Soil CO2 efflux in contrasting boreal deciduous and coniferous stands and its contribution to the ecosystem carbon balance

Similar nonsteady‐state automated chamber systems were used to measure and partition soil CO₂ efflux in contrasting deciduous (trembling aspen) and coniferous (black spruce and jack pine) stands located within 100 km of each other near the southern edge of the Boreal forest in Canada. The stands were exposed to similar climate forcing in 2003, including marked seasonal variations in soil water availability, which provided a unique opportunity to investigate the influence of climate and stand characteristics on soil CO₂ efflux and to quantify its contribution to the net ecosystem CO₂ exchange (NEE) as measured with the eddy‐covariance technique. Partitioning of soil CO₂ efflux between soil respiration (including forest‐floor vegetation) and forest‐floor photosynthesis showed that short‐ and long‐term temporal variations of soil CO₂ efflux were related to the influence of (1) soil temperature and water content on soil respiration and (2) below‐canopy light availability, plant water status and forest‐floor plant species composition on forest‐floor photosynthesis. Overall, the three stands were weak to moderate sinks for CO₂ in 2003 (NEE of ?103, ?80 and ?28 g C m^?2 yr^?1 for aspen, black spruce and jack pine, respectively). Forest‐floor respiration accounted for 86%, 73% and 75% of annual ecosystem respiration, in the three respective stands, while forest‐floor photosynthesis contributed to 11% and 14% of annual gross ecosystem photosynthesis in the black spruce and jack pine stands, respectively. The results emphasize the need to perform concomitant measurements of NEE and soil CO₂ efflux at longer time scales in different ecosystems in order to better understand the impacts of future interannual climate variability and vegetation dynamics associated with climate change on each component of the carbon balance.     

Competition and facilitation between tree species change with stand development

Processes governing tree interspecific interactions, such as facilitation and competition, may vary in strength over time. This study tried to unveil them by performing dendrometrical analyses on black spruce Picea mariana, trembling aspen Populus tremuloides and jack pine Pinus banksiana trees from pure and mixed mature boreal forest stands in the Clay Belt of northwestern Quebec and on the tills of northwestern Ontario. We cored 1430 trees and cut 120 for stem analysis across all stand composition types, tree species and study regions. Aspen annual growth rate was initially higher when mixed with conifers, but then progressively decreased over time compared to pure aspen stands, while jack pine growth rate did not differ with black spruce presence throughout all stages of stand development. When mixed with aspen, black spruce showed a contrary response to aspen, i.e. an initial loss in growth but a positive gain later. On the richer clay soil of the Quebec Clay Belt region, however, both aspen and spruce responses in mixed stands reversed between 37 and 54 years. Overall, our results demonstrate that interspecific interactions were present and tended to change with stand development and among species. Our results also suggest that the nature of interspecific interactions may differ with soil nutrient availability.     

The Contrasting Effects of Aspen and Jack Pine on Soil Nutritional Properties Depend on Parent Material

Abstract The influence of forest stand composition on soil was investigated by comparing the forest floor (FH) and upper mineral soil (0–20 cm) nutritional properties of jack pine and aspen stands on two soil types of contrasting fertility, a coarse-textured and a fine-textured deposit, in a replicated design. The studied tree species are pioneers that are found after major disturbances in the southern boreal forest of western Quebec and that differ in their nutrient requirements but not in their growth rate. Soil organic matter as well as total and available N, P, K, Ca, Mg contents were determined and the relationships with nutrient accumulation in tree biomass were studied. On both soil types a greater total and available nutrient accumulation in the forest floor layer was observed in aspen than in jack pine whereas such differences between stand types could not be detected in the mineral soil. Differences in FH nutrient content between stand types were larger on coarse deposits than on fine-textured soils. These results support the hypothesis that tree species with greater nutrient requirements cause an enrichment of the surface soil at least in the short term. The modulation of tree species effect by soil type was contrary to the pattern observed in other studies since a greater expression of this effect was observed on poorer soils. Differences in soil nutrient content were related to levels of organic matter accumulation.     

