Western Carpathian mountain spruce forest after a windthrow: Natural regeneration in cleared and uncleared areas

A catastrophic windstorm called “bora” occurred in The Tatra National Park in Slovakia in 2004 and destroyed an extensive area of mountain spruce forests. Part of the windthrow area was left uncleared for natural development, but most of the area was cleared. We analyzed natural regeneration in cleared and uncleared windthrows, and in reference closed canopy forest in 2007 and 2008 with the aim of assessing the regeneration capability of the windthrown forests and proposing the best method for their restoration.     

The influence of windthrow microsites on tree regeneration and establishment in an old growth mountain forest

The Rothwald forest in Lower Austria represents one of the few never logged old growth forests in Central Europe. Here, the influence of windthrow pits and mounds on tree regeneration in a spruce-fir-beech mountain forest was studied. Two main factors for recolonisation of sites after disturbance were investigated: (i) the trapping ability of microsites for seeds and (ii) the ability of particular tree species to establish and grow on different microsites. On two one hectare permanent sample plots, pits resulting from windthrow with existing tree trunks were recorded. At each windthrow three microsites were determined: pit, pitwall and undisturbed forest floor. If the windthrow plate already collapsed and formed a mound, this microsite was recorded as well. Furthermore, a control site was established at each windthrow. For two years the development of seedlings at four different microsites and the according control sites were monitored. To describe the variability of environmental factors possibly influencing seedling establishment and survival, litter, humus and soil attributes, herb vegetation cover as well as light availability (GSF) were assessed. In addition, soil temperature, chemical soil properties and water content were recorded at randomly selected windthrows. The distribution of the seedlings was assessed with a Redundancy Analysis, its occurrence modelled with a logistic regression and the mortality analysed with the Kaplan-Maier lifetime function. In contrast to the assumption of seed accumulation in concave microsites because of secondary dispersal, the pit microsites show comparable seed rain to the control site. However, pits have the lowest seedling numbers and by far the highest mortality rates of seedlings. The thickness of leaf litter was identified as a major driver of suitability of a microsite for recolonisation. Through its thickness, it inhibits germination and smothering of seedlings poses a particular high hazard during the winter period. Also different establishment patterns of the investigated tree species were detected. The results indicate that windthrows create microsite heterogeneity and plant species richness but limit tree population size and spatial distribution of the occurring tree species.     

The impact of windthrow on carbon sequestration in Switzerland: a model-based assessment

Carbon sequestered in biomass is not necessarily stored infinitely, but is exposed to human or natural disturbances. Storm is the most important natural disturbance agent in Swiss forests. Therefore, if forests are taken into account in the national carbon budget, the impact of windthrow on carbon pools and fluxes should be included. In this article the forest scenario model MASSIMO and the soil carbon model YASSO were applied to assess the effect of forest management and an increased storm activity on the carbon sequestration in Swiss forests. First, the soil model was adapted to Swiss conditions and validated. Second, carbon fluxes were assessed applying the two models under various forest management scenarios and storm frequencies. In particular, the influence of clearing after a storm event on the carbon budget was analyzed. The evaluation of the model results showed that the soil model reliably reproduces the amount of soil carbon at the test sites. The simulation results indicated that, within the simulated time period of 40 years, forest management has a strong influence on the carbon budget. However, forest soils only react slightly to changes in the above-ground biomass. The results also showed that a storm frequency increase of 30% has a small impact on the national carbon budget of forests. To develop effective mitigation strategies for forest management, however, longer time periods must be regarded.     

