Sage Scrub Research Articles

Potential soil extracellular enzyme activity is altered by long-term experimental nitrogen deposition in semiarid shrublands

Semi-arid shrublands in southern California are exposed to high levels of atmospheric dry nitrogen (N) deposition, which can alter soil microbial activity, and thus, ecosystem carbon (C) and N cycling and storage. We used archived soil samples to assess the relative effects of experimental N enrichment on the potential activity of key C, N, and phosphorus (P) cycling enzymes in chaparral and coastal sage scrub (CSS) shrublands over a 14 year period. Chaparral plots exposed to N exhibited a significant decline in β-glucosidase and phosphatase activity and an increase in peroxidase activity over time, while N exposure had no consistent effect on N-acetylglucosaminidase (NAGase) activity. In contrast, N exposure did not significantly affect the activity of any of the measured enzymes in CSS. However, ANCOVA results revealed that the potential response of some enzymes to cumulative N input was affected by annual rainfall, because significantly more N accumulated in the soil during times of low rainfall. These results indicate that chronic dry season N inputs to semi-arid shrublands can fundamentally alter potential enzyme activity, but the impact depends on climate and vegetation type.

Exotic grass litter modulates seasonal pulse dynamics of CO2 and N2O, but not NO, in Mediterranean-type coastal sage scrub at the wildland-urban interface

Mediterranean shrublands adjacent to urbanization experience nitrogen deposition and exotic grass invasions which likely have downstream consequences for carbon and nitrogen emissions from soils. We tested a hypothesis that soil wetting produces trace gas pulses modified by seasonal wetting history, grass litter availability, and cover type in these systems. Over two seasons, we conducted 48-h wetting experiments and measured CO2, N2O, and NO pulses at an invaded, polluted California shrubland following grass litter addition to sites dominated by either an invasive grass (Schismus barbatus) or a native shrub (Erioginum fasciculatum). CO2 and N2O pulses consistently appeared 15 min post-wetting and diminished within 12 h; NO peaked later and remained elevated at 24 h. All pulses were stronger in the dry season than wet season. Grass litter amendments increased CO2 and dry-season N2O pulses without significantly modifying NO pulses. Grass cover reduced CO2 pulses compared to shrub cover. Our results support the hypothesis that shrubland soils produce stronger pulses of CO2, N2O, and NO during the dry season when wetting is less frequent. We show that invasive grass litter can provide a labile C source that stimulates CO2 and N2O, but not NO, emissions from shrubland soils.

Shoot and root biomass production in semi-arid shrublands exposed to long-term experimental N input

Anthropogenic nitrogen (N) deposition has affected the primary production of terrestrial ecosystems worldwide; however, ecosystem responses often vary over time because of transient responses, interactions between N, precipitation, and/or other nutrients, and changes in plant species composition. Here we report N-induced changes in above- and below-ground standing crop and production over an 11-year period for two semi-arid shrublands, chaparral and coastal sage scrub (CSS), of Southern California. Shrubs were exposed to 50 kgN ha−1 in the fall of each year to simulate the accumulation of dry N deposition, and shoot and root biomass and leaf area index (LAI) were measured every 3 months to assess how biomass production responded to chronic, dry N inputs. N inputs significantly altered above- and below-ground standing crop, production, and LAI; however, N impacts varied over time. For chaparral, N inputs initially increased root production but suppressed shoot production; however, over time biomass partitioning reversed and plants exposed to N had significantly more shoot biomass. In CSS, N inputs caused aboveground production to increase only during wet years, and this interaction between added N and precipitation was due in part to a highly flexible growth response of CSS shrubs to increases in N and water availability and to a shift from slower-growing native shrubs to fast-growing introduced annuals. Together, these results indicate that long-term N inputs will lead to complex, spatially and temporally variable growth responses for these, and similar, Mediterranean-type shrublands.

Leaf traits and performance vary with plant age and water availability in Artemisia californica.

