The regeneration of whitebark pine in the timberline ecotone of the Beartooth Plateau, Montana and Wyoming
Pinus albicaulis ist eine nordamerikanische Hochlagenkiefer die sich durch schwere, flügellose Samen und einer zoochoren Verbreitung durch den nordamerikanischen Tannenhäher (Nucifraga columbiana) auszeichnet. Diese Arbeit beschäftigt sich mit der Verjüngung von Pinus albicaulis im Waldgrenzökoton des Beartooth Plateaus, Montana und Wyoming. Die Beziehung zwischen zoochorer Samenverbreitung, räumlicher Verteilung und den jeweiligen Standortfakoren von Pinus albicaulis wurde mit Hilfe von Kartierungen und Keimungsexperimenten untersucht. Die Ergebnisse der Untersuchungen zeigen, daß eine Verjüngung von Pinus albicaulis im Waldgrenzökoton möglich ist, und in lokalklimatisch günstigen Geländebereichen auch stattfindet. Jungwuchs kommt vorwiegend leewärts von Baumgruppen und in konkaven Hangabschnitten mit mittlerer bis langer Schneebedeckung und relativ hoher Bodenfeuchte auf. Exponierte, schneearme Standorte sind im Untersuchungsgebiet aufgrund des kontinentalen Klimacharakters ungünstig für eine erfolgreiche Verjüngung. Die Verüngung von Pinus albicaulis oberhalb der aktuellen Waldgrenze konnte nicht nachgewiesen werden.
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, site characteristics, and tree distribution 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. The composition and structure of ecotonal forest and woodland stands were different in the two areas studied. Engelmann spruce (Picea engelmannii) was the dominant tree species on Tibbs Butte, where it formed decumbent tree islands. Whitebark pines were usually growing inside or leeward of tree islands and were younger than Engelmann spruce. In the Wyoming Creek study area, whitebark pine was the dominant tree species, forming an open woodland leeward of a minor ridge. Regeneration of Engelmann spruce and subalpine fir (Abies lasiocarpa) was poor in both study areas. No young fir or spruce seedlings (£ 3 years) were located during the study. 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. Germination of whitebark pine seeds generally occurred two years after planting. Between 1992 and 1994, 24% of all planted seeds germinated in 38% of all seed caches. Germination rates in experiment sites varied between 0 and 84% of planted seeds, with no germination on Tibbs Butte or on sites with the earliest snow melt. Higher maximum July surface temperatures and late snow melt corresponded with a higher germination probability of whitebark pine. Environmental conditions favorable for whitebark pine germination were not necessarily favorable for seedling establishment. Seedlings were most vulnerable to injury or death immediately after germination. Insolation damage and drought were judged to be major causes of mortality during the first growing season. Eight years after the major germination event, 14% of all seedlings and 20% of germinated seedling clusters (8% of planted seed caches) were still alive. High survival rates were correlated with high topographic moisture indices, presence of shade, and low covers of coniferous litter. Plant composition was a suitable indicator for site conditions favorable for whitebark pine regeneration. All experiment sites with Salix glauca vegetation and dry Geum rossii turf had good germination and survival success. 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. Descriptive and experimental data show that regeneration of whitebark pine does occur in parts of the timberline ecotone. Recruitment in exposed sites or areas at higher elevation was unsuccessful. The lack of sexual reproduction of tree species on Tibbs Butte suggest that the timberline stand in this area may be a relict reflecting past climatic conditions. The results of this study do not provide any evidence that whitebark pine is invading alpine meadows above the current timberline ecotone.