The 2006 results are presented here in terms of crown densities rather than the crown density reductions reported for the Forest Condition Surveys undertaken between 1987 and 2001. For an explanation of this change, and of how to convert the current figures to crown density reductions, the report of the 2002 survey should be consulted (Hendry et al. 2003).
The marked effect of using a local reference tree rather than an ideal tree as a standard for comparison can be seen in Table 1, where the results obtained in 2006 using both methods of crown density assessment are presented. A greater proportion of trees receive low density scores when compared with an 'ideal' rather than an ‘local’ standard. This difference can largely be accounted for by variations in the growth habit between the reference photographs of ideal trees (Innes, 1990) and the trees in and around the plots to be assessed, from among which a local reference tree is chosen. For example, young trees of all species tend to have a more open appearance (i.e. a lower crown density) than the older trees illustrated in Innes (1990). Some older oaks and spruces also have a naturally open structure. The crown density scores allotted to trees like these are much lower when compared to an ideal tree than when judged against local trees of the same age and form.
Table 1: Percentages of trees in each crown density class for five species in 2006. Each 10% class represents the density of the tree’s crown compared either to an 'ideal tree' (I), i.e. a tree with the maximum possible amount of foliage, or to a 'local tree' (L), i.e. an actual tree with full foliage under local conditions.
Figure 1 shows the changes in crown condition that have taken place since 1987. A downward gradient in this figure indicates a deterioration in crown condition. Alterations in condition compared to last year were minor for all of the surveyed species except beech, the crown density of which improved markedly compared with 2005. Beech displays no long-term trend for deterioration or improvement, however, with relatively large short-term deteriorations in condition tending to be counterbalanced by equally rapid improvements of the same magnitude. Similarly, no long-term trends in the crown densities of either Sitka spruce or Scots pine are apparent.
Figure 1: Changes in crown density since 1987 for five species surveyed annually. The crown density compared to that of an 'ideal' tree with a completely opaque crown is shown for each species.
Analysis of the 1987-2006 data indicates that statistically significant deteriorations in the crown densities of both Norway spruce and oak have occurred over the duration of the survey. However, the time series are relatively short and the indicated rates of change are small, with apparent reductions in crown density of 0.37% per annum in oak and 0.27% per annum in Norway spruce. The magnitudes of past increases in the crown density of Norway spruce suggest that a single year of improvement could nullify the trend currently displayed by this species. For oak, a trend for deterioration has been apparent since 1999 (Redfern, Boswell and Proudfoot, 2000) and a number of seasons of improvement in condition would be required to negate the established pattern of decline.
The condition scores of Scots pine and Norway spruce have changed little since 1991 and have displayed only minor inter-annual changes during that period. The average difference between the crown density scores recorded in successive years has been less than 1% for either species and a change of more than 2% has occurred on only one occasion. Over the same period, greater fluctuations in crown density have occurred in Sitka spruce, with sharp declines in condition having been recorded in 1997 and 2002 but minor improvements having occurred in most other years. The decline in the condition of oak since 1987 similarly reflects a pattern of relatively large reductions in crown density between particular years and smaller increases in the intervening periods.
Figure 2 shows the geographical variation in crown density for each of the assessed species. The condition of beech was variable but was poorer than in 2005 across much of the East Midlands and Yorkshire & Humber regions. However, there was little change in Scotland where crown densities were similar to those recorded last year. Oaks in central Scotland displayed no improvement in condition this year and a reduction in the average crown density of trees in the north and northeast of the country extended the area in which the condition of the species was poor to most of the region north of the Firth of Forth. However, an increase in the crown density of oaks in northwest England and the eastern reaches of the Yorkshire and Humber region occurred in 2006. As in previous years, crown densities in Scots pine tended to be higher in the region south of the Humber-Mersey line but trees in northwestern England and east central Scotland were also in relatively good condition during the 2006 growing season. The condition of Sitka spruce was generally good, though the species continued to display a lower average crown density in southwest England, northeast England and northeast Scotland than elsewhere. Norway spruce exhibits considerable local variation but its condition was notably poorer than in 2005 in the southern part of West Wales and the East of England region.
Factors affecting crown condition in 2006
Following relatively warm and dry conditions in January and February, March of 2006 was both colder and wetter than average across the UK, although the associated snowfalls in mid-month were not sufficiently heavy to be injurious to trees in most cases. April was changeable with occasional heavy snow in the south of the country and storms in the north but overall both temperature and rainfall were close to the long-term average for the month. Mild and extremely wet weather predominated in May and damaging spring frosts were consequently rare, though temperatures as low as –4ºC occurred in some Highland regions as late as 23rd of the month. Both June and July were dry and extremely hot but in spite of this few acute symptoms of water deficit were noted on trees during the course of the summer. With rainfall near or slightly above average and temperatures being mostly warm, conditions during the remainder of the 2006 growing season were generally good for tree growth.
