Forest Condition 2002
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 5
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 a
long-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 longterm
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 time
series 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 excercised 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 any 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 between
1999 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.
Factors affecting crown condition in 2002
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 (Figure
1). 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.
Conclusions
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
seditiosumand 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.
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