Leaf functional traits are strongly tied to growth strategies and ecological processes across species, but few efforts have linked intraspecific trait variation to performance across ontogenetic and environmental gradients. Plants are believed to shift towards more resource-conservative traits in stressful environments and as they age. However, uncertainty as to how intraspecific trait variation aligns with plant age and performance in the context of environmental variation may limit our ability to use traits to infer ecological processes at larger scales. We measured leaf physiological and morphological traits, canopy volume and flowering effort for Artemisia californica (California sagebrush), a dominant shrub species in the coastal sage scrub community, under conditions of 50, 100 and 150 % ambient precipitation for 3 years. Plant age was a stronger driver of variation in traits and performance than water availability. Older plants demonstrated trait values consistent with a more conservative resource-use strategy, and trait values were less sensitive to drought. Several trait correlations were consistent across years and treatments; for example, plants with high photosynthetic rates tended to have high stomatal conductance, leaf nitrogen concentration and light-use efficiency. However, the trade-off between leaf construction and leaf nitrogen evident in older plants was absent for first-year plants. While few traits correlated with plant growth and flowering effort, we observed a positive correlation between leaf mass per area and performance in some groups of older plants. Overall, our results suggest that trait sensitivity to the environment is most visible during earlier stages of development, after which intraspecific trait variation and relationships may stabilize. While plant age plays a major role in intraspecific trait variation and sensitivity (and thus trait-based inferences), the direct influence of environment on growth and fecundity is just as critical to predicting plant performance in a changing environment.

Plant-Soil Feedback Effects on Germination and Growth of Native and Non-Native Species Common across Southern California

Changes in plant assemblages can influence biotic and abiotic soil conditions. These changes can cause plant–soil feedbacks that can inhibit or facilitate plant germination and growth. Here, we contribute to a growing literature examining plant–soil feedbacks in the endangered sage scrub ecosystem by examining the germination and growth of Artemisia californica, the dominant native shrub species in the ecosystem, in soil conditioned by two widespread plant invaders (Brassica nigra, Bromus madritensis ssp. rubens), and the germination and growth of these invasive species in conspecific and heterospecific soils. Our findings suggest that: (i) A. californica soils can limit establishment of some species (B. nigra) but not others (B. madritensis), (ii) A. californica soil conditions reduce growth of all plant species, and (iii) non-natives are negatively impacted by soil microbes, but in some contexts can do better in heterospecific soil. As our findings were often incongruent with other studies that examined interactions among similar species at other sites, we suggest that we are at our infancy of understanding these complex interactions, and that developing a predictive framework for understanding plant soil feedbacks in the sage scrub ecosystem involves understanding how various plant species respond in different soil contexts within the ecosystem.

Identifying Mechanisms for Successful Ecological Restoration with Salvaged Topsoil in Coastal Sage Scrub Communities

Although aboveground metrics remain the standard, restoring functional ecosystems should promote both aboveground and belowground biotic communities. Restoration using salvaged soil—removal and translocation of topsoil from areas planned for development, with subsequent deposition at degraded sites—is an alternative to traditional methods. Salvaged soil contains both seed and spore banks, which may holistically augment restoration. Salvaged soil methods may reduce non-native germination by burying non-native seeds, increase native diversity by adding native seeds, or transfer soil microbiomes, including arbuscular mycorrhizal fungi (AMF), to recipient sites. We transferred soil to three degraded recipient sites and monitored soil microbes, using flow cytometry and molecular analyses, and characterized the plant community composition. Our findings suggest that salvaged soil at depths ≥5 cm reduced non-native grass cover and increased native plant density and species richness. Bacterial abundance at recipient sites were statistically equivalent to donor sites in abundance. Overall, topsoil additions affected AMF alpha diversity and community composition and increased rhizophilic AMF richness. Because salvaged soil restoration combines multiple soil components, including native plant and microbial propagules, it may promote both aboveground and belowground qualities of the donor site, when applying this method for restoring invaded and degraded ecosystems.