Soil and tree chemistry reflected the cumulative impact of acid deposition in Pinus banksiana and Populus tremuloides stands in the Athabasca oil sands region in western Canada

The spatial variability of soil chemistry and Ca/Al ratios of soil solution and fine roots were investigated in jack pine (Pinus banksiana) and trembling aspen (Populus tremuloides, aspen) stands to assess the impact of chronic acid deposition on boreal forest ecosystems in the Athabasca oil sands region (AOSR) in Alberta, Canada. Available SO₄²⁻ (as the sum of soluble and adsorbed SO₄²⁻) accumulated in the soil near tree boles of both species, reflecting the influence of canopy intercepted SO₄²⁻. In jack pine stands, pH and soluble base cation concentrations decreased towards tree boles due to increased SO₄²⁻ leaching; the reverse was found in aspen stands due to deposition of base cations leached from the canopy. As a result, Ca/Al ratios in the soluble fraction in soils near jack pine boles were 5–20 times lower than that near aspen boles. The Ca/Al ratio did not reach the critical limits of 1.0 for soil solution (ranged from 1.0 to 4.1) or 0.5 for fine roots (0.7–7.9) in the studied watersheds. However, Alⁿ⁺ concentrations in the soil solution ranged from 0.2 to 4.1 mg L⁻¹ in NE7 and from 0.1 to 8.5 mg L⁻¹ in SM8 that can inhibit the growth of white spruce (Picea glauca) seedlings that commonly succeed aspen in upland sites in the AOSR. We suggest that the spatial variation caused by tree canopies/stems will affect forest regeneration and the effect of acid deposition on forest succession in the AOSR should be further studied.     

Carbon and Nitrogen Dynamics of Boreal Jack Pine Stands With and Without a Green Alder Understory

We compared the species composition, structure and selected components of the carbon (C) and nitrogen (N) budgets of similar-aged, mature boreal jack pine (Pinus banksiana Lamb.) forests with and without green alder [Alnus crispa (Ait.) Pursh.] in two different boreal environments. The C and N content of the overstory biomass components (for example, stem, branch, and foliage), total vegetation, forest floor, and mineral soil were greater (P= 0.05 to P= 0.10) for jack pine with alder (JPA) stands than for jack pine without alder (JP) stands at both study areas. Jack pine foliage N isotopic discrimination (δ¹⁵N) and annual litterfall N content were significantly greater (P < 0.05) in the JPA than the JP stands at both study areas, suggesting that alder was fixing N and that N availability was greater in the JPA than the JP stands. The greater leaf area index (LAI) and overstory C accumulation in the JPA than the JP stands (P < 0.05) is likely because of the greater N availability in the JPA stands, but the effect of soil texture discontinuity on water availability in the JPA stands can not be dismissed. Percent ground cover by feathermoss varied among the jack pine communities and was positively correlated with overstory LAI (r ²= 0.83, P< 0.05). One index of N-use efficiency (NUE), defined as aboveground net primary productivity (ANPP) per litterfall N, was significantly greater (P < 0.05) for the JP than the JPA stands, but a second index of NUE, ANPP/N uptake, did not differ between the two jack pine communities. Jack pine trees growing without alder produced more organic matter per unit of N, but percent N retranslocation from senescing foliage and N mean residence time in the overstory did not differ between the JPA and the JP stands. A conceptual model is presented that illustrates the potential influence of alder on the species composition, structure, and function of boreal jack pine forests. Received 6 January 1998; accepted 15 April 1998.     

The effects of genetic diversity,climate and defoliation events on trembling aspen growth performance across Canada

Tree genetic makeup may provide an important control of growth dynamics; however, no studies have previously attempted to evaluate its effects in natural trembling aspen stands. In this study, we examined the relative contribution of genetics (i.e. clonal diversity, observed heterozygosity) and environmental conditions (i.e. insects, climate) on aspen growth as represented by mean inter-tree correlation (RBAR), tree basal area increment (TBAI) and inter-annual growth variability (MS). We sampled 440 trees in 22 even-aged natural stands dominated by aspen along an east-west continental gradient of decreasing annual precipitation in the Canadian boreal forest. Linear and mixed-effect models tested the relationships between tree growth, genetics and environmental factors. We showed that clonal diversity and number of years with forest tent caterpillar (FTC) defoliation (N_FTC) reduced and increased the level of growth synchronicity (RBAR), respectively. Clonal diversity explained 30 % of variation in RBAR among sites. TBAI was positively influenced by high moisture conditions while N_FTC and climate explained the variation in MS among trees for each site. No genetic effect could explain either TBAI or the MS variation. Climate and N_FTC drive annual growth variability in trembling aspen at stand and subcontinental scales. Tree genetic makeup contributed to these dynamics, the annual growth dynamics of multi-clonal stands being less homogeneous than those of monoclonal stands. Maintaining diverse aspen stands may ensure a wider range of growth responses to environmental variability, which in turn may help maintain resilience of aspen stands under future climate.     

