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Duval B.D.,Merriam Powell Center for Environmental Research | Duval B.D.,Arizona State University | Dijkstra P.,Merriam Powell Center for Environmental Research | Natali S.M.,University of Florida | And 7 more authors.
Environmental Science and Technology | Year: 2011

The distribution of contaminant elements within ecosystems is an environmental concern because of these elements' potential toxicity to animals and plants and their ability to hinder microbial ecosystem services. As with nutrients, contaminants are cycled within and through ecosystems. Elevated atmospheric CO2 generally increases plant productivity and alters nutrient element cycling, but whether CO2 causes similar effects on the cycling of contaminant elements is unknown. Here we show that 11 years of experimental CO2 enrichment in a sandy soil with low organic matter content causes plants to accumulate contaminants in plant biomass, with declines in the extractable contaminant element pools in surface soils. These results indicate that CO2 alters the distribution of contaminant elements in ecosystems, with plant element accumulation and declining soil availability both likely explained by the CO2 stimulation of plant biomass. Our results highlight the interdependence of element cycles and the importance of taking a broad view of the periodic table when the effects of global environmental change on ecosystem biogeochemistry are considered. © 2011 American Chemical Society.


Duval B.D.,Northern Arizona University | Duval B.D.,Merriam Powell Center for Environmental Research | Duval B.D.,University of Illinois at Urbana - Champaign | Blankinship J.C.,University of California at Merced | And 4 more authors.
Plant Ecology | Year: 2012

Elevated CO 2 is expected to lower plant nutrient concentrations via carbohydrate dilution and increased nutrient use efficiency. Elevated CO 2 consistently lowers plant foliar nitrogen, but there is no consensus on CO 2 effects across the range of plant nutrients. We used meta-analysis to quantify elevated CO 2 effects on leaf, stem, root, and seed concentrations of B, Ca, Cu, Fe, K, Mg, Mn, P, S, and Zn among four plant functional groups and two levels of N fertilization. CO 2 effects on plant nutrient concentration depended on the nutrient, plant group, tissue, and N status. CO 2 reduced B, Cu, Fe, and Mg, but increased Mn concentration in the leaves of N 2 fixers. Elevated CO 2 increased Cu, Fe, and Zn, but lowered Mn concentration in grass leaves. Tree leaf responses were strongly related to N status: CO 2 significantly decreased Cu, Fe, Mg, and S at high N, but only Fe at low N. Elevated CO 2 decreased Mg and Zn in crop leaves grown with high N, and Mn at low N. Nutrient concentrations in crop roots were not affected by CO 2 enrichment, but CO 2 decreased Ca, K, Mg and P in tree roots. Crop seeds had lower S under elevated CO 2. We also tested the validity of a "dilution model." CO 2 reduced the concentration of plant nutrients 6. 6% across nutrients and plant groups, but the reduction is less than expected (18. 4%) from carbohydrate accumulation alone. We found that elevated CO 2 impacts plant nutrient status differently among the nutrient elements, plant functional groups, and among plant tissues. Our synthesis suggests that differences between plant groups and plant organs, N status, and differences in nutrient chemistry in soils preclude a universal hypothesis strictly related to carbohydrate dilution regarding plant nutrient response to elevated CO 2. © 2011 Springer Science+Business Media B.V.


Cushman S.A.,Rocky Research | Max T.,Northern Arizona University | Max T.,University of Idaho | Meneses N.,Northern Arizona University | And 7 more authors.
Ecological Applications | Year: 2014

Fremont cottonwood (Populus fremonti) is a foundation riparian tree species that drives community structure and ecosystem processes in southwestern U.S. ecosystems. Despite its ecological importance, little is known about the ecological and environmental processes that shape its genetic diversity, structure, and landscape connectivity. Here, we combined molecular analyses of 82 populations including 1312 individual trees dispersed over the species' geographical distribution. We reduced the data set to 40 populations and 743 individuals to eliminate admixture with a sibling species, and used multivariate restricted optimization and reciprocal causal modeling to evaluate the effects of river network connectivity and climatic gradients on gene flow. Our results confirmed the following: First, gene flow of Fremont cottonwood is jointly controlled by the connectivity of the river network and gradients of seasonal precipitation. Second, gene flow is facilitated by mid-sized to large rivers, and is resisted by small streams and terrestrial uplands, with resistance to gene flow decreasing with river size. Third, genetic differentiation increases with cumulative differences in winter and spring precipitation. Our results suggest that ongoing fragmentation of riparian habitats will lead to a loss of landscape-level genetic connectivity, leading to increased inbreeding and the concomitant loss of genetic diversity in a foundation species. These genetic effects will cascade to a much larger community of organisms, some of which are threatened and endangered. © 2014 by the Ecological Society of America.


Trotter III R.T.,U.S. Department of Agriculture | Whitham T.G.,Northern Arizona University | Whitham T.G.,Merriam Powell Center for Environmental Research
Journal of Sustainable Forestry | Year: 2011

As forested systems are impacted by both natural and anthropogenic factors such as climate change, the biodiversity supported by those forests is likely to change. Quantifying that change, however, remains a difficult task due to variations in the sizes and conditions of forested systems. Species accumulation curves are a commonly used tool to scale estimates of species richness and provide an avenue for comparing biodiversity among habitats through rarefaction. However, we found that the ranked biodiversity among forested systems depends on the sample unit used, and there is a need to integrate landscape heterogeneity in spatially scaleable estimates of biodiversity. Both of these biodiversity assessment issues can be addressed using a new approach we term the Integrated Accumulation Function (IAF), a method based on combining component species accumulation curves. Using this approach on communities of canopy arthropods found in pinyon pine forests in the southwestern United States, we found three major patterns. First, in small stands, trees growing under low environmental stress support the greatest species richness. Second, when stands are large, stands growing under higher environmental stress support greater species richness, and species richness is resilient to change over a broad range of the stress gradient. Third, there are threshold levels of stress at both ends of the stress spectrum beyond which species are rapidly lost. This analysis reveals unexpected patterns and suggests that conservation practices should consider the inclusion of forests growing under suboptimal conditions to maximize the preservation of biodiversity. © 2011 Copyright Taylor and Francis Group, LLC.


