Consequences of disturbance on epiphytic lichens in boreal and near boreal forests

This paper provides a quantitative review of the consequences of disturbance on epiphytic lichens in boreal and near boreal forests, focusing on disturbance effects on lichen persistence, growth, and colonization, and on the development (recovery rates) of lichen diversity over time. Lichen persistence, growth, and colonization were examined by reviewing studies on e.g. edge-effects, lichen transplants, and logging experiments. Recovery rates were examined by reviewing studies on relationships between lichen diversity and stand age. The results from the reviewed studies were analysed by various meta-analysis methods. The review showed e.g. overall negative effects of disturbance on lichen persistence, but that persistence depend on the magnitude of the disturbance (e.g. forest edge orientation and clearcut size). However, many lichens seem to persist disturbance, and growth rates were as good in disturbed as in intact forest. The review also showed that lichen populations may need long time to recover, but that species richness does not necessarily increase over time. However, disturbance origin (logging or natural disturbance) is important to explain lichen diversity but is often confounded with stand age. The results are promising for the potential to restore and maintain lichen diversity by forest management methods based on natural disturbance regimes. Further research is needed to e.g. separate the effects of time since disturbance and disturbance type, to examine edge-effects under varying edge conditions, and to examine the influence of landscape context on stand level lichen diversity.     

Functional implications of soil fauna diversity in boreal forests

This paper is a review of recent experiments dealing with the role of soil fauna in decomposition, mineralisation and primary production in coniferous forest soils. The experiments have been grouped according to the degree and nature of the ‘diversity gradient' between the ‘more diverse' community and its control: single animal species or an uncontrolled mixture of species versus microbiota only, several known animal species of the same trophic group versus one species only (species diversity), two or more functional groups versus one only, and food chains with predators versus microbes and microbivores only. The evidence available at present suggests that taxonomic diversity and predation have no consistent effects on the process rates in soil, while adding to the ‘functional' or ‘trophic group diversity' results in a more predictable enhancement in mineralisation. Especially the enchytraeid Cognettia sphagnetorum seems to be a keystone species in boreal forest soils. However, there are only few experiments in which species diversity per se has been taken as a separate factor, without a simultaneous change in the number of trophic groups or in total decomposer biomass.     

Resilience of bryophyte communities to clear-cutting of boreal stream-side forests

We asked if short-term changes in bryophyte communities in response to clear-cutting of boreal stream-side forests are persistent and whether species with low resilience may survive in narrow riparian buffer strips. To assess short-term changes and the function of buffer strips we compared the bryophyte community in permanent 0.1 ha plots of mature forest before and after clear-cutting. Persistent changes were inferred by pair-wise comparisons of 0.1 ha plots of mature forests with carefully matched 0.1 ha plots in stands established after clear-cutting 30-50 years earlier. Total bryophyte species richness did not respond significantly to clear-cutting. However, richness changed in many subgroups defined by phylogenetic, habitat or substrate affinity. Numbers of both liverwort and forest species were significantly reduced on clear-cuts and these differences remained significant, although smaller, 30-50 years after clear-cutting. In contrast, there were short-term increases in richness of mosses and of species growing on disturbed mineral soil, but these species numbers returned to mature-forest levels in the young stands. Number of species associated with convex substrates, especially woody debris species, was strongly reduced by clear-cutting and showed no significant recovery after 30-50 years. Hence, most of the negative effects of clear-cutting on bryophyte species persist almost halfway into the next forestry rotation period. However, narrow buffer strips (10 m on each side) prevented most of the short-term extirpations of species with low resilience on clear-cuts. Buffer strips may thus be effective in conserving the bryophyte flora of stream-side forests, but their long-term function as refugia and their contribution to population recovery in other parts of the landscape need further evaluation.     

