Modelling the risks of natural stand closure release with ageing in pure beech (Fagus sylvatica) and spruce (Picea abies) stands

Background In the development of pure beech and pure spruce pristine forests, we assume there should be a continuous transition from full stocking to successive disintegration and, finally, total replacement of the first generational wave at the end of its lifetime, dependent on mortality due to overcrowding as well as ageing, leading newly recruited cohorts to grow up from below. Material and methods A survey of stem number distribution from permanent plots measured repeatedly in six pristine beech reserves and four spruce reserves, mainly in Slovakia, was considered as well as surveys from yield plots in untouched stands in Switzerland on comparable site conditions. From 64 (54, respectively) surveys in virgin forests, we separated (if necessary) the original regeneration waves from successive waves for calculation of stand density development. Comparison with a previous model for full stand closure according to Reineke's density rule allowed the risk of stand release to be evaluated and determined. Results The risk of stand closure release is substantially higher for spruce than for beech because of differences in longevity and in vulnerability to storms. Decay does not seem to be the decisive factor, until advanced ageing is reached. Consequences for the understanding of natural forest dynamics, especially reinitiation through successive waves of regeneration are drawn on the basis of silvicultural expertise from continuous cover systems. The density threshold limit for the accession of a second growth in the upper storey is attained at an age of about 180 years for beech and 140 years for spruce. This corresponds to about 70% or 40% of the life cycle, respectively. Conclusion Vulnerability to stand release should be considered as a gradual (discrete) rather than a continuous phenomenon that depends on the complex relationship between the collective cohesion within a stand and a propensity for weakening factors. This suggests that the establishment of large disclosure patches could be the driving force in the renewal of these virgin forests.