Developing the silviculture of continuous cover forestry : using the data and experience collected from the Glentress Trial Area

Abstract

Continuous Cover Forestry (CCF) has become increasingly popular since the early 1990s. CCF utilises several silvicultural techniques in order to promote and enhance forest structural diversity and favours natural regeneration. As CCF is relatively new to the UK there are still areas of knowledge regarding management interventions that need to be improved upon. This study utilises simple models, seedling physiology and a hybrid gap model and applies them to the Glentress Trial Area which has been under transformation from even-aged forestry since 1952. These efforts have led to an improved understanding of thinning interventions and the effects they may have on future stand structure. Since the formation of the Forestry Commission in 1919, clearfell-replant forestry has been the main form of management practiced in the UK. CCF management differs in several respects and is commonly practiced using expert knowledge in Continental Europe. In the UK the knowledge-base is still growing and therefore simple models can prove useful for guiding management. This study investigated the use of the idealised reverse-J and the Equilibrium Growing Stock (EGS). This study found that the reverse-J shaped diameter distribution is maintained at the Trial, Block and sub- Block scale indicating that an irregular structure is being approached. In addition, the diminution coefficient, a parameter of the reverse-J distribution, falls within values typical of continental Europe. Comparison of the actual diameter-frequency distribution against an ideal reverse-J distribution can inform both thinning intensity and which diameter classes to target. The EGS, which is a volume–diameter distribution, examines standing volume and how that volume is distributed across three broad diameter classes. Typical distributions from the Swiss Jura indicate that percentage volume should be split 20:30:50 across diameter classes. The EGS analysis showed that standing volume in the Trial Area is much lower than European values at just 174 m3 ha-1. In addition, the classic 20:30:50 percent split was not observed. The 1990 data set showed a 49:43:8 distribution but by 2008 it was 40:41:19. As natural regeneration is favoured in CCF a better understanding of seedling physiology is essential. This study established open (15-35 m2 ha-1) and closed canopy plots (>35 m2 ha-1). Plot characteristics were recorded and then seedlings were selected for physical measurements, chlorophyll fluorescence and gas-exchange measurements. There were clear differences between the physical characteristics with a mean Apical Dominance Ratio (APR) of 1.41 for the open plots and 0.9 for the closed plots which is consistent with previous studies suggesting an APR of 1 is needed for successful regeneration. The chlorophyll fluorescence measurements showed a linear relationship with PAR. However, although the results of the gas-exchange measurements showed an increase in photosynthetic rates with PAR for open plots, there was no obvious relationship in the closed plots. As a result, the study did not find a linear relationship between photosynthetic rate and chlorophyll fluorescence. Finally a complex, hybrid gap model was used to investigate the effects of management on predicted future stand structure. The hybrid gap model, PICUS v1.41, was parameterised for Sitka spruce. The model was used to explore different management scenarios on stand structure over two time periods; 1954-2008 and 1952- 2075. The output from the group selection with underplanting scenario, which resembled the actual management, produced realistic output that was comparable to the stand characteristics measured during the 2008 assessment. The output from the 1952-2075 runs suggested that thinning to a residual basal area suitable to allow natural regeneration (<30 m2 ha-1) or a group selection with underplanting were the best management options for maintaining structural diversity.