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The dark taiga of Siberia is a boreal vegetation dominated by Picea obovata, Abies sibirica, and Pinus sibirica during the late succession. This paper investigates the population and age structure of 18 stands representing different stages after fire, wind throw, and insect damage. To our knowledge, this is the first time that the forest dynamics of the Siberian dark taiga is described quantitatively in terms of succession, and age after disturbance, stand density, and basal area. The basis for the curve–linear age/diameter relation of trees is being analyzed. (1) After a stand-replacing fire Betula dominates (4,000 trees) for about 70 years. Although tree density of Betula decreases rapidly, basal area (BA) reached >30 m2/ha after 40 years. (2) After fire, Abies, Picea, and Pinus establish at the same time as Betula, but grow slower, continue to gain height and eventually replace Betula. Abies has the highest seedling number (about 1,000 trees/ha) and the highest mortality. Picea establishes with 100–400 trees/ha, it has less mortality, but reached the highest age (>350 years, DBH 51 cm). Picea is the most important indicator for successional age after disturbance. Pinus sibirica is an accompanying species. The widely distributed “mixed boreal forest” is a stage about 120 years after fire reaching a BA of >40 m2/ha. (3) Wind throw and insect damage occur in old conifer stands. Betula does not establish. Abies initially dominates (2,000–6,000 trees/ha), but Picea becomes dominant after 150–200 years since Abies is shorter lived. (4) Without disturbance the forest develops into a pure coniferous canopy (BA 40–50 m2/ha) with a self-regenerating density of 1,000 coniferous canopy trees/ha. There is no collapse of old-growth stands. The dark taiga may serve as an example in which a limited set to tree species may gain dominance under certain disturbance conditions without ever getting monotypic. [ABSTRACT FROM AUTHOR]

Vaccinium vitis-idaea L. plants were grown in open-top chambers in northern Finland under CO2 enrichment (600–700 ppm) with or without elevated (double ambient) O3. Half of the plants received ammonium nitrate fertilizer equivalent to 200 kg N ha–1 year–1. Frost hardiness during the period of frost hardening was determined three times after the end of the experiment, at the beginning of August, September and October. The results indicate that frost hardening of the Vaccinium vitis-idaea is accelerated by N fertilization. However, CO2 and O3 enrichment had no measurable effect on frost hardiness. [ABSTRACT FROM AUTHOR]

A few studies have reported a recent and rapid decline in NO deposition in eastern North America. Whether this trend can be observed at remote boreal sites with low rates of N deposition and how it could impact canopy uptake (CU) of N remain unknown. Here we report trends between 1997/1999 and 2012 for precipitation, throughfall N deposition as well as inorganic N CU for two boreal forest sites of Quebec, Canada, with contrasted N deposition rates and tree species composition. NO bulk deposition declined by approximately 50 % at both sites over the studied period while no change was observed for NH. As a result, the contribution of NH to inorganic N deposition changed from ~33 % to more than 50 % during the study period. On average, 52-59 % of N deposition was intercepted by the canopy, the retention being higher for NH (60-67 %) than for NO (45-54 %). The decrease in NO bulk deposition and the increase in the NH:NO ratio had important impacts on N-canopy interactions. The contribution of NH CU to that of total inorganic N CU increased at both sites but the trend was significant only at Tirasse (lowest N deposition). At this site, absolute NO CU significantly decreased (as did total N CU) during the study period, a consequence of the strong relationship ( r = 0.88) between NO bulk deposition and NO CU. Our data suggest that N interactions with forest canopies may change rapidly with changes in N deposition as well as with tree species composition. [ABSTRACT FROM AUTHOR]

The early responses of the field layer to changes in biotic and abiotic conditions are key determinants of the future composition and structure of forests where sustained heavy browsing pressure has depauperated the shrub understory. We investigated the relationships between white-tailed deer density and field layer plant community dynamics in boreal forests managed for wildlife and timber production. We hypothesized that the growth and reproduction of field layer plants are either: (H1) directly proportional to deer density, or (H2) related to deer density through nonlinear relationships or (H3) through nonlinear relationships with thresholds. We tested these hypotheses using data from a controlled browsing experiment involving a gradient of deer densities (0, 7.5, 15, 27 and 56 deer km−2) in interaction with timber harvesting conducted on Anticosti Island, Canada. In recent clearcuts, the dominant responses of the field layer plants were exponential recovery in growth and reproduction with decreasing deer densities. The abundance of browse-tolerant species such as grasses was positively related to deer density, suggesting an apparent competitive gain. These results support the prediction from our second hypothesis, although the presence of ecological thresholds should not be ruled out. Rapid changes in the early successional stages have potentially long-term consequences on successional patterns through processes such as the modulation of germination and early establishment success of seedlings from later successional species. Quantitative data as those presented here are essential for the development of ecosystem management prescriptions. On Anticosti Island, reduction of local deer densities to levels <15–7.5 deer km−2 in the first 3 years following timber harvesting appears to be compatible with the regeneration dynamics of this system although lower levels of deer densities may be required for the conservation of browse-sensitive plant species. [ABSTRACT FROM AUTHOR]

