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Insect pests cost billions of dollars per year globally, negatively impacting food crops and infrastructure, and contributing to the spread of disease. Timely information regarding developmental stages of pests can facilitate early detection and control, increasing efficiency and effectiveness. In 2018, the U.S. National Phenology Network (USA-NPN) released a suite of ‘Pheno Forecast’ map products relevant to science and management. The Pheno Forecasts include real-time maps and short-term forecasts of insect pest activity at management-relevant spatial and temporal resolutions and are based on accumulated temperature thresholds associated with critical life-cycle stages of economically important pests. Pheno Forecasts indicate, for a specified day, the status of the insect’s target life-cycle stage in real time across the contiguous United States. The maps are available for 12 pest species including the invasive emerald ash borer (Agrilus planipennis Fairmaire [Coleoptera: Buprestidae]), hemlock woolly adelgid (Adelges tsugae Annand), and gypsy moth (Lymantria dispar Linnaeus [Lepidoptera: Erebidae]). Preliminary validation based on in-situ observations for hemlock woolly adelgid egg and nymph stages in 2018 indicated the maps to be ≥93% accurate depending on phenophase. Since their release in early 2018, these maps have been adopted by tree care specialists and foresters across the United States. Using a consultative mode of engagement, USA-NPN staff have continuously sought input and critique of the maps and delivery from end users. Based on feedback received, maps have been expanded and modified to include additional species, improved descriptions of the phenophase event of interest, and e-mail-based notifications to support management decisions.

ContextLandscape science relies on foundational concepts of landscape ecology and seeks to understand the physical, biological, and human components of ecosystems to support land management decision-making. Incorporating landscape science into land management decisions, however, remains challenging. Many lands in the western United States are federally owned and managed for multiple uses, including recreation, conservation, and energy development.ObjectiveWe argue for stronger integration of landscape science into the management of these public lands.MethodsWe open by outlining the relevance of landscape science for public land planning, management, and environmental effects analysis, including pertinent laws and policies. We identify challenges to integrating landscape science into public land management, including the multijurisdictional nature and complicated spatial pattern of public lands, the capacity of agencies to identify and fill landscape science needs, and perceptions about the meaning of landscape approaches to management.ResultsWe provide several recent examples related to landscape monitoring, restoration, reclamation, and conservation in which landscape science products were developed specifically to support decision-making.ConclusionsWe close by highlighting three actions—elevating the importance of science-management partnerships dedicated to coproducing actionable landscape science products, identifying where landscape science could foster efficiencies in the land-use planning process, and developing scenario-based landscape models for shrublands—that could improve landscape science support for public land planners and managers.

Citizen science involves a range of practices involving public participation in scientific knowledge production, but outcomes evaluation is complicated by the diversity of the goals and forms of citizen science. Publications and citations are not adequate metrics to describe citizen-science productivity. We address this gap by contributing a science products inventory (SPI) tool, iteratively developed through an expert panel and case studies, intended to support general-purpose planning and evaluation of citizen-science projects with respect to science productivity. The SPI includes a collection of items for tracking the production of science outputs and data practices, which are described and illustrated with examples. Several opportunities for further development of the initial inventory are highlighted, as well as potential for using the inventory as a tool to guide project management, funding, and research on citizen science.

Among the many effects of climate change is its influence on the phenology of biota. In marine and coastal ecosystems, phenological shifts have been documented for multiple life forms; however, biological data related to marine species' phenology remain difficult to access and is under-used. We conducted an assessment of potential sources of biological data for marine species and their availability for use in phenological analyses and assessments. Our evaluations showed that data potentially related to understanding marine species' phenology are available through online resources of governmental, academic, and non-governmental organizations, but appropriate datasets are often difficult to discover and access, presenting opportunities for scientific infrastructure improvement. The developing Federal Marine Data Architecture when fully implemented will improve data flow and standardization for marine data within major federal repositories and provide an archival repository for collaborating academic and public data contributors. Another opportunity, largely untapped, is the engagement of citizen scientists in standardized collection of marine phenology data and contribution of these data to established data flows. Use of metadata with marine phenology related keywords could improve discovery and access to appropriate datasets. When data originators choose to self-publish, publication of research datasets with a digital object identifier, linked to metadata, will also improve subsequent discovery and access. Phenological changes in the marine environment will affect human economics, food systems, and recreation. No one source of data will be sufficient to understand these changes. The collective attention of marine data collectors is needed—whether with an agency, an educational institution, or a citizen scientist group—toward adopting the data management processes and standards needed to ensure availability of sufficient and useable marine data to understand marine phenology.

Changes in the timing of plant and animal life cycle events, in response to climate change, are already happening across the globe. The impacts of these changes may affect biodiversity via disruption to mutualisms, trophic mismatches, invasions and population declines. To understand the nature, causes and consequences of changed, varied or static phenologies, new data resources and tools are being developed across the globe. The USA National Phenology Network is developing a long-term, multi-taxa phenological database, together with a customizable infrastructure, to support conservation and management needs. We present current and potential applications of the infrastructure, across scales and user groups. The approaches described here are congruent with recent trends towards multi-agency, large-scale research and action.

