Exotic Annual Species Research Articles

Multi-Species Inference of Exotic Annual and Native Perennial Grasses in Rangelands of the Western United States Using Harmonized Landsat and Sentinel-2 Data

The invasion of exotic annual grass (EAG), e.g., cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae), into rangeland ecosystems of the western United States is a broad-scale problem that affects wildlife habitats, increases wildfire frequency, and adds to land management costs. However, identifying individual species of EAG abundance from remote sensing, particularly at early stages of invasion or growth, can be problematic because of overlapping controls and similar phenological characteristics among native and other exotic vegetation. Subsequently, refining and developing tools capable of quantifying the abundance and phenology of annual and perennial grass species would be beneficial to help inform conservation and management efforts at local to regional scales. Here, we deploy an enhanced version of the U.S. Geological Survey Rangeland Exotic Plant Monitoring System to develop timely and accurate maps of annual (2016–2020) and intra-annual (May 2021 and July 2021) abundances of exotic annual and perennial grass species throughout the rangelands of the western United States. This monitoring system leverages field observations and remote-sensing data with artificial intelligence/machine learning to rapidly produce annual and early season estimates of species abundances at a 30-m spatial resolution. We introduce a fully automated and multi-task deep-learning framework to simultaneously predict and generate weekly, near-seamless composites of Harmonized Landsat Sentinel-2 spectral data. These data, along with auxiliary datasets and time series metrics, are incorporated into an ensemble of independent XGBoost models. This study demonstrates that inclusion of the Normalized Difference Vegetation Index and Normalized Difference Wetness Index time-series data generated from our deep-learning framework enables near real-time and accurate mapping of EAG (Median Absolute Error (MdAE): 3.22, 2.72, and 0.02; and correlation coefficient (r): 0.82, 0.81, and 0.73; respectively for EAG, cheatgrass, and medusahead) and native perennial grass abundance (MdAE: 2.51, r:0.72 for Sandberg bluegrass (Poa secunda)). Our approach and the resulting data provide insights into rangeland grass dynamics, which will be useful for applications, such as fire and drought monitoring, habitat suitability mapping, as well as land-cover and land-change modelling. Spatially explicit, timely, and accurate species-specific abundance datasets provide invaluable information to land managers.

Shrinking opportunities for establishment of native annual forbs in fragmented grassy woodlands

AbstractQuestionsRepeat vegetation surveys in fragmented woodlands in southeast Australia show native and exotic annual plant species have different trajectories of abundance; native annuals are declining while exotic annuals are increasing. We ask: is the divergent species origin response due to fundamental differences in germination cues and/or “safe‐sites” for establishment?LocationTemperate grassy woodlands, southern Victoria, Australia.MethodsWe quantified patch type frequency in the field. Diurnal temperature effects on four native annual forbs and three exotic annual grasses were assessed in a laboratory germination trial. We used a glasshouse experiment to assess the effects of patch type (bare ground, tree litter, grass litter, moss mat) on the emergence and growth of two native and two exotic annuals.ResultsExotic annuals typically had higher and more rapid germination across a wider range of diurnal temperatures than native annuals. Emergence of native and exotic annuals was highest on bare ground patches but was suppressed by leaf litter and moss mat. Native annuals did not emerge from under a moss mat and germinated much less than exotics in all patch types. Seedling growth of native annuals was suppressed by tree litter whereas growth of exotic species was not suppressed by litter or the moss mat.ConclusionsThe regeneration niche of the native annual forb species we examined was typically narrower than that of exotic annual grasses. Many of the differences in response between native and exotic species may be due to seed traits such as seed mass; native annuals have much smaller seeds than exotic annual species. This likely affects the persistence, germination cues, co‐existence and restoration potential of native annuals in fragmented grassy woodlands that have lost their endogenous disturbance agents.

