California Coastal Sage Scrub Research Articles

Long-term drought promotes invasive species by reducing wildfire severity.

Anthropogenic climate change has increased the frequency of drought, wildfire, and invasions of non-native species. Although high-severity fires linked to drought can inhibit recovery of native vegetation in forested ecosystems, it remains unclear how drought impacts the recovery of other plant communities following wildfire. We leveraged an existing rainfall manipulation experiment to test the hypothesis that reduced precipitation, fuel load, and fire severity convert plant community composition from native shrubs to invasive grasses in a Southern California coastal sage scrub system. We measured community composition before and after the 2020 Silverado wildfire in plots with three rainfall treatments. Drought reduced fuel load and vegetation cover, which reduced fire severity. Native shrubs had greater prefire cover in added water plots compared to reduced water plots. Native cover was lower and invasive cover was higher in postfire reduced water plots compared to postfire added and ambient water plots. Our results demonstrate the importance of fuel load on fire severity and plant community composition on an ecosystem scale. Management should focus on reducing fire frequency and removing invasive species to maintain the resilience of coastal sage scrub communities facing drought. In these communities, controlled burns are not recommended as they promote invasive plants.

Increase in heat tolerance following a period of heat stress in a naturally occurring insect species.

Climate warming is the defining environmental crisis of the 21st century. Elucidating whether organisms can adapt to rapidly changing thermal environments is therefore a crucial research priority. We investigated warming effects on a native Hemipteran insect (Murgantia histrionica) that feeds on an endemic plant species (Isomeris arborea) of the California coastal sage scrub. Experiments conducted in 2009 quantified the temperature responses of juvenile maturation rates and stage-specific and cumulative survivorship. The intervening decade has seen some of the hottest years ever recorded, with increasing mean temperatures accompanied by an increase in the frequency of hot extremes. Experiments repeated in 2021 show a striking change in the bugs' temperature responses. In 2009, no eggs developed past the second nymphal stage at 33°C. In 2021, eggs developed into reproductive adults at 33°C. Upper thermal limits for maturation and survivorship have increased, along with a decrease in mortality risk with increasing age and temperature, and a decrease in the temperature sensitivity of mortality with increasing age. While we cannot exclude the possibility that other environmental factors occurring in concert could have affected our findings, the fact that all observed trait changes are in the direction of greater heat tolerance suggests that consistent exposure to extreme heat stress may at least be partially responsible for these changes. Harlequin bugs belong to the suborder Heteroptera, which contains a number of economically important pests, biological control agents and disease carriers. Their differential success in withstanding warming compared to beneficial holometabolous insects such as pollinators may exacerbate the decline of beneficial insects due to other causes (e.g. pollution and pesticides) with potentially serious consequences on both biodiversity and ecosystem functioning.

Three-way species interactions reverse the positive pairwise effects of two natives on an exotic invader

The disruptive effects of tertiary species on otherwise positive pairwise species interactions (e.g. context-dependent parasitism in pollinator syndromes) is well-known. However, few—if any—studies have investigated how invasive plants affect interactions between facilitative plants and their native plant communities. Further, if tertiary invasive species can change interactions among native species from positive to negative, then a tertiary native should be capable of the same phenom for pairwise interactions between natives and invasives. Our previous research indicates invasive black mustard (Brassica nigra) changes interaction signs for otherwise positive species interactions between the dominant, native facilitator California buckwheat (Eriogonum fasciculatum) and its co-dominant beneficiary California sagebrush (Artemisia californica) in semi-arid California coastal sage scrub habitat. Here, E. fasciculatum and A. californica seedlings increased B. nigra shoot growth in pairwise species interactions in the greenhouse. However, in three-way species interactions, E. fasciculatum and A. californica together reduced B. nigra SLA, height, and reproductive potential while not increasing shoot DW. In three-way species interactions, B. nigra did not significantly reduce E. fasciculatum facilitation of A. californica. Also surprisingly, light competition with B. nigra resulted in an increase in A. californica height, which reduced the negative effects of A. californica light competition on shade-intolerant E. fasciculatum. In an additive field experiment, A. californica protected E. fasciculatum from facilitating germination and growth of B. nigra when water competition was minimized. Taken together, this study demonstrates the importance of studying species interactions between competitive, native perennials in the current ecological context of invaded ecosystems.

