Modelling the distribution of a threatened habitat: the California sage scrub

Aim Using predictive species distribution and ecological niche modelling our objectives are: (1) to identify important climatic drivers of distribution at regional scales of a locally complex and dynamic system – California sage scrub; (2) to map suitable sage scrub habitat in California; and (3) to distinguish between bioclimatic niches of floristic groups within sage scrub to assess the conservation significance of analysing such species groups. Location Coastal mediterranean‐type shrublands of southern and central California. Methods Using point localities from georeferenced herbarium records, we modelled the potential distribution and bioclimatic envelopes of 14 characteristic sage scrub species and three floristic groups (south‐coastal, coastal–interior disjunct and broadly distributed species) based upon current climate conditions. Maxent was used to map climatically suitable habitat, while principal components analysis followed by canonical discriminant analysis were used to distinguish between floristic groups and visualize species and group distributions in multivariate ecological space. Results Geographical distribution patterns of individual species were mirrored in the habitat suitability maps of floristic groups, notably the disjunct distribution of the coastal–interior species. Overlap in the distributions of floristic groups was evident in both geographical and multivariate niche space; however, discriminant analysis confirmed the separability of floristic groups based on bioclimatic variables. Higher performance of floristic group models compared with sage scrub as a whole suggests that groups have differing climate requirements for habitat suitability at regional scales and that breaking sage scrub into floristic groups improves the discrimination between climatically suitable and unsuitable habitat. Main conclusions The finding that presence‐only data and climatic variables can produce useful information on habitat suitability of California sage scrub species and floristic groups at a regional scale has important implications for ongoing efforts of habitat restoration for sage scrub. In addition, modelling at a group level provides important information about the differences in climatic niches within California sage scrub. Finally, the high performance of our floristic group models highlights the potential a community‐level modelling approach holds for investigating plant distribution patterns.     

Xerie Mediterranean-type shrubland associations of Alta and Baja California and the community/continuum debate

A survey of the xeric shrublands of Pacific coastal North America from San Francisco to El Rosario (Mexico), including the inner Channel Islands, was conducted using 99 sample sites of 0.063 ha size. TWINSPAN classification and DECORANA ordination confirmed the existence of two plant formations, distinguishable physiognomically: coastal sage scrub and coastal succulent scrub. Within coastal sage scrub, four floristic associations were recognized: Diablan, Venturan, Riversidian and Diegan. Within coastal succulent scrub, two floristic associations were defined: Martirian and Vizcainan. These associations occur in distinct geographical regions following the coastline, with the Riversidian association occurring in the basin inland from Venturan and Diegan regions. Their locations are strongly correlated with differences in evapotranspirative stress regimes. The Channel Island sites show affinities to several of the mainland associations. The Venturan association can be further subdivided floristically into two subassociations, dominated by Salvia mellifera and S. leucophylla respectively. These subassociations which are coextensive geographically at a regional scale, typically do not intermingle at a local scale but often meet along sharp boundaries in the landscape. The dominant species segregate by moisture preference, S. mellifera preferring coarser-texture soils and more southerly aspects than S. leucophylla. Richness and equitability of these sites are depressed relative to other xeric shrubland sites, reflecting the fact that the two subassociations partition the Venturan flora into substantially non-overlapping subsets of species. This segregation of associates between the two Salvia dominance types strongly suggests biotic influence of the dominants on subordinate species, perhaps mediated by allelopathy. This biotic interaction, leading to relatively strong floristic subassociations segregating independently in the landscape, would provide an example of the holistic community structure referred to by Clements and his followers, embedded within a larger pattern of continuity in species distributions.Nomenclature follows Munz & Keck (1959), Munz (1974) and Wiggins (1980).I am grateful to the following for research assistance: S. Coon, E. Hobbs, S. Lavinger, J. F. O'Leary, K. R. Preston, B. Rich and A. Troeger. I also thank the numerous public and private land owners who permitted access to the study sites. This research is based upon work supported by the National Science Foundation under grant DEB 76-81712.     

