Forest-lines and tree-lines
Subalpine forests, and forest and tree-lines are important indicators of environmental changes because they are natural or artificial features, highly sensitive to environmental conditions.
Knowledge of forest dynamics at the upper altitude limits is thus a key element in the investigation of global change, i.e. the accelerating changes to global environmental systems that result from the collective activities of humanity on the surface of the earth, the most striking aspect of which is climate change.
Studying how global change, especially climate fluctuations, has affected growth dynamics at the upper tree-line in the European Alps, can be extremely complicated. In this region anthropogenic disturbances, which affect forests even at the highest altitudes, are far more intense and frequent than in other mountain chains. Records show that direct human activity has been going on at the forest-lines and tree-lines for thousands of years: slash and burn and deforestation started more than 6000 BP. Subsequent human activity radically changed subalpine forests in various ways: lowering the upper forest limit to create pasture or as the result of indiscriminate logging; the natural composition of forests was modified; the natural structure was changed by logging and thinning and tending; forest stands were eliminated to provide wood and fuel for mining and other industrial activities, and forest litter was collected from time to time. It is thus extremely difficult to distinguish the action of ecological factors and climate fluctuations from the anthropogenic effects. Human activities can mask the impact of global climate change and they are a source of confusion in studies attempting to forecast the temperature trend.
How climate fluctuations interfere with the growth dynamics of tree populations at the upper tree limits can be found out on the one hand by measuring fluctuations in the upper tree limits; and on the other by measuring the growth rate (e.g. using Basal Area Increments) of trees within subalpine forests. As a tool which can be used to reconstruct the response of the tree-site complex to stand disturbances and climate fluctuations to an accuracy of within a year or a season, dendroecology has an important role to play in the investigation of these phenomena. Fluctuations in the upper limits of forests and trees may be due to destructive events or to gradual changes in environmental conditions. Moreover they can be originated by natural events or by directly and indirectly human activity.
In order to study the effect of global changes in such a context it is essential to be aware of the present and past uses to which the territory has been subject, and hence to take into account multiple lines of evidence.