Positive associations with native shrubs are intense and important for an exotic invader but not the native annual community across an aridity gradient

AbstractAim and LocationPositive interactions influence the assembly of plant communities globally, particularly in stressful environments such as deserts. However, few studies have measured the intensity and relative importance of positive interactions involving native and invasive species along aridity gradients. These measures are essential for predicting how dryland communities will respond to biological invasions and environmental change. Here, we measured the intensity and importance of positive associations formed between native shrubs and the annual plant community, which included highly invasive Bromus madritensis ssp. rubens (“B. rubens”) and native neighbours, along an aridity gradient across the Mojave and San Joaquin Deserts.MethodsAlong the gradient, we sampled metrics of abundance and performance for invasive B. rubens, native annual species (pooled), exotic annual species (pooled) and all annual species (pooled) during peak flowering at 120 pairs of shrub and open microsites.ResultsAcross the gradient, B. rubens occurred at far greater abundance, cover, biomass and fitness near shrubs than away from shrubs. When Larrea tridentata was the focal shrub, positive effects on B. rubens abundance and cover were least intense at the most arid sites under the shortest shrubs. The native annual community occurred at greater abundance, cover and species richness away from shrubs, regardless of relative aridity or shrub traits. Community‐level species richness was greatest away from shrubs, but exotic species richness was similar in shrub and open microsites.Main conclusionsAcross two deserts, B. rubens formed intense and important positive associations with native shrubs that consistently improved its abundance, cover, biomass and fitness, and for abundance and cover, the intensity of B. rubens–L. tridentata associations depended upon relative aridity and shrub height. By strongly facilitating a dominant invader but not native‐ or community‐level biodiversity, native shrubs provided the wrong kind of help to the annual plant community.

Exotic herbaceous species interact with severe drought to alter soil N cycling in a semi-arid shrubland

Mediterranean-type ecosystems are increasingly threatened by climate change and exotic annual species, jeopardizing the native communities and their global biodiversity. In these systems, soil nitrogen (N) limits net primary production, and its availability can be influenced by both of these stressors. To understand the interactive effects of droughts and exotic herbaceous species on soil N, we monitored the temporal variability of soil inorganic N, net N mineralization, net nitrification, and NO3- leaching under native- and exotic-dominated stands exposed to rainfall manipulation plots in a Mediterranean-type shrub-dominated community. Increasing drought severity resulted in the accumulation of soil NH4+ and NO3-, with a more pronounced increase in exotic-dominated plots. Increased net N mineralization and net nitrification and reduced leaching losses were observed as mechanisms of inorganic N accumulation. In comparison to soils under native plants, soils under exotic plants had enhanced leaching losses upon soil rewetting. We propose that distinct traits of exotic annual herbaceous species associated with higher N inputs, faster turnover, and reduced temporal uptake determine the changes in N cycling in response to droughts. Severe droughts and exotic plants may produce a larger, more vulnerable pool of N that is prone to losses while providing a competitive advantage to promote exotic growth in these N-limited ecosystems.

Effects of native bryophytes on exotic grass invasion across an environmental gradient

AbstractUnderstanding the role of native biodiversity in controlling exotic species invasion is a critical goal in ecology. In terrestrial plant communities, most research has focused on the effects of native vascular plants on invasion by exotic vascular plants. However, in many ecosystems, native bryophytes and other non‐vascular plants are common and can affect the establishment, survival, and growth of vascular plants. A more complete picture of how native biodiversity affects exotic plant invasion demands that more studies measure the effects of native bryophytes on exotic vascular plants. Moreover, there is growing realization that the effects of native species on invaders can range from negative to positive and that a complete picture of interactions between native and exotic plants requires measuring interactions in multiple environments. We used both observational and experimental studies to quantify the effects of native bryophytes on vascular plants along a 200‐m environmental gradient in a coastal dune in northern California. We found a positive association between vascular plants and bryophytes across the environmental gradient. Our experiments with two exotic annual grass species showed the effects of bryophytes to be species‐specific and to vary with environmental context. Bryophytes facilitated the survival of one exotic grass species, Vulpia bromoides, at both ends of the environmental gradient. Bryophytes reduced the survival and inflorescence production of the other exotic grass, Bromus diandrus, at one end of the environmental gradient and had no effect at the other end. Our findings provide a test of the effects of native bryophytes on exotic vascular plant invasion and show that these effects can vary dramatically even across local environmental gradients.

