Assessing trends and vulnerabilities in the mutualism between whitebark pine (Pinus albicaulis) and Clark’s nutcracker (Nucifraga columbiana) in national parks of the Sierra-Cascade region

Abstract

Dispersal of whitebark pine (Pinus albicaulis Engelm.), a keystone species of many highelevation ecosystems in western North America, depends on Clark's nutcracker (Nucifraga columbiana Wilson), a seed-caching bird with an affinity for whitebark seeds. To the extent that this dependence is mutual, declines in whitebark seed production could cause declines in nutcracker abundance. Whitebark pine is in decline across much of its range due to interacting stressors, including the non-native pathogen white pine blister rust (Cronartium ribicola J. C. Fisch.). We used avian point-count data and tree surveys from four national park units to investigate whether trends in whitebark pine can explain trends in Clark's nutcracker. Spatial trends were modeled using recent data from two parks, while temporal trends were modeled using longer time-series of nutcracker and whitebark data from two additional parks. To assess the potential dependence of nutcrackers on whitebark, we linked a model of nutcracker density (accounting for detection probability) with a model of whitebark trends, using a Bayesian framework to translate uncertainty in whitebark metrics to uncertainty in nutcracker density. In Mount Rainier National Park, temporal models showed dramatic declines in nutcracker density concurrent with significant increases in whitebark crown mortality and trees infected with white pine blister rust.

Clark's nutcracker forest community visitation: Whitebark pine maintains a keystone seed disperser

“…Our primary study aim was to compare predicted nutcracker density among predefined forest community types found within Yellowstone National Park. The magnitude of nutcracker forest community use during the late summer seed harvesting and caching period was previously related to metrics of seed resources and abundance (Barringer et al, 2012;Ray et al, 2020;Williams et al, 2020).…”

“…Threats to whitebark pine forests, especially white pine blister rust and recent large-scale mountain pine beetle outbreaks, lead to a downward spiral of both birds and whitebark pine regeneration, as observed in areas such as Glacier and Mt. Rainier national parks (Barringer et al, 2012;Ray et al, 2020;Shanahan et al, 2016;Shepherd et al, 2018;Smith et al, 2011).…”

“…They also utilize as seed resources the other species listed in Figure 1, depending on local availability and annual cone production, and move from one seed resource to another following cone ripening phenology (e.g., Ray et al, 2020;Samano & Tomback, 2003;Tomback, 1978;Williams et al, 2020). During years of low cone production for the earlier ripening whitebark pine and limber pine, nutcrackers converge on later-ripening resources such as ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) (e.g., Williams et al, 2020).…”

“…Across their range, Clark's nutcrackers prefer the most energetically rewarding seed sources—usually conifers with large, wingless seeds, including whitebark pine (Tomback, 1978), limber pine (Benkman et al., 1984; Tomback & Kramer, 1980; Vander Wall, 1988), southwestern white pine (Benkman et al., 1984; Samano & Tomback, 2003), Colorado pinyon ( Pinus edulis ) (Vander Wall & Balda, 1977), and single‐leaf pinyon ( Pinus monophylla ) (Tomback, 1978; Vander Wall, 1988). They also utilize as seed resources the other species listed in Figure 1, depending on local availability and annual cone production, and move from one seed resource to another following cone ripening phenology (e.g., Ray et al., 2020; Samano & Tomback, 2003; Tomback, 1978; Williams et al., 2020). During years of low cone production for the earlier ripening whitebark pine and limber pine, nutcrackers converge on later‐ripening resources such as ponderosa pine ( Pinus ponderosa ) and Douglas‐fir ( Pseudotsuga menziesii ) (e.g., Williams et al., 2020).…”

“…Ray et al. (2020) analyzed multiyear point count data from four national parks in the Sierra‐Cascade region, which suggested a decline in nutcracker abundance over time (Ray et al., 2017). They found that nutcracker density was positively related to whitebark pine cover in Sequoia‐Kings Canyon and Yosemite National Parks, and that trends in Mount Rainier National Park indicated declines in nutcracker density as WPBR incidence increased.…”