Growth patterns, competition and coexistence in gap-phase regeneration under close-to-nature silviculture

The dominant European forest policy objective is to create multifunctional, mixed-species and uneven-aged forests. This objective includes the nature‐oriented conversion of monospecific plantations. To reach this objective, close-to-nature silviculture is applied. This generally entails natural regeneration in canopy gaps. In the Netherlands, experience with gap-phase regeneration is limited, making it difficult to asses the effect of close-to-nature management on forest development. In this thesis, this problem is addressed by investigating species coexistence between the four dominant species that occur in naturally regenerated canopy gaps in Douglas fir forest in the Netherlands. These species are Silver birch (Betula pendula Roth.), Japanese larch (Larix kaempferi Carr.), Scots pine (Pinus sylvestris L.) and Douglas fir (Pseudotsuga menziesii Mirb. Franco). During gap‐phase regeneration, saplings undergo self‐thinning. This affects species composition of the regeneration unit. Important factors in self‐thinning are species‐specific morphological growth patterns, emergent stand characteristics, the effect of stand characteristics on competitive relationships, and the effect of light availability on growth and mortality. Growth patterns differed between species and resulted in differences in the achieved height per unit biomass. Differences in mass‐based heights subsequently caused a vertical stratification in the regeneration unit which, in turn, affected the interspecific competition for light. However, results demonstrated a competitive response rather than a competitive effect. Surrounding saplings formed a functionally equivalent neighborhood, and target individuals responded mainly to their position in the canopy. Canopy position affected the radial growth of saplings. Mortality probabilities depended on radial growth, but did not differ between species despite a wide range in light demand. Scots pine did not show a relation between growth and mortality, even though it is highly responsive to light. Under continued autogenic development, a low-density top stratum of Silver birch and Japanese larch will develop, overtopping a declining number of Scots pine and a large number of increasingly dominant Douglas fir. This implies that Scots pine will be outcompeted by the other species, and Douglas fir will gain dominance in the future. Forest conversion by natural regeneration will therefore not lead to the desired mixed-species composition, but stands will eventually revert back to Douglas fir forest. Maintaining a diverse forest thus means the need for interventions in the early developmental stage.