Ventenata and Other Coexisting Exotic Annual Grass Control and Plant Community Response to Increasing Imazapic Application Rates

Ventenata (Ventenata dubia [Leers] Coss.) is an exotic annual grass that can invade intermountain rangeland plant communities, where it can form monotypic stands, degrade wildlife habitat, and reduce livestock forage. There is limited information on ventenata control in rangelands as it has only recently been identified as a substantial problem. Imazapic is a pre-emergent herbicide commonly used to control other exotic annual grasses and, therefore, is likely to control ventenata in rangelands. We evaluated five application rates of imazapic (0−175 g ae∙ha−1) on ventenata and other exotic annual grass control and plant community response at two rangeland sites in 2 yr (2014 and 2015). Imazapic reduced exotic annual grass (largely ventenata) cover and density, with greater control with increasing imazapic rates. Exotic annual grass density at the highest levels of control (82%−94%) was 184−299 plants∙m−2 the first yr after imazapic application. Exotic annual grasses fully recovered in the second or third yr after imazapic application. Bare ground generally increased with imazapic application. However, density of perennial vegetation (grasses and forbs) did not vary among treatments. Perennial vegetation cover generally did not increase with imazapic control of ventenata and other exotic annual grasses. Imazapic can control ventenata; however, even at the highest rates, control was not enough to shift the dominance from exotic annual species to perennial species. Integrating other treatments with imazapic application may be a strategy to improve ventenata control and increase perennial vegetation and will require further investigation. The difficulty and likely expense of achieving substantial and lasting control of ventenata suggest, similar to other exotic annual grasses, that preventing ventenata invasion and dominance should be a high management priority.

Drought in Southern California coastal sage scrub reduces herbaceous biomass of exotic species more than native species, but exotic growth recovers quickly when drought ends

Semi-arid regions with Mediterranean-type climates harbor exceptional biodiversity, but are increasingly threatened by invading exotic annual species and climatic changes, including drought. In semi-arid ecosystems, antecedent conditions often influence plant growth, but the role of antecedent conditions for drought response and recovery of native versus exotic species remains largely unexplored. From 2013 to 2016, we imposed experimental rainfall treatments (average rainfall, moderate or severe drought) in plots under a native shrub canopy and in inter-spaces dominated by herbaceous vegetation, and quantified growth (peak biomass) and abundance (cover) of native and exotic herbaceous species. The following year, we quantified recovery from the drought treatments (2017). Exotic biomass was less resistant to drought (declined more than native biomass), but was more resilient (increased more than native biomass in the year following drought), especially in unshaded inter-spaces between shrubs. These responses were associated with life history; annual species responded more negatively to drought in the inter-spaces than perennial species. Current years' rainfall was a better predictor of biomass than prior rainfall, but antecedent factors were also important. After four years of rainfall treatments, exotic species had the highest growth recovery in the severe drought treatment, while growth of natives had the opposite response. In contrast, litter was positively associated with plant growth regardless of origin. This study demonstrates that when native and exotic species differ in life history, as they do in Mediterranean climate ecosystems, they may respond differently to antecedent factors, and hence differ in recovery from climate extremes such as drought.

