The Role of Habitat Suitability Modeling and Tree Shelters in Coastal Sage Scrub Restoration by Seed

Abstract

Slope and aspect are important factors in determining where native plant communities occur in seasonally dry climates, and yet are underutilized in locating sites for restoration of native species in many semi-arid ecosystems such as California coastal sage scrub (CSS), an important and threatened habitat. Habitat suitability modeling aims to identify high priority sites where abiotic conditions may be more favorable for native plant re-establishment in heavily disturbed and degraded landscapes. As “high suitability” sites (for example, north-facing, moderate-sloped areas) may not be accessible in all restoration projects, the use of artificial tree shelters may also improve abiotic conditions and facilitate native plant recruitment in a range of stressful conditions. Tree shelters have been used traditionally to provide herbivory protection and understanding of their value as a beneficial microclimate for seed germination and seedling establishment is more recent. Very few studies of either restoration seeding or restoration of semi-arid shrubland communities have utilized tree shelters. Furthermore, the importance of identifying techniques to improve native plant restoration from seed is growing, especially as an alternative to nursery propagation which may be costly, time-consuming, and carries the risk of spreading nursery-borne pathogens to wildlands. The primary objectives of this thesis were to investigate the effect of microclimates produced by both natural topographic landscape features, plastic tree shelters, and the interaction of the two, on the germination, establishment, and growth of four native CSS species representing diverse life-history traits and growth strategies. A two-year restoration experiment was established within a disturbed CSS habitat in southern California to test for differences in native plant establishment from seed in high and low suitability sites, both in and out of plastic tree shelters. Seeds were sown directly in the field within experimental plots, and survival and growth data were collected along with abiotic factors including solar radiation, soil moisture, and sediment erosion. Shelters had lower solar radiation and sediment erosion levels, improved germination of all species, and had a greater impact on the survival of woody species compared to herbaceous species. High Suitability sites had increased soil moisture and lower soil temperatures and sediment erosion rates, and also improved shrub survival secondarily to shelters. As sediment erosion was the abiotic variable with the most consistent responses to experimental treatments (Shelters limited all sediment movement, and within the No Shelter treatments High Suitability plots always had lower sediment movement rates than Low Suitability plots), major results of this study are in highlighting the role of sediment erosion in preventing seedling establishment, and in demonstrating the effectiveness of tree shelters in retaining seed and inhibiting seedling burial in steep, exposed sites. Additionally, shelters reduced dessicating solar radiation levels and should continue to provide an increasingly important abiotic microclimate for seedling regeneration under climate change scenarios of more stressful environmental conditions in Mediterranean and other ecosystems.