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

Abstract

Clark's nutcrackers (Nucifraga columbiana) are obligate seed dispersers for whitebark pine (Pinus albicaulis), but they frequently use other conifer seed resources because of annual variability in cone production or geographic variation in whitebark pine availability. Whitebark pine is declining from several threats including white pine blister rust, leading to potential population declines in the nutcracker and the pine. We hypothesize that where there are few additional seed resources, whitebark pine becomes the key and limiting resource supporting nutcracker populations. We investigated how nutcrackers use coniferous forest community types within Yellowstone National Park to determine potential seed resources and the importance of whitebark pine. We established sites representing five forest community types, including whitebark pine, lodgepole pine (P. contorta), Engelmann spruce (Picea engelmannii), limber pine (P. flexilis), and Douglas-fir (Pseudotsuga menziesii). Each transect annually generated nutcracker point counts, conifer cone production indices, community composition data, and seed resource use observations. We compared hierarchical distance sampling models, estimating nutcracker density and its relationship to forest community type, seed harvesting time-period, year, study site, and cone seed energy. We found cone production varied across years indicating annual variability in energy availability. Nutcracker density was best predicted by forest community type and survey time-period and was highest in whitebark pine stands during the mid-harvesting season. Nutcracker density was comparatively low for all other forest community types. This finding underscores the importance of whitebark pine as a key seed resource for Clark's nutcracker in Yellowstone National Park. The decline of whitebark pine potentially leads to a downward spiral in nutcrackers and whitebark pine, arguing for continued monitoring of nutcrackers and implementation of restoration treatments for whitebark pine.

Keywords: Clark's nutcracker; Yellowstone National Park; cone production; forest communities; hierarchical distance sampling model; seed caching whitebark pine; white pine blister rust.

FIGURE 1

Seed resources used by Clark's nutcracker (Nucifraga columbiana) across its range, according to the cited studies for each region. The reliable range of the nutcracker ends south of the Sacramento Mountains and Mogollon Plateau, New Mexico, and at its northern edge at about 55° (Tomback, 1998).

FIGURE 2

Forest types of Yellowstone National Park overlaid on a topographical map. Data accessed from the National Park Service IRMA database shows a simplified version of forest habitat types (1992). Areas on map designated as “Water” represent the major lakes within the park. We added locations of our two known limber pine stands on the forest types map layer, indicated by yellow circles; these forest communities are too spatially limited to be visible on the map at this scale. Limber pine also has a scattered distribution within the park along stream and river banks and on steep, rocky slopes (D. F. Tomback, T. H. McLaren, W. Wehtje, L. E. Walker, D. W. Smith, personal observations).

FIGURE 3

Prediction plot of average nutcracker density within each forest community type within our study during the three survey periods. Early‐, mid‐, and late‐harvesting time periods are presented from left to right. Intervals represent 95% confidence intervals around each estimate. Nutcracker density confidence intervals for whitebark pine did not overlap with confidence intervals for other forest types, whereas confidence intervals for all other forest types overlapped with one another.

FIGURE 4

Left: Detection function scale parameter in response to the density of DBH stems (per 500 m²) from community assessment surveys. Blue lines indicate 95% confidence intervals. Smaller values of sigma, the scale parameter for the half‐normal detection function, result in a lower detectability estimate for nutcrackers. Right: Estimated detection probability at minimum, midpoint, and maximum stem density values.

FIGURE 5

Model‐based predictions from negative binomial regression of cone count data. PIAL, whitebark pine; PICO, lodgepole pine; PIEN, Engelmann spruce; PIFL, limber pine; PSME, Douglas‐fir. Intervals represent 95% confidence intervals around predicted values. Whitebark pine and lodgepole pine had relatively few cones per tree in comparison to Engelmann spruce, limber pine, and Douglas‐fir. For all conifer species, cones per tree were estimated as lower in 2019 than in 2020 and 2021. Engelmann spruce study sites were first established in 2020, and as a result, there are no cone count data for Engelmann spruce for 2019.