Chaparral Shrub Species Research Articles

Drivers of chaparral photosynthetic rate reduction under modern drought conditions

Terrestrial vegetation communities are experiencing rapid and novel changes to photosynthetic rates under the changing climate. Chaparral, a semi-arid shrubland ecosystem of the Southwestern United States and Northern Mexico, is projected to experience substantial increases in aridity and stochastic precipitation. This study identifies the primary meteorological drivers of photosynthesis for three widespread chaparral shrub species—Adenostoma sparsifolium, Adenostoma fasciculatum, and Ceanothus perplexans—from 2019 to 2021. Monthly leaf-level carbon exchange rates, water potentials (WPs), and meteorological conditions were collected for each species. Average monthly primary productivity (n = 25) demonstrated vapor pressure deficit (VPD) as a significant limit to photosynthetic rates for A. sparsifolium and A. fasciculatum. VPD was also the most influential predictor of WP for all three species. These results suggest increasing atmospheric dryness as a key predictor for reduction in chaparral primary productivity, particularly for deeply-rooted, resprouting species. There are additional indications that VPD could exacerbate drought-related mortality for C. perplexans and A. sparsifolium by pushing WP to novel extremes. This study concludes that atmospheric dryness, across 3 years of differing soil water stress levels, was consistently a substantial physiological limitation for three common, chaparral species. Although this experiment occurred over a limited window and cannot assess climatic response trends, acute increases in air temperature and VPD within the region would exacerbate photosynthetic limitation for these species and may contribute to declining primary productivity in broader chaparral ecosystems.

Xylem structure and hydraulic function in roots and stems of chaparral shrub species from high and low elevation in the Sierra Nevada, California.

Xylem structure and hydraulics were compared between individuals at lower and upper elevation distribution limits for five chaparral shrub species along a steep transect in the southern Sierra Nevada, California, USA. Higher-elevation plants experienced frequent winter freeze-thaw events and increased precipitation. We hypothesized that environmental differences would lead to xylem trait differences between high and low elevations, but predictions were complicated because both water stress (low elevation) and freeze-thaw events (high elevation) may select for similar traits, such as narrow vessel diameter. We found significant changes in the ratio of stem xylem area to leaf area (Huber value) between elevations, with more xylem area required to support leaves at low elevations. Co-occurring species significantly differed in their xylem traits, suggesting diverse strategies to cope with the highly seasonal environment of this Mediterranean-type climate region. Roots were more hydraulically efficient and more vulnerable to embolism relative to stems, potentially due to roots being buffered from freeze-thaw stress, which allows them to maintain wider diameter vessels. Knowledge of the structure and function of both roots and stems is likely important in understanding whole-plant response to environmental gradients.

Spatial and temporal strategies of resprouting and seeding in a chaparral shrub species.

There is no conflict of interest. Seedling and resprout data (Keeley, 2023) are available in the USGS ScienceBase Catalog at https://doi.org/10.5066/P9713QT4. Fire history data were from annual reports known as Redbooks obtained from CAL FIRE downloaded from https://www.fire.ca.gov/stats-events/.

Starch storage capacity of sapwood is related to dehydration avoidance during drought.

The xylem tissue of plants performs three principal functions: transport of water, support of the plant body, and nutrient storage. Tradeoffs may arise because different structural requirements are associated with different functions or because suites of traits are under selection that relate to resource acquisition, use, and turnover. The structural and functional basis of xylem storage is not well established. We hypothesized that greater starch storage would be associated with greater sapwood parenchyma and reduced fibers, which would compromise resistance to xylem tensions during dehydration. We measured cavitation resistance, minimum water potential, starch content, and sapwood parenchyma and fiber area in 30 species of southern California chaparral shrubs (evergreen and deciduous). We found that species storing greater starch within their xylem tended to avoid dehydration and were less cavitation resistant, and this was supported by phylogenetic independent contrasts. Greater sapwood starch was associated with greater parenchyma area and reduced fiber area. For species without living fibers, the associations with parenchyma were stronger, suggesting that living fibers may expand starch storage capacity while also contributing to the support function of the vascular tissue. Drought-deciduous species were associated with greater dehydration avoidance than evergreens. Evolutionary forces have led to an association between starch storage and dehydration resistance as part of an adaptive suite of traits. We found evidence for a tradeoff between tissue mechanical traits and starch storage; moreover, the evolution of novel strategies, such as starch-storing living fibers, may mitigate the strength of this tradeoff.

