Bo Långström

The influences of forest stand management on biotic and abiotic risks of damage

Abstract• This article synthesizes and reviews the available information on the effects of forestry practices on the occurrence of biotic and abiotic hazards, as well as on stand susceptibility to these damaging agents, concentrating on mammal herbivores, pest insects, pathogenic fungi, wind and fire.• The management operations examined are site selection, site preparation, stand composition, regeneration method, cleaning and weed control, thinning and pruning, and harvesting. For each of these operations we have examined how they influence the occurrence of biotic and abiotic damaging agents, the susceptibility of European forests, and describe the ecological processes that may explain these influences.• Overall, we find that the silvicultural operations that have the largest influence on both biotic and abiotic risks to European forest stands are closely related to species composition and the structure of the overstorey. Four main processes that drive the causal relationships between stand management and susceptibility have been identified: effect on local microclimate, provision of fuel and resources to biotic and abiotic hazards, enhancement of biological control by natural enemies and changes in individual tree physiology and development.• The review demonstrates an opportunity to develop silvicultural methods that achieve forest management objectives at the same time as minimising biotic and abiotic risks.Résumé• Cette revue bibliographique s’intéresse aux effets de la sylviculture sur la sensibilité des peuplements forestiers aux principaux agents de dégâts biotiques et abiotiques que sont les mammifères herbivores, les insectes ravageurs, les champignons pathogènes, le feu et les vents forts.• Les pratiques forestières analysées sont la sélection et la préparation des sites de reboisement, la définition de la composition en essences et le choix du matériel génétique, les méthodes de régénération et d’entretien, les modalités d’éclaircie et d’élagage, le mode de récolte finale. L’influence de chacune de ces opérations sur l’occurrence des agents de dégâts biotiques et abiotiques et sur la sensibilité des peuplements est examinée ainsi que les processus écologiques sous-jacents.• Les opérations sylvicoles qui se révèlent les plus déterminantes pour la sensibilité des forêts en Europe sont celles qui affectent la composition et la structure de la strate arborée. Quatre principaux processus écologiques semblent expliquer la relation entre sylviculture et sensibilité des peuplements : la modification du micro-climat, l’apport de ressources ou de combustible aux agents de dégâts, l’amélioration du contrôle biologique par les ennemis naturels et l’altération de la physiologie et du développement des arbres.• Cette revue permet donc d’envisager le développement de méthodes de gestion des peuplements forestiers qui permettent d’atteindre les objectifs de production tout en minimisant les risques de dégâts sanitaires.

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.

Fungi Vectored by the Bark Beetle Ips typographus Following Hibernation Under the Bark of Standing Trees and in the Forest Litter

The bark beetle Ips typographus has different hibernation environments, under the bark of standing trees or in the forest litter, which is likely to affect the beetle-associated fungal flora. We isolated fungi from beetles, standing I. typographus-attacked trees, and forest litter below the attacked trees. Fungal identification was done using cultural and molecular methods. The results of the two methods in detecting fungal species were compared. Fungal communities associated with I. typographus differed considerably depending on the hibernation environment. In addition to seven taxa of known ophiostomoid I. typographus-associated fungi, we detected 18 ascomycetes and anamorphic fungi, five wood-decaying basidomycetes, 11 yeasts, and four zygomycetes. Of those, 14 fungal taxa were detected exclusively from beetles that hibernated under bark, and six taxa were detected exclusively from beetles hibernating in forest litter. The spruce pathogen, Ceratocystis polonica, was detected occasionally in bark, while another spruce pathogen, Grosmannia europhioides, was detected more often from beetles hibernating under the bark as compared to litter. The identification method had a significant impact on which taxa were detected. Rapidly growing fungal taxa, e.g. Penicillium, Trichoderma, and Ophiostoma, dominated pure culture isolations; while yeasts dominated the communities detected using molecular methods. The study also demonstrated low frequencies of tree pathogenic fungi carried by I. typographus during its outbreaks and that the beetle does not require them to successfully attack and kill trees.

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.

Induced Terpene Accumulation in Norway Spruce Inhibits Bark Beetle Colonization in a Dose-Dependent Manner

Background Tree-killing bark beetles (Coleoptera, Scolytinae) are among the most economically and ecologically important forest pests in the northern hemisphere. Induction of terpenoid-based oleoresin has long been considered important in conifer defense against bark beetles, but it has been difficult to demonstrate a direct correlation between terpene levels and resistance to bark beetle colonization. Methods To test for inhibitory effects of induced terpenes on colonization by the spruce bark beetle Ips typographus (L.) we inoculated 20 mature Norway spruce Picea abies (L.) Karsten trees with a virulent fungus associated with the beetle, Ceratocystis polonica (Siem.) C. Moreau, and investigated induced terpene levels and beetle colonization in the bark. Results Fungal inoculation induced very strong and highly variable terpene accumulation 35 days after inoculation. Trees with high induced terpene levels (n = 7) had only 4.9% as many beetle attacks (5.1 vs. 103.5 attacks m−2) and 2.6% as much gallery length (0.029 m m−2 vs. 1.11 m m−2) as trees with low terpene levels (n = 6). There was a highly significant rank correlation between terpene levels at day 35 and beetle colonization in individual trees. The relationship between induced terpene levels and beetle colonization was not linear but thresholded: above a low threshold concentration of ∼100 mg terpene g−1 dry phloem trees suffered only moderate beetle colonization, and above a high threshold of ∼200 mg terpene g−1 dry phloem trees were virtually unattacked. Conclusion/Significance This is the first study demonstrating a dose-dependent relationship between induced terpenes and tree resistance to bark beetle colonization under field conditions, indicating that terpene induction may be instrumental in tree resistance. This knowledge could be useful for developing management strategies that decrease the impact of tree-killing bark beetles.

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.

Fungal infection and mechanical wounding induce disease resistance in Scots pine.

Scots pine trees (Pinus sylvestris) recovering from a 90–100% defoliation 2–3 years previously were pretreated with small mechanical wounds or inoculations with the blue-stain fungi Leptographium wingfieldii and Ophiostoma canum. Pretreated trees were less susceptible to a subsequent massive inoculation with L. wingfieldii than untreated control trees, which were extensively colonised by the mass-inoculation. A low pretreatment dosage of L. wingfieldii was somewhat more effective in inducing disease resistance than a higher dosage. Pretreatment with L. wingfieldii, O. canum, and mechanical wounding were about equally effective inducers of resistance in Scots pine, even though L. wingfieldii is known to produce much more extensive phloem necrosis than the other pretreatments. Thus, the strength of the induced resistance response did not depend on the amount of host tissues that was destroyed by the pretreatment. Previously, induced disease resistance has been demonstrated in Norway spruce (Picea abies), and the present study shows that similar responses can be activated in Scots pine.

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.

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.