Restoration of a Native Shrubland Impacted by Exotic Grasses, Frequent Fire, and Nitrogen Deposition in Southern California

Natural ecosystems globally are often subject to multiple human disturbances that are difficult to restore. A restoration experiment was done in an urban fragment of native coastal sage scrub vegetation in Riverside, California that has been subject to frequent fire, high anthropogenic nitrogen deposition, and invasion by Mediterranean annual weeds. Hand cultivation and grass‐specific herbicide were both successful in controlling exotic annual grasses and promoting establishment of seeded coastal sage scrub vegetation. There was no native seedbank left at this site after some 30 years of conversion to annual grassland, and the only native plants that germinated were the seeded shrubs, with the exception of one native summer annual. The city green‐waste mulch used in this study (C:N of 39:1) caused short‐term N immobilization but did not result in decreased grass density or increased native shrub establishment. Seeding native shrubs was successful in a wet year in this Mediterranean‐type climate but was unsuccessful in a dry year. An accidental spring fire did not burn first‐year shrubs, although adjacent plots dominated by annual grass did burn. The shrubs continued to exclude exotic grasses into the second growing season, suggesting that successful shrub establishment may reduce the frequency of the fire return interval.     

The Importance of Community Classification to Mitigation and Restoration of Coastal Sage Scrub

Community classification has emerged from ecology textbooks into the arena of environmental impact evaluation and mitigation. This development is especially apparent in southern California, where the fate of a community called coastal sage scrub is being decided. Regional plans for development, mitigation, preservation, and restoration are being formulated that will permanently affect the natural landscape. This paper demonstrates the potential for the name of a plant community to affect tradeoffs in planning processes that are intended to offset removal of habitat by development. The discussion focuses on two types of classification systems. One system is hierarchical, with established nomenclatorial rules that allow natural variation to refine the community definitions. Names of plant associations directly reflect the dominant species in the association, as in Artemisia californica—Eriogonum fasciculatum (California sagebrush—California buckwheat). The second system is nonhierarchical, in which observations of natural landscapes are fit into established definitions of community types, such as Diegan coastal sage scrub. Advantages and disadvantages of both types of systems are discussed and illustrated by two examples of problems in classifying coastal sage scrub. The discussion concludes with the point that the name of a community has the potential to significantly affect the extent to which equal tradeoffs between community types are actually achieved in mitigation and restoration efforts.     

Post-fire regeneration strategies of Californian coastal sage shrubs

Summary Regeneration methods for coastal sage srub vegetation after fire were studied in the coastal Santa Monica Mountains of southern California. Six sites were sampled two years after a large fire of fall, 1978. The intensity of fire varied. Foliar cover and flowering incidence were recorded for individuals regenerating by resprouting or from seed. Resprouting plants contributed most to post-fire recovery, comprising 95% of the relative foliar shrub cover; 84% of resprout and 47% of seedling cover had flowered. An ANOVA of reproductive mode and fire intensity indicates that resprout total cover and individual size are significantly greater than those of seedlings, regardless of fire intensity. Among sites the average foliar cover of resprouts exceeded that of seedlings by factors ranging from 9 to 63. All coastal sage species examined resprout, although the potential vigor of resprouting appears to vary widely within genera (e.g. Encelia, Eriogonum, and Salvia) and even within species. In the second growing season following fire seedling density increased due to seeds shed by resprouted shrubs. Most of the cover on these stands of coastal sage scrub is destined to be either crown-sprouted individuals or their progeny.     

