California Annual Research Articles

Rapid responses to drought in a rare California annual (San Francisco collinsia, Collinsia multicolor)

Rapid responses to drought in a rare California annual (San Francisco collinsia, Collinsia multicolor)

Compost Amendment to a Grazed California Annual Grassland Increases Gross Primary Productivity Due To a Longer Growing Season

AbstractCompost amendment to rangelands is a proposed nature‐based climate solution to increase plant productivity and soil carbon sequestration. However, it has not been evaluated using quasicontinuous ecosystem‐scale measurements. Here, we present the first study to utilize eddy covariance and footprint partitioning to monitor carbon exchange in a grassland with and without compost amendment, monitoring for 1 year before and 1 year after treatment. Compost amendment to an annual California grassland was found to enhance net ecosystem removal of carbon. Our study confirmed that compost‐amended grasslands, similar to nonamended grasslands, are net carbon sources to the atmosphere; however, the amendment appears to be slowing down the rate at which these ecosystems lose carbon by 0.71 Mg C ha−1 per growing season. Digital repeated imagery of the canopy revealed that compost‐amended grasslands experienced an earlier green‐up, resulting in an overall longer growing season by >60 days. Notably, we did not detect significantly higher amounts of soil carbon in compost‐amended soils. High variability in soil carbon demands greater sampling replication in future studies. A longer growing season and higher productivity are hypothesized to be a result of greater availability of macronutrients and micronutrients in the top layer of soil (specifically nitrogen, phosphorus, and zinc).

Managing Medusahead Using Dormant Season Grazing in the Northern Great Basin

The invasive annual grass, medusahead, infests rangelands throughout the West, from the Columbia Plateau to the California Annual Grasslands and the Great Basin. Dominating secondary succession in the sagebrush steppe, medusahead can degrade the habitat of threatened species such as the greater sage-grouse. This research explores the potential of dormant season grazing as an applied management strategy to reduce the negative impacts of medusahead while promoting recovery of perennial vegetation at the landscape scale. In particular, it assessed grazing with four treatments from 2018 to 2020: traditional grazing (May–October), dormant season grazing (October–February), traditional + dormant season grazing (May–February), and no grazing. After 2 yr of grazing treatments, biomass, density, cover, and fuel continuity did not differ between treatments (P > 0.05). However, biomass measurements were significantly different between years, which is likely due to greater than normal precipitation in 2019 and 2020. Between 2018 and 2019, annual grass biomass increased by 81% (666–1 212 kg ha−1) and perennial grass biomass increased by 165% (118–313 kg ha−1). Litter biomass decreased by approximately 15% in every year since 2018 (2 374, 2 012, and 1 678 kg ha−1 in 2018–2020). There were not significant differences in cover or density of annual and perennial grasses between treatments and years. Our results indicate that 2 yr may not be adequate time for dormant season grazing treatments to be effective in reducing the abundance of medusahead and that after 2 yr of treatments, dormant season grazing does not have a detrimental effect on perennial vegetation.

Evaluating Multimodel Ensemble Seasonal Climate Forecasts on Rangeland Plant Production in the California Annual Grassland

Seasonal precipitation and temperature directly affect total plant production in the California Annual Grassland (CAG). Technological advances have resulted in skillful seasonal climate forecasts (i.e., significant correlations between actual and forecasted climate), which could be input into plant production models to inform stocking and other rangeland management decisions. This study presents a procedure for forecasting plant production in the CAG ecosystem to predict annual plant production for grazing, restoration, or other rangeland management practices using a combination of historical gridMET climate data and seasonal hindcasts (i.e., retrospective forecasts, from the North American Multi-Model Ensemble program). The results of this study first confirmed high forecast skill, throughout the growing season at all sites. We also identified skillful plant production forecasts across most of the growing season at two sites and in three of the seven forecasting months at one study site. Forecasting climate and end-of-year plant production across the growing season at three CAG sites allowed us to identify the places and times in the growing season when forecasting might be most helpful in informing management decisions. Integrating plant production forecasting into rangeland management practices could significantly improve rangeland management outcomes. These procedures provide a user guide for creating plant production forecasts for any given area of interest and may be applicable across a wide range of other agricultural and rangeland management systems.

