The 2005 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 2005 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 2005. Each 10% class represents the density of the tree's crown compared either with an 'ideal' tree, i.e. a tree with the maximum possible amount of foliage, or with a 'local' tree, i.e. a 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 1987-2005 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 0.40% per annum in oak and 0.29% 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.
Although the mean crown density of Sitka spruce has decreased by only 1.0% since 1987, its condition has varied considerably over the 18-year survey period. Typically, marked deteriorations in crown density during particular growing seasons have been followed by incremental improvements in the condition of the species over several following years. The notable decrease in the crown density of Sitka spruce which occurred in 2002, followed by the recovery in condition noted in 2004 (Hendry et al., 2005) and maintained by a further minor increase in crown density this year, is consistent with this established pattern. In the last decade Norway spruce and Scots pine have, contrastingly, displayed only slight changes in condition from year to year. In spite of a minor improvement in the condition of Scots pine this year and a minor deterioration in that of Norway spruce, the crown densities of both species are currently within 1% of the values which they displayed in 1995.
Figure 2 shows the geographical variation in crown density for each of the assessed species. The condition of beech was variable but was better than in 2004 across much of central and southwest England. However, there was little improvement in Scotland where crown densities were largely unchanged since last year. In central Scotland, where trees have suffered from high levels of insect defoliation for several consecutive years, the poor condition of oak noted in previous surveys was again evident in 2005. Average crown densities in oak were also low in the northwest of Scotland and Northwest England this year, extending the area in which the condition of the species was poor to most of northern Britain. The condition of Sitka spruce was generally good but slightly poorer than elsewhere in Yorkshire and the Humber, the southern reaches of Northwest England and the far north of Scotland; a pattern which has been maintained over the last three years. Crown densities in Scots pine tended to be higher in the region south of the Humber-Mersey line than elsewhere, although trees in northwestern England and northeastern Scotland were also in relatively good condition. Whilst the condition of Norway spruce appeared to be poorer in the western half of Southeast England and in the East Midlands than in the remainder of southern Britain, this impression is created by relatively few plots and there is considerable local variation in the condition of the species.
Factors affecting crown condition in 2005
Climatic conditions during the 2005 growing season and the preceding winter were variable, being drier than average in the south of the country and wetter than average in the north. Although January was mild, a prolonged stormy period at the beginning of the month resulted in physical damage to the crowns of both conifers and broadleaves in northern Britain. However, the snowfalls which occurred over much of the country in late February were not sufficiently heavy to be injurious to trees in most cases. Following a warm and generally dry March, the mild and wet weather which predominated in April was punctuated by occasional extremes: the temperature in one part of northern Scotland rising from an overnight minimum of -5°C to a daytime maximum of 17°C on the 25th April. May was generally cool and overnight frosts of sufficient severity to cause damage to conifer shoots and foliage were recorded in northern Britain as late as 18th of the month. With rainfall near or slightly below average for the remainder of the 2005 growing season and temperatures being mostly warm, conditions for tree growth were generally good.
The increase in crown density recorded for beech this year was almost sufficient to reverse the deterioration in condition which the species displayed in 2004 (Figure 1). As noted in previous years (Hendry et al. 2001) crown density in beech is generally greatest when fruiting is light or absent whilst poor crown condition is often associated with heavy mast production. Whereas masting was heavy in 2004, fruiting was recorded as absent or scarce on 97% of the trees assessed this year. Less mast production has only been recorded on one previous occasion in the last 16 years. Insect damage due to the beech leaf miner Rhynchaenus fagi was more widespread and severe in 2005 than at any time since 1994, being common or abundant on 26.9% of the assessed trees. Although trees which are attacked by R. fagi are not defoliated, their leaves suffer from conspicuous discolouration and the increase in such attacks was reflected by an increase in the proportion of beech trees with browned leaves from 29% in 2004 to 39% in 2005.
The condition of Sitka spruce improved slightly compared to last year. Although the presence of new damage by green spruce aphid (Elatobium abietinum) was reported from 51% of the plots assessed, attacks were minor both in terms of the numbers of trees affected and in the degree of defoliation caused. However, the percentage of trees retaining their needles for 7 or more years remained unchanged between 2004 and 2005 at 55% suggesting that the recovery of Sitka spruce from the severe Elatobium defoliation which it suffered in 2002 may be protracted. In spite of the unusually cold conditions which occurred in late May there were few reports of climatic damage to Sitka spruce, with frost damage reported from only a single plot. Galls on current year’s shoots caused by the insect Adelges cooleyi were recorded on trees in three plots although the level of damage caused was insufficient to affect crown condition.
The condition of oak improved slightly in 2005, the species now having displayed increases in crown density for three years in succession and for 5 of the last 6 years. However, its crown density is currently the same as in 1999 due to the large deterioration in condition which it suffered in 2002. The most important damage to oaks in 2005 was caused by defoliating and leaf-mining insects, the actions of which were recorded in all 86 of the plots assessed. Damage was notably more severe than last year, with insect attack recorded as common or abundant on 30.4% of the assessed trees compared 17.5% in 2004. Heavy or severe attacks by larvae of the winter moth Operophtera brumata and mottled umber moth Erannis defoliaria were reported from 29% of plots in 2005 compared with 15% of plots in 2004. However, production of lammas growth was more widespread and abundant in 2005 than in recent years, being noted on trees in 36% of plots. The production of such new foliage accounts for the apparent anomaly of an increase in the crown density of the species when increased levels of defoliation were also evident.
Although the increase in mean crown density of 1.2% which Scots pine displayed this year was relatively modest, greater improvements in the condition of the species have only been recorded on 3 previous occasions in the survey’s 19 year history. The needle retention of surveyed trees in 2005 was generally good, with 62.8% retaining needles for 3 or more years. Whilst insect damage affected 15.8% of Scots pines assessed this year compared with 13.1% of trees in 2004, the severity of such damage in 2005 was generally slight. Attacks by the pine shoot beetle Tomicus piniperda were evident in only 32.5% of plots compared with 38.1% of plots last year, with death or loss of more than a few shoots occurring in only 9.5% of assessed trees. New wind damage attributable to the storms of early January 2005 was noted in 8 plots, mostly located in northern Britain. No other forms of damage were significant on Scots pine this year.
Rainfall levels were near or slightly below average for the majority of the 2005 growing season and temperatures were mostly warm providing good conditions for tree growth. Severe gales in early January and late frosts in May resulted in localised damage to the tree species assessed in the survey. Changes in condition were minor in oak, Norway spruce, Sitka spruce and Scots pine this year but beech exhibited a sharp increase in crown density. A slight improvement in the condition of Scots pine was largely related to a reduction in the severity of insect damage this year. Both the incidence and severity of insect damage to oak increased this year but many trees produced more foliage via late-season lammas growth and the crown density of the species thereby increased slightly. In common with previous years, the crown density of Norway spruce fluctuated only slightly in 2005 and its condition has remained virtually unchanged since 1991. A minor improvement in the condition of Sitka spruce in 2005 indicates that recovery from the severe defoliations by green spruce aphid which the species suffered in 2002 and 2003 may be protracted. The notable improvement in the condition of beech which occurred this year represents a recovery from the sharp decline in condition which occurred in 2004 due to heavy mast production and is therefore not necessarily an indication of improving health.