Measures of resilience: the response of coastal sage scrub to fire

Measures of four components of resilience are developed and used to quantify the response of coastal sage scrub to fire in southern California: (1) elasticity (rate of recovery following disturbance), (2) amplitude (threshold of disturbance beyond which recovery to the original state no longer occurs), (3) malleability (extent of alteration of the new stable-state from the original) and (4) damping (extent and duration of oscillation in an ecosystem parameter following disturbance). Vegetation and soil properties measured before fire, and for the first 5–6 yr after fire on four coastal (Venturan association) and four inland (Riversidian association) sites of coastal sage were used to follow changes. In addition, results from a simulation model of post-fire succession in Venturan coastal sage scrub (the FINICS model of Malanson) were used to examine resilience behavior over a 200 yr period. Resilience behavior of coastal sage scrub is critically influenced by the presence of a competitive mix of inherently strongly and weakly resprouting species. Sites dominated by weak resprouters exhibit lower elasticity and less damping of year-to-year fluctuations in composition in the early post-fire years. Sites with a mixture of weak and strong resprouters have a lower threshold of disturbance (amplitude) before species extirpation occurs, a result intensified by a higher frequency of disturbance. Malleability is also greater in these systems under higher disturbance frequency.Nomenclature follows. P.A. Munz, 1974. A Flora of Southern California. Univ. California Press.     

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

Aims: 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.     

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 C(1) 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 10(2) to 10(5) 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.     

Soil moisture and fungi affect seed survival in California grassland annual plants

Survival of seeds in the seed bank is important for the population dynamics of many plant species, yet the environmental factors that control seed survival at a landscape level remain poorly understood. These factors may include soil moisture, vegetation cover, soil type, and soil pathogens. Because many soil fungi respond to moisture and host species, fungi may mediate environmental drivers of seed survival. Here, I measure patterns of seed survival in California annual grassland plants across 15 species in three experiments. First, I surveyed seed survival for eight species at 18 grasslands and coastal sage scrub sites ranging across coastal and inland Santa Barbara County, California. Species differed in seed survival, and soil moisture and geographic location had the strongest influence on survival. Grasslands had higher survival than coastal sage scrub sites for some species. Second, I used a fungicide addition and exotic grass thatch removal experiment in the field to tease apart the relative impact of fungi, thatch, and their interaction in an invaded grassland. Seed survival was lower in the winter (wet season) than in the summer (dry season), but fungicide improved winter survival. Seed survival varied between species but did not depend on thatch. Third, I manipulated water and fungicide in the laboratory to directly examine the relationship between water, fungi, and survival. Seed survival declined from dry to single watered to continuously watered treatments. Fungicide slightly improved seed survival when seeds were watered once but not continually. Together, these experiments demonstrate an important role of soil moisture, potentially mediated by fungal pathogens, in driving seed survival.     

Granivory in California sage scrub: implications for common plant invaders and ecosystem conservation

Seed predation may influence community assembly and invasion dynamics when seed predators preferentially select some seed species over others. However, the role of different seed predators and their preferences for seeds in multiple ecological contexts, including the endangered California sage scrub ecosystem, remain unresolved making predictions about which processes limit or promote invasions difficult. In addition, selection criteria, specifically how seed size and biogeographic origin (native versus invasive) influences selection, requires further research across multiple granivore guilds. We examined seed predator preference for common native and invasive seeds across small and large seed classes using seed dish experiments with motion-sensor video observation. We also quantified the relative importance of ants, birds, and mammals as seed predators in sage scrub. Community-wide, granivores preferred the small-seeded invasive mustard Brassica nigra, avoided the large-seeded invasive grass Bromus rubens and the native large-seeded shrub Encelia californica, and did not show significant selection or avoidance for the small-seeded native shrub Salvia apiana. Birds, notably California towhees (Melozone crissalis), were the most frequent granivore while rodents and ants rarely removed seeds. This study reveals that birds are important seed predators in California sage scrub and have the potential to contribute to biotic resistance to mustard invasions (Brassica nigra and potentially others).

