Ambient Plots Research Articles

Positive feedback of elevated CO<sub>2</sub> on soil respiration in late autumn and winter

Abstract. Soil respiration of terrestrial ecosystems, a major component in the global carbon cycle is affected by elevated atmospheric CO2 concentrations. However, seasonal differences of feedback effects of elevated CO2 have rarely been studied. At the Gießen Free-Air CO2 Enrichment (GiFACE) site, the effects of +20% above ambient CO2 concentration have been investigated since 1998 in a temperate grassland ecosystem. We defined five distinct annual seasons, with respect to management practices and phenological cycles. For a period of 3 years (2008–2010), weekly measurements of soil respiration were carried out with a survey chamber on vegetation-free subplots. The results revealed a pronounced and repeated increase of soil respiration under elevated CO2 during late autumn and winter dormancy. Increased CO2 losses during the autumn season (September–October) were 15.7% higher and during the winter season (November–March) were 17.4% higher compared to respiration from ambient CO2 plots. However, during spring time and summer, which are characterized by strong above- and below-ground plant growth, no significant change in soil respiration was observed at the GiFACE site under elevated CO2. This suggests (1) that soil respiration measurements, carried out only during the growing season under elevated CO2 may underestimate the true soil-respiratory CO2 loss (i.e. overestimate the C sequestered), and (2) that additional C assimilated by plants during the growing season and transferred below-ground will quickly be lost via enhanced heterotrophic respiration outside the main growing season.

The ecological role of small rainfall events in a desert grassland

AbstractThe frequency, magnitude and intensity of precipitation events are known to influence dryland ecosystems; yet, the majority of these rain events are small (<5 mm), and their role in driving biotic and abiotic processes remains unclear. To explore the effects of small rain events that occur between larger events on ecosystem processes, we experimentally manipulated small rainfall events in a mixed Bouteloua eriopoda and Bouteloua gracilis desert grassland. We experimentally removed all events <3.8 mm from treatment plots during a dry monsoon (2012), and to assess potential legacy effects, we added a similar magnitude of small rainfall events in 3.8‐mm increments to the same treatment plots during a subsequent wet monsoon (2013). No difference in aboveground productivity occurred between ambient and treatment plots when small events were eliminated in 2012. However, soil moisture, soil organic carbon, available nitrogen and extracellular enzyme activity for phosphate mobilization were all lower in treatment relative to ambient plots. In 2013, treatment plots that received supplemental small events had lower aboveground productivity, soil moisture and soil N availability than ambient plots. We hypothesize that legacy effects from the removal of small events in 2012 limited the ability of these plots to respond to higher, supplemented rainfall in 2013. Therefore, a reduction in small precipitation events – which may be caused by changing rainfall properties or increasing rates of evaporation in a warming climate – may intensify some deleterious effects of dry monsoons, and inhibit grassland recovery in subsequent years with higher rainfall. Copyright © 2015 John Wiley & Sons, Ltd.

Recognizing cross‐ecosystem responses to changing temperatures: soil warming impacts pelagic food webs

The energy and materials that move across ecosystem boundaries influence food web structure and key ecosystem functions. Despite the acknowledged importance of such ecological subsidies, surprisingly little information is available regarding the role of environmental temperature in influencing subsidy quality and the response of the recipient ecosystem. We evaluated the impacts of temperature‐mediated changes in leaves from deciduous trees, an important subsidy from terrestrial to freshwater ecosystems, on both the producer‐based and detritivore‐based components of a pelagic pond food web in a field mesocosm experiment. We hypothesized that variation in leaf chemistry driven by increased soil temperature would alter both the quality of leaf subsidies and the pond response. We collected red maple Acer rubrum leaves from heated and ambient temperature plots from the long‐term soil warming experiment at the Harvard Experimental Forest and added them to 167‐l field mesocosms containing established plankton communities, creating ‘no leaf’, ‘ambient leaf’ and ‘heated leaf’ treatments during autumn 2012. We then monitored physical, chemical, and biological responses to treatments until the mesocosms froze six weeks later. Experimental soil warming altered the chemical composition of deciduous leaves, the physical and chemical environment of the aquatic ecosystems to which leaves were added, and the pelagic pond food webs as measured by community composition. Compared to leaves from ambient‐temperature soils, leaves from warmed soils initially resulted in lower water column phosphorus and dissolved organic carbon, reducing bacterial densities. However, the diminished carbon and phosphorus resulting from soil warming also increased light availability that ultimately stimulated cladoceran zooplankton relative to ambient‐temperature leaves. Our results suggest that changes in temperature can alter ecological subsidies in unanticipated ways, and suggest that accurately predicting the potential consequences of climate change will require conducting research across ecosystem boundaries.

