Claes Hellqvist

Effects of different pruning regimes on growth and sapwood area of Scots pine

Abstract Scots pine trees (about 25 years old) were deprived of 50–75% of their needle biomass by three different pruning regimes, to study the allocation of growth, heartwood formation and homeostatic adjustment of the stem sapwood area to the reduced tree crown. After four growing seasons (including that of the pruning), differential effects in radial and basal area growth were observed at different stem heights. Unilateral stem pruning reduced growth all along the bole, whereas stem pruning from below caused greater reductions in the lower stem, and pruning from above actually increased radial growth above and decreased growth below the pruned crown fraction. Concurrent with the increase in radial growth, the branch basal area (representing the needle biomass) of the developing new whorls increased in the trees pruned from above, but not in other treatments. Height growth was only slightly affected by the treatments. The pruned trees had a total volume growth loss during the study period in the range 24–33%, compared with the controls. The pruning treatments resulted in a decrease in conducting sapwood area and an increase in non-conducting ‘heartwood’ (i.e. immobilized sapwood rather than true heartwood) area at breast height, but the homeostatic adjustment was still incomplete after four growing seasons, except for the upper whorls developed after the prunings. Thus, reliable needle biomass estimates cannot be derived from sapwood basal area data during the process of homeostatic adjustment after a sudden loss in needle biomass.

Spatial distribution of crown damage and growth losses caused by recurrent attacks of pine shoot beetles in pine stands surrounding a pulp mill in southern Sweden

Crown damage and growth losses caused by the shoot‐feeding of Tomicus piniperda (L.) and T. minor (Hart.) (Col., Scolytidae) were studied in pine forests surrounding a timber yard established in 1977 at a pulp mill in southern Sweden. During 1978–1982, the level of beetle attack was monitored by counting beetle‐pruned shoots on sample plots along survey lines extending 1800 m away from the timber yard. In 1983, increment cores were taken from a total of 324 pine and 63 spruce trees growing on the same sample plots.

Tree Mortality, Needle Biomass Recovery and Growth Losses in Scots Pine Following Defoliation by Diprion pini (L.) and Subsequent Attack by Tomicus piniperda (L.)

Tree mortality and growth losses following insect defoliation are poorly documented in Scandinavia. In 1990-1991, Diprion pini (L.) caused extensive defoliation to Scots pine in Lauhanvuori national park and surrounding areas in south-western Finland. Most trees lost all their foliage in 1990. In 1991, the outbreak area was sprayed with diflubenzuron (Dimilin®), except in the national park, where trees were severely defoliated again. No further defoliation occurred in 1992. In spring 1993, sprayed trees had ca 30% foliage, whereas unsprayed trees on average carried less than 10% of full foliage. The latter trees were susceptible to attack by Tomicus piniperda (L.), whereas the former largely escaped beetle attack. Beetle attacks peaked in 1993, and depletion of suitable host trees probably terminated the beetle outbreak in the area. Two years of severe defoliation resulted in substantial tree mortality and growth losses. In spring 1997, these unsprayed stands had suffered a ca. 50% loss in basal area which was mainly because of mortality, and about half of the dead trees had been attacked by T. piniperda. Surviving trees had ca 50% of full foliage, and radial growth had still not recovered. Basal area growth was reduced by 40-70%, depending on the amount of foliage left after the second year of defoliation. In contrast, tree mortality and beetle attack in the sprayed stands were negligible, and these trees had recovered full foliage and radial growth by spring 1997. Thus, one year of total defoliation resulted in an estimated loss in basal area growth of approximately 30% during ca. 5 yrs. In conclusion, the spraying operation was economically justified, as it prevented substantial tree mortality and reduced growth losses.

Induced and spontaneous attacks by pine shoot beetles on young Scots pine trees: tree mortality and beetle performance

Ovipositional attacks by the pine shoot beetles, Tomicus piniperda (L.) und T. minor (Hart.) (Col., Scolytidae) were induced on 72 young Scots pine trees (Pinus sylvestris L.) by baiting the trees with split pine bolts. Despite high attack densities of T. piniperda and reduced tree vigour due to repeated shoot‐feeding prior to attack, only eight trees died as a result of the induced attack. In the next year, spontaneous beetle attacks resulting in hundreds of dead trees occurred along a new stand edge. Ca 80 % of 106 dead trees grew within 2 m of the new stand edge. Twentyfive of the dead trees were felled together with adjacent living counterparts of the same size, and carefully inspected for beetle attack. All felled trees had been heavily attacked by T. piniperda. T. minor occurred in low numbers on some of the baited as well as spontaneously attacked trees. Killed trees of both categories were significantly more attacked by T. piniperda. Among baited trees, crown length, diameter and radial growth were larger for surviving trees than for killed ones. This was not the case for the pairs of spontaneously attacked trees. In both batches of trees, the critical attack density was ca 300 egg galleries per m2 on the lower stem. The high attack density required to overcome the resistance of the trees resulted in short galleries and a brood production below unity in all but three trees. Thus, both Tomicus‐species are dependent on dead or dying host material for their reproduction, and outbreaks on living trees cannot become selfperpetuating, not even in severely weakened stands.