Structure of the Microbial Communities in Coniferous Forest Soils in Relation to Site Fertility and Stand Development Stage

Abstract The structure, biomass, and activity of the microbial community in the humus layer of boreal coniferous forest stands of different fertility were studied. The Scots pine dominated CT (Calluna vulgaris type) represented the lowest fertility, while VT (Vaccinium vitis-idaéa type), MT (Vaccinium myrtillus type), and OMT (Oxalis acetocella–Vaccinium myrtillus type) following this order, were more fertile types. The microbial community was studied more closely by sampling a succession gradient (from a treeless area to a 180-years-old Norway spruce stand) at the MT type site. The phospholipid fatty acid (PLFA) analysis revealed a gradual shift in the structure of the microbial community along the fertility gradient even though the total microbial biomass and respiration rate remained unchanged. The relative abundance of fungi decreased and that of bacteria increased with increasing fertility. The structure of the bacterial community also changed along the fertility gradient. Irrespective of a decrease in fungal biomass and change in bacterial community structure after clear-cutting, the PLFA analysis did not show strong differences in the microbial communities in the stands of different age growing on the MT type site. The spatial variation in the structure of the microbial community was studied at a MT type site. Semivariograms indicated that the bacterial biomass, the ratio between the fungal and bacterial biomasses, and the relative amount of PLFA 16:1ω5 were spatially autocorrelated within distances around 3 to 4 m. The total microbial and fungal biomasses were autocorrelated only up to 1 m. The spatial distribution of the humus microbial community was correlated mainly with the location of the trees, and consequently, with the forest floor vegetation. Received: 9 November 1998; Accepted: 26 April 1999     

Tropical montane forest ecotones: climate gradients, natural disturbance, and vegetation zonation in the Cordillera Central, Dominican Republic

Aim We examined relationships between climate–disturbance gradients and patterns of vegetation zonation and ecotones on a subtropical mountain range. Location The study was conducted on the windward slopes of the Cordillera Central, Dominican Republic, where cloud forest appears to shift in a narrow ecotone to monodominant forest of Pinus occidentalis. Methods Climate, disturbance and vegetation data were collected over the elevation range 1100–3100 m and in 50 paired plots along the ecotone. Aerial photographs were georeferenced to a high‐resolution digital elevation model in order to enable the analysis of landscape‐scale patterns of the ecotone. Results A Shipley–Keddy test detected discrete compositional ecotones at 2200 and 2500 m; the distributions of tree species at lower elevations were continuous. The elevation of the ecotone determined with aerial photographs was fairly consistent, namely ± 164 m (SD) over its 124‐km length, but it exhibited significant landscape variation, occurring at a lower elevation in a partially leeward, western zone. The ecotone also occurred significantly lower on ridges than it did in drainage gullies. Ecotone forest structure and composition differed markedly between paired plots. In pine paired plots, the canopy height was 1.7 times higher and the basal area of non‐pine species was 6 times lower than in the cloud forest directly below. Fire evidence was ubiquitous in the pine forest but rare in the abutting cloud forest. Mesoclimate changed discontinuously around the elevation of the ecotone: humidity and cloud formation decreased markedly, and frost frequency increased exponentially. Main conclusions The discreteness of the ecotone was produced primarily by fire. The elevational consistency of the ecotone, however, resulted from the overarching influence of mesoclimate on the elevational patterns of fire occurrence. Declining temperature and precipitation combine with the trade‐wind inversion to create a narrow zone where high‐elevation fires extinguish, enabling fire‐sensitive and fire‐tolerant taxa to abut. Once established, mesotopography and contrasting vegetation physiognomy probably reinforce this boundary through feedbacks on microenvironment and fire likelihood. The prominence of the pine in this study – and of temperate and fire‐tolerant taxa in subtropical montane forests in general – highlights the importance of climate‐disturbance–biogeography interactions in ecotone formation, particularly where fire mediates a dynamic between climate and vegetation.     