Competition reverses the response of shrub seedling mortality and growth along a soil moisture gradient

Abstract Predicting species responses to climate change involves understanding both the direct effects of environmental change, as well as indirect effects mediated by altered interspecific interactions. Indirect effects may be particularly important for understanding native species responses in systems invaded by highly competitive exotic species. For instance, Mediterranean climate regions are predicted to experience more frequent drought, and are increasingly invaded by exotic annual plants. For native shrubs in these regions, seedling establishment is episodic, and associated with high rainfall years. However, exotic annual plants also often increase in abundance with high rainfall, suggesting competition from exotic annual species could alter the relationship between rainfall and shrub seedling establishment. Theories such as the stress gradient hypothesis predict competition intensity should increase with resource supply, but there have been few evaluations of competitive interactions across experimental gradients of soil moisture availability. Here we examined how competition from an exotic annual influenced native shrub establishment, across an experimental soil moisture gradient. Seedlings of two native shrub species (Encelia californica and Eriogonum fasciculatum) were grown with and without an exotic grass competitor (Avena fatua) across eight water availability levels, and monitored for growth and survival. These species are common and abundant in the Mediterranean climate region of coastal Southern California, where climate change projections include long‐term drought for the coming decades. Without competition, shrub seedlings achieved higher growth and survival at high water availability levels. However, when grown in competition with the exotic grass, shrub seedlings had higher growth and/or survival under relatively dry conditions, suggesting competition can modify and even reverse species responses to changing rainfall patterns, compared to predictions made in the absence of competitors. The exotic grass strongly reduced soil nitrate and water availability when it was planted with the native shrub seedlings, and the exotic grass responded with positive but saturating growth with increasing levels of water addition. Synthesis. This experiment demonstrates that competition from invasive species can alter native species responses to climate change; consistent with ecological theories predicting a positive association between the supply of limiting resources and the intensity of competition.

Trade-off between early emergence and herbivore susceptibility mediates exotic success in an experimental California plant community.

Ecological trade‐offs are fundamental to theory in community ecology; critical for understanding species coexistence in diverse plant communities, as well as the evolution of diverse life‐history strategies. Invasions by exotic species can provide insights into the importance of trade‐offs in community assembly, because the ecological strategies of invading species often differ from those present in the native species pool. Exotic annual species have invaded many Mediterranean‐climate areas around the globe, and often germinate and emerge earlier in the growing season than native species. Early‐season growth can enable exotic annual species to preempt space and resources, competitively suppressing later‐emerging native species; however, early‐emerging individuals may also be more apparent to herbivores. This suggests a potential trade‐off between seasonal phenology and susceptibility to herbivory. To evaluate this hypothesis, we monitored the emergence and growth of 12 focal species (six each native and exotic) in monoculture and polyculture, while experimentally excluding generalist herbivores both early and later in the growing season. Consistent with past studies, the exotic species emerged earlier than native species. Regardless of species origin, earlier‐emerging species achieved greater biomass by the end of the experiment, but were more negatively impacted by herbivory, particularly in the early part of the growing season. This greater impact of early‐season herbivory on early‐active species led to a reduction in the competitive advantage of exotic species growing in polyculture, and improved the performance of later‐emerging natives. Such a trade‐off between early growth and susceptibility to herbivores could be an important force in community assembly in seasonal herbaceous‐dominated ecosystems. These results also show how herbivore exclusion favors early‐active exotic species in this system, with important implications for management in many areas invaded by early‐active exotic species.

Diverse outcomes of species interactions in an invaded annual plant community

AimsDespite acknowledgement that interactions among native and exotic species are important for determining the structure and diversity of novel communities, directed experiments using multiple exotics from the same system are rare. Recent observational studies have highlighted distinct ways that exotic species interface with resident natives across invaded communities. The correlative nature of these studies, however, has provided few details about the mechanisms driving distinct interaction outcomes within the same communities. Our aim was to determine how three exotic annual plant species with distinct relationships with local plant diversity impact the performance of a co-occurring native annual in the York gum-jam woodlands of Western Australia.MethodsWe grew species in experimental communities in growth chambers at varying total planting densities to assess how interaction outcomes varied among natives and exotics across a gradient of competition intensity. We measured a variety of performance responses, including survival, biomass and population-level and individual-level reproductive investment.Important FindingsOverall, the effects of interspecific versus intraspecific competition on performance varied with the identity and density of exotic competitors. The exploitative exotic grass Bromus madritensis was dominant in polyculture, whereas the diminutive grass Pentameris airoides conferred weak intraspecific competition and interspecific facilitation on native Waitzia nitida. The exotic broadleaf forb, Hypochaeris glabra, suppressed growth and survival of W. nitida, while W. nitida had weakly negative, neutral or positive effects on all exotics. These outcomes highlight the complexity of interactions impacting the diversity, stability and structure of novel plant communities. As few of these communities contain a single exotic species, understanding the competitive dynamics occurring in diverse novel communities is critical for their conservation and restoration.