Comparison of soil CO2 flux between uncleared and cleared windthrow areas in Estonia and Latvia

Storms can turn a great proportion of forests’ assimilation capacity into dead organic matter because of windthrow and thus its role as a carbon sink will be diminished for some time. However, little is known about the magnitude or extent to which storms affect carbon efflux. We compared soil CO₂ fluxes in wind-thrown forest stands with different time periods since a storm event, and with different management practices (deadwood cleared or left on-site). This study examined changes in soil CO₂ efflux in two windthrow areas in north-eastern Estonia and one area in north-western Latvia, which experienced severe wind storms in the summers of 2001, 2002 and 1967, respectively. We measured soil CO₂ fluxes in stands formerly dominated by Norway spruce (Picea abies L. Karst.) with total and partial canopy destruction (all trees or roughly half of the trees in stand damaged by storm), in harvested areas (material removed after the wind storm) and in control areas (no damage by wind). Removal of wind-damaged material decreased instantaneous CO₂ flux from the soil surface. The highest instantaneous fluxes were measured in areas with total and partial canopy destruction (0.67 g CO₂ m⁻² h⁻¹ in both cases) compared with fluxes in the control areas (0.51 g CO₂ m⁻² h⁻¹), in the new storm-damaged areas where the material was removed (0.57 g CO₂ m⁻² h⁻¹) and in the old storm-damaged area where wood was left on site (0.55 g CO₂ m⁻² h⁻¹). The only factor affecting soil CO₂ flux was location of the measuring collar (plastic collar with diameter 100 mm, height 50 mm) - either on undamaged forest ground or on the uprooted tree pit, where the mineral soil was exposed after disturbance. New wind-thrown stands where residues are left on site would most likely turn to sources of CO₂ for several years until forest regeneration reaches to substantial assimilation rates. New wind-thrown stands where residues are left on site would most likely tend to have elevated CO₂ fluxes for several years until forest regeneration reaches to substantial assimilation rates. However, forest managers might be concerned about the amounts of CO₂ immediately released into the atmosphere if the harvested logs are burned.     

Natural development and regeneration of a Central European montane spruce forest

Montane Norway spruce forests of Central Europe have a very long tradition of use for timber production; however, recently there has been increasing concern for their role in maintaining biological diversity. This concern, coupled with recent severe windstorms that led to wide-spread bark beetle outbreaks, has brought the management of montane spruce forests to the forefront of public policy discussions in Central Europe. In order to shed light on the natural development and current structure of mature montane spruce forests, we established four 0.25 ha research plots in a semi-natural montane spruce forest in the Šumava Mountains (The Bohemian Forest), Czech Republic. We mapped all trees, extracted increment cores for age and growth-pattern analyses, and inventoried all current tree regeneration, including the substrates on which it was found. Stands were characterized by uni-modal tree diameter distributions and high basal areas (56.6 m² ha⁻¹ on average), indicating a natural transition from the stem exclusion phase towards the understory reinitiation phase. The stands showed largely single-cohort recruitment age structures, however, with recruitment spanning seven decades. Our analyses suggest that this cohort existed as advance regeneration prior to major disturbances in the late 1800s, which included post-bark beetle salvage logging. Spatial pattern analyses of living and dead stems combined, showed an increase in uniformity of living trees, pointing to the role of natural density-dependent mortality. However, past growth patterns and historical documentation suggest that low intensity canopy disturbances (wind and snow) also caused mortality and diversified canopy structure. Because the stands developed naturally over the past 120+ years and thus escaped thinning operations, high volumes of coarse woody debris (94 m³ ha⁻¹) and snag densities (546 stems ha⁻¹) have accrued. Advance spruce regeneration was quite abundant and existed primarily on deadwood substrates, even though these occupied only a small percent of stand area. Because of salvage logging in the late 1880s, these stands do not qualify, according to the traditional paradigm, as natural spruce forests. As a result, they are recently subject to active management practices including salvage logging that remove dead and dying trees. Given the importance of deadwood for forest regeneration and recovery from disturbance, as demonstrated in this study, we argue that dead wood removal may limit future natural regeneration in these stands. Thus, the purported benefits of removing dead and dying trees from semi-natural forests must be carefully weighed against the potential detrimental impacts on natural spruce forest regeneration and biodiversity.     