Declines in native forb richness of an imperiled plant community across an anthropogenic nitrogen deposition gradient

AbstractAnthropogenic nitrogen (N) deposition is known to reduce plant diversity in ecosystems worldwide; however, effects on the diversity of Mediterranean‐type ecosystems—global hotspots of biodiversity—are relatively unexplored. In California, elevated N deposition due to air pollution has a multitude of ecological effects including the facilitation of nonnative plant invasion and altered ecosystem functioning, but impacts on plant richness have been inadequately quantified. We addressed this research gap by evaluating patterns of plant richness in coastal sage scrub (CSS), a severely threatened, highly diverse Mediterranean‐type shrubland, across the Santa Monica Mountains National Recreation Area. This is the largest urban national park in the United States and experiences a strong gradient of N deposition due to its proximity to urban Los Angeles. We measured soil N, plant cover, and richness at 30 CSS sites across this gradient and used regression analyses to explore relationships between richness, N deposition, and other environmental variables. We observed significant declines in plant richness across a steep gradient of soil N availability that paralleled patterns of N deposition, primarily due to decreases in native forb species. Our analyses identified soil N as the best predictor of patterns of native forb richness, but other factors, including nonnative plant cover and aridity, may also drive reduced richness. In addition to the marked decline in the number of native forb species, increasing N deposition was also associated with lower native shrub richness per area and increased cover of nonnatives. These results highlight the threat posed by N deposition to the conservation of this already imperiled ecosystem under continued environmental change.

Protecting the Wildland-Urban Interface in California: Greenbelts vs Thinning for Wildfire Threats to Homes

This study utilized native chaparral and sage scrub shrubs planted in lightly irrigated greenbelts around homes to evaluate the impact on live fuel moisture content (LFMC) and predicted fire behavior. As to be expected LFMC varied markedly throughout the year being over 100% in winter in all species and treatments that included adjacent thinned native shrublands and untreated control shrublands. However, in the summer and fall there were marked differences between treatments. For most species lightly irrigated plants had the highest LFMC in the summer and fall, followed by thinned treatments and controls. These differences in moisture content coupled with structural differences in the vegetation contributed to expected differences in flame length and rate of spread. Lightly irrigated native shrubs planted around homes can reduce fire hazard while possessing other desirable features of utilizing native vegetation.

A multi-model approach to guide habitat conservation and restoration for the endangered San Bernardino kangaroo rat

The San Bernardino kangaroo rat (Dipodomys merriami parvus) is a federally listed endangered species endemic to Southern California and limited to three remaining populations. Its native habitat of alluvial fan sage scrub faces many anthropogenic threats, including urban and agricultural development, and the resulting flood control and fire suppression. With the loss of natural processes such as scouring or burning from floods and fires, the mosaic of seral stages across the landscape has shifted to dense vegetation, and active restoration may be necessary to provide suitable habitat for the San Bernardino kangaroo rat. Species distribution modeling using the partitioned Mahalanobis distance method on all occurrence points collected in the past 16 years revealed that alluvial scrub cover and fluvent soils were most strongly associated with San Bernardino kangaroo rat occupancy. Through surveys at 14 locations across the species’ range, we identified non-native grass cover, shrub cover, bare ground and sandy soils as microhabitat features related to San Bernardino kangaroo rat abundance. We also calculated the optimal range of cover for each habitat type that was correlated with higher kangaroo rat abundance. The results of this multiple-model approach can be used by the agencies to assess the value of conserved habitat, set targets for microhabitat enhancement to facilitate population growth and expansion, or identify receiver sites should translocation be required for recovery. This work lays the foundation for more coordinated and strategic restoration efforts, given the compressed and rigid timelines of development projects that continue to impact remaining San Bernardino kangaroo rat populations.

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.

Quantifying Drought Sensitivity of Mediterranean Climate Vegetation to Recent Warming: A Case Study in Southern California

A combination of drought and high temperatures (“global-change-type drought”) is projected to become increasingly common in Mediterranean climate regions. Recently, Southern California has experienced record-breaking high temperatures coupled with significant precipitation deficits, which provides opportunities to investigate the impacts of high temperatures on the drought sensitivity of Mediterranean climate vegetation. Responses of different vegetation types to drought are quantified using the Moderate Resolution Imaging Spectroradiometer (MODIS) data for the period 2000–2017. The contrasting responses of the vegetation types to drought are captured by the correlation and regression coefficients between Normalized Difference Vegetation Index (NDVI) anomalies and the Palmer Drought Severity Index (PDSI). A novel bootstrapping regression approach is used to decompose the relationships between the vegetation sensitivity (NDVI–PDSI regression slopes) and the principle climate factors (temperature and precipitation) associated with the drought. Significantly increased sensitivity to drought in warmer locations indicates the important role of temperature in exacerbating vulnerability; however, spatial precipitation variations do not demonstrate significant effects in modulating drought sensitivity. Based on annual NDVI response, chaparral is the most vulnerable community to warming, which will probably be severely affected by hotter droughts in the future. Drought sensitivity of coastal sage scrub (CSS) is also shown to be very responsive to warming in fall and winter. Grassland and developed land will likely be less affected by this warming. The sensitivity of the overall vegetation to temperature increases is particularly concerning, as it is the variable that has had the strongest secular trend in recent decades, which is expected to continue or strengthen in the future. Increased temperatures will probably alter vegetation distribution, as well as possibly increase annual grassland cover, and decrease the extent and ecological services provided by perennial woody Mediterranean climate ecosystems as well.