Modeling Weather Effects on Plant Production in the California Annual Grassland

Weather drives plant community assembly and seasonal plant production in most rangeland ecosystems. Previous models in the California Annual Grassland have indicated that plant production is largely determined by the thermal accumulation, or degree days, between the start and end of the growing season. These models, however, were relatively limited in spatial and temporal scope and did not include seasonal parameters commonly used in more current production-system models. The purpose of this study is to develop models that account for the annual and seasonal effects of weather inputs on plant production. Weather variables for these models included seasonal effects of accumulated degree days, total precipitation, average temperature, season length, past year's precipitation, and past year's plant production. Our results indicated that sorting precipitation years into low, moderate, and high groupings resulted in better production-model fit compared with a full-suite analysis of all precipitation years. In addition, we found that precipitation, temperature, past year's precipitation, and past plant production significantly improved model fit over models that considered only accumulated degree days and season length. Linking these models to seasonal forecasting applications across western rangeland ecosystems could significantly enhance management of grazing production systems in this environment.

Forage Quantity and Quality Dynamics Due to Weathering over the Dry Season on California Annual Rangelands

Livestock obtain forage by grazing on rangeland. In California annual rangelands, residual dry matter is commonly used to determine proper grazing levels. Rangeland forage biomass and quality can degrade dramatically during the dormant summer period. We examined 25 sites across an annual rainfall gradient (183–492 mm) over 3 contrasting rainfall yr (2015–2017) that varied from 57% to 152% of average annual precipitation. Overall fractional biomass loss was 54.4% (range = 46.5–61.5%) with greater fractional losses occurring in dry years. Biomass losses were related to the amount of peak standing crop and plant composition—both a function of annual precipitation. Fractional seasonal losses from the peak standing biomass in 2015 = 962 kg/ha (61.5% seasonal; 9.7% monthly), 2016 = 1 541 kg/ha (55.0% seasonal; 8.7%monthly) and 2017 = 1 923 kg/ha (46.5% seasonal; 7.3%, monthly). Forage quality metrics were strongly affected by summer weathering processes. Crude protein concentrations decreased by 33.6%, 27.7%, and 21.0% in 2015, 2016, and 2017, respectively. In contrast, relative concentrations of fiber and lignin (acid detergent fiber [ADF] = cellulose + lignin) and in the weathered biomass showed increases for ADF: 44.6% (2015), 32.2% (2016), and 24.1% (2017). Increased lignin varied: 3.4% in 2015, 23.9% in 2016, and 28.0% in 2017. While ADF and lignin concentrations (weathered biomass, kg/ha) increased during the weathering process, the standing stock decreased by 39.3% (ADF) and 46.6% (lignin), compared with overall weathered biomass loss of 54.4% and CP loss of 67.1%. The significant loss of aboveground biomass and forage quality as weathering processes occurred throughout the dry summer period affects livestock grazing strategies. Forage biomass and nutrient losses through the dry season should be considered when determining grazing strategies to achieve proper residual dry matter levels and nutrient supplementation regimes before the onset of the rainy season.

Managed Grazing on California Annual Rangelands in the Context of State Climate Policy

Approximately one third of California is grazed by livestock, with most forage produced on annual rangelands, the common term for rangelands with a significant annual herbaceous component. Given the state's interest in mitigating climate change and growing public attention on grazing systems that enhance carbon sequestration, we investigate the impact of grazing management on soil carbon cycling on California annual rangelands, drawing on soil science, rangeland management, and policy analysis literature. We conclude that using managed grazing for augmenting soil organic carbon on California annual rangelands presents significant challenges. Challenges include the heterogeneity, biogeochemical characteristics, and nonequilibrium nature of California's Mediterranean region, where ecological site conditions, soil type and texture, and climate moderate carbon sequestration. Enduring unknowns in the science underlying soil carbon and the dearth of relevant California-based studies further obscure the potential climate change mitigation effects of grazing systems. Given this, grazing management on California annual rangelands should not be prioritized as a climate change mitigation strategy, unless it is for the purposes of data collection and research. Alternative climate change mitigation opportunities on these landscapes include preventing rangeland conversion and enhancing soil carbon stocks through the suite of range management practices known to augment soil organic carbon or prevent erosion, including marginal cropland restoration, riparian restoration, organic amendments, and silvopasture. In this review, we argue that single-purpose management is generally not fitting for the diverse portfolio of social-ecological services produced on California's vast and varied rangelands. When assessing the value of grazing systems for augmenting soil organic carbon, policymakers, landowners, and other decision makers should consider the potential impacts on the numerous ecosystem services supported by the landscape. The multidisciplinary method presented in this review provides a critical framework for evaluating the appropriateness of working lands carbon policies as Natural Climate Solutions for climate change mitigation are developed in California and other geographies.