An analysis of future carbon budgets of Canadian boreal forests

The Canadian boreal forest covers over 300 Mha of land area. Its dynamics are largely influenced by fires and insect-induced stand mortality and to a much lesser extent by forest management. This paper analyses six scenarios of future (1990–2040) carbon (C) budgets of the Canadian boreal forest, each based on different assumptions about natural disturbances, rates of reforestation of disturbed land, and conversion of non-stocked to productive forest stands. The objective of these scenarios is to explore the range of responses to different management options. The results indicate an overall inertia of a system whose dynamics are strongly influenced by a recent 20-year period (1970–1989) of large-scale forest disturbances by fire and insects. The 50-year C budget of the six scenarios ranges from an estimated net source of 1.4 Pg C to a net sink of 9.2 Pg C. These estimates indicate the range of response to the management of the Canadian boreal forest. Although a full-scale implementation of the management activities examined here is not likely given ecological and economic realities in the Canadian boreal forest, the analyses explore the relative merits of reducing forest disturbance rates, regeneration delays, and the area of non-stocked forest land.     

Gross nitrogen mineralisation and fungi-to-bacteria ratios are negatively correlated in boreal forests

In terrestrial ecosystems, gross nitrogen mineralisation is positively correlated to microbial biomass but negatively to soil organic matter C-to-N ratios; the influence of the microbial community structure is less well known. Here, we relate rates of gross N mineralisation to fungi-to-bacteria ratios in three natural forest types of contrasting N availability and in a long-term N-loading experiment in a boreal forest. We report, for the first time, a strong negative correlation between gross N mineralisation and the fungi-to-bacteria ratio ( = 0.91, P = 0.0005, N = 7). There was also a negative correlation between gross N mineralisation and the C-to-N ratio ( = 0.89, P = 0.001, N = 7), but a weaker positive correlation between gross N mineralisation and soil pH ( = 0.64, P = 0.019, N = 7). Our analysis suggests that soil fungi-to-bacteria and C-to-N ratios are interrelated and that they exert strong influences on soil N cycling in boreal forests.     

Dynamics of the dead wood carbon pool in northwestern Russian boreal forests

Our study examines dead wood dynamics in a series of permanent plots established in closed, productive second-growth forest stands of north-west Russia and in temporary plots that represent different successional stages and types of disturbance. Dead wood stores measured on 63 plots 0.2–1.0 ha in size range from 1–8 Mg C ha^?1 in young to mature intensively managed stands, 17 Mg C ha^?1 in an old-growth forest, 20 Mg C ha^?1 on a clear-cut, and 21–39 Mg C ha^?1 following a severe windthrow. A total of 122 logs, snags, and stumps aged by long-term plot records was sampled for decay rates and to develop a system of decay classes. Annual decomposition rates are: 3.3% for pine, 3.4% for spruce, and 4.5% for birch. Based on these decay rates the average residence time of carbon (C) in the dead wood pool is 22–30 years. The mortality input on the permanent plots was 23–60 Mg C ha^?1 over 60 years of observation or 15–50% of the total biomass increment. This data suggests a dead wood mass of 10–22 Mg C ha^?1 would be expected in these mature forests if salvage had not occurred. In old-growth forests, dead wood comprised about 20% of the total wood mass, a proportion quite similar to the larger, more productive forests of the Pacific Northwest (USA). If this proportioning is characteristic of cool conifer forests it would be useful to estimate potential dead wood mass for old-growth forests without dead wood inventories. However, the use of a single live/dead wood ratio across the range of successional stages, a common practice in C budget calculations, may substantially over-or under-estimate the dead wood C pool depending upon the type of disturbance regime. Intensive forest management including short harvest rotations, thinning and wood salvage reduces dead wood C stores to 5–40% of the potential level found in undisturbed old-growth forest. In contrast, natural disturbance increases dead wood C pool by a factor of 2–4.     

Sensitivity of forest-floor mosses in boreal forests to nitrogen and sulphur deposition

The response of forest-floor mosses to deposition of nitrogen (N) and sulphur (S) was examined in field conditions in a 60-year-old Norway spruce (Picea abies Karst.) stand in southern Finland. The experimental plots received nitrogen (25 kg N ha^–1) and sulphur (30 kg S ha^–1) as ammonium sulphate once a year for 4 years.The dominant moss species on the site were Pleurozium schreberi (Mitt.) and Dicranum polysetum (Sw.). The biomass of the dominant moss species was decreased significantly by N and S deposition during the study period. Due to the addition of N and S, the biomass of Pleurozium schreberi was decreased by 60% and the biomass of Dicranum polysetum by 78%.     