Urbanization alters belowground invertebrate community structure in semi-arid regions: A comparison of lawns, biofilters and sage scrub

As the proportion of the human population living in cities continues to increase, understanding the factors affecting urban biodiversity is crucial to enhancing conservation efforts, urban ecosystem function, and human wellbeing. Here we present the results of the first comparative study of belowground invertebrate communities in urban soils (lawns and biofilters) and remnant patches of native coastal sage scrub (CSS) in a semi-arid urban environment. Among 12 sites sampled in La Jolla, California, USA, invertebrate community composition was most homogeneous among lawns, which differed significantly in composition from both biofilters and CSS. In almost all other aspects of invertebrate community diversity metrics (richness, evenness, diversity) as well as environmental factors (moisture, organic matter, plant cover and richness), lawns differed significantly from native CSS habitats, but biofilters were intermediate and not significantly different from other habitat types. Lawn soil invertebrate evenness and diversity were significantly lower, but density was highest in lawns, with higher densities of oligochaetes (Megadrilacea, Enchytraeidae), fly larvae (Sciaridae, Chironomidae), mites (Oribatida, Mesostigmata), and springtails (Poduromorpha, Symphypleona) in lawns relative to CSS. Differences in soil moisture, organic matter, and plant, tree, and building cover explained 40–62% of the variability in invertebrate density, richness, diversity, and evenness among sites. These findings suggest a shift in belowground invertebrate community structure that may be common among urbanizing semi-arid environments. The amendment of urban dryland soils with organic matter and moisture alters the presence and population density of several invertebrate taxa, and may cause a transition toward dominance by desiccation-intolerant litter transformers.

Caloric Values of Selected Wetland and Coastal Sage Scrub Vascular Plant Seeds

Caloric Values of Selected Wetland and Coastal Sage Scrub Vascular Plant Seeds

Type conversion from native California sage scrub to non-native grassland accelerates decomposition processes

Predicting how plant invasions influence decomposition processes is difficult, as multiple factors change with plant community alteration. Here, we examined the importance of various abiotic and biotic factors (litter quality (C:N), UV radiation, and macroinvertebrate access) in driving early decomposition processes in native California sage scrub (CSS) and in adjacent non-native grassland habitats. Many findings from our experiment were consistent with studies from semi-arid regions (e.g., UV radiation drove decomposition processes and high quality litter decayed more rapidly). However, the acceleration of litter decomposition in the non-native grassland relative to native California sage scrub (CSS) was unexpected and contrasts with studies of partially invaded CSS systems. Our results highlight that studies of invaded systems that still retain native species diversity should not be extrapolated to type-converted systems. We hypothesize that high grass cover in the type-converted grassland created conditions that enhanced soil moisture, bacterial abundance, and macroinvertebrate influence relative to the CSS, accelerating decomposition and reducing regional C storage.

Facilitation and herbivory during restoration of California coastal sage scrub

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

No guts, no glory: Gut content metabarcoding unveils the diet of a flower‐associated coastal sage scrub predator

AbstractInvertebrate generalist predators are ubiquitous and play a major role in food‐web dynamics. Molecular gut content analysis (MGCA) has become a popular means to assess prey ranges and specificity of cryptic arthropods in the absence of direct observation. While this approach has been widely used to study predation on economically important taxa (i.e., pests) in agroecosystems, it is less frequently used to study the broader trophic interactions involving generalist predators in natural communities such as the diverse and threatened coastal sage scrub communities of Southern California. Here, we employ DNA metabarcoding‐based MGCA and survey the taxonomically and ecologically diverse prey range of Phymata pacifica Evans, a generalist flower‐associated ambush bug (Hemiptera: Reduviidae). We detected predation on a wide array of taxa including beneficial pollinators, potential pests, and other predatory arthropods. The success of this study demonstrates the utility of MGCA in natural ecosystems and can serve as a model for future diet investigations into other cryptic and underrepresented communities.