Identifying and Remediating Soil Microbial Legacy Effects of Invasive Grasses for Restoring California Coastal Sage Scrub Ecosystems

As invasive grasses encroach upon native ecosystems, they have the potential to transform intact California coastal sage scrub (CSS) into non-native grasslands. This occurs—in part—due to legacy effects: changes in soil microbial composition elicited by grass invasions with long-term impacts on soil and native vegetation. Along with direct effects on CSS vegetation, legacy effects may alter microbial communities which may in turn impact native plant growth. To tease apart these factors, we monitored growth in three CSS species inoculated with either uninvaded soil or sterilized uninvaded soil that were subsequently planted into a site characterized by observed microbial legacy effects. Our findings indicate native plant growth can be explained by changes in soil microorganisms. Specifically, native Artemisia californica and Baccharis pilularis grew larger in plots with unsterilized uninvaded soil inoculum, which contained a larger abundance of Gemmatimonadetes and Glomus, compared to the sterilized soil inoculum plots. Therefore, microbial augmentation may not only improve restoration in post-invasive sites but long-term changes in soil microbial communities may be linked to native plant growth. Furthermore, adding uninvaded soil replete with native microbes has the potential to support restoration of invaded sites by promoting native plant survival and establishment in these restored ecosystems.

Facilitation at early growth stages results in spatial associations and stable coexistence in late growth stages of two long‐lived, dominant shrubs

While models of species coexistence largely focus on how competition defines biological communities, over recent decades, a number of studies show positive plant–plant species interactions (facilitation) can also promote stable coexistence. The long‐lived, co‐dominant shrubs California buckwheatEriogonum fasciculatumand California sagebrushArtemisia californicashare a well‐documented positive association at the habitat level in their native California coastal sage scrub ecosystem, but mechanisms underlying their interactions remain unclear at finer spatial scales. Here, a hypothesis thatE. fasciculatumacidifies CSS's alkaline soils and facilitatesA. californicathrough amelioration of alkalinity stress is tested in a greenhouse experiment and association tests in the field. Greenhouse results demonstrate facilitation at early growth stages. In late growth stages, water competition is known to determine the shrubs' interactions with each other, but here, field observations of the shrubs in late growth stages show positive associations betweenA. californicaandE. fasciculatumthat have a positive linear relationship to increasing soil pH. These results highlight the importance of understanding lifecycle‐long interactions among species in evaluating facilitation's impacts on community structure.

Facilitation and herbivory during restoration of California coastal sage scrub

Using nurse plants to facilitate native plant recruitment in degraded habitats is a common restoration practice across various arid and semiarid environments. Living trees or shrubs are typically considered nurse plants, whereas dead shrubs left in the landscape from prolonged drought are understudied prospective facilitators for native plant recruitment. The interaction between nurse plants and biotic pressures, such as herbivory, on native recruitment is also not well understood in semiarid plant communities. We investigated the effects of facilitation and herbivory on native seedling germination, growth, and survival in the restoration of degraded coastal sage scrub (CSS) habitat. Native shrub seedlings (Artemisia californica and Salvia mellifera) were planted, and native annual species (Amsinckia intermedia, Deinandra fasciculata, Phacelia distans, and Pseudognaphalium californicum) were sown in three Shrub Type treatments (live shrub, dead shrub, and exposed areas), with a nested Cage treatment (no cage and cage) in each Shrub Type treatment. Annual species grew equally well in all Shrub Type treatments; shrub seedlings grew largest in exposed areas. While there was little evidence of facilitation for all species tested, there were strong positive effects of caging on growth and establishment of all species. Caging palatable native species or planting species with anti‐herbivory traits around target plants may be more strategic approaches compared to using nurse plants in restoring degraded CSS after extended drought.