Response of vegetation and birds to severe wind disturbance and salvage logging in a southern boreal forest

Vegetation and birds were inventoried on the same plot before and after a severe windstorm in 1999 disturbed a mature black spruce (Picea mariana)–jack pine (Pinus banksiana) forest in northern Minnesota. Following the storm, another plot was established in an adjacent portion of the forest that was salvage-logged. Birds were inventoried on both plots through 2002. The original unsalvaged plot was prescribed-burned in 2004, but vegetation was surveyed through 2003, and through 2005 on the salvaged plot. We examined the effects of wind disturbance by comparing the pre-storm bird and vegetation communities with those developing afterwards through 2002 and 2003, respectively, and the effects of salvage logging by comparing vegetation and the bird community on the unsalvaged plot with those in the salvaged area. Wind reduced the canopy of the forest by over 90% with a temporary increase in the shrub layer, mostly resulting from tip-ups. Several plant species, including jack pine and beaked hazel (Corylus americana), appeared temporarily in the ground layer (<1 m height), but did not persist through 2003. Quaking aspen (Populus tremuloides) root sprouts were abundant in 2001, but decreased dramatically by 2003. Delayed mortality of tipped trees resulted in reduction of the shrub layer to pre-storm levels, and release of advanced regeneration black spruce and balsam fir (Abies balsamea). Bird species using the forest changed from dominance by canopy-foraging species to ground-brush foraging species, with an overall increase in bird diversity. Salvage logging resulted in significant reduction in coarse woody debris, and successful recruitment of jack pine seedlings. Quaking aspen sprouts were nearly 30 times more abundant in the salvage-logged area compared to the unsalvaged control. Ruderal species, especially red raspberry (Rubus ideaus), fringed bindweed (Polygonum cilinode), and several sedges (Carex spp.), were significantly more abundant after salvage logging. The bird community, on the other hand, was greatly diminished by salvage logging, with a reduction in diversity, density, and overall richness of species.     

Effects of treefall gaps created by windthrow on bat assemblages in a temperate forest

Determining the way in which spatial distribution and diversity of forest-dwelling mammals varies with natural disturbance is essential to understanding the spatial dynamics of mammal assemblages in forests. Bats are the only forest-dwelling mammals capable of true flight. At a local scale, bat flight ability, which may be related to ecomorphological traits, is an important factor influencing spatial distribution. We tested two postulates: (1) the spatial distribution of bats is affected by sizes of forest gaps created by natural disturbances and (2) species-specific responses can be predicted from bat ecomorphological traits (aspect ratio (AR) and wingtip shape index (WT)) that influence bat flight ability. We found that sizes of forest gaps affected the occurrence of each bat species and species richness of bats at local scales; species-specific responses were related to the ecomorphological traits of individual species. Bat species with high AR and low WT were not affected by variation in canopy gap size. In contrast, bat species with low AR and high WT responded negatively to gap size, and those with intermediate AR and WT responded positively to canopy gap size at sites with small-sized gaps but responded negatively to large-sized gaps. Overall bat species richness responded negatively to gap size. Thus, ecomorphological traits may be important determinants of bat spatial distributions and species diversity at local scales in disturbed habitats. In this study, forest edges might have been undersampled due to the location of bat detectors. However, this potential undersampling should not have affected the interpretation of occurrence patterns of bat species responding to gap size. Our results imply that bat conservation efforts in forest lands should take into consideration specific responses related to ecomorphological traits of species inhabiting an area. The results also suggest that quantifying the effects of natural disturbances on bat assemblages may provide a knowledge base for forest management to minimize the impacts of unavoidable anthropogenic disturbances on bat species diversity. Rare or infrequent natural disturbances can provide models for forest management aimed at maintaining bat species diversity.     

