Foliar nitrogen isotope (δsup.15N) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential ...importance of the direct foliar uptake of gaseous reactive N on foliar δsup.15N. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δsup.15N in potted red maple (Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δsup.15N values in red maple seedlings from 0.06 to 0.74 % across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δsup.15N, and mycorrhizae on foliar δsup.15N in adult stands of A. rubrum, American beech (Fagus grandifolia), black birch (Betula lenta), and red oak (Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO.sub.2) concentration explained 0, 69, 23, and 45 % of the variation in foliar δsup.15N in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δsup.15N. There was no correlation between foliar δsup.13C and foliar %N with increasing atmospheric NO.sub.2 concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δsup.15N in several dominant species occurring in temperate forest ecosystems. Keywords Nitrogen isotope composition * Foliar uptake * Nitrogen cycling * Reactive nitrogen * Temperate forest
Fluctuations in mean annual precipitation (MAP) will strongly influence the ecology of dryland ecosystems in the future, yet, because individual precipitation events drive growth and resource ...availability for many dryland organisms, changes in intra-annual precipitation may disproportionately influence future dryland processes. This work examines the hypothesis that intra-annual precipitation changes will drive dryland productivity to a greater extent than changes to MAP. To test this hypothesis, we created a physiology-based model to predict the effects of precipitation change on a widespread biocrust moss that regulates soil structure, water retention, and nutrient cycling in drylands. First, we used the model to examine moss productivity over the next 100 years driven by alterations in MAP by ±10, 20 and 30 %, and changes in intra-annual precipitation (event size and frequency). Productivity increased as a function of MAP, but differed among simulations where intra-annual precipitation was manipulated under constant MAP. Supporting our hypothesis, this demonstrates that, even if MAP does not change, changes in the features of individual precipitation events can strongly influence long-term performance. Second, we used the model to examine 100-year productivity based on projected dryland precipitation from published global and regional models. These simulations predicted 25–63 % reductions in productivity and increased moss mortality rates, declines that will likely alter water and nutrient cycling in dryland ecosystems. Intra-annual precipitation in model-based simulations was a stronger predictor of productivity compared to MAP, further supporting our hypothesis, and illustrating that intra-annual precipitation patterns may dominate dryland responses to altered precipitation in a future climate.
Reducing soil nutrient availability has been proposed as a strategy to favor native vs. non‐native invasive plant species and represents a potential alternative to traditional manual removal or ...chemical control methods. We implemented a field experiment in invaded dry and wet montane Hawaiian ecosystems to test responses of soil and dominant plant species to three soil nutrient treatments (Control = no nutrient manipulation; Carbon = C substrate added to reduce nutrients; Fertilizer = fertilizer added to increase nutrients) and two non‐native plant treatments (Weeds removed; Weeds present) in a fully factorial experiment in each ecosystem over 18 months. Carbon amendments reduced soil inorganic nutrient availability by 60–70% in dry shrubland and 30–50% in wet forest. Fertilizer amendments increased soil inorganic nutrient availability by >20‐fold. Altered nutrient availability did not impact gross mineralization or nitrification rates in either ecosystem. In dry shrubland, neither C amendments nor weed removal altered growth or reproduction, but fertilizer increased woody growth and forb/grass reproduction in both natives and non‐natives. In wet forest, weed removal but not C amendments increased growth and survival of native woody seedlings, while fertilizer decreased native seedling survival and increased non‐native woody seedling growth. Overall, growth and reproduction of native and non‐natives responded similarly to altered nutrient availability, indicating that for the tropical ecosystems and species examined, manipulating nutrient availability does not favor native versus non‐native invasive plants in the first 18 months. In contrast, weed removal had positive effects on native plant growth, likely mediated through changes in other resources.