Foliar litter chemistry mediates susceptibility to UV degradation in two dominant species from a semi-arid ecosystem

Abiotic processes such as photodegradation play important roles in litter decomposition in semi-arid ecosystems. However, little is known about whether UV degradation responds similarly to factors controlling biotic decomposition rates, such as soil moisture and plant litter chemistry. Here, we evaluated the relative importance of UV degradation versus biotic decomposition in contrasting precipitation regimes for two species. We manipulated ultraviolet (UV) radiation under two rainfall treatments (ambient and added precipitation) and measured decomposition rates of leaf litter from two abundant species in a southern California coastal sage scrub ecosystem: a native shrub (Salvia mellifera) and an exotic annual grass (Avena fatua). The influence of UV radiation exposure on decomposition did not vary with rainfall treatment, and UV radiation exposure only increased litter mass loss of the A. fatua litter, not S. mellifera. This pattern was driven by accelerated loss of hemicellulose and cellulose litter fractions when the exotic grass litter was exposed to UV radiation. The greatest influence of UV radiation was observed after 18 months. The sensitivity of hemicellulose and cellulose litter fractions to UV radiation suggests that shifting plant species composition which results in altered litter chemistry could change ecosystem sensitivity to UV photodegradation.

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.

Rapid root responses of seedlings exposed to a postdrought water pulse.

Mediterranean-type climate ecosystems experience significant variability in precipitation within and across years and may be characterized by periods of extreme drought followed by a brief, high-intensity precipitation pulse. Rapid root growth could be a key factor in effective utilization of precipitation pulses, leading to higher rates of seedling establishment. Changes in root growth rate are rarely studied, however, and patterns in seedling root traits are not well explored. We investigated the influence of an extreme postdrought precipitation event on seedlings that occur in southern California coastal sage scrub. We measured root elongation rate, root tip appearance rate, new leaf appearance rate, and canopy growth rate on 18 mediterranean species from three growth forms. Root elongation rate responded more strongly to the precipitation pulse than did root tip appearance rate and either metric of aboveground growth. The majority of species exhibited a significant change in root growth rate within 1 week of the pulse. Responses varied in rapidity and magnitude across species, however, and were not generally predictable based on growth form. While the majority of species exhibited shifts in belowground growth following the pulse, the direction and magnitude of these morphological responses were highly variable within growth form. Understanding the implications of these different response strategies for plant fitness is a crucial next step to forecasting community dynamics within ecosystems characterized by resource pulses.

Urbanization-induced habitat fragmentation erodes multiple components of temporal diversity in a Southern California native bee assemblage.

Despite a large number of ecological studies that document diversity loss resulting from anthropogenic disturbance, surprisingly few consider how disturbance affects temporal patterns of diversity that result from seasonal turnover of species. Temporal dynamics can play an important role in the structure and function of biological assemblages. Here, we investigate the temporal diversity patterns of bee faunas in Southern California coastal sage scrub ecosystems that have been extensively fragmented by urbanization. Using a two-year dataset of 235 bee species (n = 12,036 specimens), we compared 1-ha plots in scrub fragments and scrub reserves with respect to three components of temporal diversity: overall plot-level diversity pooled over time (temporal gamma diversity), diversity at discrete points in time (temporal alpha diversity), and seasonal turnover in assemblage composition (temporal beta diversity). Compared to reserves, fragments harbored bee assemblages with lower species richness and assemblage evenness both when summed across temporal samples (i.e., lower temporal gamma diversity) and at single points in time (i.e., lower temporal alpha diversity). Bee assemblages in fragments also exhibited reduced seasonal turnover (i.e., lower temporal beta diversity). While fragments and reserves did not differ in overall bee abundance, bee abundance in fragments peaked later in the season compared to that in reserves. Our results argue for an increased awareness of temporal diversity patterns, as information about the distinct components of temporal diversity is essential both for characterizing the assemblage dynamics of seasonal organisms and for identifying potential impacts of anthropogenic disturbance on ecosystem function through its effects on assemblage dynamics.

Effects of temperature and resource variation on insect population dynamics: the bordered plant bug as a case study.