     

RANGE CHARACTERISTICS OF PACIFIC COAST BOTTLENOSE DOLPHINS (TURSIOPS TRUNCATUS) IN THE SOUTHERN CALIFORNIA BIGHT

Boat-based photoidentification surveys of bottlenose dolphins (Tursiops truncatus) were conducted from 1982 to 1989 in three discrete coastal study areas within the Southern California Bight: (1) Santa Barbara, California; (2) Orange County, California; (3) Ensenada, Baja California, Mexico. A total of 207 recognizable dolphins were identified in these three “secondary” study areas. These individuals were compared to 404 dolphins identified from 1981 to 1989 in our “primary” study area, San Diego, California, to examine the coastal movement patterns of bottlenose dolphins within the Southern California Bight. A high proportion of dolphins photographed in Santa Barbara (88%), Orange County (92%), and Ensenada (88%) were also photographed in San Diego. Fifty-eight percent (n= 120) of these 207 dolphins exhibited back-and-forth movements between study areas, with no evidence of site fidelity to any particular region. Minimum range estimates were 50 and 470 km. Minimum travel-speed estimates were 11-47 km/d, and all dolphin schools sighted during the study were within 1 km of the shore. These data suggest that bottlenose dolphins within the Southern California Bight are highly mobile within a relatively narrow coastal zone. Home-range dimensions and movement patterns for many vettebrate species are influenced, in part, by variation in food resources. The unique range characteristics documented during this study may reflect the highly dynamic nature of this coastal ecosystem and the associated patchy distribution of food resources available to these bottlenose dolphins.     

Dynamic landscapes shape post‐wildfire recolonisation and genetic structure of the endangered Hermes copper (Lycaena hermes) butterfly

1. The coastal sage scrub vegetation community experiences frequent fires, so the long‐term survival of species depends on the rate of recolonisations exceeding the rate of local extinctions. Recolonisation of these post‐wildfire habitats probably requires long‐distance dispersal events. These movements can also counter detrimental impacts associated with inbreeding. 2. The Hermes copper (Lycaena hermes) is an extremely rare butterfly inhabiting coastal sage scrub adjacent to San Diego, California, USA. Habitat loss due to urbanisation and impacts of recent wildfires has greatly restricted its range, prompting the United States Fish and Wildlife Service to list the Hermes copper as a Candidate Species in 2011. 3. Surveys for Hermes copper butterflies in 2010–2013 documented only two recolonisation events following wildfires in 2003 and 2007. Larger populations were readily detected each year, but detection of smaller populations was inconsistent. 4. Amplified fragment length polymorphism was used to identify potential genetic discontinuities within this species across the landscape. Results indicated that movements across much of the landscape were possible historically. However, individuals from three peripheral populations exhibited a higher degree of differentiation, suggesting more restricted dispersal in these regions. 5. From the results, it can be concluded that historically Hermes copper butterflies were able to move among habitat patches prior to recent changes in the landscape. However, low post‐fire recolonisation rates suggest limited contemporary dispersal, probably due to recent habitat fragmentation. This fragmentation is a relatively new event, as the human population in San Diego County experienced substantial growth in the late 20th Century.     

Impacts of invasive annuals on soil carbon and nitrogen storage in southern California depend on the identity of the invader

Non‐native plant invasions can alter nutrient cycling processes and contribute to global climate change. In southern California, California sage scrub (hereafter sage scrub), a native shrub‐dominated habitat type in lowland areas, has decreased to <10% of its original distribution. Postdisturbance type‐conversion to non‐native annual grassland, and increasingly to mustard‐dominated invasive forbland, is a key contributor to sage scrub loss. To better understand how type‐conversion by common invasive annuals impacts carbon (C) and nitrogen (N) storage in surface soils, we examined how the identity of the invader (non‐native grasses, Bromus spp.; and non‐native forbs, Brassica nigra), microbial concentrations, and soil properties interact to influence soil nutrient storage in adjacent native and invasive habitat types at nine sites along a coast to inland gradient. We found that the impact of type‐conversion on nutrient storage was contingent upon the invasive plant type. Sage scrub soils stored more C and N than non‐native grasslands, whereas non‐native forblands had nutrient storage similar to or higher than sage scrub. We calculate that >940 t C km^?2 and >60 t N km^?2 are lost when sage scrub converts to grass‐dominated habitat, demonstrating that grass invasions are significant regional contributors to greenhouse gas emissions. We found that sites with greater total C and N storage were associated with high cation exchange capacities and bacterial concentrations. Non‐native grassland habitat type was a predictor of lower total C, and soil pH, which was greatest in invasive habitats, was a predictor of lower total N. We demonstrate that modeling regional nutrient storage requires accurate classification of habitat type and fine‐scale quantification of cation exchange capacity, pH, and bacterial abundance. Our results provide evidence that efforts to restore and conserve sage scrub enhance nutrient storage, a key ecosystem service reducing atmospheric CO₂ concentrations.     

