Using plastic tree shelters to improve post-fire ecological restoration of California chaparral and coastal sage scrub communities

Background/Question/Methods

Southern California shrubland communities are experiencing increased invasion by non-native grasses due to increasing fire frequency and slow recovery of native shrubs following fire. Recovery tends to be even slower on steep slopes and south-facing aspects due to the aridity of these areas. For these reasons, restoration methods that are rapid, relatively low-cost, and that can improve native plant recruitment are needed to improve restoration outcomes in these communities. Plastic tree shelters, typically used to protect seedlings from herbivory, have been shown to ameliorate physical stresses experienced by seedlings and provide them with a favorable growing microclimate. This study examined how tree shelters affect seedling germination and establishment in a restoration of a highly invaded California native mixed chaparral and coastal sage scrub community in the Copper Fire area of the Angeles National Forest. The study incorporated 30 sites, which were selected to encompass the broadest range of slopes and aspects available in order to test the effectiveness of tree shelters across the various growing conditions exhibited by these landscape features. At each site, locally-harvested seed of native chaparral species (Eriogonum fasciculatum, Salvia leucophylla, and Salvia mellifera) was seeded in exposed, caged, and sheltered treatments. A nursery outplant treatment of the same chaparral species was also added to compare the cost and success of seeding in shelters versus outplanting.

Results/Conclusions

Overall, the sheltered treatment had 34 percent more subplots with seedlings than the caged treatment and 64 percent more subplots with seedlings than the exposed treatment. E. fasciculatum germinated at a rate that was more than four times higher in the sheltered treatment and nearly three times higher in the caged treatment compared to the exposed. S. leucophylla had a germination rate that was eighteen times higher in the sheltered treatment and nine times higher in the caged treatment compared to the exposed. S. mellifera germinated at a rate that was six times higher in the sheltered treatment and three times higher in the caged treatment compared to the exposed. Trends showed that the sheltered treatment had more live seedlings present on steeper slopes than the other two treatments, and north-facing aspects had a higher proportion of subplots with live seedlings compared to south-facing aspects; however, these patterns were not statistically significant. We found that sheltered subplots had consistently higher germination and seedling survival rates than exposed subplots, which was even more apparent on steep slopes.