Questions Woody plant encroachment into former grasslands currently represents a major physiognomic shift globally. Seedling establishment is a critical demographic bottleneck and is considered to be alleviated by increases in water availability and negatively impacted by interactions with grasses, particularly when water stress increases. However, interactions with grasses that are not actively competing for resources (‘passive interactions’ when grasses are dead) has seldom been considered. Could the transition from a live to a dead grass (litter) canopy favour recruitment of woody seedlings in a semi-arid grassland of the American SW? How does the sign and intensity of grass–seedling interactions change across drastically different summer precipitation regimes with and without passive interactions? Location Sonoran Desert shrub savanna at the Santa Rita Experimental Range, near Tucson, AZ, US. Methods Four cohorts of Prosopis velutina seeds were planted annually (2002–2005) under rainout shelters that intercepted all incoming precipitation on a soil with sandy loam texture. Summer precipitation was manipulated to simulate either a 50% increase or decrease in the long-term mean, and cover was manipulated to simulate a grassland dominated by the C4 bunchgrass Heteropogon contortus or left unvegetated. Emergence and survival of P. velutina was monitored and compared across cover types, along with monitoring of soil water content and light interception. Results Strong active competition was observed with live grasses, under both summer drought and also under ample summer water supply. However, the pattern was reversed and strong passive facilitation of P. velutina was observed over time when grass canopies died and remained in place. This passive facilitation under dry summers was so strong that recruitment under dead grass conditions was comparable to that observed when ample water supply removed the effects of competition on unvegetated plots. Conclusions After significant mortality of standing grass canopies, which typically compete for limited soil moisture resources, rates of recruitment by shrubs may increase even with significant seasonal drought. This work extends our understanding of interactions among co-located organisms and their effects on plant community dynamics, and introduces a new hypothesis on how grass litter facilitates woody plant encroachment during seasonal droughts.

The increasing spread and abundance of an invasive perennial grass, buffelgrass (Pennisetum ciliare), represents a critical threat to the native vegetation communities of the Sonoran desert in southern Arizona, USA, where buffelgrass eradication is a high priority for resource managers. Herbicidal treatment of buffelgrass is most effective when the vegetation is actively growing, but the remoteness of infestations and the erratic timing and length of the species’ growth periods confound effective treatment. The goal of our research is to promote buffelgrass management by using remote sensing data to detect where the invasive plants are located and when they are photosynthetically active. We integrated citizen scientist observations of buffelgrass phenology in the Tucson, Arizona area with PRISM precipitation data, eight-day composites of 250-m Moderate-resolution Imaging Spectroradiometer (MODIS) satellite imagery, and aerially-mapped polygons of buffelgrass presence to understand dynamics and relationships between precipitation and the timing and amount of buffelgrass greenness from 2011 to 2013. Our results show that buffelgrass responds quickly to antecedent rainfall: in pixels containing buffelgrass, higher correlations (R2 > 0.5) typically occur after two cumulative eight-day periods of rain, whereas in pixels dominated by native vegetation, four prior 8-day periods are required to reach that threshold. Using the new suite of phenometrics introduced here—Climate Landscape Response metrics—we accurately predicted the location of 49% to 55% of buffelgrass patches in Saguaro National Park. These metrics and the suggested guidelines for their use can be employed by resource managers to treat buffelgrass during optimal time periods.

Continental-scale monitoring is vital for understanding and adapting to temporal changes in seasonal climate and associated phenological responses. The success of monitoring programs will depend on recruiting, retaining, and managing members of the public to routinely collect phenological observations according to standardized protocols. Here, we trace the development of infrastructure for phenological monitoring in the US, culminating in the USA National Phenology Network, a program that engages scientists and volunteers.

Large increases in the density of woody plants in former grasslands have been documented globally over the past two centuries. Prosopis velutina (velvet mesquite) has recently expanded on over 38 million ha in the grasslands of the American southwest. Mesquite establishment and expansion is potentially influenced by competitive interactions with grasses, particle size distribution (texture) of soil and changes in the amount and/or seasonality of precipitation, amongst other factors. To investigate the relative importance of precipitation seasonality, grass competition, and soil texture on mesquite seedling and establishment, we experimentally manipulated seasonality of precipitation across 72 1.5 m × 1.7 m plots that we planted with two grasses from contrasting functional groups (or left unvegetated) across two highly contrasting soil textures within a semi-desert grassland in Arizona, USA, wherein we followed the demography of 8640 planted mesquite seeds between 2002 and 2006. Up to 70% of the variance in seedling emergence was explained by soil water content, and seedling mortality was more dependent on summer than on winter precipitation. Mortality of less drought tolerant grasses after drought in coarse textured soils created conditions most favorable for seedling establishment, whereas lower rates of seedling establishment were observed in finer textured soils or in plots dominated by more drought tolerant grasses. We conclude that mesquite encroachment is episodic, based on discontinuous patterns of seedling establishment, favored on coarse textured soils and associated with unique conditions of high summer water availability when precipitation is not limiting and when competition from grasses is lowered.

Atmospheric and climatic change can alter plant biomass production and plant community composition. However, we know little about how climate change-induced alterations in biomass production affect plant species composition. To better understand how climate change will alter both individual plant species and community biomass, we manipulated atmospheric [CO2], air temperature, and precipitation in a constructed old-field ecosystem. Specifically, we compared the responses of dominant and subdominant species to our climatic treatments, and explored how changes in plant dominance patterns alter community evenness over 2 years. Our study resulted in four major findings: (1) all treatments, elevated [CO2], warming, and increased precipitation increased plant community biomass and the effects were additive rather than interactive, (2) plant species differed in their response to the treatments, resulting in shifts in the proportional biomass of individual species, which altered the plant community composition; however, the plant community response was largely driven by the positive precipitation response of Lespedeza, the most dominant species in the community, (3) precipitation explained most of the variation in plant community composition among treatments, and (4) changes in precipitation caused a shift in the dominant species proportional biomass that resulted in lower community evenness in the wet relative to dry treatments. Interestingly, compositional and evenness responses of the subdominant community to the treatments did not always follow the responses of the whole plant community. Our data suggest that changes in plant dominance patterns and community evenness are an important part of community responses to climatic change, and generally, that such compositional shifts can alter ecosystem biomass production and nutrient inputs.