Disturbance history and dynamics of an old-growth mixed species mountain forest in the Slovenian Alps

A long history of human presence in the Alps has made studies of natural forest structure, composition, and disturbance processes difficult. In the Slovenian Alps, we identified a mixed European beech (Fagus sylvatica L.)-silver fir (Abies alba Mill.)-Norway spruce (Picea abies (L.) Karst.) stand with old-growth characteristics that escaped past cutting. We reconstructed the dynamics of the stand using a combination of stand and age structure analyses, dendroecological evidence of past canopy disturbance, and historical evidence of past human activities. The disturbance reconstruction revealed a stand-scale disturbance in the 1850s. Based on the presence of pit-and-mound topography throughout much of the stand and the absence of logging activities found in historical records, we assume this event was caused by strong winds. The current structure and composition of the stand are largely a result of this event. Post-disturbance forest development was dominated by the release of shade tolerant regeneration, especially F. sylvatica. The windthrow also created appropriate conditions (i.e. increased light and exposed mineral soil) for recruitment of less shade tolerant species, including larch (Larix decidua Mill.) and maple (Acer pseudoplatanus L.). The results suggest that periodic, intermediate severity wind disturbances, similar to the event documented here, may have had an important influence on pre-settlement forest dynamics in mixed mountain forests of the Alps, and add to the limited available information regarding the historic range of natural variability of disturbance processes. This information may be useful for forest management that attempts to emulate natural processes.     

Effects of uprooting tree on herbaceous species diversity, woody species regeneration status and soil physical characteristics in a temperate mixed forest of Iran

We conducted a study to examine the pattern of development of herbaceous plant species, woody species regeneration and soil physical characteristics after tree uprooting in 20-ha areas of Experimental Forest Station of Tarbiat Modares University located in a temperate forest of Mazandaran province in the north of Iran. Soil bulk density, soil texture and moisture from pit and mound (PM) were measured in the laboratory. Results show that the soil bulk density was most in soil deeper layers at mound top, and the soil moisture content was most in soil deeper layers at Pit bottom. Our study supports that the micro-topography of PM (pit and mound) topography will create a mosaic of environmental conditions. This environmental heterogeneity could be responsible for the diversity of herbaceous plant species and regeneration of woody species. It is recommend that the fallen trees with PM structure should remain in the protected area without clearing as the best option for forest restoration. This information can be useful for forest management that attempts to emulate natural processes.     

Restoration fire and wood-inhabiting fungi in a Swedish Pinus sylvestris forest

A growing awareness of the negative consequences of efficient fire prevention in boreal Fennoscandia has resulted in an increasing use of fire as a restoration method. The primary purpose of restoration fire is to recreate features of natural forests that have been lost during long periods of fire suppression. We used the occurrence of fruiting bodies from wood-inhabiting fungi to assess the conservation value of and gain ecological information about restoration fire in a Pinus sylvestris dominated forest. The general pattern for the majority of the species was a drastic decline the first two years after the restoration fire. However, our results clearly demonstrate that most of the species that declined the first years after the fire rebounded after four years and were frequently found on charred wood. Species that increased after the fire and often occurred on charred logs were: Antrodia sinuosa, Botryobasidium obtusisporum, Galzinia incrustans, Phlebia subserialis and Tomentella spp. In addition, three threatened, red-listed and fire-favored species were also found on heavily charred logs: Antrodia primaeva, Dichomitus squalens and Gloeophyllum carbonarium. Our results indicate that fire disturbance creates a unique type of dead wood important for fungal species richness. The results also support the use of restoration fires in maintaining forest biodiversity.     

Habitat factors for land snails in European beech forests with a special focus on coarse woody debris

