McLaren Thomas H, Tomback Diana F, Grevstad Nels, Wunder Michael B, Wehtje Walter, Walker Lauren E, Smith Douglas W
Department of Integrative Biology University of Colorado Denver Denver Colorado USA.
Present address: Klamath Bird Observatory Ashland Oregon USA.
Ecol Evol. 2023 Dec 21;13(12):e10813. doi: 10.1002/ece3.10813. eCollection 2023 Dec.
Clark's nutcrackers () are obligate seed dispersers for whitebark pine (), 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 (), Engelmann spruce (), limber pine (), and Douglas-fir (). 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.
克拉克星鸦（Nucifraga columbiana）是白皮松（Pinus albicaulis）的专性种子传播者，但由于球果产量的年度变化或白皮松可利用性的地理差异，它们经常利用其他针叶树种子资源。白皮松因包括白松疱锈病在内的多种威胁而数量减少，这可能导致星鸦和松树的种群数量下降。我们假设，在几乎没有其他种子资源的地方，白皮松成为支持星鸦种群的关键和限制性资源。我们研究了星鸦如何利用黄石国家公园内的针叶林群落类型，以确定潜在的种子资源以及白皮松的重要性。我们设立了代表五种森林群落类型的地点，包括白皮松、黑松（Pinus contorta）、恩氏云杉（Picea engelmannii）、柔枝松（Pinus flexilis）和花旗松（Pseudotsuga menziesii）。每个样带每年都进行星鸦点数统计、针叶树球果产量指数、群落组成数据以及种子资源利用观察。我们比较了分层距离抽样模型，估计星鸦密度及其与森林群落类型、种子收获时期、年份、研究地点和球果种子能量的关系。我们发现球果产量随年份变化，表明能量可利用性存在年度差异。星鸦密度最好由森林群落类型和调查时期预测，并且在收获中期白皮松林分中最高。所有其他森林群落类型的星鸦密度相对较低。这一发现强调了白皮松作为黄石国家公园克拉克星鸦关键种子资源的重要性。白皮松的减少可能导致星鸦和白皮松数量的螺旋式下降，这表明需要持续监测星鸦并实施白皮松恢复措施。
