The 2004 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 2004 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 a '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 with an ideal tree than when judged against local trees of the same age and form.
Figure 1 shows the changes in crown condition that havetaken place since 1987. A downward gradient in thisfigure indicates a deterioration in crown condition.Alterations in condition compared with last year wereminor for all of the surveyed species except beech, thecrown density of which deteriorated markedly comparedwith 2003. However, the crown condition of beech hasfluctuated widely over the entire 18-year survey periodand the species displays no overall trend for deteriorationor improvement. Similarly, no long-term trends in thecondition of either Sitka spruce or Scots pine are apparent.
Table 1: Percentages of trees in each crown density class for five species in 2004. Each 10% class represents the density of the treeÌs 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: 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.
Analysis of the 1987 to 2004 data indicates that statisticallysignificant deteriorations in the crown densities of both2Norway spruce and oak have occurred over the durationof the survey. The time series are relatively short and theindicated rates of change are small, however, with averagereductions in crown density of 0.45% per annum in oakand 0.31% per annum in Norway spruce. The magnitudesof past increases in the crown density of Norway sprucesuggest that a single year of improvement could nullify thetrend currently displayed by this species. In contrast, anumber of seasons of improvement in condition would berequired to negate the long-term trend for deterioration inoak which has been apparent since 1999 (Redfern,Boswell and Proudfoot, 2000). Caution should beexercised when interpreting the indicated deterioration,however, since it is heavily influenced by the high crowndensity values recorded for oak in the period 1987 to1990 when the number of survey plots of this species wasrelatively low.
Since 1991 the mean crown densities of Norway spruce,Sitka spruce and Scots pine have increased by 0.2%, 1.7%and 2% respectively. In the cases of Norway spruce andScots pine, these relatively minor improvements incondition are a reflection of the very small inter-annualfluctuations in crown density which have occurred overthe past 13 years. However, Sitka spruce has displayedmuch greater variations in condition over the same period.Incremental improvements in crown condition over anumber of consecutive years have been punctuated bymarked deteriorations in particular growing seasons. Thelast notable decrease in the crown density of Sitka spruceoccurred in 2002 and a further slight deterioration in itscondition occurred in 2003. In 2004, this decline wasreversed and the species displayed its largest annualincrease in crown density since 1996. The condition ofoak also improved slightly this year but it has yet torecover from the marked deterioration in condition whichoccurred in 2002.
Figure 2 shows the geographical variation in crowndensity for each of the assessed species. The condition ofbeech was variable but was poorer than in 2003 acrossmuch of central England. Crown densities in the southernparts of the East Midlands and the western parts of eastEngland were notably lower than in recent years. Thecondition of Scots pine tended to be in better in the regionsouth of the Humber-Mersey line than elsewhere,although trees in northwestern England and northeasternScotland also displayed relatively high crown densities.Whilst the condition of Norway spruce appeared to bepoorer in the western half of southeast England and in theEast Midlands than in the remainder of southern Britainthis impression is created by relatively few plots and there is considerable local variation in the condition of thespecies. Sitka spruce displayed no clear pattern. As inprevious years, the crown density of oak displayedconsiderable variation but was poor in southern andcentral Scotland, south Wales and the western parts ofEast England. The condition of trees was particularly poorin central Scotland, where high levels of insect defoliationhave been recorded in oak for several consecutive surveys.
Factors affecting crown condition in 2004
The late summer and autumn of 2003 were very dry, withrainfall in October being only 60% of the long-termaverage. Conditions from November until mid-February2004 were milder and wetter than normal in most parts ofthe country, although heavy snowfalls and blizzardsoccurred in northern Britain in late December and earlyJanuary resulting in physical damage to the crowns ofconifers at certain locations. Widespread and occasionallydamaging snowfalls also occurred towards the end ofFebruary. Following a dry and warm but occasionallystormy March, mild and wet weather predominated inApril and early May and damaging spring frosts wereconsequently rare. With rainfall near or above average forthe remainder of the 2004 growing season andtemperatures being generally warm, conditions for treegrowth were good. Localised extremes of weather thatwere injurious to trees did occur during this period,however. Notably, the heavy hail storms which occurredin the southeast of England in mid-July and early Augustwere sufficiently severe to cause foliar damage to a rangeof broadleaved trees.