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.     

Positive Effects of Non-Native Grasses on the Growth of a Native Annual in a Southern California Ecosystem

Fire disturbance is considered a major factor in the promotion of non-native plant species. Non-native grasses are adapted to fire and can alter environmental conditions and reduce resource availability in native coastal sage scrub and chaparral communities of southern California. In these communities persistence of non-native grasses following fire can inhibit establishment and growth of woody species. This may allow certain native herbaceous species to colonize and persist beneath gaps in the canopy. A field manipulative experiment with control, litter, and bare ground treatments was used to examine the impact of non-native grasses on growth and establishment of a native herbaceous species, Cryptantha muricata. C. muricata seedling survival, growth, and reproduction were greatest in the control treatment where non-native grasses were present. C. muricata plants growing in the presence of non-native grasses produced more than twice the number of flowers and more than twice the reproductive biomass of plants growing in the treatments where non-native grasses were removed. Total biomass and number of fruits were also greater in the plants growing in the presence of non-native grasses. Total biomass and reproductive biomass was also greater in late germinants than early germinants growing in the presence of non-native grasses. This study suggests a potential positive effect of non-native grasses on the performance of a particular native annual in a southern California ecosystem.     

Foraging ecology of the California gnatcatcher deduced from fecal samples

The California gnatcatcher is a threatened species essentially restricted to coastal sage scrub habitat in southern California. Its distribution and population dynamics have been studied intensely, but little is known about its diet. We identified arthropod fragments in 33 fecal samples of the California gnatcatcher to gain insight into its foraging ecology and diet. Fecal samples were collected from adult males, adult females, fledglings, and nestlings. Leaf- and planthoppers (Homoptera) and spiders (Araneae) predominated numerically in samples. Spider prey was most diverse, with eight families represented. True bugs (Hemiptera) and wasps, bees, and ants (Hymenoptera) were only minor components of the gnatcatcher diet. Gnatcatcher adults selected prey to feed their young that was larger than expected given the distribution of arthropod size available in their environment, and chicks were provisioned with larger prey items and significantly more grasshoppers and crickets (Orthoptera) and spiders than adults consumed themselves. Both adults and young consumed more sessile than active prey. Further studies are needed to determine whether arthropods sampled in coastal sage scrub that are common in fecal samples are good indicators of California gnatcatcher habitat. Received: 30 December 1998 / Accepted: 28 April 1999     

Comparative water relations of adjacent california shrub and grassland communities

Summary Much of the coastal mountains and foothills of central and southern California are covered by a mosaic of grassland, coastal sage scrub, and evergreen sclerophyllous shrubs (chaparral). In many cases, the borders between adjacent plant communities are stable. The cause of this stability is unknown. The purpose of our study was to examine the water use patterns of representative grasses, herbs, and shrubs across a grassland/chaparrel ecotone and determine the extent to which patterns of water use contribute to ecotone stability. In addition, we examined the effects of seed dispersal and animal herbivory. We found during spring months, when water was not limited, grassland species had a much higher leaf conductance to water vapor diffusion than chaparral plants. As the summer drought progressed, grassland species depleted available soil moisture first, bare zone plants second, and chaparral third, with one chaparral species (Quercus durata) showing no evidence of water stress. Soil moisture depletion patterns with depth and time corresponded to plant water status and root depth. Rabbit herbivory was highest in the chaparral and bare zone as indicated by high densities of rabbit pellets. Dispersal of grassland seeds into the chaparral and bare zone was low. Our results support the hypothesis that grassland species deplete soil moisture in the upper soil horizon early in the drought, preventing the establishment of chaparral seedlings or bare zone herbs. Also, grassland plants are prevented from invading the chaparral because of low seed dispersability and high animal herbivory in these regions.     

