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Summary: Uncertainty persists within trait‐based ecology, partly because few studies assess multiple axes of functional variation and their effect on plant performance.For 55 species from two semiarid grasslands, we quantified: (1) covariation between economic traits of leaves and absorptive roots, (2) covariation among economic traits, plant height, leaf size, and seed mass, and (3) relationships between these traits and species' abundance.Pairs of analogous leaf and root traits were at least weakly positively correlated (e.g. specific leaf area (SLA) and specific root length (SRL)). Two pairs of such traits, N content and DMC of leaves and roots, were at least moderately correlated (r > 0.5) whether species were grouped by site, taxonomic group and growth form, or life history. Root diameter was positively correlated with seed mass for all groups of species except annuals and monocots. Species with higher leaf dry matter content (LDMC) tended to be more abundant (r = 0.63). Annuals with larger seeds were more abundant (r = 0.69).Compared with global‐scale syntheses with many observations from mesic ecosystems, we observed stronger correlations between analogous leaf and root traits, weaker correlations between SLA and leaf N, and stronger correlations between SRL and root N. In dry grasslands, plant persistence may require coordination of above‐ and belowground traits, and dense tissues may facilitate dominance. [ABSTRACT FROM AUTHOR]

Pinus aristata Engelm., Rocky Mountain bristlecone pine, has a narrow core geographic and elevational distribution, occurs in disjunct populations, and is threatened by rapid climate change, white pine blister rust, and bark beetles. Knowledge of genetic diversity and population structure will help guide gene conservation strategies for this species. Sixteen sites across four mountain ranges in the core distribution of P. aristata were sampled and genetic diversity was assessed with 21 isozyme loci. Low species and population level genetic diversity ([H.sub.e] = 0.070 and 0.062, respectively) occurred with moderate among-population differentiation ([F.sub.ST] = 0.131). Genetic diversity correlated with longitude, latitude, and elevation and a strong mountain island effect may contribute to substructuring and isolation. Using multiple complementary analyses, sampled trees were assigned to three genetic lineages that varied in diversity and admixture and were associated with different climatic factors. The distribution of genetic diversity and substructuring of P. aristata may be an outcome of a combination of restricted gene flow due to geographic and phenological isolation, random processes of genetic drift, life history traits, natural selection, and postglacial migrations. The combination of low genetic diversity, moderate population isolation, and a protracted regeneration dynamic puts populations at risk for extirpation by novel stresses. Resume: Pinus aristata Engelm., le pin ariste, possede une distribution centrale restreinte, tant geographique qu'altitudinale. L'espece se retrouve en populations isolees et est menacee par la rapidite des changements climatiques, la rouille vesiculeuse du pin blanc et les scolytes. La connaissance de la diversite genetique et de la structure de populations aidera a orienter les strategies de conservation des ressources genetiques chez cette espece. Les auteurs ont echantillonne 16 populations reparties dans les quatre chaines de montagnes se retrouvant au coeur de la distribution centrale de P. aristata. La diversite genetique a ete evaluee pour 21 loci d'isoenzymes. Une diversite genetique faible fut remarquee, tant pour l'espece que dans les populations ([H.sub.e] = 0,070 et 0,062, respectivement). La differenciation de populations etait moderee ([F.sub.ST] = 0,131). La diversite genetique etait correlee avec la longitude, la latitude et l'altitude, et un fort effet insulaire du aux montagnes pourrait contribuer a la sous-structuration et a l'isolement. En recourant a des analyses multiples complementaires, les arbres echantillonnes pouvaient etre assignes a trois lignees genetiques qui variaient quant a leur diversite et leur composition et qui etaient associees a differents facteurs climatiques. La distribution de la diversite genetique et la sous-structuration de P. aristata pourraient etre la resultante d'une combinaison de facteurs incluant le flux genique restreint en raison de l'isolement geographique ou phenologique, les processus aleatoires decoulant de la derive genetique, les caracteristiques du cycle vital, la selection naturelle et les migrations postglaciaires. La combinaison de la faible diversite genetique, de l'isolement modere des populations et d'une dynamique de regeneration prolongee resulte augmente les risques que les populations disparaissent a la suite de nouveaux stress. [Traduit par la Redaction], Introduction Ex situ gene conservation of wild species is of growing interest, as natural populations are increasingly being challenged by introduced pests and pathogens and a changing climate (Guerrant et [...]