Soil Microbes Generate Stronger Fitness Differences than Stabilization among California Annual Plants.

AbstractSoil microorganisms influence a variety of processes in plant communities. Many theoretical and empirical studies have shown that dynamic feedbacks between plants and soil microbes can stabilize plant coexistence by generating negative frequency-dependent plant population dynamics. However, inferring the net effects of soil microbes on plant coexistence requires also quantifying the degree to which they provide one species an average fitness advantage, an effect that has received little empirical attention. We conducted a greenhouse study to quantify microbially mediated stabilization and fitness differences among 15 pairs of annual plants that co-occur in southern California grasslands. We found that although soil microbes frequently generate negative frequency-dependent dynamics that stabilize plant interactions, they simultaneously generate large average fitness differences between species. The net result is that if the plant species are otherwise competitively equivalent, the impact of plant-soil feedbacks is to often favor species exclusion over coexistence, a result that becomes evident only by quantifying the microbially mediated fitness difference. Our work highlights that comparing the stabilizing effects of plant-soil feedbacks to the fitness difference they generate is essential for understanding the influence of soil microbes on plant diversity.

Disentangling the effects of external perturbations on coexistence and priority effects

Abstract A major challenge in ecological research is to identify the tolerance of ecological communities to external perturbations. Modern coexistence theory (MCT) has been widely adopted as a framework to investigate the tolerance to perturbations in relative reductions of per capita growth rates, often using metrics that explicitly eliminate the independent role of intrinsic growth rates. More recently, the structural approach (SA) was introduced to investigate the tolerance of communities to perturbations in intrinsic growth rates as a function of the strength of intraspecific and interspecific competition. Because the external perturbations are likely to happen in both intrinsic growth rates and competition strengths, no framework alone can fully disentangle the effects of external perturbations. Here we combine MCT and SA to disentangle the tolerance in coexistence and priority effects of a pair of competing species when subject to perturbations in intrinsic growth rates and competition strengths. Through this combination, we reveal the emergence of a key trade‐off: increasing the tolerance to perturbations in intrinsic growth rates typically decreases the tolerance in competition strengths, and vice versa. Furthermore, this trade‐off is stronger under coexistence than under priority effects. We test this combined framework on competing pairs of 18 California annual plant species. For both coexistence and priority effects, we find that the tolerance to perturbations in intrinsic growth rates is maximized instead of that to perturbations in competition strengths in the studied annual plant communities. Synthesis. Our combined framework of modern coexistence theory and structural approach illustrates that it is possible to disentangle the impact of different external perturbations on the persistence of species. Importantly, our findings show that species interactions may reveal whether communities are dominated either by changes in intrinsic growth rates or by competition strengths. Overall, this combined framework can open a new perspective to understand and predict the response of populations to changing environmental conditions.

Vegetation mapping using hierarchical object‐based image analysis applied to aerial imagery and lidar data

AbstractAims: The primary objective of this study is to map the distribution and quantify the cover of vegetation alliances over the entirety of San Clemente Island (SCI). To this end, we develop and evaluate the mapping method of hierarchical object‐based classification with a rule‐based expert system.Location: San Clemente Island, California, USA.Methods: We developed and tested an approach based on hierarchical object‐based classification with a rule‐based expert system to effectively map vegetation communities on SCI following the Manual of California Vegetation classification system. In this mapping approach, the shrub species defining each vegetation community and non‐shrub growth forms were first mapped using aerial imagery and lidar data, then used as input in an automated mapping rule set that incorporates the percent cover rules of a field‐based mapping rule set.Results: The final vegetation map portrays the distribution of 19 vegetation communities across SCI, with the largest areas comprised of California Annual and Perennial Grassland (35%) and three types of coastal sage scrub and maritime succulent scrub, comprising a combined 53% of the area. Map accuracy was assessed to be 79% based on fuzzy methods and 61% with a traditional accuracy assessment. The accuracy of tree identification was assessed to be 81%, but species‐level tree accuracy was 45%.Conclusions: Semi‐automated approaches to vegetation community mapping can produce repeatable maps over large spatial extents that facilitate ecological management efforts. However, some low‐statured shrub community types were difficult to differentiate due to patchy canopies of co‐occurring species including abundant non‐native grasses characteristic of complex disturbance histories. Species‐level tree mapping accuracy was low due to the difficulty of identifying species within poorly illuminated canyons, resulting from sub‐optimal image acquisition timing.