Modelling Variable Fire Severity in Boreal Forests: Effects of Fire Intensity and Stand Structure

It is becoming clear that fires in boreal forests are not uniformly stand-replacing. On the contrary, marked variation in fire severity, measured as tree mortality, has been found both within and among individual fires. It is important to understand the conditions under which this variation can arise. We integrated forest sample plot data, tree allometries and historical forest fire records within a diameter class-structured model of 1.0 ha patches of mono-specific black spruce and jack pine stands in northern Québec, Canada. The model accounts for crown fire initiation and vertical spread into the canopy. It uses empirical relations between fire intensity, scorch height, the percent of crown scorched and tree mortality to simulate fire severity, specifically the percent reduction in patch basal area due to fire-caused mortality. A random forest and a regression tree analysis of a large random sample of simulated fires were used to test for an effect of fireline intensity, stand structure, species composition and pyrogeographic regions on resultant severity. Severity increased with intensity and was lower for jack pine stands. The proportion of simulated fires that burned at high severity (e.g. >75% reduction in patch basal area) was 0.80 for black spruce and 0.11 for jack pine. We identified thresholds in intensity below which there was a marked sensitivity of simulated fire severity to stand structure, and to interactions between intensity and structure. We found no evidence for a residual effect of pyrogeographic region on simulated severity, after the effects of stand structure and species composition were accounted for. The model presented here was able to produce variation in fire severity under a range of fire intensity conditions. This suggests that variation in stand structure is one of the factors causing the observed variation in boreal fire severity.     

Effect of forest canopy composition on soil nutrients and dynamics of the understorey: mixed canopies serve neither vascular nor bryophyte strata

Question: The effect of overstorey composition on above‐ground dynamics of understorey vegetation is poorly understood. This study examines the understorey biomass, production and turnover rates of vascular and non‐vascular plants along a conifer–broadleaf gradient of resource availability and heterogeneity. Location: Canadian boreal forests of northwest Quebec and Ontario. Methods: We sampled mature stands containing various proportions of black spruce (Picea mariana (Mill.) BSP), trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lamb.). Above‐ground biomass of the understorey vegetation was assessed through harvesting; annual growth rates were calculated as the differences between biomass in 2007 and 2008, as estimated by allometric relationships, and turnover rates were estimated as net primary production divided by the biomass in 2007. Results: Higher aspen presence, linked to greater nutrient availability in the forest floor, was generally associated with higher vascular biomass and production in the understorey. This effect was less pronounced in sites of high intrinsic fertility. In contrast, bryophyte biomass was positively associated with conifer abundance, particularly in wet sites of the Quebec study area. Non‐linear responses resulted in total understorey biomass being lower under mixed canopies than under pure aspen or pure conifer canopies. Turnover rates did not differ with overstorey composition. Conclusions: While resource availability is a main driver of understorey productivity, resources as drivers appear to differ with differences in understorey strata components, i.e. vascular versus non‐vascular plants. Resource heterogeneity induced by a mixed canopy had overall negative effects on understorey above‐ground productivity, as this productivity seemed to rely on species adapted to the specific conditions induced by a pure canopy.     

Spatial and temporal variations in boreal forest fire frequency in northern Alberta

Abstract. Spatial and temporal variations in fire frequency in the boreal forest of Wood Buffalo National Park (WBNP) were assessed using forest stand age, fire scar and historical data. I test the hypotheses that (1) fire frequency is higher in jack pine forests and aspen forests than in black spruce forests and white spruce forests, (2) these variations in fire frequency can be related to the mean waterbreak distance (MWD) around a site and (3) fire frequency has changed over the past 300 years. The fire cycles (the time required to burn an area equal in size to the entire study area) in jack pine forests (39 years) and in aspen forests (39 years) were significantly shorter than those in black spruce forests (78 years) and in white spruce forests (96 years). The length of the fire cycle varies inversely with the MWD around a site, and the MWD was significantly higher in jack pine and aspen forests than in black or white spruce forests. It is suggested that covariations between soil type and the MWD influence, respectively, variations in forest dominant and fire frequency. A change in fire frequency at 1860 was apparent in the fire history for all of WBNP, the black spruce dominated stands, and the near and medium MWD classes. The fire cycle estimates for these classes were all significantly shorter during the period 1750 to 1859 (fire cycles = 25–49 years) than they were in the period 1860 to 1989 (fire cycles = 59–89 years). The possible roles of changes in climate and aboriginal burning practices in causing the temporal change in fire frequency are discussed.     