A plant traits approach to managing legacy species during restoration transitions in temperate eucalypt woodlands

Degraded communities often contain a subset of the species that comprised the predisturbance community. These represent an important legacy of the predisturbance state, yet restoration treatments may be detrimental to them. This study examined the potential of leaf traits and life form to predict whether restoration treatments can maintain legacy swards of Austrostipa bigeniculata (hereafter Austrostipa) while controlling exotic annuals in temperate eucalypt woodlands. Treatments included carbon addition to reduce soil nitrate, both with and without burning or pulse grazing to deplete exotic seed pools. We compared leaf traits of Austrostipa with a native grass (Themeda triandra) known to be advantaged, and 8 exotic annual species known to be disadvantaged by these treatments. Leaf traits indicated potentially greater negative impacts of carbon addition on exotic annuals compared to Austrostipa, and on Austrostipa compared to Themeda, suggesting a net restoration benefit. Similarly, burning or pulse grazing is expected to have little negative impact on perennial resprouting grasses (hemicryptophytes; Austrostipa and Themeda) compared with annual exotics (therophytes) with short-lived seed banks. Treatment responses were largely consistent with predictions: treatments that significantly reduced exotic annuals had no net disadvantage to Austrostipa swards despite significant reductions in Austrostipa seedling growth with carbon addition. Indeed by Year 3, Austrostipa mortality in untreated plots led to 46% lower Austrostipa abundance than in treated plots at one site, potentially due to litter build-up or other mechanisms. We conclude that plant traits provide a useful framework for designing restoration transitions that retain native legacy species while controlling exotics.

Bet-hedging strategies of native and exotic annuals promote coexistence in semiarid Chile

Scientists are increasingly interested in the evolutionary responses of organisms to unpredictable, variable, and extreme climate changes. In semiarid environments, inter-annual variability in the frequency and amount of rainfall affects both the growth and recruitment of plant species, especially annuals. In these inherently variable environments, individual selection should favor demographic responses that spread the risk of mortality over time and enhance long-term reproductive success (i.e., bet-hedging strategies). However, the same processes that allow the persistence and recruitment of native species could facilitate the introduction and establishment of exotics. We assessed whether native and exotic annuals in semiarid Chile displayed similar or contrasting bet-hedging traits, and discuss mechanisms of coexistence of both types of species and their demographic variation under interannual rainfall variability driven by El Niño Southern Oscillation (ENSO). We analyzed a proxy of long-term fitness, i.e., the variability of seed density over 17 years, for the two most common native and two exotic annual plant species present in the study area. We experimentally tested whether the quality of the maternal environment (soil water supply in a given year) had an extended effect (e.g. the next year) on the proportion of seed germination or on the mean and/or variability of seed size and seed dormancy. Results showed that native and exotic species in this annual plant assemblage displayed contrasting bet-hedging strategies as evolutionary responses to variable rainfall. Although rainfall variability promotes the evolution of bet-hedging strategies, the nature of these strategies varies across species, presumably to minimize competitive exclusion. In semiarid Chile, the success of two exotic ephemerals that are components of a diverse community of native annual species seems to reflect bet-hedging germination strategies that complement rather than compete with those expressed in dominant natives.

