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Forest Condition 2001
The crown density results, using both methods of
assessment, 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), and some older oaks and spruces also
have a naturally open structure. For trees like this, the
apparent reduction in crown density would therefore be
much greater when judged against an ideal tree than
when compared to local trees of the same age and form.
Figure 1 shows the changes in crown condition that
have taken place since 1987. An upward gradient in
this figure indicates a deterioration in crown condition.
Changes in crown density compared to last year were
minor for all species except beech, the condition of
which improved significantly. Analysis of the time series
for each species shows that, in general, only relatively
large changes in crown density (more than 3% to 4%)
between years are statistically significant. Short-term
changes of such magnitude, both positive and negative,
have characterised beech over the fourteen-year survey
period. However, there is no evidence of a long-term
deterioration or improvement of crown condition in
this species. Similarly, no long-term trends can be
detected in the crown densities of either Scots pine or
Sitka spruce, although the latter is currently in better
condition than at the commencement of the survey in
1987. Analysis of the 1987 to 2001 data indicates that a
statistically significant deterioration in the crown
condition of both oak and Norway spruce has occurred
over the duration of the survey. However, the time
series is of relatively short duration and the indicated
rates of change are small, with an apparent reduction in
crown density of 0.47% per annum in oak and 0.37%
per annum in Norway spruce. The magnitudes of past
increases in crown density in these species suggest that a single year of major improvement in condition could
nullify the current trends. In spite of the low rates of
change involved, any continued deterioration in the
condition of these species would warrant more detailed
investigation in order to determine whether such trends
were related to long-term changes in specific factors
affecting tree condition, such as climatic variables.
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 2001 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 began to improve
in 1999 and this recovery continued throughout 2000
and 2001. The crown density of oak also increased this year, its condition having improved in consecutive
growing seasons for the first time since 1989.
However, it is still markedly poorer than during the
late 1980s. The improvement in beech recorded in
2001 completely reversed the deterioration in its
condition which occurred in 2000.
Table 1: Percentages of trees in each crown density class for five species in 2001. 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 has shown substantially the same
pattern since 1997 when data were first presented in this
way (Redfern et al., 1998; 1999; 2000). The condition of
oak was poorest in central Scotland and north-east
England, south-west 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 showed no clear pattern.
Both spruces were poorest in the south and east than
elsewhere, but this impression is created by relatively few
plots and both species show considerable local variation.
Factors affecting crown condition in 2001
The autumn of 2000 was one of the wettest on record,
with the highest October rainfall since 1903 being
recorded. High levels of precipitation were also
recorded over the winter months, and in late February
2001 heavy snowfall occurred in Scotland and
northern England resulting in snow-break of branches
in many coniferous species, but particularly in Scots
pine. A dull April and warm May resulted in a general
absence of damaging spring frosts. In spite of hot dry
conditions in June and early July no drought damage
to forest trees was reported. The remainder of the
growing season was generally warm and wet,
providing good conditions for tree growth.
The increase in the crown density of beech recorded in
2001 was of sufficient magnitude to reverse the
deterioration in condition which the species had
displayed in the previous growing season (Figure 1). As
noted in previous years (Hendry et al. 2001) poor crown
condition in beech is often associated with heavy mast
production, whilst crown density is greatest when
fruiting is light or absent. Whereas masting was heavy
in 2000, little or no fruiting was recorded for 89% of
the 1 392 beech trees assessed in 2001. However, past
deteriorations in the condition of beech associated with
heavy masting (e.g. 1990 and 1995) have not been
followed by a complete recovery in crown density in the
following year in spite of the occurrence of light fruiting.
It is therefore likely that some other factor(s) contributed
to the large improvement in beech crown density
recorded in 2001. Beech is particularly intolerant of
water deficit, and the wet weather which prevailed at the
end of 2000 and throughout much of 2001 would have
provided favourable conditions for its rapid recovery.
Although the condition of oak continues to improve
and is now better than at any time since 1995, it
remains poorer than any of the other species surveyed.
This appears to be due largely to insect damage which
was recorded in 73 of the 82 plots assessed this year.
Plots with low crown density were generally those in
which insect activity was scored as heavy or severe,
although wind damage of sufficient extent to affect
crown density scores was recorded in five plots. Oak
dieback (Gibbs, 1999) was the primary cause of poor
condition in three plots in England.
The condition of Sitka spruce improved slightly
compared to last year. Although the presence of new
damage by green spruce aphid was reported from 24 of
the 65 plots assessed, attacks were minor both in terms
of the numbers of trees affected and in the degree of
defoliation caused. Between 2000 and 2001 the
percentage of trees retaining 7 or more needle years
rose by 9% to 60.5% suggesting that Sitka spruce is
continuing to recover from the severe defoliation which
it suffered during the widespread Elatobium outbreak
in 1997. In accord with the generally favourable
weather conditions there were few reports of climatic
damage to Sitka spruce, with frost damage reported
from only a single plot. Galls on current yearÌs shoots
caused by the insect Adelges cooleyi were recorded on
trees in six plots although the level of damage caused
was insufficient to affect crown condition.
A slight deterioration in Scots pine this year reflected
both an increase in the incidence of infection by the
fungus Lophodermium seditiosum, which was
recorded in 16% of surveyed plots, and an increase in
the incidence of male flowering which results in nonfoliated
gaps at the bases of shoots and thus in
increased crown transparency. A minor improvement
in the crown density of Norway spruce was not
associated with a decrease in the incidence or severity
of biotic or abiotic damage but may have been due in
part to an increase in secondary shoot formation,
which was recorded in the majority of trees.
Conclusions
In spite of a relatively short dry period over the
country in June and early July, rainfall was well
distributed throughout the growing season and tree
growth was generally good. Heavy snowfall in
February resulted in localised damage to Sitka spruce,
Norway spruce and Scots pine in the north of Britain
but no other forms of climatic injury were important
this year. Changes in crown density were minor in all
species except beech, the condition of which improved
significantly. However, this change represents a
recovery from the sharp decline in the crown
condition of beech in 2000 which was associated with
heavy mast production and is therefore not necessarily
an indication of improving health. Crown density in
Norway spruce has displayed only minor fluctuations
since 1991 and remained virtually unchanged in 2001.
Scots pine deteriorated slightly, largely due to an
increase in the incidence of the needle cast fungus
Lophodermium seditiosum and heavy male flowering
on many trees. A slight improvement in condition this
year suggests that Sitka spruce is continuing a gradual
recovery from the severe outbreak of Elatobium which
affected it in 1997. Oak, although in better in
condition than at any time since 1995, still displays a
lower crown density than the other surveyed species.
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