This study aims to assess biomass and area growth of 600 thalli of the old forest lichen, Lobaria pulmonaria, transplanted to three successional boreal forest stands with (1) natural rainfall regime, (2) additional moistening during dry days, and (3) additional moistening with added nutrients. Mean biomass growth during 100 days varied from 8.3% in the dark young spruce forest to 23.1% in the clear-cut area, with the old forest in between (16.0%). Additional moistening did not enhance lichen growth, probably because the transplantation period was wet. Nutrient additions slightly increased area growth compared to artificial water additions only. Growth was determined by a combination of external (forest stand, site factors) and internal factors (chlorophyll content, biomass per area). Transplants acclimated to high light by increasing thickness and chlorophyll a/b-ratio. Some visible bleaching and a strong positive correlation between chlorophyll content per area and lichen growth in clear-cuts suggest some high light-induced chlorophyll degradation. We believe that biomass growth and natural occurrence of L. pulmonaria is controlled by a delicate balance between light availability and desiccation risk, and that the species is confined to old forests due to a physiological trade-off between growth potential and fatal desiccation damage, both of which increase with increasing light. The discrepancy between potential and realized ecological niches is probably caused by a long-term risk to be killed in open habitats by high light during long periods with no rain. [ABSTRACT FROM AUTHOR]

Scots pine (Pinus sylvestris L.) forests of northern Sweden are often considered to be N limited. This limitation may have been exacerbated by the elimination of wildfire as a natural disturbance factor in these boreal forests. Phenolic inhibition of N mineralization and nitrification (due to litter and exudates of ericaceous shrubs) has been proposed as a mechanism for N limitation of these forests, but this hypothesis remains largely untested. N mineralization rates, nitrification rates, and sorption of free phenolic compounds were assessed along a fire-induced chronosequence in northern Sweden. A total of 34 forest stands varying in age since the last fire were identified and characterized. Overstorey and understorey vegetative composition and depth of humus were analysed in replicated plots at all 34 sites. Eight of the forest stands aged 3–352 years since the last fire were selected for intensive investigation in which ten replicate ionic resin capsules (used to assess net N mineralization and nitrification) and non-ionic carbonaceous resin capsules (used to assess free phenolic compounds) were installed at the interface of humus and mineral soil. A highly significant correlation was observed between site age and net sorption of inorganic N to resin capsules. Net accumulation of NH4+ and NO3– on resin capsules followed a linear decrease (R2=0.61, P<0.01) with time perhaps as a result of increased N immobilization with successional C loading. NO3– sorption to resin capsules followed a logarithmic decrease (R2=0.80, P<0.01) that may be related to a logarithmic increase in dwarf shrub cover and decreased soil charcoal sorption potential along this chronosequence. A replicated field study was conducted at one of the late successional field sites to assess the influence of charcoal and an added labile N source on N turnover. Three rates of charcoal (0, 100, and 1,000 g M–2) and two rates of glycine (0 and 50 g N as glycine M–2) were applied in a factorial design to microplots in a randomized complete block pattern. Net ammonification (as assessed by NH4+ sorption to resins) was readily increased by the addition of a labile N source, but this increase in NH4+ did not stimulate nitrification. Nitrification was stimulated slightly by the addition of charcoal resulting in similar levels of resin-sorbed NO3– as those found in early successional sites. Resin-sorbed polyphenol concentrations were decreased with charcoal amendments, but were actually increased with N amendments (likely due to decomposition of polyphenols). Net N mineralization appears to be limited by rapid NH4+ immobilization whereas nitrification is limited by the lack of an appropriate environment or by the presence of inhibitory compounds in late successional forests of northern Sweden. [ABSTRACT FROM AUTHOR]

