leaves, may also be associated with lower drought tolerance (LDMC). Senescence date and LDMC appear to be key traits in these semi‐arid grasslands, differentiating species along multiple axes of function.Synthesis. Covariation between drought, herbivory and economic traits means that, of the many potential trait combinations, few actually exist within these grasslands. Consequently, changes in land management and climate should have predictable effects on drought resistance, forage quality and productivity in the western Great Plains. [ABSTRACT FROM AUTHOR]

Species introduced into areas outside of their native range face novel biotic and abiotic conditions, which probably impose novel selection pressures. Adaptation to these new conditions may increase the ability of introduced species to establish and spread. Like many other introduced plant populations, introduced genotypes of common mullein ( Verbascum thapsus) are more successful in their introduced than in their native range, with increased growth and fecundity. These differences appear to be at least partly genetically based. The most successful introduced populations also grow in an environment that is drier and has fewer competitors than native populations. It is not known, however, whether differences between native and introduced mullein populations are related to these environmental differences between ranges., We used a common garden experiment with 23 native and 27 introduced populations of common mullein to test whether common mullein in the introduced range exhibits evolutionary shifts with respect to responses to competition, drought stress and nitrogen (N) stress. We also used choice experiments to learn whether introduced mullein is more or less resistant to a generalist herbivore than native mullein., Without competition, introduced genotypes grew larger than native genotypes under high resource availability (control) and N stress, but not water stress. Survival, however, was increased in native populations under competition and N stress. The introduced genotypes also had a lower root:shoot ratio than the native genotypes. With competition, introduced genotypes grew larger than native genotypes across all treatments, with that difference being significant under N stress. The introduced genotypes were also more resistant to a generalist herbivore., Synthesis: Together, high biomass, strong responses to high water availability and low root:shoot ratio suggest that mullein has evolved a fast-growing, weedy phenotype in its introduced range rather than adapting to a low-water environment through increased root growth. Although fast-growing plants can be more palatable to herbivores, in this case there does not appear to be a trade-off between growth and defence against a generalist herbivore. Mullein appears to have evolved to be both faster growing and better defended in the introduced range. [ABSTRACT FROM AUTHOR]

In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO2 might ameliorate these effects by improving plant water-use efficiency., We examined the effects of CO2 and water availability on seedlings of two native ( Populus deltoides spp. monilifera, Salix exigua) and three exotic ( Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO2-controlled glasshouse, using 1-m-deep pots with different water-table decline rates., Low water availability reduced seedling biomass by 70-97%, and hindered the native species more than the exotics. Elevated CO2 increased biomass by 15%, with similar effects on natives and exotics. Elevated CO2 increased intrinsic water-use efficiency (Δ13Cleaf), but did not increase biomass more in drier treatments than wetter treatments., The moderate positive effects of elevated CO2 on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO2, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species. [ABSTRACT FROM AUTHOR]