Rising concentrations of atmospheric carbon dioxide ([CO{sub 2}]) may influence forest successional development and species composition of understory plant communities by altering biomass production of plant species of functional groups. Here, we describe how elevated [CO{sub 2}] (eCO{sub 2}) affects aboveground biomass within the understory community of a temperate deciduous forest at the Oak Ridge National Laboratory sweetgum (Liquidambar styraciflua) free-air carbon dioxide enrichment (FACE) facility in eastern Tennessee, USA. We asked if (i) CO{sub 2} enrichment affected total understory biomass and (ii) whether total biomass responses could be explained by changes in understory species composition or changes in relative abundance of functional groups through time. The FACE experiment started in 1998 with three rings receiving ambient [CO{sub 2}] (aCO{sub 2}) and two rings receiving eCO{sub 2}. From 2001 to 2003, we estimated species-specific, woody versus herbaceous and total aboveground biomass by harvesting four 1 x 0.5-m subplots within the established understory plant community in each FACE plot. In 2008, we estimated herbaceous biomass as previously but used allometric relationships to estimate woody biomass across two 5 x 5-m quadrats in each FACE plot. Across years, aboveground biomass of the understory community was on average 25% greater in eCO{sub 2} more » than in aCO{sub 2} plots. We could not detect differences in plant species composition between aCO{sub 2} and eCO{sub 2} treatments. However, we did observe shifts in the relative abundance of plant functional groups, which reflect important structural changes in the understory community. In 2001-03, little of the understory biomass was in woody species; herbaceous species made up 94% of the total understory biomass across [CO{sub 2}] treatments. Through time, woody species increased in importance, mostly in eCO{sub 2}, and in 2008, the contribution of herbaceous species to total understory biomass was 61% in aCO{sub 2} and only 33% in eCO{sub 2} treatments. Our results suggest that rising atmospheric [CO{sub 2}] could accelerate successional development and have longer term impact on forest dynamics. « less

Aims The direct effects of atmospheric and climatic change factors— atmospheric [CO2], air temperature and changes in precipitation— can shape plant community composition and alter ecosystem function. It is essential to understand how these factors interact to make better predictions about how ecosystems may respond to change. We investigated the direct and interactive effects of [CO2], warming and altered soil moisture in open-top chambers (OTCs) enclosing a constructed old-field community to test how these factors shape plant communities. Materials and methods The experimental facility in Oak Ridge, TN, USA, made use of 4-m diameter OTCs and rain shelters to manipulate [CO2] (ambient, ambient + 300 ppm), air temperature (ambient, ambient + 3.5C) and soil moisture (wet, dry). The plant communities within the chambers comprised seven common old-field species, including grasses, forbs and legumes. We tracked foliar cover for each species and calculated community richness, evenness and diversity from 2003 to 2005. Important findings This work resulted in three main findings: (1) warming had speciesspecific effects on foliar cover that varied through time and were altered by soil moisture treatments; (2) [CO2] had little effect on individual species or the community; (3) diversity, evenness and richness were influenced most by soil moisture, primarily reflecting the response of one dominant species. We conclude that individualistic species responses to atmospheric and climatic change can alter community composition and that plant populations and communities should be considered as part of analyses of terrestrial ecosystem response to climate change. However, prediction of plant community responses may be difficult given interactions between factors and changes in response through time.

Climate change predictions for the desert southwestern U.S. are for shifts in precipitation patterns. The impacts of climate change may be significant, because desert soil processes are strongly controlled by precipitation inputs (“pulses”) via their effect on soil water availability. This study examined the response of soil respiration—an important biological process that affects soil carbon (C) storage—to variation in pulses representative of climate change scenarios for the Sonoran Desert. Because deserts are mosaics of different plant cover types and soil textures—which create patchiness in soil respiration—we examined how these landscape characteristics interact to affect the response of soil respiration to pulses. Pulses were applied to experimental plots of bare and vegetated soil on contrasting soil textures typical of Sonoran Desert grasslands. The data were analyzed within a Bayesian framework to: (1) determine pulse size and antecedent moisture (soil moisture prior to the pulse) effects on soil respiration, (2) quantify soil texture (coarse vs. fine) and cover type (bare vs. vegetated) effects on the response of soil respiration and its components (plant vs. microbial) to pulses, and (3) explore the relationship between long-term variation in pulse regimes and seasonal soil respiration. Regarding objective (1), larger pulses resulted in higher respiration rates, particularly from vegetated fine-textured soil, and dry antecedent conditions amplified respiration responses to pulses (wet antecedent conditions dampened the pulse response). Regarding (2), autotrophic (plant) activity was a significant source (∼60%) of respiration and was more sensitive to pulses on coarse- versus fine-textured soils. The sensitivity of heterotrophic (microbial) respiration to pulses was highly dependent on antecedent soil water. Regarding (3), seasonal soil respiration was predicted to increase with both growing season precipitation and mean pulse size (but only for pulses between 7 and 25 mm). Thus, the heterogeneity of the desert landscape and the timing or the number of medium-sized pulses is expected to significantly impact desert soil C loss with climate change.

Summary 1. A major research focus in population and community ecology is to establish a mechanistic understanding of plant interactions and demographic responses. The first step towards this mechanistic approach relies on understanding the differences in stress caused by different environmental conditions. Leaf-level photosynthetic rate ( A ) within and among plant populations provides important insight into population and community processes, but is difficult to acquire with sufficient replication under field conditions. Instead, chlorophyll fluorescence ( F v / F m ) and predawn water potential ( Ψ pd ) are often used in arid and semi-arid ecosystems. 2. F v / F m reflects the photoactivation status of photosystem II (PSII), whereas Ψ pd indicates water availability in the rhizosphere. Here we compare these indices with A in two perennial C 4 grasses (native Heteropogon contortus and invasive Eragrostis lehmanniana ) and in seedlings of the C 3 shrub Prosopis velutina growing on highly contrasting sandy loam and loamy clay soils in experimental plots. Measurements were made the day prior to and up to 7 days following a 39-mm rainfall pulse after 2 months of drought. 3. A was more sensitive across a broad range of environmental conditions, whereas F v / F m and Ψ pd only responded to periods of protracted drought. The use of these measures was further complicated because their values varied daily and we observed different time-lags in their response to precipitation pulses. 4. We suggest sampling schemes and a priori measurements to capture the value that is representative for the question of interest, and that match the pulsed biological activity in these ecosystems. Finally, we suggest the use of these measures in combination with measurements providing integration over longer time periods, such as δ 13 C, δ 18 O and N concentration in bulk leaf tissue.

