Trees die slowly. The ability to postpone the moment of death enables many trees to survive long enough to
Fig. 5.24 Norway spruce (Picea abies) clonal group at an altitude of c. 450 m in Yllastunturi Fjell, Finnish Lapland (67° N). All the lower lateral branches contacting the ground and embedded in the litter layer and moss cover have produced adventitious roots. Seedlings on this exposed site are rare but occur at high density in the closed forest below. (Photo Prof. F.-K. Holtmeier.)
outlive the duration of adverse climatic oscillations. Slow senescence is apparently a vital process for trees in marginal situations and a lag phase can therefore be expected before a period of climatic adversity removes a marginal tree population. As growth, development and senescence all take place slowly at high altitudes and latitudes it should be expected that tree stands in these regions should be of considerable age. At the treeline there can often be found individual trees that are thousands of years old. However, in the northern regions of the boreal forests ages of 200-500 years are more common. In both situations vegetative reproduction by layering can maintain populations for centuries without sexual reproduction (Fig. 5.24).
Equally evident is the lag phase between seedling advance and the establishment of a forest. In addition there are physical obstacles to the advance of the tree-line in the form of wetlands and mountains. From time to time outliers can become established, in the form of isolated or small cluster of seedlings and saplings that manage to survive beyond the limits of their parent trees as a result of periods of favourable climatic conditions (Fig. 5.16). However, these advancing colonists may not always be able to establish the extent of tree cover that would be sufficient to qualify as an advance of the treeline. It is therefore not surprising that the re-establishment of tree cover with an intact forest canopy is a vegetation change that is not readily accomplished.
Once established, forests with a well-developed canopy layer create within themselves a climatically buffered environment not only in the understorey but also in relation to soil temperature and nutrient status. The eventual disappearance of trees will take place only when conditions eventually deteriorate to such an extent as to make accretion of biomass untenable. The location where this balance is achieved varies with climate, but catastrophic events and human interference make correlation with climate tenuous and future predictions difficult (Sveinbjornsson, 2000). The establishment of trees can create both positive and negative feedbacks through the action of tree density on tree growth. Increased tree density provides shelter and leads to increased air temperatures and decreased wind damage. It also has negative aspects by lowering soil temperature, reducing nutrient availability, and increasing nutrient competition (Sveinbjornsson et al., 2002). There is even the extreme condition, already mentioned above, where forest advancing over descending permafrost eventually cools the soil so that the permafrost becomes re-established near the surface and kills the trees (Holtmeier, 2003).
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