Subspecies differentiation in an enigmatic chaparral shrub species.

Delimiting biodiversity units is difficult in organisms in which differentiation is obscured by hybridization, plasticity, and other factors that blur phenotypic boundaries. Such work is more complicated when the focal units are subspecies, the definition of which has not been broadly explored in the era of modern genetic methods. Eastwood manzanita (Arctostaphylos glandulosa Eastw.) is a widely distributed and morphologically complex chaparral shrub species with much subspecific variation, which has proven challenging to categorize. Currently 10 subspecies are recognized, however, many of them are not geographically segregated, and morphological intermediates are common. Subspecies delimitation is of particular importance in this species because two of the subspecies are rare. The goal of this study was to apply an evolutionary definition of "subspecies" to characterize structure within Eastwood manzanita. We used publicly available geospatial environmental data and reduced-representation genome sequencing to characterize environmental and genetic differentiation among subspecies. In addition, we tested whether subspecies could be differentiated by environmentally associated genetic variation. Our analyses do not show genetic differentiation among subspecies of Eastwood manzanita, with the exception of one of the two rare subspecies. In addition, our environmental analyses did not show ecological differentiation, though limitations of the analysis prevent strong conclusions. Genetic structure within Eastwood manzanita does not correspond to current subspecies circumscriptions, but rather reflects geographic distribution. Our study suggests that subspecies concepts need to be reconsidered in long-lived plant species, especially in the age of next-generation sequencing.

Seasonal patterns of growth phenology and nutrient dynamics in four matorral shrubs in Central Chile

Chile is one of five global regions exhibiting a mediterranean-type climate regime characterized by evergreen sclerophyll shrublands. These matorral shrublands which dominate the foothills and slopes of the Coastal Mountains and foothills of the Andes in central Chile have received much less study than evergreen shrublands in other mediterranean-type climate regions of the world. Phenological development, growth, and nutrient dynamics of the four widespread matorral shrub species, Lithrea caustica (Anacardiaceae), Colliguaja odorifera (Euphorbiaceae), Kageneckia oblonga (Rosaceae), and Retanilla trinervia (Rhamnaceae), were monitored in central Chile from 1971 to 1975. The four study species all demonstrated growth dynamics and nutrient relations similar to chaparral shrub species of southern California. The species exhibited a sequential development of phenological stages in leaf components following fall precipitation. Colliguaja with relatively shallow root systems showed a sharp peak of new leaf production at the beginning of summer, dropping quickly as summer drought occurred. Another shallow- rooted species while Retanilla initiated leaf production in the late winter, earlier than the other two species, and presented a growth pattern dependent on soil moisture availability. Lithrea and Kageneckia, a more deep-rooted species, initiated leaf production in late spring and continued through the summer. Overall, Chilean shrubs exhibited higher nutrient contents than those of chaparral shrubs in California, suggesting a difference in soil nutrient availability. Reproductive tissues represent a significant portion of the new biomass and nutrient allocation in all four matorral shrubs. Biomass allocation of new tissues (i.e. new leaves, new stems and reproductive tissues) ranged from 8.2% in Retanilla to 16.7% in Colliguaja. Overall, the phenology and nutrient relations of matorral shrubs were similar to those found in chaparral, shrubs indicating that the similar climate regimes can lead to convergence in leaf traits, phenology, and nutrient relationships.

Cascading effects of soil type on assemblage size and structure in a diverse herbivore community.

Soil type is understudied as a driver of herbivore community size and structure across host plants. This study extends predictions of resource availability hypotheses to understand how soil types of different resource levels alter plant resistance and structure of herbivore assemblages. In this 2-yr study we use seven dominant chaparral shrub species that grow across a natural mosaic of low and high resource soils to explore effects of soil type on plant resistance, and relate these soil-based differences in resistance to the abundance and diversity of the larval lepidopteran community. We show that growing on low-resource soils increases plant resistance, as measured by herbivore performance, both within and across host plant species, and that resistance may be driven by variation in plant nutritive and defensive traits. We then show that more resistant plants on low-resource soils host less abundant and less diverse herbivore assemblages across a natural soil mosaic in the field.

