Drivers of twoneedle pinyon (Pinus edulis) cone productivity: Implication for wildlife

Abstract

Seeds of twoneedle pinyon (Pinus edulis) serve as a nutritionally dense food resource for a myriad of wildlife. Pinyons exhibit synchronist masting with high cone production occurring every 4–7 years. Per-tree cone abundance may be influenced by tree-level and site-level features. Our goal was to identify features that influence pinyon cone abundance and determine how cone production relates to habitat selection by the Oscura Mountains Colorado chipmunk (Neotamias quadrivittatus oscuraensis), an old growth pinyon-juniper specialist and seed-caching rodent. We hypothesized increased cone abundance would be exhibited by older trees and at sites with increased water availability. We hypothesized that the chipmunk would select sites exhibiting increased 8-year cone production and annual cone abundance. We used double observer abscission scar counts on five branches per tree averaged between observers to estimate tree cone abundance for the most recent eight years. We used linear modeling to determine tree- and site-level features influencing per tree cone abundance, incorporating site as a random-intercept for tree-level models. The most influential tree-level feature was diameter, with larger diameter trees having greatest cone abundance. Percent defoliation in the living tree canopy exhibited an inverse relationship with cone abundance. Water availability of a site was represented by reproductive tree density and heat load, both of which inversely influenced per-tree cone abundance. However, reproductive tree density exhibited a positive relationship with cone production across a stand. Our data did not support the prediction that the chipmunk would select sites based on mean 8-year cone production, although there was a suggestion that it preferentially used sites with higher annual cone abundance during a high mast year. Though it requires further testing, sites that produce high cone abundance might be important for maintaining chipmunk metapopulations. We encourage management of pinyon-juniper woodlands to consider drivers of pinyon cone productivity and influence of cone production on wildlife. We urge the protection of old growth stands of pinyon-juniper woodland to support wildlife, as older trees are the most influential determinant of cone abundance and concurrently provide structural complexity important for wildlife. Lastly, our study indicates thinning of pinyon-juniper woodlands as unwarranted for the goal of increasing cone production. Maximizing pinyon cone production would be best met by maintaining dense old growth stands.