Analyses of land snails and habitat factors in acid beech forests were conducted in southern Germany (northern Bavaria). The objectives were to study the effects of habitat characteristics on snail density and species richness. Habitat structures were determined for 37 plots in one big forest. We found a significant relationship between the number of snail species and individuals and the following set of habitat factors coverage of herbaceous layer, growing stock, mean diameter at breast height of the three largest trees (DBHmax), stand age, total dead wood volume per ha, and advanced decomposed dead wood volume per ha. We use maximally selected rank statistics to estimate cutpoints separating stands with low densities, from stands with high snail densities. Here, we define cutpoints for a significant higher snail density at a stand age of 187 years, 57 m³/ha dead wood, 40 m³/ha advanced decomposed dead wood, 63 cm DBHmax and more than 1% herbaceous layer. For species richness, cutpoints are estimated at 338 m³/ha stand volume, 170 years stand age, 50 m³/ha total dead wood amount, 15 m³/ha advanced decomposed dead wood and 56 cm DBHmax. The microhabitat analysis shows a higher pH value and a higher Calcium content at the bottom of large snags and under large lying dead wood pieces in comparison to litter, upper mineral soil and at the bottom of vital living trees. Snail species and individual density are significantly linked to these patterns of chemical parameters. The identified cutpoints are a good base for ecological management decisions in forest management.     

Dynamics of soil carbon in a beechwood chronosequence forest

Accurate estimates of forest soil organic matter (OM) are now crucial to predictions of global C cycling. This work addresses soil C stocks and dynamics throughout a managed beechwood chronosequence (28–197 years old, Normandy, France). Throughout this rotation, we investigated the variation patterns of (i) C stocks in soil and humic epipedon, (ii) macro-morphological characteristics of humic epipedon, and (iii) mass, C content and C-to-N ratio in physical fractions of humic epipedon. The fractions isolated were large debris (>2000 μm), coarse particular OM (cPOM, 200–2000 μm), fine particular OM (fPOM, 50–200 μm) and the mineral associated OM (MaOM, <50 μm).     

Dynamics of large woody debris in small streams disturbed by the 2001 Dogrib fire in the Alberta foothills

We investigated the temporal dynamics of large woody debris (LWD) in five headwater streams before and after the 2001 Dogrib fire in the foothills of Alberta. The density of LWD varied from 5 to 41 logs per 50 m of stream reach and accounted for 19.4 ± 5.1 m³ ha⁻¹ (mean ± standard error) of wood in the riparian zones and 114.1 ± 30.1 m³ ha⁻¹ of wood in the bankfull margins of the stream channel. Individual logs averaged 18.9 ± 1.15 cm in diameter, 5.5 ± 0.7 m in length, and 0.2 ± 0.02 m³ in volume. Logs became significantly shorter in decay classes II–IV. Bridges were longer than partial bridges, which were longer than loose and buried LWD. Individual log volume was greatest for bridges, but not significantly different among other position classes. Bridges and loose LWD contributed little to stream morphology and function; however, 55% of partial bridges and all buried logs contributed to sediment storage, channel armouring, or riffles and pools in the stream channel.     

Mapping dead wood distribution in a temperate hardwood forest using high resolution airborne imagery

Dead wood, in the form of coarse woody debris and standing dead wood, or snags, is an essential structural component of forest ecosystems. It plays a key role in nutrient cycling, ecosystem functions and provision of habitat for a wide variety of species. In order to manage dead wood in a temperate hardwood forest, an understanding of its availability and spatial distribution is important. This research evaluates airborne digital camera remote sensing for mapping temperate forest dead wood across an area within Gatineau Park, Canada. Two approaches were evaluated: (1) direct detection and mapping of canopy dead wood (dead branches and tall snags) through the combination of three techniques in a hybrid classification: ISODATA clustering, object-based classification, and spectral unmixing, and (2) indirect modelling of coarse woody debris and snags using spectral and spatial predictor variables extracted from the imagery. Indirect modelling did not provide useful results while direct detection was successful with field validation showing 94% accuracy for detected canopy level dead wood objects (i.e. 94% of validation sites with canopy dead wood were detected correctly) and 90% accuracy for control sites (i.e. 90% of validation sites with no canopy level dead wood were identified correctly). The procedures presented in this paper are repeatable and could be used to monitor dead wood over time, potentially contributing to applications in forest carbon budget estimation, biodiversity management, and forest inventory.     