In species with complex life cycles, population dynamics result from a combination of intrinsic cycles arising from delays in the operation of negative density-dependent processes (e.g., intraspecific competition) and extrinsic fluctuations arising from seasonal variation in the abiotic environment. Abiotic variation can affect species directly through their life history traits and indirectly by modulating the species' interactions with resources or natural enemies.We investigate how the interplay between density-dependent dynamics and abiotic variability affects population dynamics of the bordered plant bug (Largus californicus), a Hemipteran herbivore inhabiting the California coastal sage scrub community. Field data show a striking pattern in abundance: adults are extremely abundant or nearly absent during certain periods of the year, leading us to predict that seasonal forcing plays a role in driving observed dynamics.We develop a stage-structured population model with variable developmental delays, in which fecundity is affected by both intra-specific competition and temporal variation in resource availability and all life history traits (reproduction, development, mortality) are temperature-dependent. We parameterize the model with experimental data on temperature-responses of life history and competitive traits and validate the model with independent field census data.We find that intra-specific competition is strongest at temperatures optimal for reproduction, which theory predicts leads to more complex population dynamics. Our model predicts that while temperature or resource variability interact with development-induced delays in self-limitation to generate population fluctuations, it is the interplay between all three factors that drive the observed dynamics. Considering how multiple abiotic factors interact with density-dependent processes is important both for understanding how species persist in variable environments and predicting species' responses to perturbations in their typical environment.

Soil nitrogen cycling is resilient to invasive annuals following restoration of coastal sage scrub

Southern California coastal sage scrub (CSS) is highly invaded by Mediterranean annual grasses and undergoing extensive restoration efforts. Exotic plant invasion alters ecosystem structure and function through plant-soil feedbacks that can be detrimental to native plants. Assessments of CSS restoration have focused on aboveground plant communities, while belowground effects have received less attention. We examined CSS soil resilience following restoration of native CSS species using ecosystem property divergence in restorations from an invaded state as a measure of exotic plant impacts. We hypothesized that exotic annual plants compete with native species for nutrients and change nutrient cycling, and exotic plant removal and native plant restoration would allow soil recovery under native plant-soil inputs. Nitrogen (N) cycling was resilient but not resistant to vegetation changes. Exotic annual plants increased N mineralization and nitrification but did not affect total soil carbon (C) and N and extractable phosphorus. Extractable N was reduced in invaded plots, and immediately increased following weeding. These changes suggest that exotic plants are directly competing with native plants for N. Impacts to N cycling were reversible after exotic plants were removed and native shrubs reestablished, which may have important implications for recovery of other ecosystems invaded by annual grasses.

Intra‐annual rainfall regime shifts competitive interactions between coastal sage scrub and invasive grasses

Changes in rainfall distribution, generally predicted by many climate models, can affect resource dynamics and ecosystem function. While little studied, intra-annual rainfall distribution may have particularly strong effects on competitive interactions. Here, we test whether increased rainfall event size and decreased frequency within a growing season can influence competitive dynamics related to the invasion of exotic annual grasses in California coastal sage scrub (CSS). We hypothesized that larger rainfall events and decreased frequency will increase the competitive ability of native CSS species: a deeper root system will permit greater water use during dry periods between pulses and enhance their resource depletion effect on more shallow-rooted grasses. We planted grass and CSS seedlings in an additive competition design under three rainfall treatments: frequent small events, infrequent large events, and infrequent small events. The first two treatments had the same total rainfall but different frequency, while the second and third treatments had the same frequency but different total rainfall. Rainfall treatment altered the competitive interactions between CSS and grasses. In the first year, the competitive effect of annual grasses on shrub seedlings was strongest under the frequent small rainfall regime where they reduced deep soil moisture and light. In year two, the established shrubs began to exert strong competitive effects on grasses, and these effects were strongest under the infrequent small rainfall regime (low total rain) where they reduced shallow soil moisture and decreased grass stomatal conductance. Results suggest that reductions in both rainfall frequency and total rainfall may be important to competitive interactions, and can alter plant community composition and invasion when species have different rooting depths and different responses to soil moisture.

The abundance of pink-pigmented facultative methylotrophs in the root zone of plant species in invaded coastal sage scrub habitat.

