Tree line population monitoring of Pinus sylvestris in the Swedish Scandes, 1973–2005: implications for tree line theory and climate change ecology

Summary Demographic trends of Pinus sylvestris L. (Scots pine) tree line populations are reported for a 32‐year monitoring period (1973–2005). Functional and projective aspects of tree line performance were analysed by relating temporal variability and change of vital population parameters, such as natality/mortality, vigour, injuries, height growth and seed viability to contemporary variations in air and soil temperatures. The size of the entire sampled population increased by 50% during the 32‐year observation period and thereby pine has become a more prominent element on the landscape. This reverses a natural multicentennial or even millennial trend of tree line decline and recession. Contrasting population trends were recorded for the subperiods 1973–87 and 1988–2005, viz. decline and increase, respectively. Mean summer temperatures (JJA) did not change perceivably over and between these intervals, although some exceptionally warm summers from 1997 onwards have contributed to population expansion by increased seed viability and seedling emergence. Winter temperatures (DJF) decreased significantly over the first subperiod and were consistently higher during the second, which has significantly lowered the mortality rates. A functional link to winter temperature conditions was particularly stressed by the aetiology of individual plant vigour, injuries and final mortality. Classical symptoms of winter desiccation correlated significantly with low winter temperatures. This negative impact occurred with a high frequency during the decline phase and virtually ceased during the expansion phase from 1988 onwards, when winter air and root zone temperatures were raised to a consistently higher level. Winter and summer temperatures in the air and soil, as well as positive feedback mechanisms and nonlinear responses, must be taken into account in the search for global or regional mechanical explanations for the tree line phenomenon. This insight helps to generate realistic tree line models for a high‐CO 2 world, when winter warming is usually predicted to be particularly large.