Assessment of crown condition in forest trees: comparison of methods, sources of variation and observer bias

In Britain, as part of an annual survey of forest ʹhealthʹ conducted throughout Europe, the condition of trees is assessed visually by comparing the transparency of crowns when viewed against the sky with standard photographs. Two standards of comparison have been used: (1) an ʹidealʹ or ʹperfectʹ tree for each species carrying the maximum possible amount of foliage, and (2) a tree with the maximum amount of foliage under local growing conditions. These are referred to as the absolute and local reference trees respectively. Results are expressed as a percentage reduction in either absolute or local crown density. Annual assessments of ~ 8500 trees based on these two standards were compared for the period 1995–2000. Reductions in crown density were smaller when trees were compared with a local reference tree than when they were compared with an absolute reference tree. Differences between results obtained by the two methods over the 6-year survey period were smaller for conifers (Scots pine 11.3%, Norway spruce 9.3%, Sitka spruce 8.0%) than for broadleaved species (oak 16.0% and beech 13.1%). Assessments were carried out by regionally-based assessors, and a proportion of plots was re-assessed by a ʹstandard observerʹ to check the consistency of scoring. A mean bias of >5% on a single 24-tree plot was significant but affected <10% of plots. There was little difference in performance by assessors on the two types of assessment. For broadleaved trees reductions in absolute crown density were significantly greater in the west of Britain than in the east and differences between results obtained by the two methods were also greater in the west than in the east. It is suggested that this may be associated, among other things, with increased exposure to wind in the west, resulting in greater crown transparency. The use of absolute reference trees enabled geographical differences in crown transparency to be detected more frequently than was possible using local reference trees. It is concluded that assessments based on an absolute (fixed) standard for each species offers the best means of detecting changes in crown condition with time and of identifying geographical differences. A relationship established between crown transparency scores based on absolute and local reference trees offers a means of adjusting scores from one basis to the other only in limited circumstances. It might not be appropriate to do this in order to facilitate international comparisons.