The crown density results, using both methods ofassessment, are presented in 10% classes in Table 1.The marked effect of using a local reference tree ratherthan an ideal tree as the basis for comparison can beseen for all species. Much of the difference can beaccounted for by variations in growth habit betweenthe 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. Forexample, young trees of all species, but particularlyScots pine, tend to have a more open appearance (i.e.a lower crown density) than the older trees illustratedin Innes (1990), and some older oaks and spruces alsohave a naturally open structure. For trees like this, theapparent reduction in crown density would therefore bemuch greater when judged against an ideal tree thanwhen compared to local trees of the same age and form.
Figure 1 shows the changes in crown condition thathave taken place since 1987. An upward gradient inthis figure indicates a deterioration in crown condition.Changes in crown density compared to last year wereminor for all species except beech, the condition ofwhich improved significantly. Analysis of the time seriesfor each species shows that, in general, only relativelylarge changes in crown density (more than 3% to 4%)between years are statistically significant. Short-termchanges of such magnitude, both positive and negative,have characterised beech over the fourteen-year surveyperiod. However, there is no evidence of a long-termdeterioration or improvement of crown condition inthis species. Similarly, no long-term trends can bedetected in the crown densities of either Scots pine orSitka spruce, although the latter is currently in bettercondition than at the commencement of the survey in1987. Analysis of the 1987 to 2001 data indicates that astatistically significant deterioration in the crowncondition of both oak and Norway spruce has occurredover the duration of the survey. However, the timeseries is of relatively short duration and the indicatedrates of change are small, with an apparent reduction incrown density of 0.47% per annum in oak and 0.37%per annum in Norway spruce. The magnitudes of pastincreases in crown density in these species suggest that a single year of major improvement in condition couldnullify the current trends. In spite of the low rates ofchange involved, any continued deterioration in thecondition of these species would warrant more detailedinvestigation in order to determine whether such trendswere related to long-term changes in specific factorsaffecting tree condition, such as climatic variables.
Since 1991 the condition scores of Scots pine andNorway spruce have changed less than those of any ofthe other species. Although Scots pine deterioratedslightly in 2001 and the condition of Norway spruceshowed a minor improvement, neither of thesechanges was significant. Following a sharp decline in1997, the condition of Sitka spruce began to improvein 1999 and this recovery continued throughout 2000and 2001. The crown density of oak also increased this year, its condition having improved in consecutivegrowing seasons for the first time since 1989.However, it is still markedly poorer than during thelate 1980s. The improvement in beech recorded in2001 completely reversed the deterioration in itscondition 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 crowndensity for the five species assessed. Variation wasgreatest in oak, which has shown substantially the samepattern since 1997 when data were first presented in thisway (Redfern et al., 1998; 1999; 2000). The condition ofoak was poorest in central Scotland and north-eastEngland, south-west England, Wales and East Anglia,and best in southern England. Scots pine also displayed apattern which was similar to that of previous years, withcrown density tending to be highest south of theHumber-Mersey line. Beech showed no clear pattern.Both spruces were poorest in the south and east thanelsewhere, but this impression is created by relatively fewplots 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 beingrecorded. High levels of precipitation were alsorecorded over the winter months, and in late February2001 heavy snowfall occurred in Scotland andnorthern England resulting in snow-break of branchesin many coniferous species, but particularly in Scotspine. A dull April and warm May resulted in a generalabsence of damaging spring frosts. In spite of hot dryconditions in June and early July no drought damageto forest trees was reported. The remainder of thegrowing season was generally warm and wet,providing good conditions for tree growth.
The increase in the crown density of beech recorded in2001 was of sufficient magnitude to reverse thedeterioration in condition which the species haddisplayed in the previous growing season (Figure 1). Asnoted in previous years (Hendry et al. 2001) poor crowncondition in beech is often associated with heavy mastproduction, whilst crown density is greatest whenfruiting is light or absent. Whereas masting was heavyin 2000, little or no fruiting was recorded for 89% ofthe 1 392 beech trees assessed in 2001. However, pastdeteriorations in the condition of beech associated withheavy masting (e.g. 1990 and 1995) have not beenfollowed by a complete recovery in crown density in thefollowing year in spite of the occurrence of light fruiting.It is therefore likely that some other factor(s) contributedto the large improvement in beech crown densityrecorded in 2001. Beech is particularly intolerant ofwater deficit, and the wet weather which prevailed at theend of 2000 and throughout much of 2001 would haveprovided favourable conditions for its rapid recovery.
Although the condition of oak continues to improveand is now better than at any time since 1995, itremains poorer than any of the other species surveyed.This appears to be due largely to insect damage whichwas recorded in 73 of the 82 plots assessed this year.Plots with low crown density were generally those inwhich insect activity was scored as heavy or severe,although wind damage of sufficient extent to affectcrown density scores was recorded in five plots. Oakdieback (Gibbs, 1999) was the primary cause of poorcondition in three plots in England.
The condition of Sitka spruce improved slightlycompared to last year. Although the presence of newdamage by green spruce aphid was reported from 24 ofthe 65 plots assessed, attacks were minor both in termsof the numbers of trees affected and in the degree ofdefoliation caused. Between 2000 and 2001 thepercentage of trees retaining 7 or more needle yearsrose by 9% to 60.5% suggesting that Sitka spruce iscontinuing to recover from the severe defoliation whichit suffered during the widespread Elatobium outbreakin 1997. In accord with the generally favourableweather conditions there were few reports of climaticdamage to Sitka spruce, with frost damage reportedfrom only a single plot. Galls on current yearÌs shootscaused by the insect Adelges cooleyi were recorded ontrees in six plots although the level of damage causedwas insufficient to affect crown condition.
A slight deterioration in Scots pine this year reflectedboth an increase in the incidence of infection by thefungus Lophodermium seditiosum, which wasrecorded in 16% of surveyed plots, and an increase inthe incidence of male flowering which results in nonfoliatedgaps at the bases of shoots and thus inincreased crown transparency. A minor improvementin the crown density of Norway spruce was notassociated with a decrease in the incidence or severityof biotic or abiotic damage but may have been due inpart to an increase in secondary shoot formation,which was recorded in the majority of trees.
In spite of a relatively short dry period over thecountry in June and early July, rainfall was welldistributed throughout the growing season and treegrowth was generally good. Heavy snowfall inFebruary resulted in localised damage to Sitka spruce,Norway spruce and Scots pine in the north of Britainbut no other forms of climatic injury were importantthis year. Changes in crown density were minor in allspecies except beech, the condition of which improvedsignificantly. However, this change represents arecovery from the sharp decline in the crowncondition of beech in 2000 which was associated withheavy mast production and is therefore not necessarilyan indication of improving health. Crown density inNorway spruce has displayed only minor fluctuationssince 1991 and remained virtually unchanged in 2001.Scots pine deteriorated slightly, largely due to anincrease in the incidence of the needle cast fungusLophodermium seditiosum and heavy male floweringon many trees. A slight improvement in condition thisyear suggests that Sitka spruce is continuing a gradualrecovery from the severe outbreak of Elatobium whichaffected it in 1997. Oak, although in better incondition than at any time since 1995, still displays alower crown density than the other surveyed species.