Convergent ecosystem responses to 23-year ambient and manipulated warming link advancing snowmelt and shrub encroachment to transient and long-term climate-soil carbon feedback.

Ecosystem responses to climate change can exert positive or negative feedbacks on climate, mediated in part by slow-moving factors such as shifts in vegetation community composition. Long-term experimental manipulations can be used to examine such ecosystem responses, but they also present another opportunity: inferring the extent to which contemporary climate change is responsible for slow changes in ecosystems under ambient conditions. Here, using 23years of data, we document a shift from nonwoody to woody vegetation and a loss of soil carbon in ambient plots and show that these changes track previously shown similar but faster changes under experimental warming. This allows us to infer that climate change is the cause of the observed shifts in ambient vegetation and soil carbon and that the vegetation responses mediate the observed changes in soil carbon. Our findings demonstrate the realism of an experimental manipulation, allow attribution of a climate cause to observed ambient ecosystem changes, and demonstrate how a combination of long-term study of ambient and experimental responses to warming can identify mechanistic drivers needed for realistic predictions of the conditions under which ecosystems are likely to become carbon sources or sinks over varying timescales.

Impact of intra- versus inter-annual snow depth variation on water relations and photosynthesis for two Great Basin Desert shrubs.

Snowfall provides the majority of soil water in certain ecosystems of North America. We tested the hypothesis that snow depth variation affects soil water content, which in turn drives water potential (Ψ) and photosynthesis, over 10 years for two widespread shrubs of the western USA. Stem Ψ (Ψ stem) and photosynthetic gas exchange [stomatal conductance to water vapor (g s), and CO2 assimilation (A)] were measured in mid-June each year from 2004 to 2013 for Artemisia tridentata var. vaseyana (Asteraceae) and Purshia tridentata (Rosaceae). Snow fences were used to create increased or decreased snow depth plots. Snow depth on +snow plots was about twice that of ambient plots in most years, and 20 % lower on -snow plots, consistent with several down-scaled climate model projections. Maximal soil water content at 40- and 100-cm depths was correlated with February snow depth. For both species, multivariate ANOVA (MANOVA) showed that Ψ stem, g s, and A were significantly affected by intra-annual variation in snow depth. Within years, MANOVA showed that only A was significantly affected by spatial snow depth treatments for A. tridentata, and Ψ stem was significantly affected by snow depth for P. tridentata. Results show that stem water relations and photosynthetic gas exchange for these two cold desert shrub species in mid-June were more affected by inter-annual variation in snow depth by comparison to within-year spatial variation in snow depth. The results highlight the potential importance of changes in inter-annual variation in snowfall for future shrub photosynthesis in the western Great Basin Desert.

Integrating ecophysiology and forest landscape models to improve projections of drought effects under climate change

Fundamental drivers of ecosystem processes such as temperature and precipitation are rapidly changing and creating novel environmental conditions. Forest landscape models (FLM) are used by managers and policy-makers to make projections of future ecosystem dynamics under alternative management or policy options, but the links between the fundamental drivers and projected responses are weak and indirect, limiting their reliability for projecting the impacts of climate change. We developed and tested a relatively mechanistic method to simulate the effects of changing precipitation on species competition within the LANDIS-II FLM. Using data from a field precipitation manipulation experiment in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) ecosystem in New Mexico (USA), we calibrated our model to measurements from ambient control plots and tested predictions under the drought and irrigation treatments against empirical measurements. The model successfully predicted behavior of physiological variables under the treatments. Discrepancies between model output and empirical data occurred when the monthly time step of the model failed to capture the short-term dynamics of the ecosystem as recorded by instantaneous field measurements. We applied the model to heuristically assess the effect of alternative climate scenarios on the piñon-juniper ecosystem and found that warmer and drier climate reduced productivity and increased the risk of drought-induced mortality, especially for piñon. We concluded that the direct links between fundamental drivers and growth rates in our model hold great promise to improve our understanding of ecosystem processes under climate change and improve management decisions because of its greater reliance on first principles.