Ecosystem water-use efficiency (eWUE; the ratio of net ecosystem productivity to evapotranspiration rate) is a complex landscape-scale parameter controlled by both physical and biological processes ...occurring in soil and plants. Leaf WUE (lWUE; the ratio of leaf CO₂ assimilation rate to transpiration rate) is controlled at short time scales principally by leaf stomatal dynamics and this control varies among plant species. Little is known about how leaf-scale variation in lWUE influences landscape-scale variation in eWUE. We analyzed approximately seven thousand 30-min averaged eddy covariance observations distributed across 9 years in order to assess eWUE in two neighboring forest communities. Mean eWUE was 19% lower for the community in which Engelmann spruce and subalpine fir were dominant, compared to the community in which lodgepole pine was dominant. Of that 19% difference, 8% was attributed to residual bias in the analysis that favored periods with slightly drier winds for the spruce-fir community. In an effort to explain the remaining 11% difference, we assessed patterns in lWUE using C isotope ratios. When we focused on bulk tissue from older needles we detected significant differences in lWUE among tree species and between upper and lower canopy needles. However, when these differences were scaled to reflect vertical and horizontal leaf area distributions within the two communities, they provided no power to explain differences in eWUE that we observed in the eddy covariance data. When we focused only on bulk needle tissue of current-year needles for 3 of the 9 years, we also observed differences in lWUE among species and in needles from upper and lower parts of the canopy. When these differences in lWUE were scaled to reflect leaf area distributions within the two communities, we were able to explain 6.3% of the differences in eWUE in 1 year (2006), but there was no power to explain differences in the other 2 years (2003 and 2007). When we examined sugars extracted from needles at 3 different times during the growing season of 2007, we could explain 3.8-6.0% of the differences in eWUE between the two communities, but the difference in eWUE obtained from the eddy covariance record, and averaged over the growing season for this single year, was 32%. Thus, overall, after accounting for species effects on lWUE, we could explain little of the difference in eWUE between the two forest communities observed in the eddy covariance record. It is likely that water and C fluxes from soil, understory plants, and non-needle tissues, account for most of the differences observed in the eddy covariance data. For those cases where we could explain some of the difference in eWUE on the basis of species effects, we partitioned the scaled patterns in lWUE into two components: a component that is independent of canopy leaf area distribution, and therefore only dependent on species-specific differences in needle physiology; and a component that is independent of species differences in needle physiology, and only dependent on species-specific influences on canopy leaf area distribution. Only the component that is dependent on species influences on canopy leaf area distribution, and independent of inherent species differences in needle physiology, had potential to explain differences in eWUE between the two communities. Thus, when tree species effects are important, canopy structure, rather than species-specific needle physiology, has more potential to explain patterns in eWUE.
The impact of grazing on C fluxes from pastures in subtropical and tropical regions and on the environment is uncertain, although these systems account for a substantial portion of global C storage. ...We investigated how cattle grazing influences net ecosystem CO₂ and CH₄ exchange in subtropical pastures using the eddy covariance technique. Measurements were made over several wet-dry seasonal cycles in a grazed pasture, and in an adjacent pasture during the first three years of grazer exclusion. Grazing increased soil wetness but did not affect soil temperature. By removing aboveground biomass, grazing decreased ecosystem respiration (R
eco) and gross primary productivity (GPP). As the decrease in R
eco was larger than the reduction in GPP, grazing consistently increased the net CO₂ sink strength of subtropical pastures (55, 219 and 187 more C/m² in 2013, 2014, and 2015). Enteric ruminant fermentation and increased soil wetness due to grazers, increased total net ecosystem CH₄ emissions in grazed relative to ungrazed pasture (27–80%). Unlike temperate, arid, and semiarid pastures, where differences in CH₄ emissions between grazed and ungrazed pastures are mainly driven by enteric ruminant fermentation, our results showed that the effect of grazing on soil CH₄ emissions can be greater than CH₄ produced by cattle. Thus, our results suggest that the interactions between grazers and soil hydrology affecting soil CH₄ emissions play an important role in determining the environmental impacts of this management practice in a subtropical pasture. Although grazing increased total net ecosystem CH₄ emissions and removed aboveground biomass, it increased the net storage of C and decreased the global warming potential associated with C fluxes of pasture by increasing its net CO₂ sink strength.
