The 2003 results are presented here in terms of crown densities rather than the crown density reduction sreported 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 beseen in Table 1, where the results obtained in 2003 using both methods of crown density assessment are presented. A greater proportion of trees receive high density scores when compared with a 'local' ratherthan an 'ideal' standard. This difference can largely beaccounted 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 tend to have amore 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 to an ideal tree than when judged against local trees of the sameage and form.
Figure 1 shows the changes in crown condition that have taken place since 1987. A 'downward' gradient inthis figure indicates a 'deterioration' in crowncondition. Alterations in condition compared with lastyear were minor for all of the surveyed species and notone of the changes was statistically significant.However, relatively large fluctuations in crowndensity (more than 3 to 4%) between years are requiredfor the statistical significance of such differences to beestablished. Short-term variations of this magnitudeare infrequent in all of the surveyed species exceptbeech, the condition of which has been characterisedby large inter-annual changes in crown density overthe 15-year survey period. However, there is noevidence of a long-term trend for deterioration orimprovement in the crown density of beech; similarly,no trend in the crown density of either Sitka spruce orScots pine is apparent. For Scots pine, this represents anegation of the trend for deterioration which wasreported in 2002 (Hendry et al., 2003) and hasresulted from a single year of minor improvement incondition. In contrast, a minor deterioration in thecrown density of Norway spruce in 2003 has re-establishedthe trend for deterioration of the specieswhich was evident in 2000 and 2001 but not in 2002(Hendry et al., 2001, 2002, 2003).
Analysis of the 1987-2003 data also indicates that astatistically significant deterioration in the crowncondition of oak has occurred over the duration of thesurvey. However, the time series is of relatively shortduration and the indicated rate of change is small,with an apparent reduction in crown density of0.48% per annum. The trend is heavily influenced bythe high crown density values recorded in the period1987 to 1990 when the number of survey plots of thisspecies was relatively low. Caution should thereforebe exercised in the interpretation of this result. Themagnitudes of past increases in crown density in oaksuggest that a single year of major improvement incondition could nullify the current trend.
The condition scores of Scots pine and Norway sprucehave changed little since 1991 and have displayedonly minor inter-annual changes during that period.The average difference between the crown densityscores recorded in successive years is less than 1% foreither species and a change of more than 2% hasoccurred on only one occasion. Over the same period,greater fluctuations in crown density have occurred inSitka spruce. A sharp decline in the condition of this species in 1997 was followed by successive minor improvements between 1999 and 2001. However, the recovery was completely reversed in 2002 and, after a further slight deterioration in 2003, its condition is currently at the poorest level recorded since 1991. The crown density of oak increased slightly in 2003, partially offsetting the deterioration recorded in 2002. The condition of beech also improved in 2003 and it is currently the only surveyed species with a higher crown density than it displayed at the inception of the survey in 1987.
Table 1: Percentages of trees in each crown density class for five species in 2003. Each 10% class represents a reduction in crown density compared either to an 'ideal tree', 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 crowndensity for the five species assessed. The condition ofoak was highly variable but poor in Scotland,northeast England, mid-Wales and the central belt ofeast England. Crown densities were particularly lowacross central Scotland where high levels of defoliationhave been recorded in oak for several consecutivesurveys. As in previous years, the condition of Scotspine tended to be in better in the region south of theHumber-Mersey line. No clear pattern was evident forNorway spruce. Sitka spruce was poorest in southwestEngland, but this impression is created by relativelyfew plots and there is considerable local variation inthe condition of the species. Although the condition ofbeech was generally good, lower crown densities wererecorded in southern and central Scotland, the EastMidlands and the southern reaches of east Englandthan elsewhere. A reasonable correspondence can bedetected between the 2003 condition map for beechshown here and the areas identified as climaticallysuitable for the growth of this species in Britain byRay et al. (2002).
Factors affecting crown condition in 2003
The autumn of 2002 was generally wet and, althoughOctober was cool, temperatures were warmer thanaverage over the entire UK in November. Decembercontinued wet in most parts of the country. Whilerainfall and temperature levels in January 2003 wereclose to their long-term averages in most localities, theperiod from February to April was unusually warm, dryand sunny. May of 2003 was warm but unseasonablywet and the dull conditions which characterised theearly part of the month contributed to a general lackof damaging spring frosts. Summer was unusually hot,with the mean June temperature being the highestrecorded in England and Wales since 1976. However, rainfall in June and July was close to average and it wasonly in August that the exceptionally dry conditionswhich prevailed until the end of October began. As aresult, few acute symptoms of water deficit were notedon trees during the course of the growing season.
