A demographic equilibrium approach to stocking control in mixed, multiaged stands in Bialowieża Forest, northeast Poland

Abstract

Manifold benefits and advantages of mixed tree stands have been known for many decades. However, while developing and maintaining multiaged and mixed-species forests with complex stand structures has often been advocated and has recently become a worldwide priority, it is not a simple task. One important reason for this is a lack of appropriate methods for controlling long-term development of such stands. Here, we present a possible solution for this problem by applying the demographic equilibrium approach to guide stocking and structure regulation in mixed stands. We tested the practical usefulness of this approach using permanent-plot inventories from 2002, 2011 and 2018, in mixed tree stands in the Experimental Sustainability Unit (ESU) Browsk 28C, in the managed part of Białowieża Forest (NE Poland). For our analysis, ESU Browsk 28C (about 30 ha) was treated as a silvicultural planning unit. First, on the basis of target stand types defined in Polish Silvicultural Guidelines (2012) and forest sites occurring at the study site, desirable tree species composition was determined. Eleven tree species of varying life histories and ecological attributes were included: aspen, birch, alder, pine, oak, maple, ash, elm, spruce, lime, and hornbeam. In the second step equilibrium diameter distributions were constructed for each species, taking into account species-specific growth and mortality rates, as well as targeted proportional representation. Next, theoretical species-specific equilibrium diameter distributions were visually and quantitatively compared with empirical distributions from permanent sample plots. Finally, departures of empirical from theoretical distributions served as a basis for silvicultural recommendations and prescriptions. We prioritized silvicultural measures (such as patch cuts opening larger gaps in forest canopy, proper site treatment, planting, protection against browsing by large herbivores), promoting compositionally diverse forest reproduction and recruitment as essential to long-term maintenance of demographic sustainability in mixed forest stands. We conclude that the demographic equilibrium approach is particularly valuable for creating and maintaining complex forest structures in multi-species stands.