Multiple resistance to glyphosate, paraquat and ACCase-inhibiting herbicides in Italian ryegrass populations from California: confirmation and mechanisms of resistance.

Glyphosate, paraquat and acetyl CoA carboxylase (ACCase)-inhibiting herbicides are widely used in California annual and perennial cropping systems. Recently, glyphosate, paraquat, and ACCase- and acetolactate synthase (ALS)-inhibitor resistance was confirmed in several Italian ryegrass populations from the Central Valley of California. This research characterized the possible mechanisms of resistance. Multiple-resistant populations (MR1, MR2) are resistant to several herbicides from at least three modes of action. Dose-response experiments revealed that the MR1 population was 45.9-, 122.7- and 20.5-fold, and the MR2 population was 24.8-, 93.9- and 4.0-fold less susceptible to glyphosate, sethoxydim and paraquat, respectively, than the susceptible (Sus) population. Accumulation of shikimate in Sus plants was significantly greater than in MR plants 32 h after light pretreatments. Glyphosate resistance in MR plants was at least partially due to Pro106-to-Ala and Pro106-to-Thr substitutions at site 106 of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS). EPSPS gene copy number and expression level were similar in plants from the Sus and MR populations. An Ile1781-to-Leu substitution in ACCase gene of MR plants conferred a high level of resistance to sethoxydim and cross-resistance to other ACCase-inhibitors. Radiolabeled herbicide studies and phosphorimaging indicated that MR plants had restricted translocation of 14 C-paraquat to untreated leaves compared to Sus plants. This study shows that multiple herbicide resistance in Italian ryegrass populations in California, USA, is due to both target-site and non-target-site resistance mechanisms. © 2017 Society of Chemical Industry.

Longevity of a Controlled Burn’s Impacts on Species Composition and Biomass in Northern California Annual Rangeland During Drought

Controlled burning timed in early summer can dramatically change the species composition of annual rangeland the following season. Although this has been well documented, the longevity of these shifts has not. Presented is a case study of a single 200-ha burn to begin to understand how long plant communities and biomass production remain diverged between burned and unburned annual rangeland. Species composition and biomass production were monitored before and for 3 yr after burning. Burning drastically reduced medusahead (Taeniatherum caput-medusae; P < 0.01) the following year from 69% in the control to 4% cover in the area burned. In the same year, filaree (Erodium spp.; P < 0.01) filled in the area left vacant, subsequently lessening production (P < 0.01) in the burn area by over half that of the control. No difference existed in the occurrence of native wildflower species due to fire. Three consecutive drought years following the burn shifted the control from medusahead dominance to filaree in a linear fashion. At the same time, in the burned area medusahead cover increased fourfold between 1 and 3 yr after the burn. By 3 yr post burning, the area had 4% more medusahead cover than the control and was equal in filaree, rose clover (Trifolium hirtum), and soft brome (Bromus hordeaceus) cover. Our results suggest that a controlled burn followed by drought can cause the divergence in species composition and production to become void in as little as 3 yr after a well-timed burn in a low-elevation annual rangeland system.

Cattle as Dispersal Vectors of Invasive and Introduced Plants in a California Annual Grassland

Plant invasions are a threat to rangelands in California. Understanding how seeds of invasive plants are dispersed is critical to developing sound management plans. Domestic livestock can transport seeds long distances by ingesting and passing seeds in dung (endozoochory) or by the attachment of seeds to skin and fur (epizoochory). Our objective was to characterize the role of cattle as seed dispersers of both invasive and noninvasive species via endozoochory and epizoochory in a Sierra foothills rangeland. To quantify endozoochory, we sampled dung from two dry-season grazing periods and evaluated seed content by growing dung for 3 months in a greenhouse. To quantify epizoochory, we collected seeds directly from the fur of 40 cattle. We categorized the invasion status and functional groups of all species found and quantified landscape-scale vegetation composition in order to determine whether dispersal mode was associated with functional group, invasion status, or vegetation composition. Finally, we evaluated the potential for the noxious weed medusahead (Taeniatherum caput-medusae [L.] Nevski) to travel long distances on cattle fur using a detachment experiment with a model cow. We found that forbs were more likely to be dispersed by endozoochory, and invasive species were more likely to be dispersed by epizoochory. Medusahead was dispersed exclusively by epizoochory and was able to travel up to 160 m on a model cow. Our results suggest that cattle may be an important dispersal vector for both invasive and noninvasive plants.