Pink-pigmented facultative methylotrophic bacteria (PPFMs) are associated with the roots, leaves and seeds of most terrestrial plants and utilize volatile C1 compounds such as methanol generated by growing plants during cell division. PPFMs have been well studied in agricultural systems due to their importance in crop seed germination, yield, pathogen resistance and drought stress tolerance. In contrast, little is known about the PPFM abundance and diversity in natural ecosystems, let alone their interactions with non-crop species. Here we surveyed PPFM abundance in the root zone soil of 5 native and 5 invasive plant species along ten invasion gradients in Southern California coastal sage scrub habitat. PPFMs were present in every soil sample and ranged in abundance from 102 to 105 CFU/g dry soil. This abundance varied significantly among plant species. PPFM abundance was 50% higher in the root zones of annual or biennial species (many invasives) than perennial species (all natives). Further, PPFM abundance appears to be influenced by the plant community beyond the root zone; pure stands of either native or invasive species had 50% more PPFMs than mixed species stands. In sum, PPFM abundance in the root zone of coastal sage scrub plants is influenced by both the immediate and surrounding plant communities. The results also suggest that PPFMs are a good target for future work on plant-microorganism feedbacks in natural ecosystems.

Transient host–parasitoid dynamics illuminate the practice of biological pest control

image [ The harlequin bug (Murgantia histrionica; top left) is a specialist herbivore on Isomeris arborea, an endemic plant species of the California coastal sage scrub ecosystem (top right). The bug's eggs (bottom center) are parasitized by two parasitoid natural enemies, Trissolcus murgantiae (bottom left) and Ooencyrtus johnsonii (bottom right). The harlequin bug and its parasitoids exhibit many attributes of pest‐enemy systems in general, and of Homopteran pest systems in particular. First, the bug is a specialist herbivore on a long‐lived host plant, similar to many Homopteran pests that attack long‐lived crop plants (e.g., citrus, olive, stone fruits), and its only natural enemies are the two specialist parasitoids. Second, the bug and parasitoids exhibit the same life history and population dynamics as Homopteran pest systems, with an invulnerable adult stage of the herbivore and a persistent host‐parasitoid interaction with host densities maintained at low levels. Third, the parasitoids engage in the same types of interactions as the natural enemies of Homopteran pests (exploitative competition and multiparasitism). Fourth, the bug is a minor pest of crucifers in the southeastern U.S. and the parasitoids are potential control agents. These attributes make this community an ideal model system for investigating the efficacy of natural enemies in pest suppression. ]Kidd, D. & Amarasekare, P. (2012) The role of transient dynamics in biological pest control: insights from a host–parasitoid community. Journal of Animal Ecology, 81, 47–57.Our current understanding of host–parasitoid relationships has been strongly influenced by the quest for stability and persistence, and consequently, is based on a large body of theory that has focused on the assumption of equilibrium dynamics. In an elegant analysis of a host–parasitoid model parameterized from laboratory observations, Kidd & Amarasekare broaden our understanding by contrasting the parameters that influence transient versus equilibrium dynamics. Their study highlights the importance of parasitoid handling time, competitive exclusion and intraspecific interference in the transient dynamics of a model host–parasitoid community.

Seasonal priority effects: implications for invasion and restoration in a semi‐arid system

Summary1. The timing of seasonal activity (i.e. phenology) may play an important role in plant invasions. In ecosystems characterized by seasonal rainfall, early‐active exotic species may pre‐empt resources and attain competitive dominance via a seasonal ‘priority advantage’. Exotic annual grasses in California are often active earlier than native species, potentially because they possess greater germination plasticity. While these problematic invaders may usually benefit from having early phenology, their flexible germination cues might be manipulated as a restoration strategy to germinate seeds far in advance of favourable growing conditions, leading to a ‘priority disadvantage’.2. We manipulated the start of the growing season in an invaded California coastal sage scrub community characterized by a Mediterranean‐type climate to (i) identify whether early‐season phenology confers a performance advantage and (ii) test whether rainfall timing could be manipulated to favour native species. We compared the performance of seeded native and exotic focal species under ambient rainfall timing (winter rains) vs. with a pre‐growing season (late‐summer) watering event.3. Under ambient rainfall timing, exotic annual grasses and forbs germinated earlier and reached higher levels of abundance than native species, consistent with a seasonal priority advantage. Many exotic annual grasses germinated with pre‐season watering, but none survived until the onset of natural rains. Observations suggest that early‐germinating seedlings suffered mortality via herbivory. The watering pulse thus depleted the exotic seedbank, fewer exotic individuals germinated with winter rains, and exotic species attained lower abundance than under the natural rainfall timing.4. Native species, whether annual or perennial, did not germinate with the pre‐season watering pulse, suggesting they may have more constrained germination cues than the exotic species.5. Synthesis and applications. Our results suggest that phenology is an important factor influencing invasion success, and that this could be manipulated to favour native species. Manipulation of the start of the growing season, for example through a pre‐growing season watering event, could be a successful restoration strategy for native species in some ecosystems.

