Opportunities and strategies of transforming regular forests to irregular forests

Transformation from a regular forest to a full irregular one is a long and difficult task which needs time and frequent intervention. Looking back over Swiss experience allows us to analyse the reasons for success and failure. One of the main reasons for transformation failure is that differentiation has been established too evenly over the entire surface and has been begun too late. The tendency towards forming over-regular two-storied stands increases if it is not possible to maintain adequate crown cover of the upper storey over the entire transformation period. This is linked to the ageing of the first generation main trees and to good crown formation. It is possible to establish concrete rules about transformation management and decision-making during the different transformation stages. As with plant succession, transformation follows a set sequence of stages which must occur in the correct order. The process must also start at the correct time and in the correct manner. A decision must be made whether to attempt transformation on the present generation of trees or on the subsequent one. Otherwise, one runs the risk of unwittingly opening the cover when mature trees die, before transformation has been achieved. For such purposes, we use a specific hierarchy of objectives in the decision-making process. These are principles such as stability; longevity of cover-building trees; optimising regeneration with regard to recruitment and its conforming to an irregular vertical structure. This hierarchical order must be followed. The activation of regeneration and recruitment is of utmost importance in the subsequent transformation stages. (C) 2001 Elsevier Science B.V. All rights reserved.