Effects of Aminopyralid on California Annual Grassland Plant Communities

AbstractAminopyralid is the most commonly used herbicide for the control of yellow starthistle and other invasive thistles in annual grasslands of California. Although the effects of aminopyralid on native plant communities over a 2-yr period have been evaluated in prairies dominated by perennial species in the northern central states, similar evaluations have not been conducted in grassland communities of California, which are generally composed of a high diversity of native and nonnative annual species. In this study we monitored the effects of 53 and 123 g ae aminopyralid ha−1on individual species cover and species richness over three growing seasons in two locations on California annual grassland. Treated plots were compared to untreated plots in randomized complete-block designs. Results were largely consistent between the two trials. In the first season after treatment, both rates of aminopyralid reduced dicot cover significantly, particularly members of the Asteraceae and Fabaceae. Treated plots also showed reduced species richness. However, these differences were less pronounced in the second season after treatment, particularly at the low rate. By the third season after treatment in both sites, there were no longer any significant effects on cover or species richness at the low herbicide rate. On California annual grasslands, winter applications of low rates of aminopyralid have been shown to give excellent control of yellow starthistle, providing long-term benefits to grassland ecosystems. Results of the current study suggest that negative impacts of aminopyralid on the desirable native forb community are transitory.

Medusahead Ecology and Management: California Annual Grasslands to the Intermountain West

AbstractThe spread of medusahead across the western United States has severe implications for a wide range of ecosystem services. Medusahead invasion reduces biodiversity, wildlife habitat and forage production, and often leads to increased fire frequency and restoration costs. Medusahead is problematic in the Intermountain West and California Annual Grasslands. The last review of medusahead ecology and management was completed 20 years ago. Since the last review, there have been scientific advances in medusahead management suggesting a significant need to develop an up-to-date synthesis. Medusahead continues to pose a serious threat to rangeland ecosystems. In this synthesis, we present new information regarding the ecology of medusahead, suggest a framework for managing medusahead based on invasion level, and identify research needs to further improve management of this invasive annual grass. Success of different management practices varies between the Intermountain West and California Annual Grasslands, signifying that the best management practices are those specifically tailored with consideration of climate, soil, plant community characteristics, and management objectives. Prevention and control treatments that are useful in the Intermountain West may not be practical or effective in the California Annual Grasslands and vice-versa.

Phylogenetic relatedness and the determinants of competitive outcomes.

Recent hypotheses argue that phylogenetic relatedness should predict both the niche differences that stabilise coexistence and the average fitness differences that drive competitive dominance. These still largely untested predictions complicate Darwin's hypothesis that more closely related species less easily coexist, and challenge the use of community phylogenetic patterns to infer competition. We field parameterised models of competitor dynamics with pairs of 18 California annual plant species, and then related species' niche and fitness differences to their phylogenetic distance. Stabilising niche differences were unrelated to phylogenetic distance, while species' average fitness showed phylogenetic structure. This meant that more distant relatives had greater competitive asymmetry, which should favour the coexistence of close relatives. Nonetheless, coexistence proved unrelated to phylogeny, due in part to increasing variance in fitness differences with phylogenetic distance, a previously overlooked property of such relationships. Together, these findings question the expectation that distant relatives should more readily coexist.

Effects of Brassica nigra and plant–fungi interactions on the arthropod community of Deinandra fasciculata

While invasive plants are widely studied for their effects on native plants, we questioned how plant invasions affect higher trophic levels. We investigated the effects of the invasive plant, Brassicanigra on the multi-trophic arthropod community residing on the native California annual Deinandra fasciculata. In a common garden experiment, we planted D. fasciculata without B. nigra or with one of 45 B. nigra half-sib genetic families. We in turn crossed this B. nigra treatment with the suppression of soil fungi to both test for effects of arbuscular mycorrhizal fungi (AMF) on arthropods, and elucidate the mechanisms of B. nigra’s below ground effects. B. nigra had no effect on D. fasciculata traits (biomass, inflorescence number, root colonization by AMF), arthropod community composition or predator density, but increased herbivore density. While B. nigra families varied 18-fold in size, there was no genetic variation for effects on D. fasciculata or its arthropods. Soil fungi suppression had no effect on D. fasciculata traits, herbivore density or herbivore community composition, but increased predator density and altered predator community composition. While the exact mechanisms of B. nigra effects are unclear, they do not appear to have been mediated by altered plant performance or reductions in root colonization by AMF. Our experiment shows that invasive plants such as B. nigra may affect higher trophic levels even when they do not measurably affect native plant performance.

