Basic features of a group selection system modification aimed to sustian regular-uneven-aged stand structure

Abstract

Proposed modification of group selection system is aimed to successfully combine its elements with biological peculiarities (such as sympodial growth, distinct heliotropism and crown growth plasticity, heavy seeds and difficult dissemination, etc.) of shade-intolerant and mid-tolerant tree species, stands of which may be managed under the same system. Principles, on which the modification is based, are: (i) application of well founded extended rotations in order to increase the proportion of tree groups above 90-100 years age in the stand, where upon the height increment and crown side spread by most of the main tree species in Bulgary drop to considerably lower level, and (ii) optimum systematic spatial positioning of openings in the stand canopy (and tree groups respectively) over the stand area, so that new gaps created by consecutive selection cuttings adjoin to a minimum number of middle aged (40-80 y. age) tree groups. Such clumps of vigorous trees with high growth rates, heliotropism and plasticity carry certain risks of spoiling the target regular-unevenaged stand structure. Factors of significance are discussed by determining the rotation age, size of gaps opened in the stand canopy, selection cutting cycle, a regularuneven- aged stand structure design and the minimum area that enables realisation of full selection system cycle. Definitions and determination of selection system elements in their mutual connections are presented in consecutive steps when designing a particular application of group selection system for exemplary pure stand of common beech (Fagus sylvatica L.). By the proposed group selection system modification the amount of combinations of various factors having impact upon processes in the tree groups are diminished compared to that by random spatial arrangement of the gaps over the stand area. Silvicultural activities aimed to maintain a stable unevenaged stand structures are thus getting more predictable, easier to schedule and efficient.