Native and exotic plant species show differential growth but similar functional trait responses to experimental rainfall

In the coming decades, rainfall in many Mediterranean‐type ecosystems is expected to decline overall, but also become more variable interannually. Historically plant communities in these regions have been dominated by native shrubs, but are becoming increasingly invaded by exotic annual species, making it important to evaluate how native versus exotic species will respond to shifting rainfall patterns associated with climate change. Exotic species are often found to possess a suite of traits associated with fast growth and resource uptake, as well as high plasticity in traits. Hence, we hypothesized that exotic species would benefit disproportionately over native species under high rainfall conditions, while native species with more conservative growth strategies would better tolerate drought. We evaluated these expectations by manipulating rainfall (five levels, from total exclusion to a doubling of ambient rainfall) over plots containing a mix of herbaceous exotic species, mature native shrubs, and planted shrub seedlings, and measured functional trait and growth responses of focal species. We found significant variation among groups in their response to variation in rainfall quantity: growth of exotic herbaceous species was higher in response to high rainfall, and lower in response to drought, compared with native adult shrubs, with juvenile shrubs having an intermediate response. In contrast with these growth responses, functional traits of native and exotic species responded similarly to experimental variation in rainfall. Further, although we found exotic species possessed thinner leaves and higher photosynthetic capacity, neither mean trait values nor trait plasticity were predictive of species‐level responses to altered rainfall in this system. We conclude that while some functional traits often predict community composition across large environmental gradients, traits may not predict species responses to environmental change at local scales with limited species pools.

Invasive spread dynamics of Anthriscus caucalis at an ecosystem scale: propagule pressure, grazing disturbance and plant community susceptibility in canyon grasslands

Several factors may influence invasive spread dynamics once a species becomes naturalized. Propagule pressure generated from established populations is likely mediated by local scale factors such as plant community characteristics and disturbance regimes as well as ecosystem scale factors such as landscape features that shape dispersal patterns and propagule availability. Using a multi-scale observational approach, we explore how propagule pressure generated from naturalized populations of Anthriscus caucalis (M.-Bier) influences invasive spread dynamics in the Snake River canyon grassland ecosystem of the Intermountain Pacific Northwest, USA. In this study, we utilize demographic insights into A. caucalis naturalization across plant communities and a companion ecosystem scale distribution survey to elucidate invasive spread dynamics using spatially-explicit SDMs. We modeled A. caucalis occurrence as a function of plant community type, grazing disturbance, and propagule pressure generated from high-shrub communities. A. caucalis was almost uniformly present in low- and high-shrub (>90 %) communities. Within bunchgrass communities, A. caucalis occurrence was significantly lower within grazed paddocks (3 %) compared to ungrazed paddocks (52 %). The probability of A. caucalis occurrence in bunchgrass communities within ungrazed paddocks also increased at a faster rate as the relative area of high-shrub communities increased. These results suggest propagule pressure generated from high-shrub communities strongly influence invasive spread into bunchgrass communities. Our results also suggest that livestock disturbance slows invasive spread by decreasing propagule supply from high-shrub communities. Plant community composition also contributed to A. caucalis occurrence patterns within bunchgrass communities. A. caucalis occurrence declined as native bunchgrass cover increased, whereas occurrence increased with increasing cover of exotic annual species. As native bunchgrass cover declined, A. caucalis occurrence increased in locations with greater relative area of high-shrub communities. These results suggest that undisturbed bunchgrass communities are resistant to A. caucalis invasion regardless of propagule supply. In disturbed bunchgrass communities dominated by exotic annuals, A. caucalis invasion levels were positively associated with increased propagule pressure generated from high-shrub communities. In conclusion, the coupling of comparative demographic observations with spatially-explicit SDMs yielded significant insights into invasive spread dynamics. This approach should be applicable for elucidating invasive spread dynamics of other invading plant species and ecosystems.