The world passed the halfway point of oil supply in 2005. World demand for oil likely will severely outstrip supply in 2008, leading to increasingly higher oil prices. Consequences are likely to include increasing gasoline prices, rapidly increasing inflation, and subsequently a series of increasingly severe recessions followed by a worldwide economic depression. Consequences may include, particularly in industrialized countries such as the United States, massive unemployment, economic collapse, and chaos.

Invasive species are a global threat to biodiversity and the functioning of natural ecosystems. Here, we report on a two-year experiment aimed at elucidating the combined and relative effects of three key controls on plant invasions: propagule supply, soil nitrogen (N) availability, and herbivory by native insects. We focus on the exotic species Lespedeza cuneata, a Rank 1 invasive species. Propagule supply and soil N-availability interacted to control the density and foliar cover of L. cuneata. In low N plots, density and foliar cover of L. cuneata were higher in the propagule addition plots than in the plots to which propagules were not added. Surprisingly, this interaction was significant only when the abundance of herbivores was experimentally reduced. This experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species.

Arid and semi-arid ecosystems of the southwestern US are undergoing changes in vegetation composition and are predicted to experience shifts in climate. To understand implications of these current and predicted changes, we conducted a precipitation manipulation experiment on the Santa Rita Experimental Range in southeastern Arizona. The objectives of our study were to determine how soil surface and seasonal timing of rainfall events mediate the dynamics of leaf-level photosynthesis and plant water status of a native and non-native grass species in response to precipitation pulse events. We followed a simulated precipitation event (pulse) that occurred prior to the onset of the North American monsoon (in June) and at the peak of the monsoon (in August) for 2002 and 2003. We measured responses of pre-dawn water potential, photosynthetic rate, and stomatal conductance of native (Heteropogon contortus) and non-native (Eragrostis lehmanniana) C(4) bunchgrasses on sandy and clay-rich soil surfaces. Soil surface did not always amplify differences in plant response to a pulse event. A June pulse event lead to an increase in plant water status and photosynthesis. Whereas the August pulse did not lead to an increase in plant water status and photosynthesis, due to favorable soil moisture conditions facilitating high plant performance during this period. E. lehmanniana did not demonstrate heightened photosynthetic performance over the native species in response to pulses across both soil surfaces. Overall accumulated leaf-level CO(2) response to a pulse event was dependent on antecedent soil moisture during the August pulse event, but not during the June pulse event. This work highlights the need to understand how desert species respond to pulse events across contrasting soil surfaces in water-limited systems that are predicted to experience changes in climate.

Quality and quantity of publications are among the most important measures determining the success of ecologists. The past 50 years have seen a steady rise in the number of researchers and collaborative manuscripts, and a corresponding increase in multi-authored articles. Despite these increases, there remains a shortage of useful and definitive guidelines to aid ecologists in addressing authorship issues, leading to a lack of consistency in what the term “author” really means. Deciding where to draw the line between those who have earned authorship and those who are more appropriately credited in the acknowledgments may be one of the more challenging aspects of authorship. Here, we borrow ideas from other scientific disciplines and propose a simple solution to help ecologists who are making such decisions. We recommend improving communication between co-authors throughout the research process, and propose that authors publish their contributions to a manuscript in a separate byline.

• The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall ('pulses') on carbon and water exchange in water-limited ecosystems are poorly quantified. • In the present study, we estimated net ecosystem exchange of CO₂ (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased ('wet') or decreased ('dry’) monsoon-season (July–September) rainfall during 2002 and 2003. • Following a June 2003 39-mm pulse, species treatments had similar NEE and ET dynamics including 15-d integrated NEE (NEEpulse). Contrary to predictions, PMSD increased net C uptake during June in plots of both species. Greater flux rates after an August 2003 39-mm pulse reflected biotic activity associated with the North American Monsoon. Furthermore, August NEEpulse and ecosystem pulse-use efficiency (PUE(e) = NEE(pulse)/ET(pulse)) was greatest in Heteropogon plots. • PMSD and rainfall seasonal timing may interact with bunchgrass invasions to alter NEE and ET dynamics with consequences for PUE(e) in water-limited ecosystems.

Summary 1 In water-limited ecosystems, discrete precipitation events trigger brief but important episodes of biological activity. Differential responses of above- and below-ground biota to precipitation may constrain biogeochemical transformations at the ecosystem scale. 2 We quantified short-term dynamics of whole ecosystem response to 39 mm irrigation events (precipitation pulses) during June 2002 and 2003 using plant physiological and ecosystem gas-exchange measurements as state variables in a principal components analysis (PCA). Experimental plots consisted of either native ( Heteropogon contortus L.) or non-native ( Eragrostis lehmanniana Nees) bunchgrasses planted in monoculture on two distinct geomorphic surfaces in a semi-arid grassland. 3 For 15 days, treatments followed similar, non-linear trajectories through state variable space with measurement periods forming distinct clusters; PCA axes 1 and 2 combined to explain 80.7% of the variation during both 2002 and 2003. 4 During both years, bunchgrass species interacted with soil type such that there was a reduction in ecosystem functional resistance in plots planted with the non-native bunchgrass species on the fine-textured clay geomorphic surface. 5 System-level hysteresis, emerging as a result of independent responses of photosynthesis, respiration and evapotranspiration to precipitation, indicated the potential for alternative functional states. 6 Quantifying the frequency and duration of ecosystem alternative functional states in response to individual precipitation events within a season will provide insights into the controls of species, soils and climate on ecosystem carbon and water cycles.