Cost-effectiveness of conservation strategies implemented in boreal forests: The area selection process

To protect land from commercial exploitation is a common conservation practice. However, this requires large financial resources and it is therefore important to evaluate the cost-effectiveness of different strategies used in the selection of these conservation areas. In this study we compare four strategies and relate the differences in cost-effectiveness to differences in the selection process. We measure conservation benefits both as the amount of three tree structures and as the number of species in three species groups. We also estimate both the information cost associated with selecting conservation areas and the opportunity cost. We found the key habitat strategy to be the over-all most cost-effective. In this strategy, the areas have a flexible size and are selected by the authorities in a national field survey. The least cost-effective strategy was one where the selection was based only on forest classes in a satellite map. Intermediate were the retention group strategy, where small areas are left by the forest owner at harvesting, and the nature reserve strategy, where large areas are selected by the authorities. We emphasize that the differences we found are associated with the selection process and that other aspects, such as long-term survival of species, may rank the strategies differently. We conclude that the cost-effectiveness of a selection strategy depends on the size of the planning area for selection of conservation areas, the size of the conservation areas, the objective of the agent making the selection, and the amount and type of information on which the selection is based.     

Hotspots in cold climate: Conservation value of woodland key habitats in boreal forests

The concept of Woodland Key Habitats (WKH, small-scaled presumed hotspots of biodiversity) has become an essential component of biodiversity conservation in Fennoscandian and Baltic forests. There have been debates over the importance of WKHs in relation to the conservation of biodiversity in production forests. We applied a systematic review protocol and meta-analysis to summarize knowledge on comparisons of biodiversity qualities, such as dead wood and species richness, between WKHs and production forests in relevant countries. We also summarized the knowledge on the impact of edge effects by comparing WKHs surrounded by production forests to WKHs surrounded by clear cuts. Studies had been conducted in Finland, Norway and Sweden. Based on our meta-analysis, WKHs seem to be relative hotspots for dead wood volume, diversity of dead wood, number of species and number of red-listed species. There were some differences also between countries in these biodiversity qualities. Only two studies compared WKHs surrounded by production forests and clear cuts, respectively. Hence, the capability of WKHs to maintain their original species composition and support species persistence over time remains to be addressed, as well as their role in relation to other conservation tools.     

The impact of clearcutting in boreal forests of Russia on soils: A review

Data on the impact of tree logging in boreal forests of Russia on soils are systematized. Patterns of soil disturbances and transformation of microclimatic parameters within clearcutting areas are discussed. Changes in the conditions of pedogenesis in secondary forests are analyzed. It is suggested that the changes in forest soils upon reforestation of clearcutting areas might be considered as specific post-logging soil successions. Data characterizing changes in the thickness of litter horizons and in the intensity of elementary pedogenic processes, acidity, and the content of exchangeable bases in soils of clearcutting areas in the course of their natural reforestation are considered. The examples of human-disturbed (turbated) soil horizons and newly formed anthropogenic soils on clearcutting areas are described. It is suggested that the soils on mechanically disturbed parts of clearcutting areas can be separated as a specific group of detritus turbozems.     

Assessing the extinction vulnerability of wood-inhabiting fungal species in fragmented northern Swedish boreal forests

Fragmentation of old-growth forests and greatly reduced amounts of coarse dead wood in managed forests threat the persistence of many saproxylic species in boreal Fennoscandia. Individual old-growth forest remnants may lose species over time as they pay off their extinction debt. We tested this by comparing the observed site occupancy of individual wood-inhabiting fungal species in isolated old-growth stands (i.e. woodland key habitats; WKHs) with statistical predictions of their occupancy assuming potential extinction debt had already been paid off. The occupancy of species was analysed in two sets of WKHs differing in time since isolation (i.e. recent and old isolates).Few species occurred more frequently than expected in WKHs. However, patterns across species and across all WKHs masked important differences among species in their risk of facing future extinction. The site occupancy decreased significantly between recent and old isolates for a group of annual, red-listed specialist fungal species, suggesting that an extinction debt in WKHs may exist among specific species confined to coarse dead wood and old-growth forest habitat. Generalist species that also occur in the surrounding matrix showed no negative trends, or actually increased in site occupancy, making future extinctions less likely. Thus, continuing loss of threatened species are likely if not preservation of WKHs are combined with other conservation efforts in managed forest landscapes. Natural forest landscapes may serve as important references when aiming to identify species in risk of future extinction but more detailed knowledge about the biology of the most vulnerable species is also required.     

