Can Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) sustainably grow in complex forest structures?

Abstract

The sustainability of shade-intermediate tree species such as Douglas fir in complex forest structures (with more than two distinct storeys) has been controversially discussed for a long time and traditionally shade-tolerant tree species have been preferred in plenter forest management. The most critical question in this context is whether recruitment trees of a particular species can survive in the shady conditions of the understorey and whether they can contribute to a demographic continuity. Using data from the unique, replicated monitoring site Artist’s Wood in North Wales (UK) we investigated the recruitment dynamics of Douglas fir to address the question whether this species can grow in complex forest structures on a long-term basis. Then we compared the results with those from Norway spruce and silver fir plenter forests in the Swiss Jura. Demographic continuity in complex structures is ensured if all forest development stages are represented in appropriate demographic proportions. We quantified these development stages in terms of stem diameter classes and analysed the corresponding diameter growth, competition and the loss of trees due to mortality and forest management. Based on this analysis we determined demographic equilibrium conditions for Artist’s Wood. The results confirmed that Douglas fir at Artist’s Wood can indeed grow sustainably in complex forest structures at a stand density of 27.4m2 ha−1 basal area which corresponds to a standing volume of 342m3 ha−1. However, equilibrium conditions for Douglas fir can only be achieved at the expense of a lower overall stand density of approximately 85% of that of Norway spruce. Despite this the corresponding Douglas fir stand volume increment of 15.3m3 ha−1 year−1 is greater than that of Norway spruce. Thus Douglas fir plenter forests are not only possible but are also appealing considering the broad site amplitude of the species in conjunction with ongoing climate change and the potential of the species for CO2 retention.