Bumble bee use of post‐fire chaparral in the central Sierra Nevada

ABSTRACTBumble bees (Bombus spp.) are declining across many regions in the Northern Hemisphere, leading to a need for management actions that will protect and enhance their habitats. In the Sierra Nevada of California, USA, montane chaparral is prevalent across the landscape, particularly after forest fires, and may provide important floral resources for pollinators. However post‐fire montane chaparral is often targeted for removal during reforestation efforts, to reduce competition with young trees. In 2015 and 2016, we conducted non‐lethal bumble bee surveys within 2 areas in the Sierra Nevada that burned in forest fires in 2004. Our goals were to describe bumble bee abundance and species richness in a post‐fire landscape, to compare results from chaparral‐dominated upland vegetation with results from interspersed patches of riparian vegetation, and to identify characteristics of individual chaparral stands that might make some stands more valuable to bumble bees than others. We captured 2,494 bumble bees of 12 species, and used Bayesian hierarchical modeling to determine that bumble bee abundance was substantially greater in riparian plots (modeled capture rate = 1.10 ± 0.31 [SD] bees/survey in 2015, and 2.96 ± 0.83 bees/survey in 2016) than in upland plots ( = 0.47 ± 0.07 bees/survey in 2015, and 1.27 ± 0.18 bees/survey in 2016), which comprised a mix of chaparral shrubs and associated herbaceous plants. Modeled species richness was also greater in riparian plots, with an average mean richness of 4.1 ± 1.8 bumble bee species in riparian plots versus 2.3 ± 1.3 species in upland plots across the 2 years of the study. Within upland and riparian areas, plots dominated by herbaceous vegetation had greater abundance and species richness. One chaparral shrub species, bearclover (Chamaebatia foliolosa), was foraged on preferentially over all other shrub species and over all but 1 forb taxon, and was associated with increased occupancy probability in the Vosnesensky bumble bee (Bombus vosnesenskii), the most abundant bumble bee species on our study plots. A complex of closely related herbaceous species in the genus Phacelia, commonly associated with upland chaparral in our study area, was the plant taxon most frequently used by bumble bees, and appeared to be particularly important during mid‐summer after bearclover flowers became scarce. Our findings suggest that post‐fire chaparral communities are generally less intensively used by bumble bees than nearby riparian vegetation but may nevertheless provide important habitat. When chaparral removal is part of post‐fire forest regeneration strategies, bumble bees will likely benefit from retention of a mosaic of upland habitat patches dominated by herbaceous vegetation and, in our study area, bearclover, which may provide foraging resources throughout the life cycle of local bumble bee colonies. Because habitat characteristics affected the occupancy of individual bumble bee species differently, managers should consider foraging preferences of target bumble bee species when making land management decisions. © 2017 The Wildlife Society.

An individual-based model of chaparral vegetation response to frequent wildfires

The Santa Monica Mountains are home to many species of chaparral shrubs that provide vegetative cover and whose deep roots contribute to the stability of the steep slopes. Recently, native chaparral have been threatened by an unprecedented drought and frequent wildfires in Southern California. Besides the damage from the wildfires themselves, there is the potential for subsequent structural losses due to erosion and landslides. In this paper, we develop a mathematical model that predicts the impact of drought and frequent wildfires on chaparral plant community structure. We begin by classifying chaparral into two life history types based on their response to wildfires. Nonsprouters are completely killed by a fire, but their seeds germinate in response to fire cues. Facultative sprouters survive by resprouting but also rely on seed germination for post-fire recovery. The individual-based model presented here simulates the growth, seed dispersal, and resprouting behavior of individual shrubs across two life history types as they compete for space and resources in a rectangular domain. The model also incorporates varying annual rainfall and fire frequency as well as the competition between plants for scarce resources. The parameters were fit using seedling and resprout survivorship data as well as point quarter sampling data from 1986 to 2014 at a biological preserve within the natural landscape of the Malibu campus of Pepperdine University. The simulations from our model reproduce the change in plant community structure at our study site which includes the local extinction of the nonsprouter Ceanothus megacarpus due to shortened fire return intervals. Our simulations predict that a combination of extreme drought and frequent wildfires will drastically reduce the overall density of chaparral, increasing the likelihood of invasion by highly flammable exotic grasses. The simulations further predict that the majority of surviving shrubs will be facultative sprouting species such as Malosma laurina.

Structural determinants of increased susceptibility to dehydration-induced cavitation in post-fire resprouting chaparral shrubs.

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem-specific hydraulic conductivity (Ks ) and resistance to cavitation (P50 ) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co-occurring post-fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in Ks . Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter-vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants.