Do Postfire Mulching Treatments Affect Plant Community Recovery in California Coastal Sage Scrub Lands?

In recent years, the use of postfire mulch treatments to stabilize slopes and reduce soil erosion in shrubland ecosystems has increased; however, the potential effects on plant recovery have not been examined. To evaluate the effects of mulching treatments on postfire plant recovery in southern California coastal sage scrub, we conducted a field experiment with three experimental treatments, consisting of two hydromulch products and an erosion control blanket, plus a control treatment. The area burned in 2007, and treatments were applied to six plot blocks before the 2008 growing season. Treatment effects on plant community recovery were analyzed with a mixed effects ANOVA analysis using a univariate repeated measures approach. Absolute plant cover increased from 13 to 90% by the end of the second growing season, and the mean relative cover of exotic species was 32%. The two hydromulch treatments had no effect on any plant community recovery response variable measured. For the erosion control blanket treatment, the amount of bare ground cover at the end of the second growing season was significantly lower (P=0.01), and greater shrub height was observed (P<0.01). We conclude that postfire mulch treatments did not provide either a major benefit or negative impact to coastal sage scrub recovery on the study area.

The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland

Many semi-arid shrublands in the western US have experienced invasion by a suite of exotic grasses and forbs that have altered community structure and function. The effect of the exotic grasses in this area has been studied, but little is known about how exotic forbs influence the plant community. A 3-year experiment in southern California coastal sage scrub (CSS) now dominated by exotic grasses was done to investigate the influence of both exotic grasses (mainly Bromus spp.) and exotic forbs (mainly Erodium spp.) on a restoration seeding (9 species, including grasses, forbs, and shrubs). Experimental plots were weeded to remove one, both, or neither group of exotic species and seeded at a high rate with a mix of native species. Abundance of all species varied with precipitation levels, but seeded species established best when both groups of exotic species were removed. The removal of exotic grasses resulted in an increase in exotic and native forb cover, while removal of exotic forbs led to an increase in exotic grass cover and, at least in one year, a decrease in native forb cover. In former CSS now converted to exotic annual grassland, a competitive hierarchy between exotic grasses and forbs may prevent native forbs from more fully occupying the habitat when either group of exotics is removed. This apparent competitive hierarchy may interact with yearly variation in precipitation levels to limit restoration seedings of CSS/exotic grassland communities. Therefore, management of CSS and exotic grassland in southern California and similar areas must consider control of both exotic grasses and forbs when restoration is attempted.

Seasonal priority effects: Implications for invasion and restoration in California coastal sage scrub

Seasonal priority effects: Implications for invasion and restoration in California coastal sage scrub

Seasonal priority effects: Implications for invasion and restoration in California coastal sage scrub

AbstractCompetition from exotic annual grasses (EAGs) threatens native plant communities in California. Coastal sage scrub communities have substantially diminished in area over the last century, in some instances by greater than 90%, while EAGs continue to proliferate. Several mechanisms may explain the success of EAGs, including the suppression of native seedlings by accumulated litter, exhaustion of soil moisture, and low seed production or dispersal of natives.The timing and magnitude of rainfall plays a significant role in determining the survival and reproduction of plant species in Mediterranean ecosystems. Populations of annuals fluctuate greatly from year to year, corresponding with variability in autumn precipitation. EAGs in California germinate quickly following small threshold rain events, whereas native species have more complex germination cues, such that exotic annual grasses become active earlier in the growing season. In this way, EAGs may exhibit a priority effect over native plant species with respect to establishment; however, they may have reduced success if their germination is induced at a time followed by a substantial drought (for instance, in the summer) so that they will not persist.The goal of this investigation is 1) to evaluate whether seasonal priority effects contribute to community-level patterns of abundance, and 2) whether late summer watering could be a feasible restoration technique for suppression of exotic annual grasses and weeds in Mediterranean ecosystems.