Factors Regulating Population Stand Structure in Blackbrush (Coleogyne ramosissima: Rosaceae), a Masting North American Desert Shrub

Abstract

Blackbrush (Coleogyne ramosissima Torr.) is the dominant shrub on three million hectares across the transition zone between the western North American warm and cold deserts. This paper presents a study of blackbrush population structure at the stand level at sixteen sites across the species range. New stand-level information is then integrated with what is already known about blackbrush population ecology to explore the stand-level consequences of several unusual features of blackbrush life history, including its masting reproductive strategy, its interactions with heteromyid rodents that are both seed predators and dispersers, and its ability to form ‘seedling banks’ of growth-suppressed individuals, often within the crowns of adult conspecifics. It complements earlier work showing that blackbrush stands are organized at both the inter-clump and intra-clump levels. Each clump represents an establishment nexus where younger individual genets replace older genets over an extended time period. Inter-clump structure is thus determined by the rate of establishment of new clumps rather than new individuals. This has resulted in contrasting stand structures in response to rodent community composition, disturbance regimes, and climatic variability at the leading and trailing edges of current blackbrush distribution in the eastern Mojave Desert and Colorado Plateau regions. Because blackbrush likely disperses too slowly to track anthropogenic climate change, assisted migration with wild-collected seeds may be necessary to promote its continued survival and dominance.