Climate change and invasions by non-native organisms are two factors of global change likely to alter the structure and function of arid and semi-arid ecosystems. We conducted a large-scale field experiment to determine how changes in amount of summer precipitation and invasions by an African grass (Eragrostis lehmanniana) may interact with soil texture to affect community and ecosystem processes in temperate grasslands and savannas of southern Arizona, USA. In particular, we investigated the response of soil moisture at several soil depths within seventy-two 2.7 m2 plots under six 9 m×18 m rainout shelters to a 3-fold difference in irrigation between February and November 2002, as well as to a single large pulse of irrigation water in June 2002. In addition, we documented the effects of the rainout shelters on microclimates under the shelters. Results indicate that relatively more water is available to plants at greater depths on sand-rich soils, whereas on clay-rich soils the greatest water availability is at the surface. Typically, soil moisture under E. lehmanniana was lower than under plots with the native grass Heteropogon contortus, although soil texture and depth modified this pattern. The precipitation shelters had minimal impacts on windspeed, soil temperature, and relative humidity; small differences in microclimatic effects between the soil types were attributed to effects of surrounding vegetation on wind velocity. Differences in soil texture, grass cover and species identity, and amount of irrigation exert interactive controls over soil moisture in this semi-arid environment, but responses are modified by time of year and depth in the soil profile.

We describe a novel method for partitioning evapotranspiration (ET) from isotopic measurements of water vapor within large (4.86 m 3 ) plot-scale gas exchange chambers. Using this approach, the short-term (15-day) dynamics of transpiration (T) and evaporation (E) in experimental replicated stands of the invasive grass Eragrostis lehmanniana and the native Heteropogon contortus were assessed following a 39-mm irrigation event in semiarid grassland in southeastern Arizona, USA. Water vapor samples (20‐40 mL each) were collected sequentially during a 6-min transient increase of vapor concentration inside the chambers and used to produce Keeling plots (isotope mixing relationships) for identification of the isotopic composition of ET and partitioning of component fluxes. The method worked well in plots free of grass cover and in the sparsely covered plots of E. lehmanniana. Keeling plot estimates of the isotopic composition of soil evaporation (dE) in bare plots closely matched modeled values, lending strong support for the validity of the chamber approach. T/ET increased in stands of E. lehmanniana from 0.35 � 0.07 on day 1 to 0.43 � 0.08 on day 3 after the irrigation pulse, but decreased to 0.22 � 0.05 by day 7 as the soil surface dried. Estimates of stand transpiration from the Keeling plot chamber method were positively correlated (Pearson’s r = 0.76, p = 0.0004, n = 17) with independent estimates based on leaf-to-canopy scaling of stomatal conductance. We were unable to calculate T/ET on days 1 and 3 in plots of H. contortus because Keeling plot intercepts did not fall within the range of soil and canopy end-member isotopevalues. This likely occurred due to unaccounted effects of a wet litter layer on the estimation of dE. Our approach is useful for partitioning ET over a dynamic wetting event in semi-arid grassland at a scale relevant for experimental ecosystem studies, but requires further validation under a wide range of vegetation structures and environmental conditions. # 2005 Elsevier B.V. All rights reserved.

The accumulation of litter or thatch can affect plant community composition by affecting the temperature, nutrient availability, and light availability of the soil environment, thereby forming a potentially important linkage between recent productivity and current ecosystem processes. To investigate the importance of litter on a fen peatland plant community, we conducted a litter addition and removal experiment in a fen in northern Minnesota, USA, between 1998 and 2001. The addition of litter had little effect on fen plant community composition or microenvironmental variables, despite a two-fold increase in litter mass compared to control plots. However, the removal of litter dramatically increased cover of Rhynchospora fusca and R. alba, and reduced cover of Carex exilis. Litter removal also increased availability of light and soil temperature and increased the phosphorus content of aboveground plant biomass. Our results indicate that litter is an important control of plant community composition in this northern fen.

Species interactions and their indirect effects on the availability and distribution of resources have been considered strong determinants of community structure in many different ecological systems. In deciduous forests, the presence of overstory trees and shrubs creates a shifting mosaic of resources for understory plants, with implications for their distribution and abundance. Determination of the ultimate resource constraints on understory vegetation may aid management of these systems that have become increasingly susceptible to invasions by non-native plants. Microstegium vimineum (Japanese grass) is an invasive annual grass that has spread rapidly throughout the understory of forests across the eastern United States since it was first observed in Tennessee in 1919. M. vimineum occurs as extensive, dense patches in the understory of eastern deciduous forests, yet these patches often exhibit sharp boundaries and distinct gaps in cover. One example of this distributional pattern was observed relative to the native midstory tree Asimina triloba (pawpaw), whereby dense M. vimineum cover stopped abruptly at the drip line of the A. triloba patch and was absent beneath the A. triloba canopy. We conducted field and greenhouse experiments to test several hypotheses regarding the causes of this observed pattern of M. vimineum distribution, including allelopathy, seed dispersal, light limitations, and soil moisture, texture, and nutrient content. We concluded that light reduction by the A. triloba canopy was the environmental constraint that prevented establishment of M. vimineum beneath this tree. Whereas overstory tree canopy apparently facilitates the establishment of this shade-tolerant grass, the interaction of overstory canopy with midstory canopy interferes with M. vimineum by reducing the availability of sunflecks at the ground layer. It is likely that other midstory species influence the distribution and abundance of other herb-layer species, with implications for management of understory invasive plant species.

We conducted a landscape-level survey of Microstegium vimineum (Trin.) A. Camus (Japanese grass, Nepal grass) at the Oak Ridge National Environmental Research Park to: (1) document the distribution of this species, (2) measure several important characteristics of the populations and invaded habitats, (3) identify biotic and abiotic environmental factors that may limit its distribution, and (4) quantify its response to environmental gradients. We found that M. vimineum exhibits the broad environmental tolerance of many “weedy” species, and it appears that no single variable explains presence or performance. Presence of M. vimineum was correlated only with soil pH, whereas M. vimineum performance was positively correlated with canopy openness and biomass of other species. Explanatory variables differed among sites, which suggests that other factors (such as disturbance or dispersal) may be locally important, or that different factors may interact to control the distribution and performance of this ...