Geographic And Seasonal Variation In Chaparral Vulnerability To Cavitation

Abstract Resistance of stem xylem to water stress-induced cavitation and embolism among chaparral shrub species in California has been extensively studied, providing the opportunity to examine broad patterns in cavitation resistance. We used previously published as well as unpublished vulnerability to cavitation curve data from 16 chaparral shrub species of southern California to examine the variability of cavitation resistance across sites, regions, and seasons. Additionally, these data provided a unique opportunity to address a recent methodological debate within the field of plant hydraulics. We found that different methods, specifically a centrifuge method and a dehydration method, produced similar results (P = 0.184). Vulnerability to cavitation varied seasonally, with species exhibiting greater susceptibility to water-stress induced cavitation during the wet season (P = 0.003). Cavitation resistance did not differ among sites that were less than 10 km apart even though these sites differed in th...

A Population Model of Chaparral Vegetation Response to Frequent Wildfires

The recent increase in wildfire frequency in the Santa Monica Mountains (SMM) may substantially impact plant community structure. Species of Chaparral shrubs represent the dominant vegetation type in the SMM. These species can be divided into three life history types according to their response to wildfires. Nonsprouting species are completely killed by fire and reproduce by seeds that germinate in response to a fire cue, obligate sprouting species survive by resprouting from dormant buds in a root crown because their seeds are destroyed by fire, and facultative sprouting species recover after fire both by seeds and resprouts. Based on these assumptions, we developed a set of nonlinear difference equations to model each life history type. These models can be used to predict species survivorship under varying fire return intervals. For example, frequent fires can lead to localized extinction of nonsprouting species such as Ceanothus megacarpus while several facultative sprouting species such as Ceanothus spinosus and Malosma (Rhus) laurina will persist as documented by a longitudinal study in a biological preserve in the SMM. We estimated appropriate parameter values for several chaparral species using 25years of data and explored parameter relationships that lead to equilibrium populations. We conclude by looking at the survival strategies of these three species of chaparral shrubs under varying fire return intervals and predict changes in plant community structure under fire intervals of short return. In particular, our model predicts that an average fire return interval of greater than 12 years is required for 50% of the initial Ceanothus megacarpus population and 25% of the initial Ceanothus spinosus population to survive. In contrast, we predict that the Malosma laurina population will have 90% survivorship for an average fire return interval of at least 6 years.

Allocation tradeoffs among chaparral shrub seedlings with different life history types (Rhamnaceae)

California chaparral shrub species have different life history types: Nonsprouters (NS) are killed by fire and persist through a fire-stimulated seed bank; facultative sprouters (FS) reestablish by a combination of vegetative sprouting and seeding; and obligate sprouters (OS) reestablish exclusively by sprouting. Nonsprouters and FS establish seedlings in open-canopy postfire environments, whereas OS establish seedlings between fires in the shady understory. We hypothesized that allocation differences among seedlings of postfire sprouters and nonsprouters and regeneration niche differences would lead to contrasting patterns in biomass accumulation (NS > FS > OS, in sun; OS > FS > NS, in shade). Seedlings of three species from each life history type were grown in sun and 75% shade. We measured net carbon assimilation and biomass accumulation after one year. Biomass accumulation was similar in the sun except FS>OS. In the shade, NS had lower biomass than FS and OS. Assimilation rates, nitrogen relations, and allocation differences could not fully explain biomass accumulation differences. Instead, biomass accumulation was inversely related to water-stress tolerance and shade tolerance. Additionally, OS and FS differed in root/shoot allocation even though both are sprouters. Seedling growth and carbon assimilation rates were divergent among three life history types and were consistent with differences in tolerance to water stress and shade or sun regeneration niches, but not tradeoffs in sprouting-related allocation differences per se.

Xylem root and shoot hydraulics is linked to life history type in chaparral seedlings

