Tools and Resources
Crown condition is assessed by estimating the ‘reduction’ in density of a tree’s crown when compared either with an ‘ideal’ tree or with the best ‘local’ tree in the vicinity of a plot. Since 1987,the crown condition data from the Forest Condition Survey have similarly been presented in terms of reductions in density. However, rather than considering the percentage of the crown which is absent, it may be easier to visualise the condition of trees on the basis of the percentage of the crown which is present when compared with a known standard. Therefore, although the methods used for assessing crown condition in 2002 remained unchanged from those employed in previous years, the results of the survey are presented here in terms of crown densities rather than crown density reductions. It should be noted that percentage scores for crown density and crown density reduction are complementary i.e. the sum of the two measures for an individual tree is always 100%. To compare the numerical results presented here with those from previous surveys, percentage crown density can be converted to percentage crown density reduction by applying the following formula:
% crown density reduction = 100 – % crown density
The crown density results obtained using both the ideal and local tree methods are presented in 10% classes in Table 1. The marked effect of using a local reference tree rather than an ideal tree as the basis for comparison can be seen for all species. Much of the difference can be accounted for by variations in 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, but particularly Scots pine, 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 densities of trees like these are much lower when compared to an ideal tree than when judged against local trees of the same age and form.
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. Changes in condition compared to last year were minor in Norway spruce, Scots pine and beech, but marked reductions in crown density occurred in Sitka spruce and oak. In spite of this, none of the changes in crown density which occurred between 2001 and 2002 were statistically significant: analysis of the time series for each species shows that relatively large fluctuations in crown density (more than 3% to 4%) between years are required for the statistical significance of such changes to be established. Short-term fluctuations of such magnitude are relatively infrequent in all of the surveyed species except beech, the condition of which has been characterised by large inter-annual changes in crown density over the 16-year survey period. However, there is no evidence of along-term trend for deterioration or improvement in the crown density of beech; similarly, no trend in the crown density of either Sitka spruce or Norway spruce is apparent. In the case of Norway spruce, this observation indicates a reversal in the trend for deterioration which has previously been reported (Hendry, Boswell & Proudfoot 2001, 2002). However, since Forest Condition Survey plots are subject to normal silvicultural operations, clearfelling of old crops in poor condition has had an effect on the overall score for the species by removal of those plots displaying the lowest crown densities. As a result, the improvement in the condition of Norway spruce indicated by the survey does not simply reflect a long term change in the health of the species per se but is at least partly attributable to a change in the nature of the population which is being sampled.
Analysis of the 1987 to 2001 data indicates that a statistically significant deterioration in the crown condition of both Scots pine and oak has occurred over the duration of the survey. However, the timeseries is of short duration and the indicated rates of change are small, with an apparent reduction in crown density of 0.24% per annum in Scots pine and 0.53% per annum in oak. For oak, the trend is heavily influenced by the high crown density values recorded in the period 1987 to 1990 when the number of survey plots of this species was relatively low and caution should therefore be exercised in the interpretation of this result. The magnitudes of past increases in crown density in both oak and Scots pine suggest that a single year of major improvement in condition could nullify the current trends for either species.
Since 1991 the condition scores of Scots pine and Norway spruce have changed less than those of the other species. Although Scots pine deteriorated slightly in 2002 and the condition of Norway spruce showed a minor improvement, neither of these changes was significant. Following a sharp decline in 1997, the condition of Sitka spruce improved between1999 and 2001, but this recovery was completely reversed in 2002. The crown density of oak also decreased this year, its condition now being as poor as in 1997. The major improvement in beech which occurred in 2001 was partially offset by a minor reduction in its crown density in 2002.
Table 1: Percentages of trees in each crown density class for five species in 2002. Each 10% class represents a reduction in crown density 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. a tree with full foliage under local conditions.
Figure 1: Changes in crown density since 1987 for five species surveyed annually. The crown density compared with that of an ‘ideal’ tree with a completely opaque crown is shown for each species.
Figure 2 shows the geographical variation in crown density for the five species assessed. Variation was greatest in oak, which continues to display a pattern substantially unchanged since 1997 when data were first presented in this way (Redfern et al., 1998; 1999;2000). The condition of oak was poorest in northern and central Scotland, north-east England, Wales and East Anglia, and best in southern England. Scots pine also displayed a pattern which was similar to that of previous years, with crown density tending to be highest south of the Humber-Mersey line. Beech and Norway spruce showed no clear pattern. Sitka spruce was poorest in southern and western England, but this impression is created by relatively few plots and the species shows considerable local variation.
The autumn of 2001 was generally warm and wet, with frequent heavy rainfalls and a record high average October temperature. Mild, wet conditions also predominated over the winter months with February being abnormally warm. Although April and May were largely dry and clear, minimum temperatures rarely approached damaging levels and there were few instances of spring frost damage to trees. Towards the end of April, severe squalls in certain parts of the country led to localised branch breakage and windblow. Rainfall was higher than average over the summer months, with July being particularly wet, and as a result no drought damage to forest trees was reported. Continuing warm weather over the remainder of the growing season provided conditions which were generally good for tree growth.
