Long-term study into the development of a plenter structure in a forest comprising a mix of European and north American species

Irregular or plenter forestry evolved in Central Europe around a small number of shade-tolerating species; however, increasing interest in developing more climatically resilient, diverse forest structures has seen a greater range of species being studied. A critical question is whether stands comprising a wide geographic mix of species can form irregular structures in the long term. Using data from a unique experimental site established in 1953 at Faskally Forest, North Scotland, temporal changes in structure, density, growth and species diversity were studied in a stand comprising a mix of European and North American species undergoing the latter stages of transformation. A 1-ha permanent sample plot was established in 1997 and data evaluated from five complete inventories carried out at six-yearly intervals (latest in 2021). Metrics describing the irregular structure – defined by negative exponential diameter distributions, q values, Lorenz curves, Gini coefficients and stand density indices – have stabilized over the last three inventories suggesting a relative balance has been achieved between recruitment and harvesting/mortality losses. Whilst shade tolerating species dominate regeneration and recruitment, species diversity has increased since the start of the study. Analyses of transition periods (average time required for all trees to progress from one diameter class into the next) and annual recruitment (average number of trees moving from each diameter class into the next higher class) highlight a growth advantage of shade tolerating species. The range of values defining the irregular stand at Faskally compare well with those from long-term plenter forests in Continental Europe highlighting the potential of forming irregular stand structures comprising geographically diverse species. The study also indicates that the silvicultural characteristics of species are more important than whether they are native or non-native. Stand characteristics reported here could aid managers of irregular/plenter forests by indicating values of such characteristics to guide the transformation of similar mixed-species stands.