Contrasting soil fungal community responses to experimental nitrogen addition using the large subunit rRNA taxonomic marker and cellobiohydrolase I functional marker.

Human activities have resulted in increased nitrogen inputs into terrestrial ecosystems, but the impact of nitrogen on ecosystem function, such as nutrient cycling, will depend at least in part on the response of soil fungal communities. We examined the response of soil fungi to experimental nitrogen addition in a loblolly pine forest (North Carolina, USA) using a taxonomic marker (large subunit rDNA, LSU) and a functional marker involved in a critical step of cellulose degradation (cellobiohydrolase, cbhI) at five time points that spanned fourteen months. Sampling date had no impact on fungal community richness or composition for either gene. Based on the LSU, nitrogen addition led to increased fungal community richness, reduced relative abundance of fungi in the phylum Basidiomycota and altered community composition; however, similar shifts were not observed with cbhI. Fungal community dissimilarity of the LSU and cbhI genes was significantly correlated in the ambient plots, but not in nitrogen-amended plots, suggesting either functional redundancy of fungi with the cbhI gene or shifts in other functional groups in response to nitrogen addition. To determine whether sequence similarity of cbhI could be predicted based on taxonomic relatedness of fungi, we conducted a phylogenetic analysis of publically available cbhI sequences from known isolates and found that for a subset of isolates, similar cbhI genes were found within distantly related fungal taxa. Together, these findings suggest that taxonomic shifts in the total fungal community do not necessarily result in changes in the functional diversity of fungi.

Warming alters potential enzyme activity but precipitation regulates chemical transformations in grass litter exposed to simulated climatic changes

Warming generally accelerates the decomposition of plant litter. However, changes in precipitation could alter the sensitivity of litter decomposition to warming, thereby affecting the formation of litter-derived soil organic matter. As grassland soils store ∼20% of Earth's soil carbon, understanding the effect of climatic changes on the decomposition dynamics of grasses is important. However, little is known about how projected changes in climate would affect litter microbial communities and enzyme activities, and the consequences of these changes for the mass loss and compound-specific degradation of grass litter that possess complex lignocellulosic chemistry. Over a period of two years, using litter of the grass Poa trivialis, we studied how mass loss, microbial enzyme activity and fine-level litter chemistry responded to a factorial combination of 4 levels of warming (up to ambient +∼4 °C) and three levels of precipitation [ambient, wet (+50%) and dry (−50%)] at the Boston-Area Climate Experiment (BACE), in Massachusetts, USA. After 393 days of decomposition, supplemental precipitation accelerated mass loss compared to the dry treatment, as a consequence of faster loss of hydroxycinnamates, which protect carbohydrates through cross-linkages with lignins. Only a third as much of the cell wall-bound ferulic and p-coumaric acids remained in litter from the supplemental precipitation treatment compared to the ambient controls. In contrast, the warming treatments did not affect mass loss until later, after 740 days, when the litter in the warmest treatment (+∼4 °C) had lost the most mass. Although warming significantly affected mass loss after 740 days, there was also a trend in the warmest treatments toward greater mass loss in the wet (78% mass loss) and ambient (68%) plots compared to dry plots (61%), possibly due to the higher activity of β-glucosidase. Though mass loss at this final time point varied with both warming and precipitation treatments, the compound-specific degradation of litter captured by diffuse reflectance infra-red Fourier transform (DRIFT) and 13C Nuclear Magnetic Resonance (NMR) spectroscopy revealed that only the precipitation treatments significantly altered the chemistry of carbon compounds in the decomposed tissue. Litter that decomposed in the dry treatment had a higher proportion of carbohydrates remaining than litter in the wet and ambient treatments. Similarly, although ergosterol content and potential activity of phenol oxidase decreased in the warmer treatments, the consequences of this response were not observed in the degradation of specific compounds in litter, which varied only with precipitation treatments. Our results suggests that mass loss and enzyme activities may not accurately capture the complexity of compound-specific degradation of litter during decomposition. Our results also identified non-linear responses of β-glucosidase and N-acetyl-β-D-glucosaminidase (NAG) activities to warming. These results thus emphasize the complexities of litter decomposition and suggest that similar changes in decomposition across other grass species could alter the carbon budget of grasslands.