Asteraceae invaders have limited impacts on the pollination of common native annual species in SW Western Australia’s open woodland wildflower communities

The York gum–jam woodlands of southwest Western Australia support diverse annual wildflower communities despite extensive habitat fragmentation, remnant isolation and the invasion of many exotic annual plant species. Few studies have explored the pollinator–plant relationships maintaining these persistently species-rich ‘novel’ communities. We examine the pollination ecology of five native species common to York gum–jam woodland annual communities to determine whether native pollinators may be mediating impacts of exotic annual plants on native wildflower species. We determined the pollination requirements of native focal species and the diversity and frequency of pollinator visitation to these focal plant species across invasion gradients. We also recorded the pollinator community of a dominant exotic herb in this system: Arctotheca calendula (cape weed). Only two of the five native species examined had significant seed set benefits attributable to insect pollination. One native plant species, Podotheca gnaphalioides, had pollinator assemblages that overlapped significantly with exotic A. calendula, with some reduction in pollinator visitation evident. One species, Waitzia acuminata, was found to benefit from insect pollination only in the larger of two surveyed remnants, which may reflect emerging reproductive polymorphism among geographically isolated populations. We highlight two mechanisms in this system that may buffer pollinator-mediated impacts of exotic species on native species: autonomous seed production, which may be increasingly prevalent in isolated populations, and segregation of pollinator resources among species. Our findings illustrate the ways that pollinator-mediated interactions can affect seed set within plant communities persisting in highly fragmented and invaded agricultural landscapes.

Site-Specific Effects of Exotic Annual Grass Control Integrated with Revegetation

Invasion by exotic annual grass species such as downy brome and Japanese brome has been implicated in ecosystem degradation in much of the western United States and strategies to restore lands dominated by these species are needed. We evaluated integration of herbicide and revegetation to restore old-field and rangeland sites dominated by annual brome species. In split-plot field studies, we compared three imazapic herbicide rates and eight seeding treatments at an old-field and a rangeland site. We evaluated brome, seeded species, and existing vegetation response to our treatments. Herbicide controlled annual brome at both sites, with > 95% decreases in brome biomass in plots treated with the high rate compared to non-sprayed controls. Control of annual brome persisted for two growing seasons at the old-field site and one season at the rangeland site. Annual brome abundance in non-treated control plots at both sites decreased by 80% between the first and second growing season. No seeded individuals established at the old-field site, but they persisted through two growing seasons at the rangeland site. Seeded species establishment at the rangeland site was impacted by both herbicide rate and seed mix, but no clear patterns emerged. Finally, while we controlled brome at both sites, existing desired vegetation increased only at the old-field site, where perennial grass biomass increased by about 30% in plots sprayed with both rates of herbicide. The differing responses of the existing and seeded plant communities across the two sites highlight the importance of integrating site-specific knowledge into restoration plans.

Short-term priority over exotic annuals increases the initial density and longer-term cover of native perennial grasses.

Temporal priority can affect individual performance and reproduction, as well as community assembly, but whether these effects persist over time remains unclear, and their demographic mechanisms have been little explored. The continued dominance of exotic annual grasses in California has been commonly attributed to their demonstrated early germination and rapid early growth relative to native perennial grasses. This advantage may play a crucial role in the structure of California exotic annual grasslands, as well as in the practice of native grassland restoration. We tested whether a two-week planting advantage under field conditions increased individual survival, growth, and reproduction for four native perennial grass species and whether these effects persisted over three years. We show that short-term priority significantly increased the establishment success of' native perennial grasses. Increased density of native grass seedlings presaged later large increases in cover that were not evident in the first year after planting. Although priority effects at the individual level may diminish over time, short differences in emergence timing can have long-lasting effects on community structure. Earlier germination and faster initial growth of exotic annual species may help explain their unprecedented invasion and continued dominance of California grasslands. Finally, these results highlight the importance of priority effects for effective exotic annual control during native grassland restoration in California: initial control can increase the establishment of native perennial seedlings, which then results in long-term control by mature native individuals.

