Temporal and energetic drivers of seed resource use by Clark's nutcracker, keystone seed disperser of coniferous forests

Abstract

AbstractClark's nutcracker (Nucifraga columbiana) functions as a keystone seed disperser and ecological mobile link for many western conifers. The bird is the primary seed disperser for limber pine (Pinus flexilis), which is an important seed resource for the bird. In the Southern Rocky Mountains, annual variation in limber pine cone production and growing threats, such as exotic disease, require that nutcrackers rely on additional conifer seed resources. We investigated the utilization of limber pine, ponderosa pine (Pinus ponderosa), and Douglas‐fir (Pseudotsuga menziesii) as seed resources by nutcrackers to determine how temporal variation in seed production and maturation drives nutcracker seed resource and habitat use decisions. Working in Rocky Mountain National Park (RMNP) during three field seasons, we examined (1) cone production in limber pine, ponderosa pine, and Douglas‐fir, and seed energy availability across the RMNP landscape; (2) timing and likelihood of nutcracker seed harvest and caching behavior for each conifer species; and (3) predictors of nutcracker visitation to the different forest stand types. Each year starting in mid‐ to late August, nutcrackers foraged on limber pine seeds. In 2014 and 2015, nutcrackers transitioned from limber pine to harvesting ponderosa pine seeds. In 2016, a year of low ponderosa pine cone production but exceptionally high Douglas‐fir cone production, they transitioned from limber pine to Douglas‐fir seeds. Cone density was a significant predictor of nutcracker counts in every negative binomial regression model in which it was used. However, the best overall model, which included interaction effects, indicated that stand type, year, and month were better predictors. We interpret these three predictors as reflecting temporal variation in patterns of cone production and maturation among the three conifers but also in habitat use by nutcrackers. In all three years of study, limber pine alone did not provide sufficient energy resources for foraging and caching, and nutcrackers used an additional conifer seed resource. With potential future limber pine losses, these other seed resources may be essential to sustain a nutcracker population. We suggest that Douglas‐fir and especially ponderosa pine will serve as increasingly important food resources as limber pine declines across the montane West.