The marked decline in the condition of oak whichoccurred in 2002 was partially offset by an increase incrown density in 2003. Although relatively modest,the improvement in condition which occurred thisyear has only been exceeded in magnitude on threeoccasions during the surveyÌs 17-year history. At thelevel of individual trees, 46.5% of the sampled oakpopulation improved in condition, 21.5% displayedno change in crown density and the condition of 32%of trees deteriorated. Of those trees which improved in condition, the majority (71%) displayed increases incrown density of only 5 to 10%. As in 2002, themagnitude of these differences was insufficient to relatethe overall improvement of the species to changes inthe incidence or severity of biotic and abiotic damageto trees.
The most important damage to oaks in 2003 wascaused by defoliating insects, the actions of which wererecorded in 59 of the 63 plots assessed. In most casessuch damage was minor in extent but in certain plots,particularly those located in northwest England andsouthern/central Scotland, attacks by the winter mothsOperophthera brumata and Erranis defoliaria weresevere. The oak plot in central Scotland with the lowestaverage crown density recorded in Britain this year(which appears within the red area of the appropriatemap in Figure 2) has suffered consecutive defoliationsof varying severities over the past 8 years. The averagecrown density for this plot has steadily declined from69.8% in 1989 to 33.4% in 2003 (Figure 3). Elsewhere,oak dieback (Gibbs, 1999) was identified as the primarycause of poor condition in five plots. However, lessdamage due to powdery mildew (Microsphaeraalphitoides) was recorded in 2003 than in recent years,a fact which may reflect the dry conditions whichpredominated throughout much of the growing season.
Figure 3: Changes in crown density since 1989 for a single oak plot in southern Scotland. Arrows indicate years in which severe defoliation by winter moth larvae has been recorded.
A marked decrease in the crown density of Sitkaspruce last year was followed by a further slightdecline in condition during 2003. After a relativelymild winter the population of Elatobium abietinum which had been responsible for much of the damageto the species in 2002 remained high and, in certainlocalities, aphids were found feeding on the currentyearÌs foliage as early as June 2003. However, theincidence and severity of damage were generallyreduced this year, with active aphid attacks reportedfrom 50% of plots compared with the 72% affected in2002. In spite of this, the recovery of spruces fromneedle loss is slow and it may take several seasons forthe crown densities of trees defoliated in the last twoyears to fully recover.
In 2003, Norway spruce was also defoliated byElatobium abietinum but it is less susceptible to attackthan Sitka spruce and damage was consequently minor.Bud blight and subsequent distortion of the branchingpattern of trees as a result of infection by the pathogenCucurbitaria piceae were more commonly recorded in2003 than in previous years, with records from 27 ofthe 54 plots assessed. Infections of sufficient severityto reduce the crown densities of trees were, however,only apparent in 5 plots. Of the conifers assessed inthe survey, only Scots pine displayed an improvementin condition in 2003. The increase in crown densityfor the species was slight and levels of damage fromliving and non-living agents were largely unchangedfrom those recorded in 2002. Attacks by the pineshoot beetle Tomicus piniperda were common butminor in extent, and symptoms of water deficit in theform of premature senescence of older needles wereconfirmed in only 2 cases.
The greatest improvement in condition during 2003occurred in beech, the crown density of whichincreased by 1.8% compared with 2002. Insectdamage, which was mostly attributable to the beechleaf miner Rhynchaenus fagi, was widespread andrecorded in 62 of the 65 plots assessed this year.However, the severity of such damage was muchlower than last year with the proportion of trees inwhich attack was scored as common or abundanthaving reduced from 23.6% in 2002 to 12.4% in2003. In beech, poor crown condition is oftenassociated with heavy mast formation while crowndensity is generally higher in years when fruiting islight or absent (Hendry et al., 2001). The relationshipbetween these factors was again evident this year, withmast production being scarce or absent in 91.6% oftrees. In spite of the sensitivity of beech to drought,overt symptoms of water deficit in the form ofpremature leaf senescence and leaf fall were recordedin only six plots located in the southeast of England.
The 2003 growing season was both drier and warmerthan average in most parts of the United Kingdom.However, May was unseasonably wet and the driestconditions of the year were not encountered untilAugust. As a result, few acute symptoms of water deficitwere noted on trees during the course of the survey.Changes in crown density with respect to 2002 wereminor in all of the assessed species but there was anoverall improvement in the condition of trees this year,due largely to reduced levels of defoliation in thebroadleaves. Slight reductions in the crown densitiesof both Sitka spruce and Norway spruce were largelyattributable to defoliation by the green spruce aphidElatobium abietinum. The insect caused severedefoliation of Sitka spruce in 2002 and continueddamage this year may be due to the survival of relativelylarge populations of the aphid through the mild winterand spring of 2003. A slight increase in the crowndensity of Scots pine was not clearly related to anychange in the levels of damage from living and nonlivingagents. Insect damage to oak was widespread in2003 but generally of low severity and the crowndensity of the species displayed improvement of amagnitude which has only been exceeded on threeprevious occasions. As a result of light mast formationand reduced levels of insect damage, the condition ofbeech improved this year and it is currently the onlysurveyed species with a higher crown density than itdisplayed at the inception of the survey in 1987.