Nematode community shifts in response to experimental warming and canopy conditions are associated with plant community changes in the temperate-boreal forest ecotone

Global climate warming is one of the key forces driving plant community shifts, such as range shifts of temperate species into boreal forests. As plant community shifts are slow to observe, ecotones, boundaries between two ecosystems, are target areas for providing early evidence of ecological responses to warming. The role of soil fauna is poorly explored in ecotones, although their positive and negative effects on plant species can influence plant community structure. We studied nematode communities in response to experimental warming (ambient, +1.7, +3.4 °C) in soils of closed and open canopy forest in the temperate-boreal ecotone of Minnesota, USA and calculated various established nematode indices. We estimated species-specific coverage of understory herbaceous and shrub plant species from the same experimental plots and tested if changes in the nematode community are associated with plant cover and composition. Individual nematode trophic groups did not differ among warming treatments, but the ratio between microbial-feeding and plant-feeding nematodes increased significantly and consistently with warming in both closed and open canopy areas and at both experimental field sites. The increase in this ratio was positively correlated with total cover of understory plant species, perhaps due to increased predation pressure on soil microorganisms causing higher nutrient availability for plants. Multivariate analyses revealed that temperature treatment, canopy conditions and nematode density consistently shaped understory plant communities across experimental sites. Our findings suggest that warming-induced changes in nematode community structure are associated with shifts in plant community composition and productivity in the temperate-boreal forest ecotones.     

Effect of forest thinning and wood quality on the short-term wood decomposition rate in a <Emphasis Type="Italic">Pinus tabuliformis</Emphasis> plantation

The effects of forest thinning and wood quality on wood decomposition in the mineral soil were investigated in a Chinese pine (Pinus tabuliformis Carriére) plantation in northern China by measuring mass loss and changes in wood properties (carbohydrates, lignin and nitrogen (N) concentrations) in wood stakes of two tree species—loblolly pine (Pinus taeda L.) and trembling aspen (Populus tremuloides Michx.). Stakes were inserted to a 20 cm soil depth in stands with three thinning levels (low, moderate, and heavy) and an unharvested control and removed after 1 year. There were significant differences in stake mass loss among the treatments. The species effect on the stake mass loss was marginally significant. Wood N content of both species increased during decomposition in all thinning treatments, and was only correlated with aspen mass loss. Wood properties of stakes placed in each stand before insertion (t?=?0) were similar, except for pine lignin concentration and aspen lignin: N ratio, but neither had any effect on thinning treatment results. Lignin concentration increased and carbohydrate concentration decreased in both aspen and pine wood stakes during decomposition across all thinning treatments, which suggests that brown-rot fungi are dominant wood-decomposers on our study site. We conclude that thinning has a significant influence on the wood decomposition in the mineral soil of this Chinese pine plantation.     

Growth and mortality of trembling aspen (Populus tremuloides) in response to artificial defoliation

To simulate the effects of forest tent caterpillar (FTC) defoliation on trembling aspen growth and mortality, an artificial defoliation experiment was performed over three years in young aspen stands of northwestern Quebec. Defoliation plots of 15 × 15 m were established on three sites, together with associated control stands of pure trembling aspen. In 2007, root collar diameters were measured and positions of all trees were mapped prior defoliation. Severe FTC defoliation was simulated for three successive years (2007–2009) by manually removing all leaves from all but 7–10% of the trees present in the defoliation plots. Yearly surveys of growth and mortality were conducted until 2010 to evaluate defoliation effects on defoliated as well as surrounding undefoliated trees. In absence of other factors, growth and mortality of trembling aspen decreased and increased, respectively, after defoliation. Our study further revealed that small diameter trees died after one year of artificial defoliation, while larger-diameter trees died after repeated defoliations. Distributions of tree mortality tended to be aggregated at small scales (<5 m), corroborating gap patterns observed in mature stands following FTC outbreaks. This experiment revealed that trembling aspen mortality can be directly attributed solely to defoliation. Repeated defoliations during FTC outbreaks have the potential to profoundly modify stand productivity and structure by reducing tree growth and increasing tree mortality in the absence of predisposing factors.     