Multiple effects of reindeer grazing on the soil processes in nutrient-poor northern boreal forests

Reindeer grazing has a great influence on the ground vegetation of nutrient-poor northern boreal forests dominated by Cladonia lichens in Fennoscandia. Grazing may influence the soil processes in these systems either by influencing the quality of plant litter, or by indirect effects through the soil microclimate. In order to investigate the mechanisms underlying the effects of reindeer on boreal forest soils, we analyzed litter decomposition, soil and microbial C and N, microbial community composition, and soil organic matter quality in three forest sites with old reindeer exclosures adjacent to grazed areas. There was no effect of grazing on soil C/N ratio, inorganic N concentrations, microbial biomass C, microbial community structure analyzed by phospholipid fatty acid (PLFA) analysis, and organic matter quality analyzed by sequential fractionation, in the soil organic layer. However, microbial N was enhanced by grazing at some of the sampling dates and was negatively correlated with soil moisture, which indicates that increased microbial N could be a stress response to drought. The effect of grazing on litter decomposition varied among the decomposition stages: during the first 1.5 months, the litter C loss was significantly higher in the grazed than the ungrazed areas, but the difference rapidly levelled out and, after one year, the accumulated litter C loss was higher in the ungrazed than the grazed areas. Litter N loss was, however, higher in the grazed areas. Our study demonstrates that herbivores may influence soil processes through several mechanisms at the same time, and to a varying extent in the different stages of decomposition.     

Responses of oribatid mites to tree girdling and nutrient addition in boreal coniferous forests

Tree girdling has been experimentally used to stop the allocation of carbohydrates from the canopy to tree roots and their associated ectomycorrhizal (EM) mycelia. We used three already established girdling experiments in northern Sweden, one in a Pinus sylvestris stand and two in Picea abies stands, to determine the effect of tree girdling on soil-living oribatid mites. These mites often feed on fungal hyphae, but it is not known to what extent they feed on EM or saprotrophic fungi. We hypothesised that a presumed decline in EM fungi after girdling would strongly reduce EM-feeding specialists and, correspondingly, reduce total abundance and species richness of oribatids in girdled plots. Tree girdling resulted in a significant decline in total abundance of oribatid mites in the two spruce stands, which was assumed to be linked to the decline in EM fungi, but not in the pine stand. Species richness decreased in girdled plots in one of the spruce forests. The decline in total abundance in girdled spruce stands was primarily dependent on a significant and consistent population decrease in Oppiella nova, which was the most abundant oribatid mite in the ungirdled spruce plots. Its abundance after girdling was only 8–18% of that in ungirdled spruce plots. In the pine stand, O. nova had much lower abundance than in the spruce stands, and despite a tendency to decline in number after girdling also in the pine stand, it had a minor effect on total abundance. These results suggest that O. nova may be dependent on EM fungi in spruce forest soils, whereas the dependence on EM fungi in pine forest soils is less evident.     

Polypore diversity in managed and old-growth boreal Picea abies forests in southern Finland

Polypore communities were compared between mature managed, overmature managed, and old-growth spruce-dominated forests in southern Finland. A total of 85 polypore species, with 6000 records, were found in 16 sample plots, each 4 ha in size. Old-growth stands had on average 80% more species than mature stands, and 38% more species than overmature managed stands. Variation in polypore species richness was best explained by diversity of dead wood and the number and volume of dead trees. The best predictor for the number of threatened polypore species was the number of cut stumps. Threatened species were practically confined to old-growth forests and to stands in which the amount of dead wood exceeded 20 m³/ha. This figure appears to represent a stand-level threshold value for the amount of dead wood, below which the persistence of threatened species becomes unlikely. Our results suggest that a significant increase in the amount of dead wood (e.g. by leaving large retention trees and even by killing trees) is needed in managed forests before they become suitable habitats for threatened polypores.     