Ground-Dwelling Beetle Responses to Long-Term Precipitation Alterations in a Hardwood Forest

It is widely predicted that regional precipitation patterns may be altered due to climate change, and these changes may affect areas with extensive forests. Therefore, studies investigating the role of this climate driver on forest floor fauna are timely. We examined the impact of precipitation alteration over 13 years on Coleoptera (specifically Family Carabidae) communities in a temperate forest by testing the effects of dry (33% precipitation interception), ambient (control), and wet (33% precipitation addition) treatments. We collected insects in pitfall traps and quantified forest-floor physical and chemical parameters. Beetle abundance and Carabidae tribe richness were significantly reduced in dry plots. Community similarity was substantially higher between wet and ambient plots compared to dry plots due to the substantial reduction of three dominant carabid tribes. Litter mass increased overall, litter nitrogen decreased, and carbon:nitrogen ratio (C:N) and total phenolics increased in the dry-plot Oi horizon. Beetle abundance and tribe richness were positively related to soil moisture, and beetle abundance was negatively related to litter mass. Microarthropod abundance was highest in the dry treatment. This study provides evidence that shifting precipitation patterns predicted with climate change could alter important ground-fauna communities in extensive ecosystems such as temperate forests.

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soilCO2efflux

Soil CO2 efflux (Fsoil ) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO2] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long-term effects of these factors on Fsoil are less clear. Expanding on previous studies at the Duke Free-Air CO2 Enrichment (FACE) site, we quantified the effects of elevated [CO2] and N fertilization on Fsoil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual Fsoil increased under elevated [CO2] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO2] reduced Fsoil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment-induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual Fsoil from potential values - estimated based on temperature alone assuming nonlimiting soil water content - by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO2]. Variability in soil N availability among plots accounted for the spatial variability in Fsoil , showing a decrease of ca. 114 g C m(-2) yr(-1) per 1 g m(-2) increase in soil N availability, with consistently higher Fsoil in elevated [CO2] plots ca. 127 g C per 100 ppm [CO2] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO2] and N fertilization on Fsoil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.

Free-air CO2 enrichment (FACE) net nitrogen fixation experiment at a paddy soil surface under submerged conditions

The aim of this study was to evaluate the cumulative net free-living nitrogen (N) fixation and its response to elevated atmospheric carbon dioxide (CO2) in the submerged surface of paddy soil. The study site was an actual paddy area in central Japan that was equipped with a free-air CO2 enrichment (FACE) facility and was composed of four bays, where each bay had an elevated CO2 plot (FACE plot) and an ambient plot. Field incubation was conducted at subplots without N fertilizer application from May 28 to August 18, 2012 (82 days). The CO2 enrichment was an average of +528 ppm during the study period at the subplots. The changes in total N (TN) content in the surface soil (0–1 cm) were determined in comparison to the initial TN content. The cumulative net N fixations during the 82-day study period were 47.1 ± 3.7 and 43.3 ± 5.8 (standard error) kg N ha−1 in the FACE and ambient plots, respectively. The difference between the FACE and ambient plots was not significant (p = 0.05). However, these values were partly affected by the charophyte blooms, which are not involved in N fixation. The results are not conclusive, and the one bay without charophyte blooming displayed a significant increase in the cumulative net N fixation (approximately 10 kg N ha−1) with CO2 enrichment (p < 0.001).