韩国森林中枯死木及地被物对小型鼠类丰富度的影响(英文)

在南韩东北部,1997-1998年间,自4月至12月对人工林和无枯死木生境及采伐后保留地被物的落叶次生林中的小型鼠类种群特征进行了调查。在两块林地中各选择1 hm2(100m100m)样地作为控制区和处理区。两块研究地的中上层林冠结构基本相似,但倒木及地被物的数量及比例却显示控制区大于处理区。两区域中共捕到两种小型鼠类,其中棕背 (Eothenomys regulus)211只,占总数的55.5%,大林姬鼠(Apodemus peninsulae)169只,占44.5%。这两种鼠类的丰富度及种群稳定性在控制区明显优于处理区。两地小型鼠类的不同捕获量主要来自两地鼠类繁殖率和居留率的不同。显然,森林地被结构对小型鼠类的生存具有重要意义。因此,采伐迹地中保留枯死木及地被物是维持小型鼠类种群所必须的。     

Stand structure of an uneven-aged fir–beech forest with an irregular diameter structure: modeling the development of the Belevine forest, Croatia

This study focuses on the problem of irregular diameter structure in a silver fir–beech selection (plenter) forest with a “surplus” of large diameter trees and a lack of natural regeneration and small diameter trees. We sampled 274 plots (900 m² each) in the Belevine research site (266.24 ha) in the mountain region Gorski Kotar (Croatia), where diameter (dbh) distribution, diameter increment, and natural regeneration were analyzed in detail. A low density of natural regeneration, weak annual recruitment of small (10 cm dbh) diameter trees (only five trees per hectare), delayed diameter growth of trees, and a low annual rate of trees reaching the next dbh class were attributed to the current irregular dbh structure. The stand development prediction for the next 50 years is based on a simulation model, which considers the current diameter structure, increment, recruitment, and future cutting regime. Intensive cutting in the first of five 10-year cutting cycles (intensity higher of 25%) is needed to initiate natural regeneration and to accelerate growth of young silver fir trees. In the next 50 years, the irregular diameter structure will be gradually improved.     

退化喀斯特森林自然恢复过程中群落动态研究

研究了贵州茂兰国家级自然保护区退化喀斯特森林自然恢复过程中群落的动态特征。群落组成结构变化体现于不同适应等级种组的变化 ,不同恢复阶段各种组的优势地位不同 ,草本群落阶段与草灌过渡阶段 ,灌丛灌木阶段与灌乔过渡阶段 ,常绿落叶阔叶林阶段与顶极群落阶段其居优势地位的种组分别为先锋种 ,次先锋种和过渡种 ,次顶极种和顶极种 ,种组替代规律是先锋种经次先锋种、过渡种最终被次顶极种和顶极种替代。随着退化喀斯特群落自然恢复 ,群落高度 (0 6～ 16 9m)、盖度 (0 2 9～ 0 79)、显著度 (0 872 0～2 1 8981cm2 ·m- 2 )逐渐提高 ,而密度早期增大之后逐渐降低 (36 32 8～ 5 6 116株·hm- 2 ,5 6 116～ 2 372株·hm- 2 ) ;物种数 (16～ 5 0 )、Shannon Wiener多样性指数 (0 99～ 4 72 )、均匀度 (0 77～ 0 83)升高而生态优势度 (0 16～ 0 0 5 )降低 ;群落生物量 (3 4 9～ 87 4 5t·hm- 2 )逐渐积累增大 ;萌生株数减少而实生株数增多 ,群落更新对策由早期阶段无性更新向后期阶段有性更新发展     

Changes in physical properties of a soil associated with logging of Eucalyptus regnan forest in southeastern Australia

Logging operations can cause soil profile disturbance and compaction. Soil profile disturbance and compaction change soil physical properties, which may reduce site productivity, increase soil erosion and degrade catchment water quality. This study was undertaken to measure the effect of logging on physical properties of the 0–100 mm surface soil in the Victorian Central Highlands, southeastern Australia. Soil physical properties were measured in the snig tracks, log landings, general logging areas (disturbed areas which were not occupied by snig tracks or log landings) and undisturbed areas. Within the general logging areas, measurements were made for three levels of soil profile disturbance: litter disturbed, topsoil disturbed and subsoil disturbed.