Acidification phenomena in beech (Fagus sylvatica) forests of Europe

When comparing the species composition of herb layers in loess beech forests of the Westphalian Bight (Northrhine-Westphalia, F.R. Germany) in the years 1976 and 1983, findings show that acidophilic plants tended to be favored during this period. Apart from many other factors, depositions have also been considered as causes for this. Where they should actually play a role, comparable locations having a higher acid deposition rate would have to show a stronger tendency towards acid-indicators than those having a lower one. It is the stemflow area of the beech tree (Fagus sylvatica L.) which attracts more precipitation than the rest of the forest area, on account of its infundibular treetop. Furthermore, stemflow water from Fagus sylvatica exhibits a higher degree of acidity than rain dripping from the treetop. Thus, the stemflow area as an area with higher acidic incidence may be compared with the normal forest area displaying minor acidic incidence. Findings from all the surveys so far conducted in Middle Europe show that soil pH recorded in the stemflow area is distinctly lower, and that there are less base- and weak acid-indicators but correspondingly more (strong) acid-indicators, than in “normal” forest soil. It must be noted that the phenomena observed need not be due to acidic precipitation. They could as well be attributable to a special feature pertaining to the stemflow area of the beech as specifically natural phenomena. In the event of natural causes being detected, these would have to be observable in the overall growth area of the beech. In Southern Europe, however, we did not find any indication whatsoever for acidification of the stemflow area. This phenomenon appears to be restricted to Central European regions affected by higher pollution.     

Soil changes induced by air pollutant deposition and their implication for forests in central Europe

A survey of leaf and needle losses of European forests in 1993 revealed that 23% of the total forested area had defoliation of more than 25%. The focus of this defoliation is in Central Europe, namely in Poland, Slowakia, Czech Republic, and Germany. The annual surveys of leaf losses and discoloration indicated only small changes during the last years for the coniferous forests in Germany. However, the increasing leaf losses of oak and beech during the last years were alarming. Evaluating the potential relation between air pollutant deposition, soil changes and forest damage, we focus here on the recent changes in deposition and soil conditions, and their implication on tree root development and drought susceptability of trees. While deposition of SO₄ ^2?, H⁺ and Ca²⁺ in many Central European forests decreased in the last decade, input of NH₄ ⁺ and NO₃ ^? remained high or even increased. The H⁺ load of many forest soils today is thus still high compared to weathering rates, but the proportion of the H⁺ load resulting from turnover of deposited N has increased. Recent effects of changing depositions on acid forest soils were: depletion of soil Al-pools, release of formerly stored soil SO₄ ^2?, accumulation of N in soil organic matter, increasing N availability to trees and decreasing concentration of Ca²⁺ in the soil solution. We hypothesise that soil acidification and increased N availability will decrease the fine root biomass of trees and shift the rooting zone to upper soil layers. Increased above ground growth, observed in many areas of Europe, will furthermore decrease the root/shoot ratio. This development will finally cause increased drought susceptability of trees and is thus of destabilizing nature. The proposed chain of events might be overlapped by other effects of air pollutants on forest ecosystems, namely direct effects of gases on leaves, nutritional inbalances, and interactions with pests.     

Different long-term and short-term responses of land snails to clear-cutting of boreal stream-side forests

Effects of clear-cutting on biodiversity have mainly been studied in the short-term, although knowledge of longer term effects are often more important for managers of forest biodiversity. We assessed relatively long-term effects of clear-cutting on litter dwelling land snails, a group with slow active dispersal and considered to be intolerant to microclimate changes. In a pair wise design we compared snail abundance, species density, and species composition between 13 old seminatural stream-side stands and 13 matched young stands developed 40-60 years after clear-cutting. Using a standardized semi-quantitative method, we identified all snail specimens in a 1.5 l subsample of a pooled litter sample collected from small patches within a 20 × 5 m plot in each stream-side stand. From the young stands a mean of 135 shells and 9.5 species was extracted which was significantly higher than the 58.1 shells and 6.9 species found in old forests. Only two of the 16 species encountered showed a stronger affinity to old than to young forests. In short-term studies of boreal stream-side forests land snail abundance is reduced by clear-cutting. Our results indicate that this decline is transient for most species and within a few decades replaced by an increase. We suggest that local survival in moist stream-side refugia makes the land snails able to benefit from the higher pH and more abundant non-conifer litter in young than in old boreal forests. Our results highlight the importance of longer term studies as a basis for management guidelines for biodiversity conservation.     