In common with previous cases of decline recorded by the survey, the decrease in the average crown density of beech which occurred in 2006 was largely associated with heavy fruiting (Hendry et al., 2005). Mast production was noted on 82.8% of the surveyed trees and was recorded as heavy (assessed by the surveyors as being “common” or “abundant”) in 63.1% of the population. However, crown thinning of beech was also related to dieback of twigs and relatively thin branches this year, with 16.3% of trees displaying a crown density which was reduced by 10% or more as a result of dieback alone. Only 18.4% of the surveyed trees displayed more than slight foliar discolouration, usually in the form of leaf browning associated with feeding damage by the beech leaf miner Rhynchaenus fagi.
Sitka spruce sustained little insect damage this year, with current attack by the green spruce aphid (Elatobium abietinum) being recorded in only 21% of plots and resulting in moderate to severe defoliation of less than 2% of surveyed trees. Concomitantly, the proportion of trees retaining their needles for 7 or more years increased by 11% compared with last year and the severity of needle discolouration noted on the species was reduced, with only 1.1% of trees exhibiting browning of their older needles in 2006. Whilst an increase in the mean crown density of Sitka spruce was associated with these changes, the degree of improvement in condition was slight and may have been influenced by an increase in shoot and twig death, which was more evident this year than at any time since 1991 and was adjudged to be common or abundant on 19% of trees. Climatic damage to Sitka spruce was rare in 2006: the effects of late frosts in May were noted in 6 plots in the north of the country but damage was confined to the lower parts of the crowns of affected trees and was not sufficiently severe to affect crown density.
Changes in the crown densities of Norway spruce and Scots pine were minor this year, continuing the pattern of little variation which has held for both species since 1991. In 2006, damage to Scots pine by the pine shoot beetle Tomicus piniperda was evident in 44% of plots compared with 32.5% of plots last year but was generally minor in extent. A marked increase in the incidence of needle discolouration was noted, however, primarily in the form of yellowing of older foliage which was recorded on 6.2% of trees this year compared with 2.4% in 2005. Such yellowing was largely due to the premature senscence of 2- and 3- year old needles which occurred on pines in certain areas of the country, particularly in Scotland, during the late summer of 2006. Defoliation of Norway spruce by Elatobium abietinum occurred in 6 plots this year but the degree of damage was slight in all cases. Fungal damage was largely restricted to cases of bud blight caused by Cucurbitaria piceae which was only adjudged to have had an adverse effect on the crown densities of a few trees. With 40.4% of the surveyed population producing cones, fruiting in Norway spruce was more common this year than at any time since 1996. However, increased production of cones has no deleterious effect on the crown condition of this species and, because cones tend to be concentrated near the apex of the crown, may even give the tops of trees an appearance of increased density.
The condition of oak improved slightly in 2006 and the species has now displayed increases in crown density for four years in succession and for 6 of the last 7 years. In spite of this, its crown density is currently lower than in 2001 due to the large deterioration in condition which it suffered in 2002. Insect damage caused by defoliating and leaf-mining insects was recorded in all 86 of the plots assessed during 2006 but the severity of such damage varied widely and was adjuged to be heavy or severe in only 27% of plots, most of which were located in the north of Britain. In the majority of cases, insect damage of sufficient magnitude to affect the crown density of the assessed trees was associated with attacks by larvae of the winter moth Operophtera brumata and mottled umber moth Erannis defoliaria. The effects of such damage were offset to some extent by the production of lammas growth, however, which was noted in 23% of plots. Other causes of poor crown density in 2006 included oak dieback (Gibbs, 1999), which was identified as the cause of long-term decline in 2 plots and of recent deterioration in the condition of trees in a further 5 plots located in central and eastern England.
The weather during 2006 was highly variable, with conditions at the start of the growing season being mild and damaging spring frosts correspondingly rare. Both June and July were dry and extremely hot but were preceded by an unseasonably wet May and few acute symptoms of water deficit were noted on trees during the course of the survey. With rainfall near or slightly above average and temperatures being mostly warm from August to October, conditions during the remainder of the 2006 growing season were generally good for tree growth. Changes in condition were minor in oak, Norway spruce, Sitka spruce and Scots pine this year but beech exhibited a marked decrease in crown density. There was no change in the crown density of Scots pine compared with last year but an increase in needle discolouration was largely attributable to premature senescence of older needles. In common with previous years, the crown density of Norway spruce fluctuated only slightly in 2006 and its condition has remained virtually unchanged since 1991. Oak displayed a slight improvement in condition again this year and has displayed increases in crown density for 6 of the last 7 years. However, recovery from the deterioration which the species suffered in 2002 is still not complete. A minor improvement in the condition of Sitka spruce in 2006 reflected low levels of insect damage and improved needle retention although increases in the incidence and severity of shoot and twig death were also noted for the species. Whilst an increase in the severity of twig dieback contributed to a reduction in the average crown density of beech in 2006, the deterioration in condition which the species displayed this year was chiefly associated with heavy mast production and is not necessarily an indication of poor health.