Disturbance of biological soil crust increases emergence of exotic vascular plants in California sage scrub

Biological soil crusts (BSCs) are comprised of soil particles, bacteria, cyanobacteria, green algae, microfungi, lichens, and bryophytes and confer many ecosystem services in arid and semiarid ecosystems worldwide, including the highly threatened California sage scrub (CSS). These services, which include stabilizing the soil surface, can be adversely affected when BSCs are disturbed. Using field and greenhouse experiments, we tested the hypothesis that mechanical disturbance of BSC increases emergence of exotic vascular plants in a coastal CSS ecosystem. At Whiting Ranch Wilderness Park in southern California, 22 plots were established and emergence of exotic and native plants was compared between disturbed and undisturbed subplots containing BSC. In a separate germination study, seed fate in disturbed BSC cores was compared to seed fate in undisturbed BSC cores for three exotic and three native species. In the field, disturbed BSCs had significantly (>3×) greater exotic plant emergence than in undisturbed BSC, particularly for annual grasses. Native species, however, showed no difference in emergence between disturbed and undisturbed BSC. Within the disturbed treatment, emergence of native plants was significantly, and three times less than that of exotic plants. In the germination study, seed fates for all species were significantly different between disturbed and undisturbed BSC cores. Exotic species had greater emergence in disturbed BSC, whereas native plants showed either no response or a positive response. This study demonstrates another critical ecosystem service of BSCs—the inhibition of exotic plant species—and underscores the importance of BSC conservation in this biodiversity hotspot and possibly in other aridland ecosystems.     

PINUS × CRITCHFIELDII,A LATE PLEISTOCENE HYBRID PINE FROM COASTAL SOUTHERN CALIFORNIA

Fossil cones buried in an old landslide at San Clemente are dated at 30,000–34,000 BP (years before present). They generally resemble cones of the natural hybrid Pinus radiata × P. attenuata at Año Nuevo, and also the artificial hybrid P. × attenuradiata. But differences in cone shape, size, and apophyses suggest that the fossil P. × critchfieldii Axelrod and T. G. Hill represents a hybrid of populations of P. radiata and P. attenuata then in coastal southern California. This agrees with population differences between these species in central and southern California during the Pleistocene. Regional comparisons of Late Pleistocene floras indicate that at San Clemente mean annual rainfall was about 580–635 mm, or twice that of today, mean annual temperature was about 2.5 C cooler (13.5 vs. 16.0 C), the mean monthly range was lower (5.9 vs. 7.7 C), the growing season was about 40 days shorter (170 vs. 210 days), and equability was higher (M 72 vs. M 67). Forest was replaced by coastal sage scrub and grassland along the San Clemente coast as drier, warmer climate developed after 12,000 BP.     

THE ORIGIN OF COASTAL SAGE VEGETATION,ALTA AND BAJA CALIFORNIA

Coastal sage is a new pioneer-type vegetation that only spread widely after the Early Quaternary, when species on the dry open borders of forest, woodland and arid tropic scrub vegetation shifted into expanding dry sites there and in adjacent grasslands. These new sites were created by a coincidence of major climatic and tectonic events and by accompanying erosion and mass movement on steep new slopes. Attaining most of its present area during the hot, dry Xerothermic, coastal sage scrub spread further as man's activities disturbed the landscape.