Coordination of leaf, root, and seed traits shows the importance of whole plant economics in two semiarid grasslands.

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.

Soil disturbance and invasion magnify CO2 effects on grassland productivity, reducing diversity.

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.

Trading water for carbon in the future: Effects of elevated CO2 and warming on leaf hydraulic traits in a semiarid grassland.

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).

Plant traits related to precipitation sensitivity of species and communities in semiarid shortgrass prairie.

Understanding how plant communities respond to temporal patterns of precipitation in water-limited ecosystems is necessary to predict interannual variation and trends in ecosystem properties, including forage production, biogeochemical cycling, and biodiversity. In North American shortgrass prairie, we measured plant abundance, functional traits related to growth rate and drought tolerance, and aboveground net primary productivity to identify: species-level responsiveness to precipitation (precipitation sensitivity Sspp ) across functional groups; Sspp relationships to continuous plant traits; and whether continuous trait-Sspp relationships scaled to the community level. Across 32 plant species, we found strong bivariate relationships of both leaf dry matter content (LDMC) and leaf osmotic potential Ψosm with Sspp . Yet, LDMC and specific leaf area were retained in the lowest Akaike information criterion multiple regression model, explaining 59% of Sspp . Most relationships between continuous traits and Sspp scaled to the community level but were often contingent on the presence/absence of particular species and/or land management at a site. Thus, plant communities in shortgrass prairie may shift towards slower growing, more stress-resistant species in drought years and/or chronically drier climate. These findings highlight the importance of both leaf economic and drought tolerance traits in determining species and community responses to altered precipitation.

Extending the osmometer method for assessing drought tolerance in herbaceous species.

Community-scale surveys of plant drought tolerance are essential for understanding semi-arid ecosystems and community responses to climate change. Thus, there is a need for an accurate and rapid methodology for assessing drought tolerance strategies across plant functional types. The osmometer method for predicting leaf osmotic potential at full turgor (πo), a key metric of leaf-level drought tolerance, has resulted in a 50-fold increase in the measurement speed of this trait; however, the applicability of this method has only been tested in woody species and crops. Here, we assess the osmometer method for use in herbaceous grassland species and test whether πo is an appropriate plant trait for understanding drought strategies of herbaceous species as well as species distributions along climate gradients. Our model for predicting leaf turgor loss point (πTLP) from πo (πTLP = 0.80πo-0.845) is nearly identical to the model previously presented for woody species. Additionally, πo was highly correlated with πTLP for graminoid species (πtlp = 0.944πo-0.611; r2 = 0.96), a plant functional group previously flagged for having the potential to cause erroneous measurements when using an osmometer. We report that πo, measured with an osmometer, is well correlated with other traits linked to drought tolerance (namely, leaf dry matter content and leaf vulnerability to hydraulic failure) as well as climate extremes linked to water availability. The validation of the osmometer method in an herb-dominated ecosystem paves the way for rapid community-scale surveys of drought tolerance across plant functional groups, which could improve trait-based predictions of ecosystem responses to climate change.

Cheatgrass is favored by warming but not CO2 enrichment in a semi-arid grassland.

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.

Water availability dictates how plant traits predict demographic rates.

A major goal in ecology is to make generalizable predictions of organism responses to environmental variation based on their traits. However, straightforward relationships between traits and fitness are rare and likely to vary with environmental context. Characterizing how traits mediate demographic responses to the environment may enhance the predictions of organism responses to global change. We synthesized 15 years of demographic data and species-level traits in a shortgrass steppe to determine whether the effects of leaf and root traits on growth and survival depended on seasonal water availability. We predicted that (1) species with drought-tolerant traits, such as lower leaf turgor loss point (TLP) and higher leaf and root dry matter content (LDMC and RDMC), would be more likely to survive and grow in drier years due to higher wilting resistance, (2) these traits would not predict fitness in wetter years, and (3) traits that more directly measure physiological mechanisms of water use such as TLP would best predict demographic responses. We found that graminoids with more negative TLP and higher LDMC and RDMC had higher survival rates in drier years. Forbs demonstrated similar yet more variable responses. Graminoids grew larger in wetter years, regardless of traits. However, in both wet and dry years, graminoids with more negative TLP and higher LDMC and RDMC grew larger than less negative TLP and low LDMC and RDMC species. Traits significantly mediated the impact of drought on survival, but not growth, suggesting that survival could be a stronger driver of species' drought response in this system. TLP predicted survival in drier years, but easier to measure LDMC and RDMC were equal or better predictors. These results advance our understanding of the mechanisms by which drought drives population dynamics, and show that abiotic context determines how traits drive fitness.