Summary 1. Shrubs in fire prone chaparral communities have evolved different life history types in response to fire. A key to understanding the evolution of life history type differences is to understand how physiological traits are linked to differences in life history type. Vascular adaptations are important for delivering an efficient and stable water supply to evergreen chaparral shrub leaves. This study tested for a link between vascular physiology and life history type in chaparral shrubs. 2. Chaparral shrub species along the south‐western coast of North America survive wildfire by three different life histories. Non‐sprouters are killed by fire and re‐establish exclusively through germination of fire‐stimulated seeds, facultative sprouters re‐establish by a combination of vegetative sprouting and fire‐stimulated seeds, and obligate sprouters re‐establish exclusively by vegetative sprouting because their seeds do not survive fire. Non‐sprouters and facultative sprouters establish seedlings in the open canopy post fire environment, whereas obligate sprouters establish seedlings in the shady understory of the mature chaparral canopy. 3. Seedlings of nine species (Rhamnaceae) representing three each of the different life history types were grown in deep containers in a common garden under treatments of sun and shade. Hydraulic conductance was measured using a high‐pressure flow meter for all organs, and a vacuum technique was used to measure conductance of fine and woody roots. We predicted that non‐sprouters would exhibit greater hydraulic efficiency than the sprouting species, and that facultative sprouters would be more efficient than the shade tolerant obligate sprouters. 4. Non‐sprouters had the greatest hydraulic conductance per unit leaf and sapwood area at the whole seedling level, whereas facultative and obligate sprouters were not different. Comparing hydraulic conductance across major organs (from fine roots to leaves) showed that the hydraulic system was well coordinated. At the whole seedling level, the root system was more of a bottleneck than the shoot system. This pattern was consistent with high resistance extraxylary pathways in roots and differences in root architecture. 5. The greater hydraulic efficiency of the non‐sprouter life history type is attributed to its post‐fire pioneering habit and may partially explain the relatively high speciation in the non‐sprouters. Lower hydraulic efficiency is associated with a sprouting life history and greater shade tolerance. The seedling root systems represent a hydraulic bottleneck that may place roots under especially intense selection.

Linkage between water stress tolerance and life history type in seedlings of nine chaparral species (Rhamnaceae)

Summary 1. Chaparral shrub species along the southwest coast of North America survive wildfire by three different life history types: non-sprouters ( NS ) are killed by fire and only re-establish through germination of fire-stimulated seeds; facultative sprouters ( FS ) re-establish by a combination of vegetative sprouting and seed germination; and obligate sprouters ( OS ) only re-establish by vegetative sprouting because their seeds do not survive fire. 2. Post-fire seedlings of NS and FS re-establish in open canopy gaps, whereas OS seedlings primarily recruit in deep shade during fire-free intervals. 3. It was hypothesized that these life history differences, compared within the same family, Rhamnaceae, would result in differential seedling resistance to water stress such that NS > FS > OS . 4. Seedlings were grown in large pots in a common garden under treatments of sun, shade, irrigation and water stress. Mortality and physiology of seedlings were monitored for about 1 year. 5. In response to the water stress treatment in full sun, life history types were not different in their dry season predawn water potentials, which were extremely negative (< ‐9 MPa). Water potential of water-stressed shade grown plants were also not different, but were less negative than the sun treatment during the dry season (about ‐6 MPa). 6. For plants grown in full sun under water stress, NS exhibited 23% mortality, FS 60% and OS 35% mortality, while irrigated controls had greatly reduced mortality across all life histories. Among shade grown plants under water stress, NS exhibited 30% mortality, FS 20% and OS 1% mortality. This pattern of differential mortality was the same for irrigated plants indicating that water stress was not the cause of mortality in the shade. 7. Greater stem and root resistance to water stress induced cavitation were strongly correlated with lower percent mortality during the summer dry season suggesting a link between these traits and fitness. 8. Synthesis . This study finds a link between life history type and water stress tolerance. Consistent with open canopy specialization, NS survived best in full sun. The FS and OS life history type had better survival in the shade. The OS was unexpectedly versatile, surviving well in the sun and shade. The evolutionary history of the life history types is discussed as an important factor in linking water stress tolerance and life history.

Do xylem fibers affect vessel cavitation resistance?

Possible mechanical and hydraulic costs to increased cavitation resistance were examined among six co-occurring species of chaparral shrubs in southern California. We measured cavitation resistance (xylem pressure at 50% loss of hydraulic conductivity), seasonal low pressure potential (P(min)), xylem conductive efficiency (specific conductivity), mechanical strength of stems (modulus of elasticity and modulus of rupture), and xylem density. At the cellular level, we measured vessel and fiber wall thickness and lumen diameter, transverse fiber wall and total lumen area, and estimated vessel implosion resistance using (t/b)(h)(2), where t is the thickness of adjoining vessel walls and b is the vessel lumen diameter. Increased cavitation resistance was correlated with increased mechanical strength (r(2) = 0.74 and 0.76 for modulus of elasticity and modulus of rupture, respectively), xylem density (r(2) = 0.88), and P(min) (r(2) = 0.96). In contrast, cavitation resistance and P(min) were not correlated with decreased specific conductivity, suggesting no tradeoff between these traits. At the cellular level, increased cavitation resistance was correlated with increased (t/b)(h)(2) (r(2) = 0.95), increased transverse fiber wall area (r(2) = 0.89), and decreased fiber lumen area (r(2) = 0.76). To our knowledge, the correlation between cavitation resistance and fiber wall area has not been shown previously and suggests a mechanical role for fibers in cavitation resistance. Fiber efficacy in prevention of vessel implosion, defined as inward bending or collapse of vessels, is discussed.

