Journal Article

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

McLaren, Thomas H., Wunder, Michael B., Walker, Lauren E., Grevstad, Nels, Wehtje, Walter, Smith, Douglas W., Tomback, Diana F.

2023

Overview

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. The coevolved mutualism between whitebark pine (Pinus albicaulis) and Clark's nutcracker (Nucifraga columbiana) is well documented, but our understanding of nutcracker use of additional conifer seed resources is lacking. Data collected over 3 years in Yellowstone National Park suggest that whitebark pine was the only seed resource that nutcrackers visited regularly and that bird density was highest during mid‐September when whitebark pine seeds ripen. In areas without additional seed resources to sustain populations of nutcrackers, we hypothesize that Clark's nutcracker populations may be more directly linked to whitebark pine seed production.

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Annual variations / Availability / Blister rust / Blistering / Clark's nutcracker / Community composition / cone production / Coniferous forests / Coniferous trees / Conifers / Density / Dispersion / Endangered & extinct species / Epidemics / Evergreen trees / Forest communities / Forests / Geographical variations / hierarchical distance sampling model / National parks / Pathogens / Picea engelmannii / Pine / Pine trees / Pinus albicaulis / Population decline / Pseudotsuga menziesii / seed caching whitebark pine / Seeds / white pine blister rust / Yellowstone National Park