Seagrass responses to nutrient enrichment depend on clonal integration, but not flow-on effects on associated biota

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 490:23-35 (2013) - DOI: https://doi.org/10.3354/meps10448 Seagrass responses to nutrient enrichment depend on clonal integration, but not flow-on effects on associated biota Fernando Tuya1,*, María A. Viera-Rodríguez1, Rayco Guedes2, Fernando Espino1, Ricardo Haroun1, Jorge Terrados3 1BIOGES, and 2Departamento de Química, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas, Canary Islands, Spain 3IMEDEA (CSIC-UIB), C/ Miquel Marquès 21, 07190 Esporles, Islas Baleares, Spain *Email: ftuya@yahoo.es ABSTRACT: We determined physiological and morphological responses of the seagrass Cymodocea nodosa and associated epiphytes and epifauna to water column nutrient enrichment, and investigated whether responses were modulated by the clonal integration of the seagrass. Nutrient levels were elevated, relative to ‘ambient’ plots, in ‘large’ (~10 to 100×) and ‛‘moderate’ (~5 to 10×) enrichment plots, in a seagrass meadow off the south coast of Gran Canaria. Clonal integration was severed in half the plots, isolating them from adjacent shoots. Seagrass shoot density, above-ground biomass and leaf surface were lower in fertilized than ambient plots when clonal integration was severed. In the plots where clonal integration was maintained, no differences in shoot density were observed between fertilized and ambient plots, while differences in above-ground biomass and leaf surface were exclusively observed between ‘large’ enrichment and ‘ambient’ plots. Seagrass above-ground tissues accumulated P, but not N, when clonal integration was maintained. The N content, but not P content, of below-ground parts increased with fertilization. The content of soluble sugars in seagrass tissues decreased under nutrient enrichment; this decay was, however, buffered when clonal integration was maintained. Grazing bites on leaves increased with nutrient enrichment regardless of the clonal integration. Epiphytic loads increased at elevated nutrient levels irrespective of the clonal integration. The abundance and richness of epifauna were larger on ‘moderate’ relative to ‘ambient’ and ‘large’ enrichment plots, independently of clonal integration. In conclusion, while the physiological and morphological responses of C. nodosa to nutrient enrichment depended on the maintenance of clonal integration, flow-on effects on epiphytes and epifauna were independent of the clonal integration of the seagrass. KEY WORDS: Seagrass · Disturbance · Eutrophication · Stress · Clonality · Epibiota · Cymodocea nodosa · Canary Islands Full text in pdf format Supplementary material PreviousNextCite this article as: Tuya F, Viera-Rodríguez MA, Guedes R, Espino F, Haroun R, Terrados J (2013) Seagrass responses to nutrient enrichment depend on clonal integration, but not flow-on effects on associated biota. Mar Ecol Prog Ser 490:23-35. https://doi.org/10.3354/meps10448 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 490. Online publication date: September 17, 2013 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2013 Inter-Research.

Interactive Effects of Simulated Nitrogen Deposition and Altered Precipitation Patterns on Plant Allelochemical Concentrations

Global environmental change alters the supply of multiple limiting resources that regulate plant primary and secondary metabolism. Through modifications in resource availability, acquisition, and allocation, global change is likely to influence plant chemical defenses, and consequently species interactions that are mediated by these compounds. While many studies focus on individual global change factors, simultaneous changes in abiotic factors may interact to influence plant allelochemicals. In this study, we examined the individual and interactive effects of nitrogen enrichment and altered precipitation patterns on chemical defense compounds (iridoid glycosides) of an invasive plant, Linaria dalmatica. Plants were grown from seed in native mixed-grass prairie for 2 years. Nitrogen and water treatments were applied in each growing season over this period. Results indicate that soil water and nitrogen availability interact to shape plant chemical defense concentrations in L. dalmatica. Nitrogen addition decreased iridoid glycoside concentrations by approximately 25% under reduced water availability, increased concentrations by 37% in ambient water plots, and had no effect on these chemical defenses for plants growing under augmented water supply. Thus, results show differing patterns of allelochemical response to nitrogen enrichment, with respect to both the magnitude and direction of change, depending on water availability. Our study demonstrates the importance of examining multiple environmental factors in order to predict potential changes in plant chemical defenses with climate change.

