Tree disc showing incremental growth
Tree growth is an indicator of how well matched the environmental requirements of a particular tree species (at a given stage in its life cycle) are with local environmental conditions.
Trees in the permanent mensuration sample plots at each Intensive Forest Monitoring site are assessed every five years, and diameter at breast height (dbh) is measured annually on twenty four trees also assessed annually for crown density. These measurements enable the relative impacts of causal agents affecting tree growth over short or extended periods of time to be evaluated.
For example, the largest inter-annual fluctuations in diameter growth are evident at the Grizedale oak site, where severe defoliation is a regular occurrence. In contrast, all four of the Sitka spruce plots also show a large degree of variation which is likely to be climate dependent.
More formal growth assessments from the Intensive Forest Monitoring mensuration plots are presented in the graph below, separately, for the first and second five-year periods of monitoring. Although clear growth trends cannot be identified from such a limited dataset, there is no evidence of a widespread downturn in productivity.
Periodic annual increment (PAI) of volume at the ten original Intensive Forest Monitoring plots for the first and second five-year periods of monitoring. Volume includes both changes in standing volume and volume removed as thinnings
Reductions at Rannoch and Loch Awe are probably due to increased exposure and wind damage. In contrast, some sites show an increase in volume, in some cases due to the effects of management intervention (thinning).
Other explanations include rising CO2 levels in the atmosphere, the fertilising effect of enhanced nitrogen deposition and global warming resulting in earlier flushing and longer growing seasons.