Water availability limits plant growth and production in almost all terrestrial ecosystems. However, biomes differ substantially in sensitivity of aboveground net primary production (ANPP) to between-year variation in precipitation. Average rain-use efficiency (RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints. Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUE(max)) that is typical of arid ecosystems. RUE(max) was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.

Summary • Rising atmospheric CO2 concentrations are likely to have direct effects on terrestrial ecosystems. Here, we describe effects of elevated concentrations of CO2 on an understory plant community in terms of production and community composition. • In 2001 and 2002 total and species-specific above-ground net primary productivity (ANPP) were estimated by harvesting above-ground biomass within an understory community receiving ambient [CO2] and elevated [CO2] at Oak Ridge National Laboratory's free-air carbon dioxide enrichment (FACE) facility. • During a wet year, community composition differed between plots receiving ambient [CO2] and elevated [CO2], but total ANPP did not differ. By contrast, during a drier year, community composition did not differ, but total ANPP was greater in elevated than ambient [CO2] plots. These patterns were driven by the response of two codominant species, Lonicera japonica and Microstegium vimineum, both considered invasive species in the south-eastern United States. The ANPP of L. japonica was consistently greater under elevated [CO2], whereas the response of M. vimineum to CO2 enrichment differed between years and mediated total community response. • These data suggest that community and species responses to a future, CO2-enriched atmosphere may be mediated by other environmental factors and will depend on individual species responses.

Biological invasion is one aspect of ecosystem function that may be controlled by the biological diversity of the invaded community, and there have been a number of recent studies that investigated relationships between diversity and invasibility. Most experimental studies report that higher species or functional group diversity increases resistance to invasion, but the role of genetic diversity is unknown. We used a model organism, Arabidopsis thaliana (Brassicaceae), to investigate relationships between genotypic richness and community invasibility by creating communities with 1, 2, 4, and 8 genotypes of A. thaliana at constant low (417 plants m−2) and high (834 plants m−2) densities, that once established, were invaded with a congener, Arabidopsis suecica. To reduce the potential effects of methodological confounding related to “sampling effects,”“variance reduction effects,” or confounding of abundance with diversity, we (1) created random communities from a relatively large pool of functionally and phenotypically similar genotypes, (2) evaluated individual and community traits across richness treatments, and (3) analyzed similarity of communities within treatments (for “quasi- replication”) and between adjacent treatments (for “nestedness”). Genotypic richness had no effect on A. suecica demography (emergence, survivorship), size (biomass, rosette area), or reproductive potential (rates of bolting and fruiting or number and size of bolts). In contrast, the density of A. thaliana genotypes had strong effects on the size and reproductive potential of A. suecica, which suggests that characteristics of the recipient community other than genotypic richness (e.g. light) form the most important determinant of community invasibility. Individual- and community-level traits of community members (cover, biomass, survivorship) did not differ among richness treatments, and within- and between-treatment similarity was reduced (relative to other recent experiments) but not eliminated. We evaluate our results vis-a-vis recent analyses of diversity-invasibility experiments, and provide directions for future investigations of genetic diversity.

Cades Cove, Great Smoky Mountains National Park, U.S.A. was historically cleared largely for pastoral purposes; it is now comprised of recently abandoned pastures dominated by non-native pasture species. To investigate the potential for reducing non-native species relative to native species, park managers initiated an experiment in 1995 that included mowing, herbicide application, planting of seed, and burning of replicate 20 × 50–m plots at each of two sites within Cades Cove. Between 1995 and 2001 we evaluated the response of the plant community (i.e., species-specific cover and frequency, biomass, diversity) to this suite of treatments and compared it with unmanipulated control plots at each site. Four years after treatment initiation abundance measures of Plantago lanceolata, Setaria geniculata, and Trifolium spp. averaged one-third lower in treated than control plots. Frequency of Festuca pratensis was lower in treated than in control plots for 2 years, but after 4 years its frequency, cover, and biomass did not differ between treated and control plots. By 2000 the cover of Sorghastrum nutans in treated plots increased to 23–47%, depending on the site. Total biomass and diversity increased in treated plots. The dominance of Lespedeza cuneata at one site apparently reduced planting success, biomass production, and diversity and evenness. Post-treatment lags in response for several species, coupled with interannual variation in response to environmental conditions, suggest that evaluations of treatment success would differ greatly depending on when the evaluation was conducted.

Climate change and biological invasions, two of the hottest topics in ecology, both have ecological and societal implications, but they have developed on separate and parallel paths. Both are likely to have global impacts, yet researchers seldom explicitly consider their interaction. We argue that changes in atmospheric concentrations of carbon dioxide (CO2) and subsequent climate change may facilitate biological invasions, both directly and indirectly, and that our research agenda to date has left us poorly prepared to predict the consequences for communities and ecosystems. Forecasting the impact of biological invaders or elevated CO2 is a challenge for ecologists, and it is therefore prudent and timely to investigate the greater challenge, namely predicting the combined effects of invaders and elevated CO2 on native ecological systems.

Boreal peatlands may be particularly vulnerable to climate change, because temperature regimes that currently constrain biological activity in these regions are predicted to increase substantially within the next century. Changes in peatland plant community composition in response to climate change may alter nutrient availability, energy budgets, trace gas fluxes, and carbon storage. We investigated plant community response to warming and drying in a field mesocosm experiment in northern Minnesota, USA. Large intact soil monoliths removed from a bog and a fen received three infrared warming treatments crossed with three water-table treatments (n = 3) for five years. Foliar cover of each species was estimated annually. In the bog, increases in soil temperature and decreases in water-table elevation increased cover of shrubs by 50% and decreased cover of graminoids by 50%. The response of shrubs to warming was distinctly species-specific, and ranged from increases (for Andromeda glaucophylla) to decreases (for Kalmia polifolia). In the fens, changes in plant cover were driven primarily by changes in water-table elevation, and responses were species- and lifeform-specific: increases in water-table elevation increased cover of graminoids – in particular Carex lasiocarpa and Carex livida– as well as mosses. In contrast, decreases in water-table elevation increased cover of shrubs, in particular A. glaucophylla and Chamaedaphne calyculata. The differential and sometimes opposite response of species and lifeforms to the treatments suggest that the structure and function of both bog and fen plant communities will change – in different directions or at different magnitudes – in response to warming and/or changes in water-table elevation that may accompany regional or global climate change.

