Swiss foresters have mapped out the light requirements of different tree species and the availability of light in forest openings. On the basis of a classification into 5 categories ranging from shade species to full light species and the availability of light as a function of the size and position within the forest opening, they deduce the optimum opening size to consider for the species required.
Species requiring ‘extreme’ light, such as Birch, Larch, Willow or Red Oak, would not be able to meet their needs in a forest opening of less than 25 ares, while light species such as Pine, Sessile and Pedunculate Oak, Mountain Ash or Aspen would require an opening of at least 16 ares. Hornbeam, sycamore and flat maple would require just 4 ares. Optimum light is found mainly in the northern part of the forest opening, and in the centre in the case of larger openings.
This theory is valid in flat conditions, surrounded by closed mature stands, located on good soils with trees of corresponding height and typical of the Swiss plateau, i.e. more of the Luzule beech forest type. Over time, the forest openings will have to be widened to maintain optimum conditions for light species and prevent shade species from taking over.
The authors of the article acknowledge that their theory is limited to the availability of light as a central factor in rejuvenation, and neglects the unfavourable ecological effects that increase with the size of the openings: alteration of the forest climate, leaching of nutrients, development of weeds, etc. The article also deals exclusively with the provision of direct light, but the authors point out that layered stands that are not too dense, allowing more diffuse light to pass through, could also present zones that are favourable to the regeneration of light-demanding species, assuming that monitoring is carried out.
Between an overly theoretical and schematic application in forest opening and a « laissez-faire » approach leading to the loss of the most demanding species, the right balance needs to be found, based on a good knowledge of the light requirements of each species and of forest dynamics.