Carbon dioxide flux from coarse woody debris (CWD) is an important source of CO2 in forests with moderate to large amounts of CWD. A process-based understanding of environmental controls on CWD CO2 flux (RCWD) is needed to accurately model carbon exchange between forests and the atmosphere. The objectives of this study were to: (1) use a laboratory incubation factorial experiment to quantify the effect of temperature (TCWD), water content (WC), decay status, and their interactions on RCWD for black spruce [Picea mariana (Mill.) BSP] CWD; (2) measure and model spatial and temporal dynamics in TCWD for a boreal black spruce fire chronosequence; and (3) validate the RCWD model with field measurements, and quantify potential errors in estimating annual RCWD from this model on various time steps. The RCWD was positively correlated to TCWD (R2=0.37, P<0.001) and WC (R2=0.18, P<0.001), and an empirical RCWD polynomial model that included TCWD and WC interactions explained 74% of the observed variation of RCWD. The RCWD estimates from the RCWD model excellently matched the field measurements. Decay status of CWD significantly (P<0.001) affected RCWD. The temperature coefficient (Q10) averaged 2.5, but varied by 141% across the 5–42°C temperature range, illustrating the potential shortcomings of using a constant Q10. The CWD temperature was positively correlated to air temperature (R2=0.79, P<0.001), with a hysteresis effect that was correlated to CWD decay status and stand leaf area index . Ignoring this temperature hysteresis introduced errors of –1% to +32% in annual RCWD estimates. Increasing TCWD modeling time step from hourly to daily or monthly introduced a 5–11% underestimate in annual RCWD. The annual RCWD values in this study were more than two-fold greater than those in a previous study, illustrating the need to incorporate spatial and temporal responses of RCWD to temperature and water content into models for long-term RCWD estimation in boreal forest ecosystems. [ABSTRACT FROM AUTHOR]

Our purpose was to characterize the relative competitive and phytotoxic potential of two closely related dwarf-shrub species, Empetrum nigrum and E. hermaphroditum, which form clones in a mosaic pattern in post-fire successions of the boreal forest of northern Sweden. We determined morphological and growth parameters of both species, performed bioassays and chemical analysis, and established field experiments to explore possible differing interference effects on trees by the two species. Both Empetrum species had very similar morphological and growth characteristics. E. hermaphroditum exerted considerably greater negative effects than E. nigrum against Pinus sylvestris and Populus tremula seed and seedlings. These negative effects were related to the different substitution of a bibenzyl in the two species. The effect on seed germination of the two bibenzyls isolated from E. nigrum and E. hermaphroditum was compared with that of other simple phenolics; the latter were found to be inactive, indicating a specific phytotoxic component in the bibenzyls. P. sylvestris seeds planted into clones of both species in the field revealed that E. hermaphroditum had much stronger inhibitory effects than did E. nigrum and the addition of activated carbon partially reversed these effects. Seed germination, biomass and survival of P. sylvestris after four seasons were significantly lower in E. hermaphroditum- than in E. nigrum-dominated plots. We conclude that while both Empetrum species are superficially very similar morphologically, they have vastly different effects on tree seed germination, seedling establishment and growth. These effects appear to be due, at least in part, to the different chemical profile of the two species. [ABSTRACT FROM AUTHOR]

The hypothesis that vegetational diversity may lessen the impact of forest insect pests by favoring natural enemies is appealing to those who seek ecologically sound solutions to pest problems. We investigated the effect of forest diversity on the impact of the spruce budworm Choristoneurafumiferana following the last outbreak, as well as the budworm's current abundance and parasitism rate, in the boreal forest of northwestern Québec. Mortality of balsam fir caused by the budworm was greater in extensive conifer stands than either in “habitat islands” of fir surrounded by deciduous forest or on true islands in the middle of a lake. Adult spruce budworm abundance, assessed by pheromone traps, did not differ significantly between the three types of sites. Larval and pupal parasitism rates were examined by transferring cohorts of laboratory-reared larvae and pupae to trees in the three site types and later collecting and rearing them. The tachinid Actiainterrupta, a parasitoid of fifth and sixth instar larvae, as well as the ichneumonid pupal parasitoids Itoplectesconquisitor, Ephialtesontario and Phaeogenesmaculicornis, caused higher mortality in the habitat islands than on true islands or in extensive stands. Exochusnigripalpistectulum, an ichneumonid that attacks the larvae and emerges from the pupae, caused greater mortality in the extensive stands of conifers. [ABSTRACT FROM AUTHOR]

We studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances. [ABSTRACT FROM AUTHOR]

Distribution and size of 38 capercaillie Tetrao urogallus leks were related to amount and configuration of old forest patches in two south-east Norwegian coniferous forests. The smallest occupied patch was 48 ha containing a solitary displaying cock. All patches larger than 1 km contained leks. Number of cocks per lek increased with increasing patch size. Number of leks per patch increased in a step-wise manner with one lek added for each 2.5-3 km increase in patch size. In large patches there was one lek per 3-5 km old forest, and density of lekking cocks was 2-2.5 per km. In small patches density of cocks varied considerably. Density of cocks was not related to patch isolation or patch shape. However, among leks surrounded by 50-60% old forest within a 1 km radius, number of cocks increased with increasing old forest fine-graininess. We argue that when old forests cover more than 50%, a fine-grained mosaic may support higher densities of lekking cocks than a coarse-grained mosaic. Conversely, when old forests cover less than 50%, a fine-grained mosaic is unfavourable, because each old forest patch becomes too small and isolated. Finally, we present a predictive model of how old forest fragmentation influences density of leks, number of cocks per lek, and total density of cocks. [ABSTRACT FROM AUTHOR]