The deteriorations in condition suffered by Sitka spruce in2002 and 2003 (Hendry et al., 2003; 2004) were partiallyoffset by an increase in the crown density of the species in2004. This improvement was largely attributable to amarked decrease in both the incidence and severity ofdamage caused by the green spruce aphid Elatobiumabietinum. Whilst signs of current or old aphid attackswere reported from 45 plots in 2003, the insect was activein only 25 survey plots in 2004. The proportion of treesdisplaying new insect damage had correspondinglydecreased from 30.5% in 2003 to 16.5% this year.Damage due to Elatobium is often manifest as a browningof the older needles and the percentage of surveyed treeswhich displayed this symptom also decreased from 22.6%in 2003 to 9.3% this year. In spite of these improvements,the recovery of trees defoliated by aphids in 2002 and2003 is unlikely to be complete for several growingseasons.
The condition of oak has improved in four of the past fiveyears but, in spite of this, its crown density is lower nowthan in 1999 due to the magnitude of the deteriorationwhich occurred in 2002. The most important damage tooaks in 2004 was caused by defoliating and leaf-mininginsects, the actions of which were recorded in 85 of the 86plots assessed. However, such damage was generally lightand insect attack was recorded as common or abundanton only 17.5% of the assessed trees. Heavy or severeattacks by the winter moths Operophtera brumata andErannis defoliaria were confined to only seven plots, all of which were in Scotland. Recent storm damage influencedthe crown condition of trees in several plots located in theareas of southern and eastern England affected by severehailstorms in July and August. Elsewhere, oak dieback(Gibbs, 1999) of long standing was identified as the causeof the poor condition of five plots and severe new diebackwas recorded in a further two plots.
In common with previous cases of decline recorded in1990, 1995, 2000 and 2002, the deterioration in thecondition of beech which occurred in 2004 was largelyassociated with heavy fruiting. Mast production wasnoted on 89.7% of the surveyed trees and was recorded asheavy (assessed by the surveyors as being 'common' or'abundant') in 78.6% of the population. A reduction inleaf size, which sometimes accompanies fruiting, wasnoted on 25.4% of trees in 2004 compared with the 3.6%of trees which displayed this attribute in 2003. Althoughpremature leaf loss was less prevalent than last year, itwas still recorded on 15.9% of the surveyed treessuggesting that recovery of beech from the abnormally drysummer of 2003 may not yet be complete. Damage frominsects was generally light this year with attack beingrecorded as absent, rare or slight on 87.5% of trees. Noother forms of damage were significant.
Changes in the crown densities of Norway spruce andScots pine were minor this year, continuing the pattern oflittle variation which has held for both species since 1991.In 2004, defoliation of Norway spruce by Elatobiumabietinum occurred in nine plots but the extent of damagewas minor in all cases. Fungal damage was largelyrestricted to cases of bud blight caused by Cucurbitariapiceae which was only adjudged to have had an adverseeffect on the crown densities of a few trees. With 24% ofthe surveyed population producing cones, fruiting inNorway spruce was more common this year than at anytime since 1996. However, increased production of coneshas no deleterious effect on the crown condition of thisspecies and, because cones tend to be concentrated nearthe apex of the crown, may even give the tops of trees anappearance of increased density. The proportion of Scotspines retaining their needles for three or more yearsincreased from 32.9% in 2003 to 64.8% in 2004 and thischange might have been expected to result in a largerimprovement in crown density than was actually exhibitedby the species. Marked increases in the incidence both ofmale flowering on the 2004 shoots and of wind damage topine crowns served to offset the effect of increased needleretention, however.
Rainfall was well distributed throughout the 2004growing season in most parts of the United Kingdom andconditions for tree growth were generally good. Heavysnowfalls in early January and late February, high windsin March and severe hailstorms in mid-July and earlyAugust all resulted in localised damage to the tree speciesassessed in the survey. Changes in condition were minorin oak, Scots pine and Norway spruce this year but beechexhibited a sharp decline, and Sitka spruce a markedincrease, in crown density. A slight improvement in thecondition of Scots pine was largely related to increasedneedle retention. Levels of insect damage to oak weregenerally low this year and the crown density of thespecies increased slightly. However, in spite of displayingimprovements in four of the past five years, it has not yetfully recovered from the sharp decline in condition whichit suffered in 2002. In common with previous years, thecrown density of Norway spruce fluctuated only slightlyin 2004 and its condition has remained virtuallyunchanged since 1991. The sharp decline in the conditionof beech which occurred this year was chiefly associatedwith heavy mast production and is not necessarily anindication of poor health. Severe defoliation by the greenspruce aphid resulted in deteriorations in the crowncondition of Sitka spruce in both 2002 and 2003.However, the incidence and severity of attacks by theinsect were reduced this year and a distinct improvementin the condition of the species resulted.