Microplastic abundance in feces of lagomorphs in relation to urbanization

The presence of microplastics (MPs) in marine environments has been extensively documented. However, studies of terrestrial species are scarce. Fecal samples (105) of lagomorphs were collected at sites with different levels of urbanization in the Baja California Chaparral and analyzed to quantify and characterize MPs found in the feces. The lagomorph species recorded in the study area are the desert cottontail rabbit (Sylvilagus audubonii), brush rabbit (Sylvilagus bachmani), and black-tailed jackrabbit (Lepus californicus), which play important roles in the food web of the chaparral ecosystem. Microplastics were identified using attenuated total reflectance Fourier transform infrared spectroscopy. Microplastics were detected in 49 % of the samples, with fibers being the dominant shape found (72 %). Most (75 %) of the MPs were <1 mm in size, with a mean length of 0.93 ± 0.99 mm (median 0.60 mm, range 0.02 - <5 mm). Polyamide was the dominant polymer (54 %), indicating that MPs are likely derived from textiles; polyethylene was also abundant (27 %). A difference was also observed in the abundance of MPs in feces from sites with different levels of urbanization, with the highest abundance in feces from the urban sites.

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.

Chronic N enrichment and drought alter plant cover and community composition in a Mediterranean-type semi-arid shrubland

Anthropogenic nitrogen (N) deposition has caused a decline in native plant species and an increase in exotic plant species in many terrestrial ecosystems; however, vegetation change depends on the rate and/or duration of N input, individual species responses, interactions with other resources, and ecosystem properties such as species richness and canopy cover, soil texture, pH, and/or disturbance regime. Native shrub and exotic forb responses to N enrichment were evaluated over a 13-year field experiment in a mature coastal sage scrub (CSS) shrubland of southern California to test the hypothesis that dry-season N input will cause a decline in native shrubs and an increase in exotic annuals. Nitrogen enrichment caused the dominant native shrubs, Artemisia californica and Salvia mellifera, to respond differently, with A. californica initially increasing with N input but declining thereafter and S. mellifera declining consistently over the 13-year-period. Both species exhibited higher canopy dieback during drought conditions, especially in N plots. Brassica nigra, an exotic annual, invaded N plots significantly more than control plots, but only after 10 years of N addition and a prolonged drought, which increased native shrub canopy dieback. These results indicate a possible synergism between N enrichment and drought on native shrub and exotic forb abundance, which would have important implications for plant diversity in semi-arid shrublands of southwest US that are anticipated to experience an increase in anthropogenic N enrichment and the frequency and duration of drought.

Use of ultra-high spatial resolution aerial imagery in the estimation of chaparral wildfire fuel loads

Development of methods that more accurately estimate spatial distributions of fuel loads in shrublands allows for improved understanding of ecological processes such as wildfire behavior and postburn recovery. The goal of this study is to develop and test remote sensing methods to upscale field estimates of shrubland fuel to broader-scale biomass estimates using ultra-high spatial resolution imagery captured by a light-sport aircraft. The study is conducted on chaparral shrublands located in eastern San Diego County, CA, USA. We measured the fuel load in the field using a regression relationship between basal area and aboveground biomass of shrubs and estimated ground areal coverage of individual shrub species by using ultra-high spatial resolution imagery and image processing routines. Study results show a strong relationship between image-derived shrub coverage and field-measured fuel loads in three even-age stands that have regrown approximately 7, 28, and 68 years since last wildfire. We conducted ordinary least square analysis using ground coverage as the independent variable regressed against biomass. The analysis yielded R ² values ranging from 0.80 to 0.96 in the older stands for the live shrub species, while R ² values for species in the younger stands ranged from 0.32 to 0.89. Pooling species-based data into larger sample sizes consisting of a functional group and all-shrub classes while obtaining suitable linear regression models supports the potential for these methods to be used for upscaling fuel estimates to broader areal extents, without having to classify and map shrubland vegetation at the species level.

Differential survival of chaparral seedlings during the first summer drought after wildfire

Big Pod Ceanothus (Ceanothus megacarpus) is an obligate seeder after fire; Laurel Sumac (Rhus laurina) is primarily a resprouter after fire. Both species commonly occur together in mixed stands and are dominant members of the coastal chaparral of southern California. We compared the mean survival of post-fire seedlings of each species during the first summer drought after fire and found C. megacarpus to have a mean survival of 54% while R. laurina had a mean survival of only 0.1%. Rooting dephs were similar between species but predawn water potentials and leaf temperatures were higher for R. laurina seedlings. Leaf temperatures for R. laurina reached a mean value of 46.8° C on hot, summer days, about 5° C higher than seedlings of C. megacarpus. By the end of the first growing season, 92% of all C. megacarpus seedlings had suffered herbivory compared to only 17% of all R. laurina seedlings. Herbivory did not appear to be the immediate cause of seedling mortality. Transect data indicated that full recovery of prefire species composition and density at our study site was likely but the mode of recovery was different for the species examined. R. laurina recovered primarily by sprouting, C. megacarpus totally by seedling establishment and a third species, Adenostoma fasciculatum (chamise), by a combination of sprouting and seedling establishment. We attribute the higher mortality of R. laurina seedlings to the greater sensitivity of its tissue to water stress. It may be that differential survival of shrub seedlings and differential modes of reestablishment after fire play an important role in maintaining species diversity in the chaparral communities of coastal, southern California.

Tissue water relations of four co-occurring chaparral shrubs

Chaparral shrubs of California have a suite of morphological and physiological adaptations to withstand the prolonged summer droughts of a mediterranean climate. Not all species of chaparral have the same rooting depth and there is some evidence that those with shallow roots have tissue that is most tolerant to water stress. We tested this notion by comparing the tissue water relations of four co-occurring chaparral shrubs: Quercus durata, Heteromeles arbutifolia, Adenostoma fasciculatum, and Rhamnus californica. We used a pressure-volume technique and a dew-point hygrometer to metsure seasonal changes in osmotic potential when plant tissue was fully hydrated and osmotic potential at predawn, midday, and the turgor loss point. We also calculated seasonal changes in the minimum daily turgor potential, saturated weight/dry weight ratio of leaf tissue, and the bulk modulus of elasticity. We had information on the seasonal water use patterns and apparent rooting depths of these same four shrubs from a previous study (Davis and Mooney 1986). All evidence indicated that Rhamnus had shallow roots and Quercus deep roots. Our results indicated that the tissue water relations of our four co-occurring chaparral shrubs were not alike. Even though Rhamnus had shallow roots, it had the least xerophytic tissue. Seasonal osmotic potential and saturated weight/dry weight ratios were relatively high and bulk modulus of elasticity and minimum daily turgor potentials were low. Furthermore, even though Quercus had deep roots and experienced no seasonal water stress at our study site, its tissue water relations indicated relatively high tolerance to water stress. We conclude that seasonal drought tolerance of stem and leaf tissue of co-occurring chaparral shrubs does not necessarily correspond to rooting depth, to soil moisture resources available to the shrub, or to the degree of seasonal water stress experienced by the shrub.