Hersch-Green E.I.,Michigan Technological University | Allan G.J.,Northern Arizona University | Whitham T.G.,Northern Arizona University | Whitham T.G.,Merriam Powell Center for Environmental Research
Tree Genetics and Genomes | Year: 2014

Cottonwoods are well known as foundation riparian trees that support diverse communities and drive ecosystem processes. Although hybridization naturally occurs when the distributions of two or more cottonwood species overlap, few cottonwood hybrid zones have been genetically characterized. We use genetic and genomic analyses to characterize patterns of admixture and introgression for a newly described hybrid zone at the intersection of three species (Populus L. Salicaceae-Populus deltoides, Populus fremontii, and Populus angustifolia) in southwestern Colorado, USA. Analysis of nuclear and chloroplast microsatellite marker data detected substantial genetic variation among individuals, revealing that (1) hybridization is occurring between two, not three, species (P. deltoides and P. angustifolia); (2) gene flow is bidirectional; (3) hybrids are not abundant (admixture detected in only 34 of 270 trees), with most being early-generation F1 hybrids; (4) cytonuclear disequilibria exists and F1 hybrids tend to retain P. deltoides-like chloroplasts; and (5) roughly 30 % of the nuclear markers deviated from a neutral pattern of introgression, suggesting that selection may play a role in shaping the genetic structure of the hybrid zone in this region. Overall, our results show that despite strong selection maintaining species divergence, transfer of allelic variants across species boundaries can occur. Our study assesses the fine-scale genetic structure of hybridization between P. angustifolia and P. deltoides and lays the foundation for examining how geographic differences in hybrid zone dynamics arise and may influence subsequent ecological and evolutionary processes. © 2014 Springer-Verlag Berlin Heidelberg.


Stone A.C.,Northern Arizona University | Stone A.C.,Merriam Powell Center for Environmental Research | Gehring C.A.,Northern Arizona University | Gehring C.A.,Merriam Powell Center for Environmental Research | And 2 more authors.
Oecologia | Year: 2010

Understanding how communities respond to extreme climatic events is important for predicting the impact of climate change on biodiversity. The plant vigor and stress hypotheses provide a theoretical framework for understanding how arthropods respond to stress, but are rarely tested at the community level. Following a record drought, we compared the communities of arthropods on pinyon pine (Pinus edulis) that exhibited a gradient in physical traits related to environmental stress (e. g., growth rate, branch dieback, and needle retention). Six patterns emerged that show how one of the predicted outcomes of climate change in the southwestern USA (i. e., increased drought severity) alters the communities of a foundation tree species. In accordance with the plant vigor hypothesis, increasing tree stress was correlated with an eight to tenfold decline in arthropod species richness and abundance. Trees that were more similar in their level of stress had more similar arthropod communities. Both foliage quantity and quality contributed to arthropod community structure. Individual species and feeding groups differed in their responses to plant stress, but most were negatively affected. Arthropod richness (r2 = 0. 48) and abundance (r2 = 0. 48) on individual trees were positively correlated with the tree's radial growth during drought. This relationship suggests that tree ring analysis may be used as a predictor of arthropod diversity, which is similar to findings with ectomycorrhizal fungi. A contrast of our findings on arthropod abundance with published data on colonization by mutualistic fungi on the same trees demonstrates that at low stress these two communities respond differently, but at high stress both are negatively affected. These results suggest that the effect of extreme climatic events such as drought on foundation tree species are likely to decrease multi-trophic diversity and shift arthropod community composition, which in turn could cascade to affect other associated taxa. © 2010 Springer-Verlag.


Smith D.S.,Northern Arizona University | Smith D.S.,Denison University | Lau M.K.,Northern Arizona University | Lau M.K.,Harvard University | And 6 more authors.
Oecologia | Year: 2015

Because introduced species may strongly interact with native species and thus affect their fitness, it is important to examine how these interactions can cascade to have ecological and evolutionary consequences for whole communities. Here, we examine the interactions among introduced Rocky Mountain elk, Cervus canadensis nelsoni, a common native plant, Solidagovelutina, and the diverse plant-associated community of arthropods. While introduced species are recognized as one of the biggest threats to native ecosystems, relatively few studies have investigated an evolutionary mechanism by which introduced species alter native communities. Here, we use a common garden design that addresses and supports two hypotheses. First, native S. velutina has rapidly evolved in the presence of introduced elk. We found that plants originating from sites with introduced elk flowered nearly 3 weeks before plants originating from sites without elk. Second, evolution of S. velutina results in a change to the plant-associated arthropod community. We found that plants originating from sites with introduced elk supported an arthropod community that had ~35 % fewer total individuals and a different species composition. Our results show that the impacts of introduced species can have both ecological and evolutionary consequences for strongly interacting species that subsequently cascade to affect a much larger community. Such evolutionary consequences are likely to be long-term and difficult to remediate. © 2015, Springer-Verlag Berlin Heidelberg.

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