Drought causes reduced growth of trembling aspen in western Canada

Adequate and advance knowledge of the response of forest ecosystems to temperature‐induced drought is critical for a comprehensive understanding of the impacts of global climate change on forest ecosystem structure and function. Recent massive decline in aspen‐dominated forests and an increased aspen mortality in boreal forests have been associated with global warming, but it is still uncertain whether the decline and mortality are driven by drought. We used a series of ring‐width chronologies from 40 trembling aspen (Populus tremuloides Michx.) sites along a latitudinal gradient (from 52° to 58°N) in western Canada, in an attempt to clarify the impacts of drought on aspen growth by using Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI). Results indicated that prolonged and large‐scale droughts had a strong negative impact on trembling aspen growth. Furthermore, the spatiotemporal variability of drought indices is useful for explaining the spatial heterogeneity in the radial growth of trembling aspen. Due to ongoing global warming and rising temperatures, it is likely that severer droughts with a higher frequency will occur in western Canada. As trembling aspen is sensitive to drought, we suggest that drought indices could be applied to monitor the potential effects of increased drought stress on aspen trees growth, achieve classification of eco‐regions and develop effective mitigation strategies to maintain western Canadian boreal forests.     

A new method to infer vegetation boundary movement from ‘snapshot’ data

Global change may induce shifts in plant community distributions at multiple spatial scales. At the ecosystem scale, such shifts may result in movement of ecotones or vegetation boundaries. Most indicators for ecosystem change require timeseries data, but here a new method is proposed enabling inference of vegetation boundary movement from one ‘snapshot’ (e.g. an aerial photograph or satellite image) in time. The method compares the average spatial position of frontrunners of both communities along the vegetation boundary. Mathematical analyses and simulation modeling show that the average frontrunner position of retreating communities is always farther away from a so‐called optimal vegetation boundary as compared to that of the expanding community. This feature does not depend on assumptions about plant dispersal or competition characteristics. The method is tested with snapshot data of a northern hardwood‐boreal forest mountain ecotone in Vermont, a forest‐mire ecotone in New Zealand and a subalpine treeline‐tundra ecotone in Montana. The direction of vegetation boundary movement is accurately predicted for these case studies, but we also discuss potential caveats. With the availability of snapshot data rapidly increasing, the method may provide an easy tool to assess vegetation boundary movement and hence ecosystem responses to changing environmental conditions.     

Archaeal Communities in Boreal Forest Tree Rhizospheres Respond to Changing Soil Temperatures

Temperature has generally great effects on both the activity and composition of microbial communities in different soils. We tested the impact of soil temperature and three different boreal forest tree species on the archaeal populations in the bulk soil, rhizosphere, and mycorrhizosphere. Scots pine, silver birch, and Norway spruce seedlings were grown in forest humus microcosms at three different temperatures, 7–11.5°C (night–day temperature), 12–16°C, and 16–22°C, of which 12–16°C represents the typical mid-summer soil temperature in Finnish forests. RNA and DNA were extracted from indigenous ectomycorrhiza, non-mycorrhizal long roots, and boreal forest humus and tested for the presence of archaea by nested PCR of the archaeal 16S rRNA gene followed by denaturing gradient gel electrophoresis (DGGE) profiling and sequencing. Methanogenic Euryarchaeota belonging to Methanolobus sp. and Methanosaeta sp. were detected on the roots and mycorrhiza. The most commonly detected archaeal 16S rRNA gene sequences belonged to group I.1c Crenarchaeota, which are typically found in boreal and alpine forest soils. Interestingly, also one sequence belonging to group I.1b Crenarchaeota was detected from Scots pine mycorrhiza although sequences of this group are usually found in agricultural and forest soils in temperate areas. Tree- and temperature-related shifts in the archaeal population structure were observed. A clear decrease in crenarchaeotal DGGE band number was seen with increasing temperature, and correspondingly, the number of euryarchaeotal DGGE bands, mostly methanogens, increased. The greatest diversity of archaeal DGGE bands was detected in Scots pine roots and mycorrhizas. No archaea were detected from humus samples from microcosms without tree seedling, indicating that the archaea found in the mycorrhizosphere and root systems were dependent on the plant host. The detection of archaeal 16S rRNA gene sequences from both RNA and DNA extractions show that the archaeal populations were living and that they may have significant contribution to the methane cycle in boreal forest soil, especially when soil temperatures rise.     