Effects of nutrient enrichment and herbivory on morphology, reproduction and chemical content of Turbinaria conoides (Phaeophyceae)

SummaryThe effects of nutrient enrichment and herbivory on resource allocation patterns among morphology, reproduction, and chemical content of the brown alga, Turbinaria conoides (J. Agardh) Kützing were tested in the shallow subtidal zone of the Gulf of Thailand. The field experimental design comprised 36 plots (50 × 50 cm2) with and without herbivores, and two nutrient levels. Cages (uncaged and fully caged plots) were used to exclude herbivorous fishes and two nutrients levels were achieved by experimental enrichment above ambient nutrient concentrations. For morphology of Turbinaria, the maximum length (holdfast to frond apex), the width of the base of the stipe, number of blades, holdfast and stipe diameter were measured. Biomass, reproductive output, and tissue nutrient (carbon: nitrogen: phosphorus) content of T. conoides were examined. Phlorotannin concentrations were examined using a modified Folin‐Ciocalteu method. The results showed that herbivory had no effect on morphology, reproduction, or phlorotannin concentrations. This could be due to the structural and morphological deterrents of alga, which might minimize grazing effects from herbivorous fishes. Nutrient enrichment had no effect on morphology and reproduction of T. conoides, possibly due to low nutrient demand in Turbinaria. However, nutrient enrichment did affect phlorotannin concentrations, as phlorotannins in the enriched plots were lower than the ambient plots. These results support, in part, the carbon–nutrient balance hypothesis that states algae will allocate fewer resources to production of anti‐herbivore chemical compounds when enriched with potential growth‐limiting nutrients.

Using leaf optical properties to detect ozone effects on foliar biochemistry

Efficient methods for accurate and meaningful high-throughput plant phenotyping are limiting the development and breeding of stress-tolerant crops. A number of emerging techniques, specifically remote sensing methods, have been identified as promising tools for plant phenotyping. These remote sensing methods can be used to accurately and rapidly relate variations in leaf optical properties with important plant characteristics, such as chemistry, morphology, and photosynthetic properties at the leaf and canopy scales. In this study, we explored the potential to utilize optical (λ=500-2,400nm) near-surface remote sensing reflectance spectroscopy to evaluate the effects of ozone pollution on photosynthetic capacity of soybean (Glycine max Merr.). The research was conducted at the Soybean Free Air Concentration Enrichment (SoyFACE) facility where we subjected plants to ambient (44nL L(-1)) and elevated ozone (79-82nL L(-1) target) concentrations throughout the growing season. Exposure to elevated ozone resulted in a significant loss of productivity, with the ozone-treated plants displaying a ~30% average decrease in seed yield. From leaf reflectance data, it was also clear that elevated ozone decreased leaf nitrogen and chlorophyll content as well as the photochemical reflectance index (PRI), an optical indicator of the epoxidation state of xanthophyll cycle pigments and thus physiological status. We assessed the potential to use leaf reflectance properties and partial least-squares regression (PLSR) modeling as an alternative, rapid approach to standard gas exchange for the estimation of the maximum rates of RuBP carboxylation (V c,max), an important parameter describing plant photosynthetic capacity. While we did not find a significant impact of ozone fumigation on V c,max, standardized to a reference temperature of 25°C, the PLSR approach provided accurate and precise estimates of V c,max across ambient plots and ozone treatments (r (2)=0.88 and RMSE=13.4μmolm(-2) s(-1)) based only on the variation in leaf optical properties and despite significant variability in leaf nutritional status. The results of this study illustrate the potential for combining the phenotyping methods used here with high-throughput genotyping methods as a promising approach for elucidating the basis for ozone tolerance in sensitive crops.

Eco-evolutionary responses of Bromus tectorum to climate change: implications for biological invasions.

