Determining the sustainable irregular condition: an analysis of an irregular mixed-species selection stand in Scotland based on recurrent inventories at 6-year intervals over 24 years

While there is considerable interest in transforming even-aged stands into species-diverse, irregular structures, the rarity of stands close to the end of the transformation process has resulted in a lack of knowledge on optimum target structures that are considered sustainable. This study examined this problem in a mixed-species selection stand that has undergone transformation for over 60 years. Complete inventories of a one-hectare permanent sample plot were carried out in 2015 and previously in 2009, 2003 and 1997. The diameter distribution remained relatively static with the q factor remaining at 1.4 since the 2009 inventory. An improvement in the number of regenerating seedlings and saplings available for recruitment from previous inventories suggests that the current structure is relatively stable although shade-tolerating conifers are gradually dominating the stand. Not all the sustainability criteria were met; however, the stand could be described as having reached a balanced condition. It is recommended that the current stocking density and basal area (similar to 350 stems ha(-1) and 26-27 m(2) ha(-1), respectively) should be maintained and that future harvesting interventions should aim to reduce the number of large diameter stems (>55 cm DBH), which represent 14 per cent of the stocking but 48 per cent of the stand basal area.