Understorey light conditions and regeneration with respect to the structural dynamics of a near-natural temperate deciduous forest in Denmark

Abstract

Suserup Skov is a near-natural temperate mixed deciduous forest dominated by Fagus sylvatica L. (beech) and Fraxinus excelsior L. (ash) with some Quercus robur L. (oak), and Ulmus glabra Huds. (elm). The forest dynamics in Suserup Skov can be described by the forest cycle, a sequential shift between a series of developmental phases: innovation, aggradation, early biostatic, late biostatic, and degradation phases. Climax microsuccession from ash to beech occurs as an integral part of the forest cycle. The spatial match to the forest cycle is a shifting mosaic of the same phases. The light conditions at the forest floor and the regeneration of beech and ash were studied with regard to the mosaic-cycle. The relative light intensity (RLI) and the variation in RLI (CV-RLI) were measured in patches representing different phases of the mosaic-cycle. RLI and CV-RLI were compared between phases by analyses of variance, the RLI-model as well as the CV-RLI-model were statistically highly significant. RLI was below 2% in all phases of the forest cycle, except in gaps (degradation and innovation phases). In gaps RLI increased to about 10%, making regeneration of ash and beech possible. CV-RLI was higher under ash dominated canopies than under beech-dominated canopies. Regeneration of ash and beech did not survive at RLI below 2%. At RLI above 3%, regeneration of both ash and beech developed successfully. Successful establishment and development of ash and/or beech regeneration only occurred in gaps. Advance regeneration of both beech and ash occurred under smaller, often temporary, canopy gaps. In an examined gap, ash established first, responding with rapid height growth at increasing light levels. Beech was established at the next mast year. Under the ash plants RLI was above 3%, which made the establishment and development of a beech understorey possible. The studied gap represents the beginning of a successional shift from ash to beech within the forest cycle. The results have practical implications for nature-based silviculture, especially concerning choice of regeneration strategy and management of stand structures in order to improve the conditions for regeneration.