How plant populations, communities, and ecosystems respond to climate change is a critical focus in ecology today. The responses of introduced species may be especially rapid. Current models that incorporate temperature and precipitation suggest that future Bromus tectorum invasion risk is low for the Colorado Plateau. With a field warming experiment at two sites in southeastern Utah, we tested this prediction over 4 years, measuring B. tectorum phenology, biomass, and reproduction. In a complimentary greenhouse study, we assessed whether changes in field B. tectorum biomass and reproductive output influence offspring performance. We found that following a wet winter and early spring, the timing of spring growth initiation, flowering, and summer senescence all advanced in warmed plots at both field sites and the shift in phenology was progressively larger with greater warming. Earlier green-up and development was associated with increases in B. tectorum biomass and reproductive output, likely due early spring growth, when soil moisture was not limiting, and a lengthened growing season. Seeds collected from plants grown in warmed plots had higher biomass and germination rates and lower mortality than seeds from ambient plots. However, in the following two dry years, we observed no differences in phenology between warmed and ambient plots. In addition, warming had a generally negative effect on B. tectorum biomass and reproduction in dry years and this negative effect was significant in the plots that received the highest warming treatment. In contrast to models that predict negative responses of B. tectorum to warmer climate on the Colorado Plateau, the effects of warming were more nuanced, relied on background climate, and differed between the two field sites. Our results highlight the importance of considering the interacting effects of temperature, precipitation, and site-specific characteristics such as soil texture, on plant demography and have direct implications for B. tectorum invasion dynamics on the Colorado Plateau.

The effects of 11 yr ofCO2enrichment on roots in aFlorida scrub‐oak ecosystem

Uncertainty surrounds belowground plant responses to rising atmospheric CO₂ because roots are difficult to measure, requiring frequent monitoring as a result of fine root dynamics and long-term monitoring as a result of sensitivity to resource availability. We report belowground plant responses of a scrub-oak ecosystem in Florida exposed to 11 yr of elevated atmospheric CO₂ using open-top chambers. We measured fine root production, turnover and biomass using minirhizotrons, coarse root biomass using ground-penetrating radar and total root biomass using soil cores. Total root biomass was greater in elevated than in ambient plots, and the absolute difference was larger than the difference aboveground. Fine root biomass fluctuated by more than a factor of two, with no unidirectional temporal trend, whereas leaf biomass accumulated monotonically. Strong increases in fine root biomass with elevated CO₂ occurred after fire and hurricane disturbance. Leaf biomass also exhibited stronger responses following hurricanes. Responses after fire and hurricanes suggest that disturbance promotes the growth responses of plants to elevated CO₂. Increased resource availability associated with disturbance (nutrients, water, space) may facilitate greater responses of roots to elevated CO₂. The disappearance of responses in fine roots suggests limits on the capacity of root systems to respond to CO₂ enrichment.

The cycling of readily available phosphorus in response to elevated phosphate in acidic temperate deciduous forests

The biogeochemical cycling of phosphorus (P) in temperate forest ecosystems is poorly understood relative to nitrogen (N) cycling because of the assumption that N is the main factor that limits plant growth. This dichotomous way of understanding nutrient regimes is likely an oversimplification of these dynamic, interacting cycles within the evermore-dynamic framework of soil development. In order to better understand P cycling in temperate deciduous forests, we sought to quantify labile (resin) P flux and determine if soil shows signs of P stress. Labile P flux was quantified in nine ambient and nine elevated P plots on unglaciated eastern deciduous forests in Eastern Ohio. Anion exchange membranes (AEMs) were deployed in situ for two weeks, in the fall and the winter. Flux values were contextualized by resin P pools, N and P mineralization, and phosphatase enzyme activities determined from soil samples collected both at the time of AEM deployment and harvest. Phosphorus flux was 0.64±0.23μgPd−1 (mean±SD) in ambient plots and 3.47±2.39μgPd−1 in amended P plots. Resin P pools were 4.16mgkg−1 and 10.26mgkg−1 in the control and elevated plots, respectively. Phosphorus turnover rate increased from one week in control plots, to 3 days in elevated P plots. Results indicate mild P stress relative to N, as N was mineralized (0.84±0.18mgNkg−1d−1) while P was immobilized (−0.22±0.09mgPkg−1d−1). The elevated P treatment did not increase P mineralization. Phosphatase enzyme activities were significantly suppressed by 34% with P amendment. Results suggest labile P is a shallow pool, but is rapidly cycling.