The decrease in the crown density of Sitka spruce recorded in 2002 was of sufficient magnitude to reverse the improvement in condition which the species had displayed between 1999 and 2001 (Figure1). This deterioration was almost entirely due to widespread defoliation by the green spruce aphid Elatobium abietinum, with attacks being reported from 43 of the 63 survey plots. However, damage was often confined to the lower areas of the crowns of the surveyed trees and was therefore not as severe as might have been anticipated from inspection of individual trees and shelter belts located near to plots, which were often completely defoliated.
In common with previous cases of decline in 1995 and 2000 (Hendry, Boswell & Proudfoot, 2001), the deterioration in the condition of beech recorded in 2002 was largely associated with heavy masting. Mast production was noted on 78% of the surveyed trees and was recorded as heavy (common or abundant) in 58% of the population. Levels of insect damage to beech in 2002 were broadly similar to those recorded in 2001, although the incidence of severe damage was slightly elevated due to heavy attack by the leaf mining insect Rhynchaenus fagi in 17 plots.
Changes in the crown densities of Norway spruce and Scots pine this year were minor, thus continuing a pattern of little variation in condition which has held since 1993. In 2002, Norway spruce was defoliated by Elatobium abietinum but it is less susceptible to attack than Sitka spruce and damage was slight. Distortion of the branching pattern of trees caused by the bud blight fungus Cucurbitaria piceae was observed in plots in northern England and southern Scotland, but this had little influence on the crown density of affected individuals. The slight deterioration in Scots pine noted this year was attributable both to elevated levels of infection by the needle pathogen Lophodermium seditiosum and to abundant production of male flowers on the 2002 shoots. The loss of foliage infected by L. seditiosum was reflected by a reduction in the percentage of trees retaining needles for 3 or more years from 62% in 2001 to 47% in 2002.
The deterioration of oak which occurred in 2002 completely reversed the improvement in condition which had occurred over the previous two years, and the crown density of the species is now as low as at any time during the 16-year survey period. However, this decline cannot be related to serious climatic injury, or to a major outbreak of a particular pest or disease in 2002, since the incidence of abiotic and biotic damage to trees was broadly similar to that recorded in 2001. Analysis of the changes in crown density which occurred between 2001 and 2002 in the 1955 trees common to both surveys indicates that 1009 trees deteriorated in condition. Of these, 68% displayed a reduction in crown density of only 5% to 10% whilst 86% displayed a reduction of 5% to 15%. The overall change in the condition of oak since last year is therefore attributable to a minor reduction in the crown density of many trees rather than a major change in the condition of a few trees. Since the classes which are used to record abiotic and biotic damage in the Forest Condition Survey are broader than those used for the recording of crown density, a minor change in the crown density of a tree would not necessarily be reflected by a change in the damage class to which it was allotted. Thus, a small but general increase in the level of insect and fungal damage to oak in 2002 may account for the recorded change in condition.
Insect damage was present in 81 of the 86 oak plots assessed in 2002 but was severe in only 9 plots where defoliation by the winter moths Operophthera brumata and Erranis defoliaria had occurred. Central Scotland was particularly affected and it is noticeable that the highest concentration of plots in poor condition was located here (Figure 2). Branch break resulting from the severe winds in April was recorded in 7 plots in northern England and Scotland but no other significant climatic damage was noted. Although not directly damaging, the warm and wet conditions which prevailed in spring and early summer are likely to have favoured the development of powdery mildew(Microsphaera alphitoides) which was more widespread and severe this year than in the recent past.
Rainfall was well distributed throughout the 2002 growing season and tree growth was generally good. High winds in April caused localised damage to all of the surveyed species but no other forms of climatic injury were important this year. Changes in condition were minor in Norway spruce, Scots pine and beech but marked reductions in crown density occurred in Sitka spruce and oak. The slight deterioration of Scots pine this year was largely due to an increase in the incidence of the needle cast fungus Lophodermium seditiosum and continued heavy male flowering on many trees. Crown density in Norway spruce has displayed only minor fluctuations since 1991 and remained virtually unchanged in 2002. A slight decline in the condition of beech was largely associated with heavy mast production and is not necessarily an indication of ill health. The sharp decrease in crown density displayed by Sitka spruce was due to widespread defoliation by the green spruce aphid Elatobium abietinum. However, attack was often confined to the lower areas of the crowns of the surveyed trees and damage was therefore not as severe as recorded after the previous outbreak of Elatobium in 1997. The deterioration in the condition of oak which occurred this year was attributable to a minor reduction in the crown density of many trees rather than a notable increase in the incidence of attack by pests or pathogens.