Weather Variability and Adaptive Management for Rangeland Restoration

Seasonal precipitation and temperature directly affect total plant production in the California Annual Grassland (CAG). Technological advances have resulted in skillful seasonal climate forecasts (i.e., significant correlations between actual and forecasted climate), which could be input into plant production models to inform stocking and other rangeland management decisions. This study presents a procedure for forecasting plant production in the CAG ecosystem to predict annual plant production for grazing, restoration, or other rangeland management practices using a combination of historical gridMET climate data and seasonal hindcasts (i.e., retrospective forecasts, from the North American Multi-Model Ensemble program). The results of this study first confirmed high forecast skill, throughout the growing season at all sites. We also identified skillful plant production forecasts across most of the growing season at two sites and in three of the seven forecasting months at one study site. Forecasting climate and end-of-year plant production across the growing season at three CAG sites allowed us to identify the places and times in the growing season when forecasting might be most helpful in informing management decisions. Integrating plant production forecasting into rangeland management practices could significantly improve rangeland management outcomes. These procedures provide a user guide for creating plant production forecasts for any given area of interest and may be applicable across a wide range of other agricultural and rangeland management systems.

Preemergent Control of Medusahead on California Annual Rangelands With Aminopyralid

Medusahead (Taeniatherum caput-medusae (L.) Nevski), the most problematic invasive grass on many California rangelands, is difficult to control selectively in grasslands. Prescribed burning, grazing, and herbicides have been tested with some success but are not practical in all situations. The selective herbicide aminopyralid, normally used for control of certain broadleaf species such as thistles, suppresses some annual grasses when applied pre- or early postemergence. In 2009-2010, we tested the efficacy of aminopyralid for medusahead control in preemergence applications at three foothill rangeland sites in northern California. We compared a rate series of aminopyralid (53, 88, 123, and 245 gha � 1 acid equivalent (ae)) with rimsulfuron (18 and 35 gha � 1 active ingredient) and imazapic (140 gha � 1 ae). Plots were 339 m with four replications at each site. Treatments were applied in early fall 2009. In May 2010, we took visual cover estimates and biomass/seedhead samples in three quadrats per plot. In regression analysis, medusahead cover was found to decrease consistently with increasing rates of aminopyralid. Medusahead control at the highest rates of aminopyralid was consistent across the three sites, averaging 89% 6 3 standard deviation (SD) with 245 gha � 1 ae and 59% 6 10 SD with 123 gha � 1 ae. Aminopyralid at lower rates, rimsulfuron, and imazapic were less consistent. Cover of other annual grasses increased in plots treated with aminopyralid at all sites. Aminopyralid has potential utility for suppressing medusahead, particularly in sites also infested with invasive members of the Asteraceae. However, the most effective rate (245 gha � 1 ae) is registered for use only as a spot application. In situations where this rate can be justifiably used, it would be expected to give season-long control of medusahead, as well as longer-term control of thistles and other susceptible species.

Environmental and dispersal controls of an annual plant’s distribution: how similar are patterns and apparent processes at two spatial scales?

At scales from microsites to entire ranges, species’ distributions reflect limited adaptation and/or limited dispersal. To what extent are specific distribution patterns and processes similar across scales? We investigated environmental effects—presumed because of adaptation—and independent spatial effects—presumed because of dispersal—on distribution at two scales (landscape patches of approximately 1,300 m2, sampled along transects, and 4-m2 cells, sampled in contiguous grids within populations) and on individual performance (water status, reproduction) in the California annual, Clarkia xantiana ssp. xantiana. Because water limitation helps set this species’ regional borders, we expected occupancy and performance at smaller scales to correlate with topographic and soil features affecting water relations. At the patch scale, environmental features associated with reduced water stress (i.e., steep slopes that face north; coarse, soft soils; igneous rather than metasedimentary parent rock) predicted occupancy. Spatial aggregation was not detected, but incomplete occupancy of apparently suitable patches indicated that dispersal limits occupancy. At the scale of small cells, relationships between environmental variables, occupancy, density, and performance varied among populations. Associations sometimes resembled those at the patch scale but sometimes opposed them. Spatial aggregation in cell occupancy and/or density occurred in all populations, implying limited dispersal, whereas spatial aggregation of water potential values in some populations might have arisen from spatially structured unmeasured environmental variables. Limited adaptation to drought and limited patch colonization appear to affect patch occupancy in C. xantiana ssp. xantiana, whereas smaller-scale patterns indicate consistent effects of limited dispersal and somewhat variable environmental effects.