Pre-exposure to drought increases the resistance of tropical forest soil bacterial communities to extended drought.

Global climate models project a decrease in the magnitude of precipitation in tropical regions. Changes in rainfall patterns have important implications for the moisture content and redox status of tropical soils, yet little is known about how these changes may affect microbial community structure. Specifically, does exposure to prior stress confer increased resistance to subsequent perturbation? Here we reduced the quantity of precipitation throughfall to tropical forest soils in the Luquillo Mountains, Puerto Rico. Treatments included newly established throughfall exclusion plots (de novo excluded), plots undergoing reduction for a second time (pre-excluded) and ambient control plots. Ten months of throughfall exclusion led to a small but statistically significant decline in soil water potential and bacterial populations clearly adapted to increased osmotic stress. Although the water potential decline was small and microbial biomass did not change, phylogenetic diversity in the de novo-excluded plots decreased by ∼40% compared with the control plots, yet pre-excluded plots showed no significant change. On the other hand, the relative abundances of bacterial taxa in both the de novo-excluded and pre-excluded plots changed significantly with throughfall exclusion compared with control plots. Changes in bacterial community structure could be explained by changes in soil pore water chemistry and suggested changes in soil redox. Soluble iron declined in treatment plots and was correlated with decreased soluble phosphorus concentrations, which may have significant implications for microbial productivity in these P-limited systems.

Modeling the effects of temperature and moisture on soil enzyme activity: Linking laboratory assays to continuous field data

Although potential enzyme activity measurements have a long history of use as an indicator of microbial activity, current methods do not provide accurate estimates of in situ activity. In the field, diffusion rates typically limit the rate at which enzymes can pair with substrates. However, the common laboratory practice of creating soil slurries removes all diffusion constraints. In addition, temperature strongly affects in situ enzyme activities, but is rarely considered in enzyme assays. To address these limitations, we developed a new protocol to measure the moisture and temperature sensitivity of enzyme activities. We incorporated sensitivity data obtained using this protocol into a model to estimate the effects of temperature and moisture on in situ β-glucosidase enzyme activity, recognizing that other factors such as substrate concentrations and diffusion constraints also affect in situ enzyme activities.Soil samples were collected from the Boston-Area Climate Experiment every two weeks over a 10-week period to track enzyme dynamics as field temperature and moisture changed. Precipitation inputs to an old-field were manipulated to produce drought (50% ambient precipitation), ambient, and wet (150% ambient precipitation) treatments. Temperature sensitivity of β-glucosidase was determined by assaying for the enzyme in soil slurries at three different temperatures (15, 25 and 35 °C). Moisture sensitivity was determined by exposing soils to different moisture levels in the lab and adding substrate to homogenized dry or moist soils instead of slurries. Temperature sensitivity was calculated as Q10 and moisture sensitivity was calculated using a linear regression for each field treatment at each sample collection date.Moisture sensitivity varied significantly among the five sample dates and treatments, whereas temperature sensitivity remained stable. At almost every time point, β-glucosidase activity responded more strongly to increased moisture in soils of drought plots than in soils of ambient and wet plots. We estimated in situ β-glucosidase activity in the fall using the temperature and moisture sensitivities. Estimates that used only temperature or only moisture sensitivity suggested that ambient plots had the highest activity, followed by wet and then drought plots. Estimates based on both temperature and moisture suggested that β-glucosidase activity responded primarily to changes in temperature, except when soils were dry, with water potentials below −1 MPa. These results demonstrate that low soil moisture can strongly limit in situ enzyme activity in soils, negating any positive effect of warming. This study provides a template for parsing out the role of specific abiotic drivers on in situ enzyme activities, which could lead to the explicit incorporation of enzymes in biogeochemical models, improving upon the ability of current models to predict rates of biogeochemical processes in dynamic environments.