Climate change, disturbance, and plant invasion threaten grassland ecosystems, but their combined and interactive effects are poorly understood. Here, we examine how the combination of disturbance and plant invasion influences the sensitivity of mixed‐grass prairie to elevated carbon dioxide (eCO2) and warming. We established subplots of intact prairie and disturbed/invaded prairie within a free‐air CO2 enrichment (to 600 ppmv) by infrared warming (+1.5°C day, 3°C night) experiment and followed plant and soil responses for 5 years. Elevated CO2 initially led to moderate increases in biomass and plant diversity in both intact and disturbed/invaded prairie, but these effects shifted due to strong eCO2 responses of the invasive forb Centaurea diffusa. In the final 3 years, biomass responses to eCO2 in disturbed/invaded prairie were 10 times as large as those in intact prairie (+186% vs. +18%), resulting in reduced rather than increased plant diversity (−17% vs. +10%). At the same time, warming interacted with disturbance/invasion and year, reducing the rate of topsoil carbon recovery following disturbance. The strength of these interactions demonstrates the need to incorporate disturbance into predictions of climate change effects. In contrast to expectations from studies in intact ecosystems, eCO2 may threaten plant diversity in ecosystems subject to soil disturbance and invasion. [ABSTRACT FROM AUTHOR]

The effects of climate change on plants and ecosystems are mediated by plant hydraulic traits, including interspecific and intraspecific variability of trait phenotypes. Yet, integrative and realistic studies of hydraulic traits and climate change are rare. In a semiarid grassland, we assessed the response of several plant hydraulic traits to elevated CO2 (+200 ppm) and warming (+1.5 to 3°C; day to night). For leaves of five dominant species (three graminoids and two forbs), and in replicated plots exposed to 7 years of elevated CO2, warming, or ambient climate, we measured: stomatal density and size, xylem vessel size, turgor loss point, and water potential (pre‐dawn). Interspecific differences in hydraulic traits were larger than intraspecific shifts induced by elevated CO2 and/or warming. Effects of elevated CO2 were greater than effects of warming, and interactions between treatments were weak or not detected. The forbs showed little phenotypic plasticity. The graminoids had leaf water potentials and turgor loss points that were 10% to 50% less negative under elevated CO2; thus, climate change might cause these species to adjust their drought resistance strategy away from tolerance and toward avoidance. The C4 grass also reduced allocation of leaf area to stomata under elevated CO2, which helps explain observations of higher soil moisture. The shifts in hydraulic traits under elevated CO2 were not, however, simply due to higher soil moisture. Integration of our results with others' indicates that common species in this grassland are more likely to adjust stomatal aperture in response to near‐term climate change, rather than anatomical traits; this contrasts with apparent effects of changing CO2 on plant anatomy over evolutionary time. Future studies should assess how plant responses to drought may be constrained by the apparent shift from tolerance (via low turgor loss point) to avoidance (via stomatal regulation and/or access to deeper soil moisture). [ABSTRACT FROM AUTHOR]

Global changes that alter soil water availability may have profound effects on semiarid ecosystems. Although both elevated CO2 (eCO2) and warming can alter water availability, often in opposite ways, few studies have measured their combined influence on the amount, timing, and temporal variability of soil water. Here, we ask how free air CO2 enrichment (to 600 ppmv) and infrared warming (+ 1.5 °C day, + 3 °C night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. We found that eCO2 and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming on soil water largely offset positive effects of eCO2. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with eCO2. These interactions constrained the combined effect of eCO2 and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, eCO2 increased soil water under drier conditions, and warming decreased soil water under wetter conditions. By increasing soil water under dry conditions, eCO2 also reduced temporal variability in soil water. These temporal patterns explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with eCO2 plus warming in autumn. They also suggest that eCO2 and warming may favor plant species that grow in autumn, including winter annuals and C3 graminoids, and species able to remain active under the dry conditions moderated by eCO2. [ABSTRACT FROM AUTHOR]

Elevated CO2 and warming may alter terrestrial ecosystems by promoting invasive plants with strong community and ecosystem impacts. Invasive plant responses to elevated CO2 and warming are difficult to predict, however, because of the many mechanisms involved, including modification of phenology, physiology, and cycling of nitrogen and water. Understanding the relative and interactive importance of these processes requires multifactor experiments under realistic field conditions. Here, we test how free-air CO2 enrichment (to 600 ppmv) and infrared warming (+1.5 °C day/3 °C night) influence a functionally and phenologically distinct invasive plant in semi-arid mixed-grass prairie. Bromus tectorum (cheatgrass), a fast-growing Eurasian winter annual grass, increases fire frequency and reduces biological diversity across millions of hectares in western North America. Across 2 years, we found that warming more than tripled B. tectorum biomass and seed production, due to a combination of increased recruitment and increased growth. These results were observed with and without competition from native species, under wet and dry conditions (corresponding with tenfold differences in B. tectorum biomass), and despite the fact that warming reduced soil water. In contrast, elevated CO2 had little effect on B. tectorum invasion or soil water, while reducing soil and plant nitrogen (N). We conclude that (1) warming may expand B. tectorum's phenological niche, allowing it to more successfully colonize the extensive, invasion-resistant northern mixed-grass prairie, and (2) in ecosystems where elevated CO2 decreases N availability, CO2 may have limited effects on B. tectorum and other nitrophilic invasive species. [ABSTRACT FROM AUTHOR]