Silvicultural tools to develop irregular and diverse forest structures

This paper discusses the dynamic definition of irregularity; it looks at conditions of sustainability and considers under which conditions irregularity could be promoted efficiently. Irregularity is not usually innate in natural forest ecosystems, the exception being in certain developmental stages of old or over-mature stands. Because of these natural tendencies, it is necessary to differentiate between temporary stand structures and sustainable irregular systems. Therefore, any assessment of irregularity needs to take the rules of demographic regulation into account. Different kinds of irregularity are discussed, as is the reference scale for implementation. This makes it possible to distinguish between genuine irregularity within the crown layer, full (vertical) irregularity on stand level (plenter system) and horizontal irregularity (for greater reference scales), by creating irregular patches (as under the irregular shelterwood system). The paper presents the differences between broadleaved species and conifers in a full irregular plenter system with a developed vertical structure. These differences are due to the degree to which tree species can support shade, without losing the ability to recover qualitative capacity and, for older stages, to use crown space efficiently. Other factors are also important: the reaction of tree species to openings, the production of epicormic branches, topology and crown expansion. Therefore, broadleaved plenter forests need much lower equilibrium standing volumes than classical conifer plenter forests and there are also losses in volume increment and stem quality. The silvicultural results produced by the different models for differentiating stands and promoting irregularity are also discussed. It is assumed that for broadleaved species, the compromise between the necessary educative steps (shaping of the stem form within tree populations) and closure control function are better in small populations (openings) than in stands with a single tree structure. Because the ultimate aim is to create not one, but several, co-existing forms of heterogeneity, in the sense of creating varied habitats, modern silviculture should make use of all silvicultural tools. This is a significant challenge for silvicultural expertise.