Leaf size, specific leaf area and microhabitat distribution of chaparral woody plants: contrasting patterns in species level and community level analyses.

We examined variation in leaf size and specific leaf area (SLA) in relation to the distribution of 22 chaparral shrub species on small-scale gradients of aspect and elevation. Potential incident solar radiation (insolation) was estimated from a geographic information system to quantify microclimate affinities of these species across north- and south-facing slopes. At the community level, leaf size and SLA both declined with increasing insolation, based on average trait values for the species found in plots along the gradient. However, leaf size and SLA were not significantly correlated across species, suggesting that these two traits are decoupled and associated with different aspects of performance along this environmental gradient. For individual species, SLA was negatively correlated with species distributions along the insolation gradient, and was significantly lower in evergreen versus deciduous species. Leaf size exhibited a negative but non-significant trend in relation to insolation distribution of individual species. At the community level, variance in leaf size increased with increasing insolation. For individual species, there was a greater range of leaf size on south-facing slopes, while there was an absence of small-leaved species on north-facing slopes. These results demonstrate that analyses of plant functional traits along environmental gradients based on community level averages may obscure important aspects of trait variation and distribution among the constituent species.

Landscape-scale patterns of shrub-species abundance in California chaparral – The role of topographically mediated resource gradients

We examine the degree to which landscape-scale spatial patterns of shrub-species abundance in California chaparral reflect topographically mediated environmental conditions, and evaluate whether these patterns correspond to known ecophysiological plant processes. Regression tree models are developed to predict spatial patterns in the abundance of 12 chaparral shrub and tree species in three watersheds of the Santa Ynez Mountains, California. The species response models are driven by five variables: average annual soil moisture, seasonal variability in soil moisture, average annual photosynthetically active radiation, maximum air temperature over the dry season (May– October), and substrate rockiness. The energy and moisture variables are derived by integrating high resolution (10 m) digital terrain data and daily climate observations with a process-based hydro-ecological model (RHESSys). Field-sampled data on species abundance are spatially integrated with the distributed environmental variables for developing and evaluating the species response models. The species considered are differentially distributed along topographically-mediated environmental gradients in ways that are consistent with known ecophysiological processes. Spatial patterns in shrub abundance are most strongly associated with annual soil moisture and solar radiation. Substrate rockiness is also closely associated with the establishment of certain species, such as Adenostoma fasciculatum and Arctostaphylos glauca. In general, species that depend on fire for seedling recruitment (e.g., Ceanothous megacarpus) occur at high abundance in xeric environments, whereas species that do not depend on fire (e.g., Heteromeles arbutifolia) occur at higher abundance in mesic environments. Model performance varies between species and is related to life history strategies for regeneration. The scale of our analysis may be less effective at capturing the processes that underlie the establishment of species that do not depend on fire for recruitment. Analysis of predication errors in relation to environmental conditions and the abundance of potentially competing species suggest factors not explicitly considered in the species response models.

Arthropods in Urban Habitat Fragments in Southern California: Area, Age, and Edge Effects

