Defining the equilibrium condition in a mixed-species uneven-aged forest in Scotland

Transforming even-aged planted forests into uneven-aged, vertically diverse structures often implies that an equilibrium or sustainable condition will eventually be reached where the stand structure and species composition remains relatively constant over time. However, identifying the equilibrium condition is complicated by the complex dynamics of diverse forest structures. In this study four ‘sustainability’ indices—stand structure, stocking density, species diversity and increment—were applied over a 20 year period in a long-term permanent sample plot within an uneven-aged forest in Scotland to determine whether the equilibrium condition has been achieved. The Shannon Index of species diversity highlighted a gradual shift towards shade tolerating species. Transition periods (average time for all trees in one diameter class to move into next higher diameter class) of shade tolerating species declined with increasing diameter (trees growing more quickly), whereas the opposite trend was found with light demanding species highlighting competition pressures. However, evaluation of the q value, Lorenz curves, Gini coefficient and Stand Density Index revealed values comparable with those found in long-established Central European uneven-aged forests and suggests that, while subtle changes in species composition are occurring, the forest is within the range of parameters indicated for the equilibrium condition.