Spatial variations in the trophic structure of soil animal communities in boreal forests of Pechora-Ilych Nature Reserve

Soil animal communities and detrital food webs are spatially compartmentalized. In old-growth boreal forests the dynamics of dominating plant species forms a considerable heterogeneity of edaphic conditions in the soil layer. We demonstrate a strong difference in total and relative abundance of main trophic groups of soil macrofauna in four microsites, i.e. under tree crowns, in gaps, in mounds and in pits created by fallen spruce trees. The variation in the functional structure of soil animal communities is likely related to different availability of key energy resources (leaf litter, roots and root deposits) in the microsites studied. However, results of the stable isotope analysis suggest that mobile litter-dwelling predators occupy very similar trophic positions in different microsites. The compartmentalization of soil invertebrate communities caused by the vegetation-induced mosaic of edaphic conditions seemingly does not lead to spatial isolation of local food webs that are integrated at the top trophic levels.     

Indices of dissolved organic nitrogen, ammonium and nitrate across productivity gradients of boreal forests

Organic nitrogen (N) uptake, rather than solely inorganic N (DIN), is considered a significant pathway for plant nutrition, especially in arctic, alpine and boreal ecosystems. Assays of plant-available N in these ecosystems might therefore be improved with measures of dissolved organic N (DON). We examined DON and DIN abundance from an in situ 5-week incubation across plant associations that represent the widest range in site potential in southern boreal forests of British Columbia, Canada. The supply of N from forest floors and mineral soils (20 cm depth) was measured separately and then combined (kg ha⁻¹) to facilitate comparisons of sites. DON was the predominant form of extractable N, and was increasingly supplemented, rather than replaced, by NH₄⁺ and NO₃⁻ on productive sites. The amount of DIN produced in the soils was very low, perhaps too small to support forest needs, and the correlation of DIN to asymptotic stand height (a measure of site potential) was significant but nonlinear. The combined amount of DON+DIN was considered a more effective index of plant-available N because it was strongly significant as a linear correlation to stand height and more typical of annual forest N uptake. The relative shift in N forms, from a predominance of DON to progressively greater ratios of DIN:DON, was consistent with the current paradigm of N forms across gradients of N availability, although the actual amounts of DON increased, rather than decreased, with site potential. Based on this, we suggest organic N uptake has the potential to contribute to plant nutrition across the entire productivity gradient of soils in southern boreal forests. Although other N indices were effective in characterizing forest productivity, a combined assay of DON+DIN production could provide new insights into functional differences in plant-available N.     

Composition of lipophilic compounds and carbohydrates in the accumulated plant litter and soil organic matter in boreal forests

Carbohydrates and lipophilic compounds constitute an important component of litter and soil organic matter in boreal forests, but are still poorly identified. We characterized needle litter and coarse tree litter (cones, seeds, bark and twigs) from coniferous trees (Pinus sylvestris L. and Picea abies Karst.), and moss litter (Pleurozium schreberi, Hylocomnium speldens), fermentation (F) and humus (H) layers in four boreal forest sites in Finland using a combination of sequential fractionation (non‐polar extractions, NPE; water‐soluble extractions, WSE; acid‐soluble fraction, AS) and detailed analyses on the soluble fractions using GC‐MS. Comparisons among the different layers of the soil organic horizon were used to assess which lipophilic compounds and carbohydrates increased in the F and H layers in proportion to their relative abundance in the litter layer and thus might have a large potential to accumulate in soil organic matter. Both concentrations and relative proportions of different compounds varied among the soil layers. Several of the fatty acids (FAs) found in the litter samples were absent in the F and the H layers. Needle and coarse tree litter contained a wide range of di‐ and triterpenes, but in the F and H layers oxidized forms of dehydroabietic acid and sterols were abundant. The large proportion of dehydroabietic acid in the lipophilic fraction in the H layer suggests that it may be poorly degradable by soil microorganisms, probably because of its anti‐microbial function in trees. The composition of the acid‐soluble fraction indicated that the proportion of cellulose in relation to hemicelluloses increased from the litter layer to the F and H layers. Put together, changes in the relative proportions of organic compounds in soluble fractions indicate that selective preservation of compounds, litter input by plant roots and microbial synthesis of compounds all contribute to the accumulation of aliphatic compounds in the H layer of boreal forests.