Characterizing Betula litwinowii seedling microsites at the alpine-treeline ecotone, central greater caucasus mountains, Georgia

Abstract

Seedling establishment is an important factor dictating the altitudinal limits of treeline species. Factors which affect seedling mortality and survival, however, have yet to be fully characterized, especially for deciduous treeline species. Here we describe microsite characteristics of successfully established Betula litwinowii seedlings at the alpine-treeline ecotone. Possible harmful effects of sky exposure on seedling physiology (i.e. photoinhibition of photosynthesis) were also examined, as well as possible facilitative effects of co-occurring Rhododendron caucasicum shrubs on northern slopes and microtopographical depressions (mainly watercourses) in ridgetop meadows. On northern slopes, seedling density was highest in newly exposed soils, with 90% of the youngest seedlings (<2 cm) occurring in patches of rocky, bare, or moss-covered soils within the Rhododendron thicket. R caucasicum was not a significant source of shade for B. litwinowii, as most seedlings were established 0.25-0.5 m away from the nearest shrub, and shade cover generated by R caucasicum was observed in only 1% of seedlings at midday. On ridgetops, density of B. litwinowii was sixfold higher inside microtopographical depressions compared to outside. Sky exposure of seedlings within depressions was similar to northern slopes, ranging from 50% to 87%. Across all microsites, seedlings were most abundant under 70-87% sky exposure. This preference for open microsites, combined with the observation that sustained photoinhibition of photosynthesis (Fv/Fm < 0.65) was observed only in the most open microsites (i.e. >80% sky exposure), suggests that sky exposure is likely not a significant factor affecting seedling mortality in B. litwinowii, in contrast to results reported for conifer and broadleaf evergreen species at treeline. A higher photosynthetic capacity and a deciduous life history may provide both tolerance and avoidance to the physiological stresses associated with high sky exposure for B. litwinowii seedlings, and other factors, such as soil moisture, more likely account for successful establishment within microtopographical depressions and R. caucasicum thickets. © 2009 Regents of the University of Colorado.