Agriculture can dramatically alter community composition, food webs, and resource availability, yet the impacts of agriculture on many ecosystems and their constituents remain unstudied. Here, we ...examined the effect of agriculture on the isotopic ecology of Horned Larks (
Eremophila alpestris
) in the Colorado Desert. Over the last century, the Imperial Valley in southeastern California has transformed from desert scrub into agricultural land, while breeding Horned Larks have persisted in the area. We compared the isotopic composition of nitrogen (δ
15
N), carbon (δ
13
C), and hydrogen (δ
2
H) from feathers of lark specimens from two time periods in the Imperial Valley: a historical series collected between 1916 and 1923 and more recent vouchers collected between 1984 and 2014. We also quantified δ
15
N and δ
13
C values from plants and soil samples in contemporary agricultural land and adjacent desert scrub to estimate isotopic baseline changes associated with agricultural intensification. Contemporary larks had lower δ
15
N and δ
13
C ratios compared to historical larks, while plant and soil δ
15
N ratios were higher at agricultural sites compared to adjacent desert sites. These opposing trends suggest temporal change in isotope values that cannot be explained by shifting baselines alone. Furthermore, isotopic niche breadth (‰
2
; δ
15
N and δ
13
C) was lower among contemporary larks. Thus, contemporary larks in the Imperial Valley may capitalize on seeds and phytophagous insects that are abundant among crops, possibly representing an opportunistic change in diet over time in response to agriculture. Finally, δ
2
H values did not differ between historical and contemporary larks, but contemporary larks had δ
2
H values that differed from historical ground water, suggesting intake of water introduced by irrigational canals. Collectively, these findings highlight the power of stable isotope analyses combined with natural history collections to examine ecological change in the Anthropocene.
Conservation science requires quickly acquiring information and taking action in order to protect species at risk of extinction. Stable isotope measurements are one way to rapidly gather data ...regarding species’ foraging ecology and habitat use, and passively collected samples limit additional stress to at-risk species. For these samples to be useful, however, we must know how representative they are of the stable isotope ratios of the entire organism. Bone tissue, often stored in museum collections or research centers, may be the most readily available tissue from rare, endangered, or extinct vertebrates, but using bone requires practitioners to understand intraskeletal stable isotope variation. We sampled the same eight skeletal elements from 72 cetacean skeletons from 14 species to evaluate intraskeletal variation in carbon and nitrogen isotope values. We found considerably more variation than anticipated. Carbon intraskeletal ranges varied from 0.4 to 7.6‰, with 84.7% (n = 61) of skeletons having a range >1‰, and 55.5% (n = 40) exhibiting a range >2‰. Similarly, nitrogen intraskeletal ranges varied from 0.4 to 5.2‰, with 59.7% (n = 43) of skeletons exhibiting a range >1‰, and 15.3% (n = 11) with a range >2‰. There were differences in which bones contributed most to intraskeletal variation; however, we advise against using humeri and mandibles as these bones presented the most consistent trends in deviation from the intraskeletal means for both isotopes. The large intraskeletal variation we observed is likely due to changes in foraging behavior or habitat use being reflected differently in bone isotope ratios due to differences in bone turnover rates. We suggest that for cetaceans, intraskeletal carbon isotope ranges >1‰ and nitrogen ranges >2‰ are ecologically relevant, and that using different bones from animals of the same population may produce false positive differences in foraging behavior or habitat within the population if intraskeletal variation is not considered. Future studies should use the same bones from each animal and conduct species-specific analyses of intraskeletal variation, if possible, when using specimens of opportunity. Failure to consider this variation could lead to erroneous conclusions regarding a species range or key habitats, jeopardizing conservation efforts.
Predators alter ecological communities by inducing changes in prey abundance and phenotypes, including elemental and isotopic composition. Climatic factors are known to often moderate predator ...effects on prey abundance, but few studies consider the combined effects of climate and predators on prey phenotype. We examined how altered precipitation moderates the effects of predators on the abundance and the chemical composition of prey, as well as the indirect effects on the basal resource: leaf litter coated in biofilm. Using an experiment with an invertebrate food web module from tank bromeliads, we manipulated the presence of an odonate predator under scenarios of 10‐fold decreased, ambient, and threefold increased precipitation and measured responses of prey and their basal resource. Predators reduced prey abundance while precipitation did not. Both precipitation and predators, either singly or interactively, affected the elemental composition of prey. Predators increased C:N ratios of detritivorous beetles under high precipitation, but reduced the C:N ratio of the basal resource. Precipitation reduced the N content of filter‐feeding mosquitoes. The observed changes in chemical composition may reflect physiological or developmental responses to stress imposed by both predators and drought. This study demonstrates that the impact of predators and precipitation can vary substantially across a food web, including additive and synergistic effects, and numerical and phenotypic responses.
in Spanish is available with online material.
