Seedling Ecology and Restoration of Blackbrush (Coleogyne ramosissima) in the Mojave Desert
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Worldwide, ecological communities are disappearing due to the joint stresses of climate change, species invasions and urbanization. In the southwestern U.S., the blackbrush (Coleogyne ramosissima) community is among the most threatened. It occurs primarily in the transition zone between the Mojave and Great Basin deserts and on the western border of the Sonoran Desert, forming vegetation bands at mid-elevation between 750 and 1,900 m from southeastern California to southwestern Colorado. Fire is the most important threat to this community, as blackbrush is slow to regenerate after fire, and fire frequency has increased due to global warming and invasion by exotic grasses, which added fine fuels. The blackbrush community could disappear entirely in the foreseeable future if steps are not taken to facilitate shrub regeneration after fire.
In this study, I tested the efficacy of several methods for promoting the germination and establishment of introduced seeds, each designed to mitigate potential recruitment bottlenecks: lack of sufficient moisture, seed and seedling predation, and high incident radiation. In addition, I tested these methods across an elevational gradient, to determine if best practices differ by location in the blackbrush range. I hypothesized that a) negative effects of moisture limitations and b) positive effects of proximity to nurse plants would decrease with elevation, while c) rates of seed or seedling predation would increase with elevation as plant productivity and rodent food availability increases.
I conducted two plot-scale experiments over two years in the Mojave Desert of southern California using a fully factorial design at each of three elevations. Factors included a) seed preparation (encapsulated in seed ball to trap moisture surrounding the seed, or bare seed), b) predator-exclusion (with or without wire cages), and c) proximity to mature shrubs (distant or near). Three elevations were chosen on the basis of local blackbrush abundance from low, at the lower boundary of its range, to high in the zone where blackbrush begins to exclude most other shrub species.
In the two years of the study, across sites and treatments, germination from bare seeds was much higher than from seed balls. Emergence and survival were highest in caged plots across the elevational gradient, and effect sizes increased with elevation. There was a positive effect of creosotebush nurse plants on emergence at low elevation, while proximity to a nurse plant interacted with the cage treatment mid- and high elevation such that there was higher predation under adult shrubs even though they provided a beneficial micro-climate in the presence of predator exclusion cages. Supporting my initial hypotheses, blackbrush regeneration was most limited by predation at the highest elevation, while positive nurse plant effects were maximized at lowest elevation. However, encapsulation of the seed had negative effects in the first growing season but permitted seeds to persist in the seed bank into the second growing season, creating a bet hedging strategy not naturally present in this species. More large seedlings were present at mid- and high elevation.
Blackbrush recruitment in the Mojave Desert depends on rare year types with higher than average precipitation and effective restoration has to take rainfall uncertainties into consideration. Use of seed balls may help blackbrush maintain a seed bank, which could be more important for achieving establishment than achieving high germination rates using bare seeds only in the year of application. Proximity to nurse plants should be a minor consideration, as the environmental benefits of germinating close to a mature shrub are partially or fully outweighed by predator attraction. My study shows that the careful examination of a species’ vulnerabilities and conditions for recruitment can help formulate sensible strategies for restoration and can be used as a model for the restoration of other threatened plant communities in a variety of contexts.