Field Performance of a Protective Collar against Damage by Hylobius abietis

The effectiveness of a plastic collar designed to protect planted seedlings against damage caused by Hylobius abietis (L.) (Col., Curculionidae) was evaluated at 63 planting sites in southern Sweden during 1979 and 1980. Nearly 10 000 collar‐protected pine and spruce plants and 10 000 controls were carefully examined for Hylobius‐damage and other injuries. In addition, the impact of some microsite factors on weevil damage and collar performance was evaluated, and the height growth was measured. The protective effect of the collar for two seasons after planting was comparable to that of insecticides and was best at sites with sparse vegetation and high weevil pressure. Proper application of the collar was crucial for good control. Collars were not observed to affect plant growth. Soil scarification reduced mortality in both collared plants and controls.

Structure–Activity Relationships of Benzoic Acid Derivatives as Antifeedants for the Pine Weevil, Hylobius abietis

Aromatic organic compounds found in the feces of the pine weevil, Hylobius abietis (L.) (Coleoptera: Curculionidae), have been shown to deter feeding behavior in this species, which is a serious pest of planted conifer seedlings in Europe. We evaluated 55 benzoic acid derivatives and a few homologs as antifeedants for H. abietis. Structure–activity relationships were identified by bioassaying related compounds obtained by rational syntheses of functional group analogs and structural isomers. We identified five main criteria of efficiency as antifeedants among the benzoic acid derivatives. By predicting optimal structures for H. abietis antifeedants, we attempted to find a commercial antifeedant to protect conifer seedlings against damage by H. abietis in regenerating forests. New, highly effective antifeedants are methyl 2,4-dimethoxybenzoate, isopropyl 2,4-dimethoxybenzoate, methyl 2-hydroxy-3-methoxybenzoate, methyl (3,5-dimethoxyphenyl)acetate, and methyl (2,5-dimethoxyphenyl)acetate. Of these, methyl 2,4-dimethoxybenzoate and isopropyl 2,4-dimethoxybenzoate have the highest antifeedant indices of all substances tested and are the best candidates for practical applications in order to protect planted seedlings in the field.

Host resistance in defoliated pine: effects of single and mass inoculations using bark beetle‐associated blue‐stain fungi

1 In 1996, 7000 ha of pine forests were defoliated by the pine looper Bupalus piniaria in south‐western Sweden.

A flexible sand coating (Conniflex) for the protection of conifer seedlings against damage by the pine weevil Hylobius abietis

1 A new method for the physical protection of conifer seedlings against feeding damage by Hylobius abietis (L.), is described and evaluated in field trials in Swedish forest plantations.

Mini-seedlings of Picea abies are less attacked by Hylobius abietis than conventional ones : is plant chemistry the explanation?

Abstract The pine weevil, Hylobius abietis (L.), is a major pest in conifer reforestation areas in the Palaearctic region. Size and chemistry of the seedlings may explain the damage rates in plantations. The performance of 10-week containerized seedlings (mini-seedlings) was compared with 1-year-old conventional seedlings of Norway spruce, Picea abies (L.), in a field experiment in central Sweden. After 2 years the weevil damage was lower for the mini-seedlings than for the conventional seedlings (3.5 vs 55%). After 3 years, the overall survival was 82 and 75%, respectively. Weevil damage was the main cause of mortality for conventional seedlings, whereas mini-seedlings mainly died from drought. Volatiles of the two seedling types were compared by solid-phase microextraction–gas chromatography–mass spectrometry (SPME-GC-MS). Unwounded mini-seedlings and conventional seedlings differed in their compositions of monoterpenes and sesquiterpenes. Mini-seedlings mainly emitted limonene, known to be repellent to the pine weevil. When wounded, green leaf volatiles were released by mini-seedlings while the pine weevil attractant α-pinene was released by conventional seedlings. Volatiles may partly explain the mini-seedlings’ resistance against weevil attack. Further studies are needed to clarify how long this mini-seedling effect remains.

Height growth recovery and crown development in top‐damaged Pinus sylvestris trees

Leader shoot damage was imposed on young Scots pines (3 m high) in four ways: by caging the uppermost whorl including the leader shoot with 10 pine shoot beetles; by caging the two uppermost whorls with 20 beetles; by topping the tree below the 1984 whorl; by topping the trees as above and removing all current shoots in whorl 1983. Six years later, top‐damaged trees were still significantly shorter than undamaged controls, although the annual height growth rate had recovered to the pre‐damage level within 3 years. Recovery occurred in two ways: lateral branches from the nearest intact whorl took dominance over the damaged leader or secondary branches developed from interfascicular buds on the damaged leader shoot. Approximately one and two years of height growth were lost in the former and latter case, respectively. Only 5 of the 34 top‐damaged trees escaped technical defects (crooks and/or vertical branches). Stem diameter and crown development were slightly affected.