Resumen
Los depredadores afectan las comunidades ecologicas a través de cambios en la abundancia y fenotipo de las pesas, incluyendo la composición de elementos e isotopica. Se sabe que las variables climaticas regulan el efecto del depredador sobre la abundancia de las presas, pero pocos estudios han considerado el efecto combinado del clima y la depredación sobre el fenotipo de las presas. Se evaluó cómo cambios en la precipitación regulan los efectos del depredador sobre la abundancia y composición química de las presas, así como los efectos indirectos sobre el recurso base, la biopelícula de la hojarasca. Se realizó un experimento utilizando la red trófica de invertebrados de las bromelias tanque, se manipuló la presencia de un depredador odonato bajo los escenarios de disminución de 10 veces la precipitación, ambiental e incremento de 3 veces la precipitación y se midió la respuesta de las presas y el recurso base. El depredador pero no la precipitación redujó la abundancia de las presas; ambos, la precipitación y el depreddor, solo o en interacción afectaron la composición química de las presas. El depredador incrementó la relación C:N de los coleópteros detritivos bajo el tratamiento de precipitación alta pero redujó la relación C:N del recurso base. La precipitación redujo el contenido de N de los mosquitos. Los cambios observados en la composición quimica pueden reflejar las respuestas fisiológicas y de desarrollo al estrés impuesto por el depredador y la sequía. Este estudio demostró que el impacto del depredador y la precipitación pueden variar sustancialmente a través de la red trofica incluyendo efetos aditivos y sinérgicos y respuestas numéricas y fenotipicas.
We designed a field experiment to test the hypothesis that altered rainfall patterns and odonate predator presence affect abundance and elemental composition of invertebrate prey (Culex sp. and Scirtes sp.) and basal resource within bromeliad food webs. We found that predators, but not precipitation, affected prey abundance. However, predators and precipitation, both alone and in combination, affected various aspects of the elemental content of surviving insect prey and remaining leaf litter. Our results suggest that the impact of predators and rainfall can vary substantially across a food web, including additive and synergistic effects and numerical and phenotypic responses.
Conservation and restoration of ecosystems impacted by nonnative ungulates increasingly involves their removal and exclusion. While the influence of nonnative ungulate removal on plant communities is ...commonly monitored, impacts on underlying ecological processes are seldom quantified. Here we examined how nonnative feral pig (
Sus scrofa
) removal from Hawaiian tropical montane wet forests affects soil physical and chemical properties. Unique to this study, measurements were taken in paired sites inside and outside of five feral pig removal units representing a ~20 year, highly constrained chronosequence where other potentially confounding variables are held constant. Additional targeted measurements were taken inside and outside of a single exclosure in areas characterized by ‘low’ versus ‘high’ feral pig activity. Overall, nonnative feral pig removal increased stable soil aggregates and porosity, and decreased bulk density, water-filled pore space, and soil moisture content. Further, feral pig removal increased soil nutrient regeneration as evidenced by increased extractable cations, increased resin available NO
3
−
and total inorganic N, and enriched foliar δ
15
N. Increasing time since feral pig removal was positively related to net nitrification and total net inorganic N mineralization, and negatively related to pH and net ammonification. Results from both the chronosequence and targeted sampling were consistent in direction and support a central role of feral pig removal in modifying soil physical and chemical properties. Changes in soil properties following ungulate removal coincided with large increases in understory vegetation cover, highlighting the need to better understand aboveground-belowground linkages following nonnative ungulate removal.
Streptomyces scabies is the best studied of those streptomycetes that cause an economically important disease known as potato scab. The phytotoxin thaxtomin is made exclusively by these pathogens and ...is required for virulence. Here we describe regulation of thaxtomin biosynthesis by TxtR, a member of the AraC/XylS family of transcriptional regulators. The txtR gene is imbedded in the thaxtomin biosynthetic pathway and is located on a conserved pathogenicity island in S. scabies, S. turgidiscabies and S. acidiscabies. Thaxtomin biosynthesis was abolished and virulence was almost eliminated in the txtR deletion mutant of S. scabies 87.22. Accumulation of thaxtomin biosynthetic gene (txtA, txtB, txtC, nos) transcripts was reduced compared with the wild-type S. scabies 87.22. NOS-dependent nitric oxide production by S. scabies was also reduced in the mutant. The TxtR protein bound cellobiose, an inducer of thaxtomin production, and transcription of txtR and thaxtomin biosynthetic genes was upregulated in response to cellobiose. TxtR is the first example of an AraC/XylS family protein regulated by cellobiose. Together, these data suggest that cellobiose, the smallest oligomer of cellulose, may signal the availability of expanding plant tissue, which is the site of action of thaxtomin.
![Foliar δ[sup.15]N is affect...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0029-8549/small "Foliar δ[sup.15]N is affected by foliar nitrogen uptake, soil nitrogen, and mycorrhizae along a nitrogen deposition gradient")
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