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Forest Condition 2000
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. In contrast to the method of presentation
used before 1998, the figure records the mean percent
reduction in crown density for each species compared
to an ideal tree. Changes in crown density compared
to last year were minor for all species except beech,
the condition of which deteriorated 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 the only species
currently in better condition than at the commencement
of the survey in 1987. Analysis of the 1987 to 2000 data
indicates that a statistically significant deterioration in
crown condition has occurred for both oak and
Norway spruce 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 0.57% per
annum in oak and 0.47% per annum in Norway
spruce. The magnitudes of past increases in crown
density in these species suggest that a single year of
improvement could nullify the current trends.
Since 1991 the condition scores of Scots pine and
Norway spruce have changed less than those of any of
the other species. Although Norway spruce displayed
no significant change in 2000, the condition of Scots
pine showed a slight improvement. Following a sharp
decline in 1997, the condition of Sitka spruce began to
improve in 1999 and this recovery continued in 2000.
Oak also improved this year, reversing the slight
deterioration recorded in 1999, and was in better
condition than at any time since 1995. However, it is
still markedly poorer than during the late 1980s. The
decline in beech recorded in 2000 completely reversed
the improvement recorded in 1998 and crown density
was reduced to the lowest level recorded since 1991.
Table 1: Percentages of trees in each crown density class for five species in 2000. 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). Oak was
again 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 that was similar to those in 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 2000
Over much of the country April and the beginning of
May were extremely wet but the consistent cloud cover
over this period contributed to the absence of damaging
spring frosts. Unprecedented high winds in mid-June
resulted in shoot loss and browning of current year
needles in two Norway spruce and four Sitka spruce
plots in the north of the UK. In spite of extremely dry
weather over much of Scotland in July, no drought
damage to forest trees was reported. The remainder of
the growing season was generally warm and wet.
The deterioration in beech this year was of similar
magnitude to former declines in 1990 and 1995 (Figure
1). As in these previous cases (Redfern et al., 1996), the
reduction in crown density in 2000 was associated with
heavy mast production (Table 2). Although the
deterioration in the condition of beech and the
increased fruiting noted in 1990 were considered as
separate manifestations of stress caused by the drought
of 1989 (Innes & Boswell, 1991), neither the 1995 nor
2000 growing season were preceded by drought years.
However, since female flowers and subsequently fruits
occupy positions which might otherwise give rise to
vegetative shoots bearing foliage, heavy fruiting alone
may bring about a reduction in crown density.
Although the condition of oak is better than for
several years, it is still poorer than any of the other
species surveyed. As in previous years, this appears to
be due largely to insect damage which was recorded in
74 of the 85 plots assessed. Plots with low crown
density were generally those in which insect activity
was scored as heavy or severe, although oak dieback
(Gibbs, 1999) appeared to be the primary cause of
poor condition in four plots in England.
The condition of Sitka spruce improved slightly
compared to last year. There were few reports of
major damage by green spruce aphid and scores for
insect damage remained at the low levels recorded in
1999, suggesting that Sitka spruce is continuing a
gradual recovery from the severe Elatobium outbreak
of 1997. Exposure injury and abrasion by wind were
reported from 13 Sitka spruce and 23 Norway spruce
plots. The condition of Norway spruce was virtually
unchanged in 2000, with only minor damage from
insects and fungi being recorded.
The improvement in Scots pine this year reflected a
marked increase in the number of trees in the lowest
defoliation classes (0–20% reduction in crown density)
despite levels of damage from the pine shoot beetle
(Tomicus piniperda) and by the fungi Lophodermium
seditiosum and Peridermium pini which were similar
to those recorded in 1999. However, counts of needle
retention demonstrated a marked increase in the number
of trees retaining needles for 2 or more years in 2000.
ConclusionsApart from a relatively short dry period over the north
of the country in July, rainfall was well distributed
throughout the growing season and tree growth was
generally good. Unseasonal high winds in June caused
localised damage to Sitka spruce and Norway spruce
in Scotland 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
deteriorated significantly. However, this change was
associated with heavy mast production, which has been
recorded during previous episodes of sharp reduction in
beech crown density, and is not necessarily an indication
of ill health. 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 better in 2000 than
for several years, is still in poorer condition than the
other surveyed species. Crown density in Norway
spruce has displayed only minor fluctuations since
1991 and remained virtually unchanged in 2000.
Scots pine improved slightly, largely due to an increase
in the number of needle years retained by trees.
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