Litter production and forest floor nutrient dynamics in pine and hardwood stands of New Brunswick, Canada

Biomass and nutrient transfer (N, P, K, Ca, Mg) of overstory (branches and leaves) and understory litter fall were examined over a two year period in four jack pine stands aged 16, 29, 49 and 57 years and four mixed hardwood stands aged 7, 17, 20 and 29 years. Relative amounts of the five nutrients in litter fall for both series of stands were N > K ≷ Ca > P = Mg. Return of mineral elements to the forest floor was generally twice as high on the hardwood stands as for similarly aged pine stands. Overall return of nutrients plotted versus stand age generally exhibited a plateau relationship, with relatively little difference among stands; however, some exceptions occurred. Understory contribution to litter fall was very important on these stands, since in most cases the nutrient mass in understory litter was usually similar to or higher than that from the tree layer. Data on forest floor biomass, nutrient distribution and turnover rates of these stands were also presented; mobility of nutrients in the forest floor was in the order K > Mg ≥ P ≥ Ca ≥ N.     

Mites (Acari,Mesostigmata) in boreal Scots pine forest floors: effect of distance to stumps

Coarse woody debris (CWD) is a basic component of forest ecosystems and it plays a crucial role in species-poor boreal forests. Generally, previous studies have focused on differences between the forest floor and decaying logs of various tree species. The impact of distance to CWD has been investigated mainly for forest-floor snails and some groups of macrofauna, but not yet for mesostigmatid mites communities. We hypothesized that the effect of CWD decreases with increasing distance from CWD. To test this hypothesis we conducted a study in relatively species-poor Finnish boreal forest (at ca. 100 km northwest of Helsinki). In total, 81 samples were collected in 2007 from nine Scots pine (Pinus sylvestris) stumps, three microhabitats (CWD, soil/litter at 0.5 m from a stump and soil/litter at 1.5 m from a stump) and in three main directions (9 stumps × 3 microhabitats × 3 directions). Overall, 1965 mesostigmatid mites were collected representing 24 species. The mean number of mite species collected was significantly different between decaying stumps and forest litter; however, there was no significant difference between the litter samples at 0.5 and 1.5 m distance. The evenness index was significantly lower for samples collected from stumps than for litter in close (0.5 m) or far (1.5 m) distance. The most frequently encountered mite species were Veigaia nemorensis, Parazercon radiatus and Zercon zelawaiensis.     

Simulating boreal forest dynamics from perspectives of ecophysiology,resource availability,and climate change

Boreal forests are under strong influences from climate change, and alterations in forest dynamics will have significant impacts on global climate-biosphere feedback as well as local to regional conservation and resource management. To understand the mechanisms of forest dynamics and to assess the fate of boreal forests, simulation studies should be based on plant ecophysiological responses onto environmental conditions. In central Canadian boreal forests, local geomorphology created by past glacial activities often generates a mosaic of very distinctive forest types. On sandy hilltop of a glacial till, due to limitations in moisture availability and short fire return intervals, drought-tolerant and fire-adapted jack pine usually becomes the dominant species. On mesic and nutrient-rich slopes, fast-growing and resource-demanding trembling aspen forms mixed forests with coniferous species. In bottomland, black spruce, slowly growing but tolerant species, is often the only species that can survive to the adult stage. These three very distinctive forest types often occur within a scale of 10 m. Simulation models of boreal forests should be able to reproduce this heterogeneity in forest structure and composition as an emergent property of plant ecophysiological responses to varying environmental properties. In this study, a process-based forest dynamics model, ecosystem demography model version 1.0, is used to mechanically reproduce the landscape heterogeneity due to edaphic variations. First, boreal tree species of northern Manitoba, Canada, are parameterized according to field observations, and, to explicitly capture interactions among tree saplings, allometric equations based on diameter at height of 0.15 m, instead of the conventional breast height of 1.37 m, is parameterized. Then, soil moisture regime and nutrient concentrations are statistically incorporated from a dataset. The resultant simulation successfully reproduces the distinctive forest dynamics influenced by the edaphic heterogeneity. The sequences of succession and the trajectories of forest development are generally consistent with the field observations. The differences in resource availability are the essential control on equilibrium values of total forest leaf area index. Next, to show the effect of anthropogenic atmospheric changes, changes in temperature and CO₂ concentrations are studied by a set of factorial experiments. The magnitude of CO₂ fertilization is largely affected by soil fertility. The temperature rise will increase the length of growing season, but can have a negative impact on forest growth by increasing aridity and autotrophic respiration. Overall, the boreal forest responses to climate change are complex due to the inherent edaphic variations and ecophysiological responses.