Regeneration of whitebark pine in the timberline ecotone of the Beartooth Plateau, U.S.A.: Spatial distribution and responsible agents

Abstract

Whitebark pine (Pinus albicaulis) is a high elevation stone pine characterized by heavy, wingless seeds that are primarily dispersed by the Clark's nutcracker (Nucifraga columbiana). The relationship between seed dispersal, spatial distribution, and site characteristics of whitebark pine was studied in the timberline ecotone of the Beartooth Plateau, Montana and Wyoming. The study focused on regeneration patterns and prevailing microsite conditions which may limit or promote germination and survival of whitebark pine at its upper elevational limit. Regeneration of Engelmann spruce (Picea engelmannii)and subalpine fir (Abies lasiocarpa) was poor in both study areas. Juvenile whitebark pines were more abundant in areas with moderate to late snow release. Only few young whitebark pine seedlings and germinants were located on Tibbs Butte, whereas in the Wyoming Creek study area, young whitebark pine seedlings clusters (≤ 3 years) were present in all transects. Regeneration densities inside and leeward of the woodland were higher than on the windward side. Regeneration results were not consistent with reported caching preferences of the Clark's nutcracker. Sites with moderate to long snow cover, leeward of tree groups or in depressions, appear unfavorable for caching, because of restricted access to stored seeds, but were favorable for germination and survival of whitebark pine. Only nutcracker caches that are not retrieved and that are established in relatively moist and protected microsites contribute to recruitment. The data of this study show that regeneration of whitebark pine does occur in parts of the timberline ecotone. Recruitment in exposed sites appears unsuccessful. Tree regeneration in timberline ecotones with continental climate character may require the additional moisture found in snowdrifts of small depressions or tree groups. © 2005 Springer-Verlag Berlin Heidelberg.