Patterns of forest structure, competition and regeneration in southwestern white pine (Pinus strobiformis) forests

Pinus strobiformis (southwestern white pine, abbr. PIST), a species that comprises a small but significant component of the mixed-conifer forests of the U.S. Southwest, faces an increasing threat from white pine blister rust (Cronartium ribicola, abbr. WPBR). This potentially lethal, non-native fungal pathogen is windborne and, therefore, virtually impossible to control. However, silvicultural strategies, such as thinning, reintroduction of natural fire regimes, and encouraging natural selection for rust-resistant genotypes, show promise for maintaining or restoring infected stands. To expand ecological knowledge of the species and guide management, we surveyed PIST in mixed-conifer stands throughout Arizona and portions of New Mexico. We examined PIST forest structure, regeneration, and competitive interactions. We found that PIST occupied lower canopy positions in Pinus ponderosa-dominated plots, thereby supporting its classification as an intermediate shade-tolerant species. Plots where PIST was a dominant component showed evidence of succession towards more tolerant conifers. PIST regeneration occurred relatively rarely and was negatively associated with increasing understory plant cover and open conditions. Although PIST faced heavy tree competition, with Pseudotsuga menziesii the most important competitor, small suppressed trees showed evidence of persistence.

► We examine structure, competition, and regeneration in Pinus strobiformis habitats.
► P. strobiformis shows evidence of moderate shade tolerance.
► P. strobiformis regeneration is comparatively uncommon and avoids open conditions.
► We found evidence that P. strobiformis faces, yet tolerates, heavy competition.
► Our results have implications for management given threats from Cronartium ribicola.