The results indicated that logging significantly increased bulk density and decreased organic carbon and organic matter content, total porosity and macroporosity on over 72% of the coupe area. However, on 35% of the coupe area, the snig tracks, log landings and subsoil disturbed areas of the general logging area, bulk densities and macroporosities reached critical levels where tree growth could be affected. On these areas, organic carbon decreased between 27 and 66%, bulk density increased between 39 and 65% and macroporosity decreased between 58 and 88%.

Saturated hydraulic conductivities decreased to critical levels for runoff to occur on over 72% of the coupe area (topsoil and subsoil disturbed areas of the general logging area, snig tracks and log landings). On this area, the reduction in saturated hydraulic conductivity varied between 60 and 95%.     

Impact of late frost events on radial growth of common beech (Fagus sylvatica L.) in Southern Germany

Phenological, temperature, and tree-ring data were used in order to identify and quantify the impact of late frosts on common beech (Fagus sylvatica L.) at different altitudes in Southern Germany during the last century. For this intention, dendroecological investigations were made upon trees at the Meteorological Observatory Hohenpeißenberg as well as from seven stands in the Bavarian Forest and 17 stands at the northern fringe of the Alps. From these locations, a considerable number of severe growth minima in the tree-ring series could be related to late frost in the days of or immediately after leaf unfolding. The frequency of frost-related growth minima increases with altitude. In individual years, radial growth can be reduced by more than 90% (stand mean) in relation to the average growth of the ten previous years. Hence, late frosts are considered as important ecological events that strongly affect beech vitality and competitiveness especially at high altitudes. Evidence of significant impacts on radial growth by late frosts distinct before leaf unfolding or with temperatures above ?3°C was not found. Also, increasing frequency and intensity of late frosts during recent decades were not ascertained. Hence, the recently observed decreased vitality of common beech accompanied by growth depressions especially at high altitude sites in Central Europe cannot be explained as a consequence of late frost damage.     

Effects of forest restoration treatments on the abundance of bark beetles in Norway spruce forests of southern Finland

Restoration of protected areas in boreal forests frequently includes creating substantial volumes of dead wood. While this benefits a wide range of dead wood dependent invertebrate species, some of these are regarded as forest pests. Therefore, the risk of elevated levels of tree mortality in surrounding commercial forests must be considered. In a large-scale field experiment in southern Finland, we studied the effects of restoration treatments on the abundance of bark beetles within and in the vicinity of restored areas, in particular focusing on Ips typographus and Pityogenes chalcographus. The treatments applied to managed Norway spruce forests were controlled burning and partial harvesting combined with retaining 5, 30 or 60 m³/ha of cut down wood. We found that the abundance of bark beetles increased by both burning and harvesting with down wood retention, being highest where burning and harvesting had been combined. The actual volume of down wood retention had no significant effect. The effect of burning on the number of bark beetles along host tree boles was negative which suggests that burnt spruces provided a less suitable resource for bark beetles than unburnt dead spruces. The abundance of bark beetles along host trees also decreased with increasing volume of down wood retention. The abundance of P. chalographus was slightly elevated up to 50 m outside restored areas but the abundance was very low compared to that within the areas. The abundance of I. typographus was extremely low outside restored areas. We conclude that restoration treatments increase the abundance of bark beetles via increased availability of resources, but that the effect of burning is likely to be counteracted by decreased resource quality. Thus, burning might be the “safest” way to produce large quantities of dead wood. Furthermore, the fact that only few beetles were collected in adjacent areas suggests that restored areas pose little threat of serving as refugia in which bark beetle populations increase in sufficient numbers to attack live trees in adjacent forests. However, restoration actions repeated at consecutive years within a small area might enable the populations to grow to outbreak levels.