Predicting fire-based perennial bunchgrass mortality in big sagebrush plant communities

Maintenance and post-fire rehabilitation of perennial bunchgrasses is important for reducing the spread of exotic annual grass species in big sagebrush plant communities. Post-fire rehabilitation decisions are hampered by a lack of tools for determining extent of fire-induced perennial grass mortality. Our objective was to correlate post-fire characteristics with perennial bunchgrass mortality at the plant and plant community scales. We recorded basal area, percent char, depth of burn and soil colour for 174 bunchgrasses across four ecological sites after a 65 000 ha wildfire in south-east Oregon and assessed plant mortality. Mortality was correlated with post-fire soil colour and ecological site; soil colours (black and grey) associated with pre-fire shrub presence had up to five-fold higher mortality than brown soils typical of interspace locations. Models incorporating depth of burn and soil colour correctly predicted mortality for 90% of individual plants; cover of brown soil explained 88% of the variation in bunchgrass mortality at the plant community scale. Our results indicate that soil colour and depth of burn are accurate predictors of bunchgrass mortality at individual plant and plant community scales and could be used to spatially allocate post-fire bunchgrass rehabilitation resources.

Priority effects vary with species identity and origin in an experiment varying the timing of seed arrival

Exotic species are sometimes phenologically distinct from native species in the invaded community, allowing them to be active when there may be reduced competition for resources. In southern California, annual species are particularly problematic invaders, and prior work has shown that these species germinate earlier in the growing season, giving them a competitive advantage over later‐germinating native species. This result begs the question, if being active earlier is advantageous, why have not native species adapted earlier cues for germination? We hypothesized native species would benefit less from earlier germination than exotic species (potentially due to slower growth following germination), thus negating potential selection for early germination. Here we manipulated planting time for common native and exotic species, growing them in all possible species pairs, to evaluate how competitive outcomes were altered by the time of arrival and the origin of competing species. In contrast to our hypotheses, the exotic species often had lower biomass when planted first, potentially due to disturbance when the second species was planted. In contrast, three out of our four native species benefited from earlier planting (a priority effect). Unlike the potential benefit of arriving early, we found no evidence that being planted one week later resulted in a competitive disadvantage, when compared to being planted simultaneously with a competitor. Further, we found that the magnitude and even direction of priority effects varied depending on the identity of the interacting species. Together these results suggest that a lack of directional selection may prevent adaptation towards earlier germination times of native species. Although this experiment was conducted with a limited suite of species, the results show that the role of seasonal priority effects varies among species, and that native species could benefit from seasonal priority effects in restoration efforts even when in competition with fast‐growing exotic annual species.

Root Inputs Influence Soil Water Holding Capacity and Differentially Influence the Growth of Native versus Exotic Annual Species in an Arid Ecosystem

AbstractInvasion by exotic annual species is increasingly impacting Southern California arid lands, altering ecosystem processes and plant community composition. With climate change, the Southwestern United States is expected to experience increasingly variable rainfall. Larger rainfall events could facilitate invasion by exotic species that can capitalize on high resource conditions. Exotic annual species also have dense shallow root systems that could create positive feedbacks to further invasion by increasing soil organic matter and water holding capacity. Alternatively, fine root inputs could create negative feedbacks to exotic plant growth if they stimulate microbial nutrient immobilization. The dual influences of rainfall regime and fine root inputs on species performance were evaluated in an experiment where native and exotic species were grown individually and in combination under varying watering regimes (large infrequent or small frequent pulses, holding total rainfall constant) and root additions (with or without sterilized exotic roots). Mean soil moisture increased with larger infrequent watering events, and also with root addition. Plant growth (both native and exotic) increased with larger watering events, but declined with root addition. Exotic species growth declined more than native species growth with root additions. Mechanistically, root addition lowered inorganic nitrogen (N) availability, and microbial N immobilization increased with soil moisture content. Together these results show that increased fine root production promotes negative feedbacks to growth of exotic species via microbial N immobilization, especially under conditions of high soil moisture. Further, our results suggest that organic carbon additions are a potentially effective strategy for suppressing growth of problematic desert invaders.