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

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

Bobcat and rabbit habitat use in an urban landscape

Mammals are known to vary in their sensitivity to urbanization. In heavily urbanized southern California, bobcats (Lynx rufus) prey on rabbits (Sylvilagus spp.) that forage for lush vegetation in developed areas. We collected data on the density of rabbits in adjacent urban and natural areas using visual line surveys. We also measured habitat use for seven GPS-collared female bobcats to determine if bobcats selected areas with higher rabbit densities. Estimated rabbit densities did not vary between urban, urban edge, and natural areas during wet or dry seasons, reflecting their consistent presence across the landscape. During daytime and crepuscular hours, bobcats used natural coastal sage scrub and grassland habitats more frequently (76–84% of the time) than urban areas. Conversely, at night, bobcats decreased their use of coastal sage scrub and doubled their time in urban areas. In summary, we found that rabbits were present across the landscape, and bobcats are likely selecting urban areas at night to prey on rabbits and avoid people.

Ecological filtering in scrub fragments restructures the taxonomic and functional composition of native bee assemblages.

Predicting the long-term consequences of habitat alteration for the preservation of biodiversity and ecosystem function requires an understanding of how ecological filters drive taxonomic and functional biodiversity loss. Here, we test a set of predictions concerning the role of ecological filters in restructuring native bee assemblages inhabiting fragmented coastal sage scrub ecosystems in southern California, USA. In 2011 and 2012, we collected native bees in scrub habitat belonging to two treatment categories: large natural reserves and small habitat fragments embedded in an urban landscape. We compared bee assemblages in reserve and fragment sites with respect to their taxonomic and functional alpha diversity, beta diversity, assemblage composition, and mean geographical range size estimated via distribution maps compiled for this study from digitized specimen records. We found multiple lines of evidence that ecological filtering drove bee diversity loss in fragments: a disproportionate loss of functional diversity relative to taxonomic diversity, shifts in assemblage composition driven largely by the preferential extirpation of reserve-associated indicator species, and disproportionate loss of range-restricted species. However, we found no evidence of taxonomic or functional homogenization across fragment bee assemblages, suggesting that filtering was not sufficiently strong to cause a subset of functional traits (and their associated species) to dominate assemblages in fragments. Our results suggest that ecological filtering altered bee assemblages in habitat fragments, even when such fragments contained well-preserved native plant assemblages, underscoring the importance of preserving large areas of natural habitat for the conservation of bees (especially range-restricted taxa) and their associated ecological functions.

Vertebrate herbivory on shrub seedlings in California sage scrub: important but understudied interactions

Herbivory is poorly characterized in the endangered California sage scrub ecosystem, because previous studies were conducted only in winter and spring. To better understand vertebrate herbivore impacts, we first examined if and when vertebrate herbivores influence Artemisia californica seedling survivorship by examining differences in mortality between caged and uncaged seedlings. Then, we examined: (1) vertebrate herbivore preference of common sage scrub shrub species, and (2) which vertebrate species constitute important herbivores in the ecosystem. We found that mortality for uncaged A. californica seedlings was twice that of caged seedlings, and occurred in the summer and early fall, when previous studies in the system were not conducted. Short leaved shrubs, Artemisia californica and Eriogonum fasciculatum, were more susceptible to herbivory than broader leaved Salvia apiana and Eriodictyon trichocalyx. However, susceptibility likely depends on the herbivore assemblage, as the three main herbivore species [Sylvilagus audubonii (desert cottontail), Zonotrichia leucophrys (white-crowned sparrow; a bird, not a mammal), and Otospermophilus beecheyi (California gray squirrel)] all had different shrub preferences. Our results contrast with previous studies that suggests that native vertebrate herbivores facilitate shrub seedling survivorship by highlighting that late-season vertebrate herbivory may negatively affect shrub seedling survivorship. We demonstrate the complex but understudied role herbivores play in the sage scrub ecosystem, and highlight that further study of these key ecological interactions is warranted.