Large-scale changes in climate may have unexpected effects on ecosystems, given the importance of climate as a control over almost all ecosystem attributes and internal feedbacks. Changes in plant community productivity or composition, for example, may alter ecosystem resource dynamics, trophic structures, or disturbance regimes, with subsequent positive or neg- ative feedbacks on the plant community. At northern latitudes, where increases in temperature are expected to be greatest but where plant species diversity is relatively low, climatically mediated changes in species composition or abundance will likely have large ecosystem effects. In this study, we investigated effects of infrared loading and manipulations of water-table ele- vation on net primary productivity of plant species in bog and fen wetland mesocosms between 1994 and 1997. We removed 27 intact soil monoliths (2.1 m2 surface area, 0.5-0.7 m depth) each from a bog and a fen in northern Minnesota to construct a large mesocosm facility that allows for direct manipulation of climatic variables in a replicated experimental design. The treatment design was a fully crossed factorial with three infrared-loading treatments, three water-table treatments, and two ecosystem types (bogs and fens), with three replicates of all treatment combinations. Overhead infrared lamps caused mean monthly soil temperatures to increase by 1.6-4.1?C at 15-cm depth during the growing season (May-October). In 1996, depths to water table averaged -11, -19, and -26 cm in the bog plots, and 0, -10, and -19 cm in the fen plots. Annual aboveground net primary production (ANPP) of bryophyte, forb, graminoid, and shrub life-forms was determined for the dominant species in the mesocosm plots based on species- specific canopy/biomass relationships. Belowground net primary production (BNPP) was esti- mated using root in-growth cores. Bog and fen communities differed in their response to infrared loading and water-table treatments because of the differential response of life-forms and species characteristic of each community. Along a gradient of increasing water-table elevation, production of bryophytes increased, and production of shrubs decreased in the bog community. Along a similar gradient in the fen community, production of graminoids and forbs increased. Along a gradient of in- creasing infrared loading in the bog, shrub production increased whereas graminoid production decreased. In the fen, graminoids were most productive at high infrared loading, and forbs were most productive at medium infrared loading. In the bog and fen, BNPP:ANPP ratios increased with warming and drying, indicating shifts in carbon allocation in response to climate change. Further, opposing responses of species and life-forms tended to cancel out the response of production at higher levels of organization, especially in the bog. For example, total net primary productivity in the bog did not differ between water-table treatments because BNPP was greatest in the dry treatment whereas ANPP was greatest in the wet treatment. The differential responses of species, life-forms, and above- and belowground biomass pro- duction to the treatments suggest that bog and fen plant communities will change, in different directions and magnitudes, in response to warming and changes in water-table elevation. Further, results of this and complementary research indicate that these peatlands may mediate their energy, carbon, and nutrient budgets through differential responses of the plant communities. Thus, predictions of the response of peatlands to changes in climate should consider differences in plant community structure, as well as biogeochemistry and hydrology, that characterize and differentiate these two ecosystems.

In contrast to documented increases in woody plant dominance of former savannas and grasslands of North America, ecotones between oak (Quercus L.) woodlands and semi-desert grasslands of the southwestern United States and northwestern Mexico have been relatively stable over the past several centuries. Soil resource partitioning, wherein shallow-rooted grasses use summer precipitation and deep-rooted woody plants use winter precipitation, may have contributed to the stable coexistence of grasses and trees that form savannas at this ecotone. Thus, predicted changes in regional precipitation patterns and soil moisture caused by anthropogenic trace gas emissions have the potential to alter interactions between woody plants and grasses with potential ramifications for their relative abundance and distribution. We used a field experiment to investigate the response of the dominant woodland and savanna tree Quercus emoryi to simulated potential scenarios of precipitation redistribution within the context of shif...

In contrast to documented increases in woody plant dominance of savannas and grasslands of North America, oak (Quercus L.) savannas that form lower tree lines in the southwestern United States and northwestern Mexico have been stable over the last several centuries. We sought to identify potential biotic and abiotic constraints on seedling recruitment of Quercus emoryi within the context of potential shifts in lower tree line. We used field surveys to describe seedling distribution at and below lower tree line, and to determine the potential for acorn dispersal from lower tree line into adjacent grassland. Field and greenhouse experiments were used to test explanatory hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspecific tree canopies within the woodland and savanna and were absent from adjacent semidesert grassland in 1993 and 1995. Seed bank surveys indicated that acorns were concentrated beneath tree canopies and were dispersed into adjacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrient amendments to subcanopy and grassland soils indicated that soil nutrients did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and grassland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade was important at all stages of seedling development investigated: the provision of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19-fold and increased recruitment rates between 30- and 60-fold. We conclude that rates of Q. emoryi recruitment within grasslands below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat-specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self-enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland–grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one-sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century-scale climatic perturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial progression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecific, biogenic safe sites.

