Seedling Recruitment at the Upper Limit of Tree Growth in the Alborz Mountains, Northern Iran: Safe Site Characteristics and Edaphic Conditions

Climate warming may enhance the upslope migration of tree species at high elevations. In this context, few studies have analyzed the requirements of tree seedlings with respect to safe sites and soil conditions, particularly in the Alborz Mountains (Iran). We collected regeneration and environmental data focusing on small seedlings and young saplings at the upper limit of tree growth in the Alborz Mountains. For each life stage, we assessed the association of seedlings with safe site conditions (rocks, stones, deadwood, and canopy shade), microhabitat substrates (vegetation, litter, bare soil, and stones), and soil variables along elevational gradients. Our findings revealed strong associations between seedlings and facilitative elements (tree shade, deadwood, rocks, and stones). Fewer associations were observed with microhabitat substrates. These associations differed between the two establishment stages. In addition, seedlings of both life stages exhibited distinct associations with soil variables (total nitrogen, sand, and bulk density). The contrasting relationships between small seedlings and young saplings with safe site conditions, microhabitat substrates, and soil variables suggest that early life stages may have different site requirements compared to later stages. Our results suggest that the potential for upslope migration of tree species may depend on safe site conditions, particularly those provided by tree shade, deadwood, rocks, and stones.