Forest dynamics in unexploited birch ( Betula pendula) stands in the Vosges (France): structure, architecture and light patterns

Abstract

Two adjacent forest stands composed of birch ( Betula pendula) and beech ( Fagus sylvatica) in the Vosges (France), aged 15 and 50 years, respectively, and representing a possible chronosequence of 35 years of forest evolution, have been studied. The objectives of the research were to compare structural and architectural changes at the level of tree population and forest ecosystem. In each forest stand, a profile of 2500 m 2 was carried out in which trees and juveniles were measured and mapped, as well as dead woody debris. Forest architecture was represented by crown projection and vertical drawings. In addition, we have drawn some typical birch and beech morphologies observed in the two plots and interpreted them on the basis of their social status: tree of the present, potential tree, suppressed tree, tree of the past. These concepts have been developed by the architectural model of [Oldeman, R.A.A., 1990. Forests: Elements of Silvology. Springer, Berlin]. Changes in canopy geometry (canopy openness, LAI), and light pattern (PPFD; R:IR) were examined with the help of hemispherical photographs and analysed by two computer programs (Gap Light Analyser, GLA and Winphot). The study of this temperate succession shows that there is a functional link between structure, tree architecture, forest architecture, light pattern and succession. Analysis of birch architecture during succession reveals a remarkable variety of architectural developments that are related to the strong infraspecific competition during site colonisation. The architecture of beech depends on the forest environment created by the birch. Beech is a very flexible species, which may grow in light as well as in alternating conditions of light and shade. In birch forest, the good light availability explains its architectural traits where growth is close to the open. Because such good light conditions are constant during the whole development phase of the birch eco-unit, beech growth does not pass through alternating periods of suppression and released growth as observed under beech canopy. Forest architecture and structure change significantly during succession, with a decrease in birch density and a global increase in tree dimensions for the birch and the understorey beech. Juveniles are more numerous among the dense herb coverage. Like young beeches, other tree species (mainly Sorbus aria and Quercus petraea) appreciate the light pattern created by birch canopy, with both good light availability and protection against wind and drought. After 50 years of dominance, the birch begins to die, creating big gaps in the canopy which favour beech growth and a rich herbaceous coverage. We predict that after 80 years, the canopy will be dominated by beeches with very broad crowns under which juveniles will grow with periods of suppression.