Birch sapling characteristics growing beyond the tree line in Norway and Sweden

Questions: How do population structure and recruitment characteristics of Betula saplings beyond the treeline vary among climatic regions, and what is the potential for development into tree-sized individuals with interacting grazing pressure? Location: Scandes Mountains. Methods: Sapling characteristics of Betula pubescens subsp. tortuosa, their topographic position above the treeline, growth habitat and evidence of recent grazing was investigated in three areas with a long continuous grazing history, along a latitudinal gradient (62-69°N). Results: Saplings were common up to 100 m above the treeline in all areas. The northern areas were characterised by small (<30 cm) and young (mean 14 years old) saplings in exposed micro-topographic locations unfavourable to long-term survival. In the southern area, broad height (2-183 cm) and age (4-95 years; mean 32 years) distributions were found in sheltered locations. Age declined with altitude in all areas. Sapling growth rate varied within and between areas, and the age x height interaction was significant only in the southern area. Growth rates decreased from south to north and indicated a considerable time required to reach tree size under prevailing conditions. Conclusions: Regional differences can be attributed to climatic differences, however, interacting biotic and abiotic factors such as micro-topography, climate and herbivory, mutually affect the characteristics of birch saplings. In view of the long time needed to reach tree size, the generally expected evident and fast treeline advance in response to climate warming may not be a likely short-term scenario. The sapling pool in the southern region possesses strongest potential for treeline advance.研究问题：林线（treeline）以上的桦木属（Betula）树苗种群结构与更新特征在不同气候区域间存在何种差异？在放牧压力的交互影响下，其发育为立木个体的潜力如何？ 研究区域：斯堪的纳维亚山脉（Scandes Mountains） 研究方法：沿纬度梯度（62°N~69°N）选取3个具备长期连续放牧历史的区域，针对曲枝毛桦（Betula pubescens subsp. tortuosa）的树苗特征、林线以上的地形位置、生长生境以及近期放牧痕迹开展调查。 研究结果：所有区域内，林线以上100米范围内均有桦树苗分布。北部区域的桦树苗多为小型（株高<30厘米）、幼龄（平均树龄14年）个体，且多分布于开阔裸露的微地形生境，不利于长期存活。南部区域的桦树苗则在庇护生境中呈现出较宽的株高（2~183厘米）与树龄（4~95年，平均32年）分布范围。所有区域内，桦树苗的树龄均随海拔升高而降低。各区域内部及区域间的树苗生长速率存在差异，且仅在南部区域中，树龄与株高的交互效应达到显著水平。生长速率从南到北逐渐降低，表明在当前环境条件下，树苗发育为立木个体需要耗费相当长的时间。 研究结论：区域间的差异可归因于气候差异，但微地形、气候与植食作用（herbivory）等生物与非生物交互因子共同调控桦树苗的特征。鉴于树苗发育为立木个体所需的周期较长，学界普遍预期的、气候变暖驱动下林线快速且显著的推进模式，在短期内可能难以实现。南部区域的树苗种群具备最强的林线推进潜力。