The distribution of non-ant arthropods was examined in 40 urban habitat fragments in coastal San Diego County, California, USA, to look for effects of fragmentation, proximity to developed edge, and the non-native Argentine ant (Linepithema humile). Arthropods were sampled with pitfall traps and by vacuum sampling from California buckwheat shrubs (Eriogonum fasciculatum). Individual arthropods were identified to order and Recognizable Taxonomic Unit (RTU), or morphospecies. At the fragment scale we looked for correlations in the point diversity and abundance of arthropods as a function of the age and area of the fragment being sampled. At the scale of the individual sample points we looked for correlations of abundance and diversity with variables that describe the species composition of the shrub vegetation and disturbance. As indicators of disturbance we used the cover of native woody and exotic non-woody vegetation, the distance to the nearest developed edge, and the abundance of Argentine ants. The following patterns were found: (1) In general, arthropods showed a fragmentation effect with point diversity and abundance positively correlated with fragment area and negatively correlated with fragment age. (2) The pitfall samples were dominated by three primarily non-native orders, Isopoda (pillbugs), Dermaptera (earwigs), and Blattaria (roaches). Over 35% of all pitfall-captured arthropods belonged to four species in these orders. Dermaptera and Blattaria increased in abundance in smaller and older fragments, respectively. Isopod abundance, in contrast, was unrelated to fragment attributes. None of these groups appeared to be associated with edges, but were distributed throughout the fragments. (3) Point diversity and abundance in ground-active spiders appears to be enhanced by fragmentation. (4) Total pitfall RTU richness and abundance, and abundance or richness in the Coleoptera (vacuum), Diptera, non-ant Hymenoptera, Hemiptera, Microcoryphia, and Acarina had significant partial negative correlations with Argentine ant abundance. The Diptera and Coleoptera had this negative partial relationship with the Argentine ants despite the fact that both they and the ants were positively associated with edges. (5) In general, diversity in most orders was higher in sampling locations dominated by coastal sage scrub habitat than in those with appreciable cover of chaparral shrub species. (6) There was a strong seasonal variation in abundance and diversity in most orders. Diversity and abundance were highest in spring, intermediate in winter, and lowest in the fall. (7) Although higher trophic levels are often considered to be more sensitive to fragmentation, two groups of arthropod predators, spiders and carabid beetles, increased in abundance in older fragments. Abundance of these predators was positively correlated with the abundance of Argentine ants and the non-native Isopods, Dermaptera, and Blattaria.

ARTHROPODS IN URBAN HABITAT FRAGMENTS IN SOUTHERN CALIFORNIA: AREA, AGE, AND EDGE EFFECTS

The distribution of non-ant arthropods was examined in 40 urban habitat fragments in coastal San Diego County, California, USA, to look for effects of fragmentation, proximity to developed edge, and the non-native Argentine ant (Linepithema humile). Arthropods were sampled with pitfall traps and by vacuum sampling from California buckwheat shrubs (Eriogonum fasciculatum). Individual arthropods were identified to order and Recognizable Taxonomic Unit (RTU), or morphospecies. At the fragment scale we looked for correlations in the point diversity and abundance of arthropods as a function of the age and area of the fragment being sampled. At the scale of the individual sample points we looked for correlations of abundance and diversity with variables that describe the species composition of the shrub vegetation and disturbance. As indicators of disturbance we used the cover of native woody and exotic non-woody vegetation, the distance to the nearest developed edge, and the abundance of Argentine ants. The following patterns were found: (1) In general, arthropods showed a fragmentation effect with point diversity and abundance positively correlated with fragment area and negatively correlated with fragment age. (2) The pitfall samples were dominated by three primarily non-native orders, Isopoda (pillbugs), Dermaptera (earwigs), and Blattaria (roaches). Over 35% of all pitfall-captured arthropods belonged to four species in these orders. Dermaptera and Blattaria increased in abundance in smaller and older fragments, respectively. Isopod abundance, in contrast, was unrelated to fragment attributes. None of these groups appeared to be associated with edges, but were distributed throughout the fragments. (3) Point diversity and abundance in ground-active spiders appears to be enhanced by fragmentation. (4) Total pitfall RTU richness and abundance, and abundance or richness in the Coleoptera (vacuum), Diptera, non-ant Hymenoptera, Hemiptera, Microcoryphia, and Acarina had significant partial negative correlations with Argentine ant abundance. The Diptera and Coleoptera had this negative partial relationship with the Argentine ants despite the fact that both they and the ants were positively associated with edges. (5) In general, diversity in most orders was higher in sampling locations dominated by coastal sage scrub habitat than in those with appreciable cover of chaparral shrub species. (6) There was a strong seasonal variation in abundance and diversity in most orders. Diversity and abundance were highest in spring, intermediate in winter, and lowest in the fall. (7) Although higher trophic levels are often considered to be more sensitive to fragmentation, two groups of arthropod predators, spiders and carabid beetles, increased in abundance in older fragments. Abundance of these predators was positively correlated with the abundance of Argentine ants and the non-native Isopods, Dermaptera, and Blattaria.