Pinus strobiformis in southwestern mixed conifer forests threatened by a non-native forest pathogen: Geographic and environmental variations in regeneration success and adaptive traits

Invasive species may induce drastic changes to native ecosystems and be selective forces that shape ecological and genetic structures of plant communities. Southwestern white pine (Pinus strobiformis Engelm., SWWP) is a high elevation five-needle pine and provides diversity, ecosystem services, and ecosystem resiliency in southwestern mixed conifer forests. Like all North American five-needle pine species, SWWP is susceptible to infection and mortality by the invasive forest pathogen that causes the disease white pine blister rust (WPBR). In order to better manage for species sustainability, it is essential to understand baseline (pre-invasion) and early-invasion effects on species' regeneration ecology and growth potential (Chapters 2 and 3), spatial patterns and interspecific dynamics (Chapter 4) and the species' adaptive traits and responses to stress (Chapter 5). Management recommendations should include results that form a complete picture of baseline ecology for sustainability (Chapter 6).;Regeneration surveys across a gradient of WPBR severities and past silvicultural treatments were conducted to determine how the disease has affected SWWP regeneration densities, what silvicultural treatments were conducive to stimulating regeneration and what variables could predict regeneration densities and height growth. Regeneration densities of SWWP varied across the six Arizona and New Mexico mountain ranges sampled, were lower in stands with severe WPBR and were generally lower in stands where recent (<20 years) silvicultural treatments had occurred. However, recently-managed stands had lower proportions of WPBR-infected SWWP regeneration and faster growing seedlings and should be used to help stimulate healthy, disease-free SWWP regeneration on the landscape. Diameter distributions of SWWP did not differ between areas with severe WPBR and areas where the disease does not occur or is considered low severity, but WPBR infections occurred across all tree size classes. Southwestern white pine regeneration densities increased with Pseudotsuga menziesii (Douglas-fir) seedling and sapling densities, and were lower in stands where shade-tolerant species dominated and high amounts of course woody debris occurred. Seedling yearly height growth increased with site index and on north-facing slopes, and decreased with increasing Pinus ponderosa (ponderosa pine) regeneration densities, percent ground cover of litter, increasing canopy closure and in the presence of a nearby microsite object.;Southwestern white pine regeneration aggregated with Douglas-fir regeneration, indicating they share similar niche spaces. No aggregated or dispersed spatial patterns were observed between SWWP and any other species of regeneration. Southwestern white pine seedlings collected from 24 seed sources were grown in a common garden study with two watering treatments. Growth and physiology traits were low to moderately differentiated between populations; among-population differentiation was larger in water-stressed seedlings compared to differentiation in ambient water treatments. Southwestern white pine seedlings acclimated to drier conditions with both physiological and morphological traits and lived 30-40 days without water, depending on population and seedling size. Populations from warmer climates grew larger, had higher stomatal densities and higher water-use efficiency. Seedlings from the northern edge of the species' distribution used water less efficiently but smaller, slower growing seedlings from northern populations may tolerate water stress better in prolonged drought. Reciprocal transplant studies, including seed sources across the species' entire range, will verify local adaptation and define seed transfer distances in a changing climate and with the threat of WPBR. These data on baseline regeneration ecology, adaptive traits and the early effects of WPBR on SWWP regeneration will hopefully save valuable time and money in proactive restoration and management activities to sustain this species in southwestern forests.