Herbivory by rodents, lagomorphs and insects may locally constrain woody plant seedling establishment and stand development. Recruitment may therefore depend either upon plant tolerance of herbivory, or low herbivore abundance, during seedling establishment. We tested potential herbivory tolerance by quantifying growth, biomass allocation, and survival of defoliated Prosopis glandulosaseedlings under optimal abiotic conditions in the absence of competition. Realized tolerance was assessed by clipping seedlings of known age grown in the field with and without herbaceous competition. At 18-d (D ‘young’) or 33-d (D ‘old’) of age, seedlings in the growth chamber were clipped just above the first (cotyledonary) node, above the fourth node, or were retained as non-clipped controls. Potential tolerance to defoliation was high and neither cohort showed evidence of meristematic limitations to regeneration. Clipping markedly reduced biomass production relative to controls, especially belowground, but survival of seedlings defoliated 5 times was still75%. Contrary to expectations, survival of seedlings defoliated above the cotyledonary node 10 times was greater ( P< 0:10) for ‘young’ (75%) than ‘old’ (38%) seedlings. Under field conditions, survival of defoliated 11-month-old P. glandulosaseedlings was59% after one defoliation and only13% after six defoliations. Results indicate P. glandulosais potentially tolerant of repeated shoot removal early in its life cycle. Seedling tolerance to defoliation under field conditions therefore appears dependent upon abiotic stresses or resource limitations rather than a lack of intrinsic adaptations for shoot replacement or a depletion of the seedlings’ bud bank. Curtailment of root growth, a consequence of top removal observed in the growth chamber experiment, may reduce the capacity of P. glandulosa seedlings to acquire soil resources needed for meristem activation and shoot growth under field conditions. The importance of resource availability to post-defoliation regeneration was implicated in the field experiment, where survival, shoot elongation, and aboveground productivity of clipped seedlings was greatest in plots without herbaceous interference. In light of the marked increases in P. glandulosaabundance in grasslands in recent history, our results suggest that (1) utilization of Prosopis seedlings by herbivores may be infrequent or sporadic, (2) periodic episodes of seed production and germination may satiate herbivore populations, and/or (3) Prosopis seedling establishment occurs during periods of low herbivore density.

Stable isotope analysis was used to determine sources of water used by coexisting trees and grasses in a temperate savanna dominated by Quercus emoryi Torr. We predicted that (1) tree seedlings and bunchgrasses utilize shallow sources of soil water, (2) mature savanna trees use deeper sources of water, and (3) trees switch from shallow to deep water sources within 1 year of germination. We found that Q. emoryi trees, saplings, and seedlings (about 2 months, 1 year, and 2 years old), and the dominant bunchgrass [Trachypogon montufari (H.B.K.) Nees.] utilized seasonally available moisture from different depths within the soil profile depending on size/age relationships. Sapling and mature Q. emoryi acquired water from >50 cm deep, 2-month-old seedlings utilized water from

Explanations for documented increases in woody plant dominance in grasslands and savannas of North America include atmospheric CO2 enrichment and changes in climate, livestock grazing, and fire regimes. However, tree/shrub encroachment has also coincided with the eradication of a once widespread native herbivore, the black-tailed prairie dog (Cynomys ludovicianus). We used field experiments and repeat aerial photography to demonstrate that prairie dogs, and the herbivores and granivores associated with their colonies, probably maintained grassland and savanna by preventing woody species such as Prosopis glandulosa (honey mesquite) from establishing or attaining dominance. Prosopis seed and pod disappearance was 3–99 times greater within prairie dog colonies. Ants were the primary agent of seed removal, whereas prairie dogs and associated vertebrates were the primary agents of pod removal. Survival of Prosopis seedlings protected from vertebrate herbivory was similar on and off prairie dog colonies (≈60%), whereas survival of unprotected seedlings was 3 times greater off- than on-colony. On-colony, prairie dogs and associated herbivores girdled and destroyed all Prosopis saplings within 2 d of planting; survival of 1-yr-old seedlings was reduced by 50% after 3 mo of exposure to on-colony herbivores. Despite high levels of woody plant seed disappearance and seedling herbivory, on-colony “seedling” reserves were substantial (950 plants/ha). Thus, prairie dogs and the fauna that occur on their colonies suppressed rather than eliminated Prosopis from the colony site. Removal of prairie dogs led to rapid development of Prosopis stands. Repeat aerial photography showed that Prosopis canopy cover on a colony eradicated in 1950 (27%) increased to a level (61%) comparable to that of off-colony Prosopis stands (65%) within 23 yr. These data illustrate how transitions from grassland to woodland vegetation can be mediated by a rodent herbivore. They further demonstrate how purposeful or inadvertent removal of native herbivores can have unforeseen effects on plant species composition and landscape physiognomy. Investigations of environmental constraints on vegetation distribution and abundance should take into account the historical role of herbivores in shaping the present system. Inconsistencies among historic accounts of woody plant distribution and abundance in semiarid western North America may be resolved by considering population dynamics of prairie dogs. Widespread eradication of this formerly abundant rodent has eliminated a significant constraint to woody plant establishment on many semiarid grassland and savanna landscapes and has thereby facilitated transitions to shrubland and woodland states. Past land management designed to remove one perceived impediment to livestock production appears to have contributed significantly to development of another management problem that is now a major detriment to sustainable livestock production.

Contrary to observations and models in which trees and herbaceous plants are viewed as competitors, we found that trees in an African savanna have positive impacts on herbaceous biomass production and compo- sition, and on soil nutrient status. In the Turkana District of northwestem Kenya, we investigated vegetation and soil gradients along equi-angular transects radiating from the boles of individual Acacia tortilis trees. Total herba- ceous biomass averaged 260 ? 17(se) g/m2 at the bole and declined to 95 ? 8 g/m2 in the tree interspaces. Soil organic carbon and total nitrogen concentrations were greatest (0.72 % and 0.083 %, respectively) in shallow soils near the bole and declined rapidly toward the inter- spaces and with increasing depth. Transects were also established between tree pairs to assess effects of differ- ential canopy proximities. Grass production averaged 220 + 21 g / m2 below overlapping canopies, 150 ? 15 g / m2 under individual canopies, and 95 ? 8 g / m2 in interstitial areas. Detrended correspondence analysis revealed that shifts in species composition were correlat- ed with distance from tree bole out to the edge of the canopy. Species response, in terms of relative cover, to increasing distance from the bole, seemed to fall into five general classes: 1) greatest at the bole, 2) increasing with distance from the bole, 3) greatest in the mid canopy zone, 4) least at the bole and 5) no response. Trees did not influence herbaceous composition beyond tree canopies. It is assumed that shade cast by the tree canopy with subsequent reductions of understory water stress and temperature and increased nutrient concentrations may be the most important factors affecting understory soil and vegetation.