Ecology and restoration of blackbrush (Coleogyne ramosissima) in the Mojave Desert

Background/Question/Methods

Blackbrush, Coleogyne ramosissima, is an emblematic and ecologically important shrub that dominates elevations between 750 and 2,500 m in the Mojave Desert, USA. The blackbrush community is currently threatened by climate change and urban encroachment, which increase the risk of fire. Having naturally low recruitment rates, it is feared that this community could disappear without intense restoration management. The goals of the study were to understand the regeneration limitations of blackbrush across its elevation range and to help develop restoration guidelines. I performed a complete factorial plot experiment (n=3) near the Mojave National Preserve in California with sown seeds to quantify seedling emergence and survivorship. Factors included a) method of seed application, b) the use of predator-exclusion cages, and c) proximity to mature shrubs. I applied seed either directly or inside “seed balls” made from a clay/soil/fiber mixture. Wire mesh cages excluded mammalian predators. Plots were located either on the north side of an adult shrub (“nurse plant”) or in the open.  The experiment was repeated at three sites along an elevation gradient (1,260 m, 1,305 m, and 1,510 m). Precipitation, soil moisture, and soil temperature were recorded at each site.

Results/Conclusions

In the year of the study, all sites had similar precipitation, but soil moisture was higher and soil temperature lower at high elevation compared to mid- and low elevation.  Across all sites and treatments, germination from bare seeds was much higher than from seed balls. Emergence was overall highest at the highest elevation, where cages had strong positive effects. At lower elevations, the effect of nurse plants was stronger than the effect of cages. For survivorship, cages had predominantly positive effects and proximity to a nurse plant had either no or negative effects. Seedling density under cages after one year was highest at high elevation, but without cages, density was higher at low and mid-elevation. Our results suggest that, in its zone of dominance at high elevation, blackbrush regeneration is highly suppressed by seed and seedling predation, whereas at lower elevations, germination rates are more limiting, presumably due to hotter and drier soil conditions. Effective restoration at high elevation would require high rates of seed application, whereas restoration at lower elevation should be conducted to coincide with high rainfall years. The use of seed balls is not recommended for this species.