Significance of old aspen (Populus tremula) trees for the occurrence of lichen photobionts

In the boreal forest landscape, aspen has been effectively selected against in favour of conifers. The decrease in aspen is of particular concern, since it has more host-specific species associated with it than any other boreal tree species. Recently, forest management systems have begun to include green-tree retention in order to maintain structural diversity. Earlier studies have focused on the importance of remnant aspen trees for lichen species prevalence. We have focused on the occurrence of free-living photobionts, i.e. cyanobacteria and green-algae, since a successful establishment of sexually dispersed lichens will depend upon the presence of the photobiont. Our study shows that the abundances of Gloeocystis, Nostoc, Scytonema and Trentepohlia increased with stand age, while the abundance of Trebouxia decreased. The response to clear-felling differed between genera. The two cyanobacterial genera were able to persist in clear-cuts, although they were more abundant on the northern side of the remnant trees. The green-algae showed no consistent pattern, Trentepohlia was affected while Trebouxia was unaffected. Our study indicates that the prerequisites for new-establishment for spore dispersed lichen species, on remnant aspen, may be fulfilled in terms of availability of free-living photobionts on the northern side of the trunks. In support of this interpretation we found that the occurrence of cyanolichens was positively correlated with the occurrence of free-living cyanobacteria in the clear-cuts. We conclude that tree retention is likely to provide a useful tool for increasing biodiversity in managed forest landscapes provided that source populations still exist in the surrounding landscape.     

Species occurrences at stand level cannot be understood without considering the landscape context: Cyanolichens on aspen in boreal Sweden

A major challenge in conservation biology is to understand species’ responses to habitat loss. In Fennoscandia, the ongoing decline in aspen in forests is of particular concern, since aspen is the boreal forest tree species that supports the most host-specific species of cryptogams and invertebrates. In order to predict the potential effects of aspen decline we compared the occurrence of three epiphytic cyanolichens in old-growth stands of the same habitat quality, in four aspen-rich and four aspen-poor landscapes. Collemacurtisporum and Collemafurfuraceum were, on average, five and six times more frequent, respectively, in the aspen-rich than in the aspen-poor landscapes. Leptogiumsaturninum was not affected by the abundance of aspen stands at the landscape level. Our data suggests that lichen species with poor dispersal abilities may be more sensitive to habitat loss than more easily dispersed species and that species with broader habitat amplitude may be less sensitive to habitat loss than more specialized species, even if they have inferior dispersal ability. We conclude that (i) predictions of species occurrences at the stand level have to take account of the amount of suitable habitat at the landscape level, and (ii) predicting the responses of individual species based on life-history traits can be crucial, but cannot be based on single traits. Thus our study shows that biological value cannot be assessed on the basis of habitat quality alone and that a landscape perspective is needed for the sustainable management of specialist species.     

The aspen parkland in western Canada: A dry-climate analogue for the future boreal forest?

Predicted future changes in regional climate under a doubling of atmospheric CO₂ concentrations were applied to the 1951–80 normals of 254 climate stations to examine future impacts on the boreal forest of western Canada. Previous analyses have indicated that in this region, the southern boreal forest is presently restricted to areas where annual precipitation (P) exceeds potential evapotranspiration (PET). The present analysis suggests that a predicted 11% increase in P would be insufficient to offset the increases in PET resulting from a predicted warming of 4–5°C. As a result, half of the western Canadian boreal forest could be exposed to a drier climate similar to the present aspen parkland zone (P < PET), where conifers are generally absent and aspen is restricted to patches of stunted trees interspersed with grassland. Future changes could result in permanent losses of forest cover following disturbance and an increase in the proportion of exposed edge habitat in remaining stands, where environmental conditions might induce additional stresses on tree growth. Thus if the predicted warming and drying occurs, productivity of aspen and other commercial species in the southern boreal forest would be greatly reduced.     

Retention patches as potential refugia for bryophytes and lichens in managed forest landscapes

Leaving small patches of forest intact at harvesting is now a standard procedure to mitigate negative effects on biodiversity. One purpose of the patches is to “life-boat” species over the forest regeneration phase, although the capacity of small forest fragments to do so is very uncertain. We investigated the survival of red-listed and indicator species of bryophytes and lichens in 74 retention patches in boreal Sweden. The patches were between 0.01 and 0.5 ha in size and of six different types with respect to tree species composition and location on the harvested area. Species presence and abundance were recorded shortly after harvest in transects covering the whole patches, and an identical inventory was carried out 6 years after the first. During this time, bryophytes generally decreased, most pronounced for liverworts. The largest decreases were found in buffer zones to streams and lakes and the smallest in tree groups dominated by deciduous trees. By contrast, among the lichens some species decreased while others increased, and there was no difference between retention patch types. Among the species abundant enough to be analyzed individually, the lichens Calicium parvum and Micarea globulosella decreased less in larger patches and the bryophyte Hylocomiastrum umbratum decreased more in patches of irregular shape. The results imply that retention patches of this size might be too small to function as refugia for sensitive bryophytes and lichens until the surrounding forest regenerates, but that some lichens appear to persist or even increase. Retention harvesting is still a young management practice and further studies on its long-term conservation benefits will be valuable.     

Forest floor microbial communities in relation to stand composition and timber harvesting in northern Alberta

With the growing interest in silvicultural techniques that more closely emulate natural disturbance regimes, there is a need to better understand how partial harvesting affects the soil microbial community in stands with varying ecological characteristics, e.g., tree species composition. Four and a half and 5.5 years post-harvest, we used phospholipid fatty acid (PLFA) and substrate-induced respiration (SIR) analyses to compare the microbial biomass and microbial community structure of forest floors from stands dominated by white spruce (Picea glauca; SPRUCE) or by trembling aspen (Populus tremuloides; ASPEN) and from mixed-species (MIXED) stands in northern Alberta, Canada, that had been clearcut, partial-cut with 20% retention, partial-cut with 50% retention or left uncut (controls). PLFA and SIR analyses revealed that ASPEN forest floors supported a larger microbial biomass with a very different community structure than MIXED or SPRUCE forest floors. The microbial community structure of these soils appeared to be strongly affected by the presence of white spruce and the composition of the understory vegetation. There were no effects of timber harvesting detected within or across stand types on any of the variables measured, with the exception of the PLFA 16:1ω5, which was relatively more abundant in the clearcuts and 50% retention treatments than in the uncut controls, perhaps in response to an increased forest floor pH and grass cover in the disturbed areas. The resilience to timber harvesting of the forest floors from these stands may be the result of efforts to minimize soil disturbance during harvesting and to allow vegetation to regenerate naturally. From the perspective of the forest floor microbial community, partial harvesting does not appear to have any benefit over clearcut harvesting at these boreal forest sites.     

Linking the abundance of aspen with soil faunal communities and rates of belowground processes within single stands of mixed aspen–black spruce

The occurrence of aspen (Populus tremuloides Michx.) patches within stands dominated by black spruce (Picea mariana Mill. BSP) has been shown to increase litter decomposition and nutrient cycling rates by improving soil physical and chemical properties. It is well known, however, that these processes are also influenced by the structure of the soil biota, but this factor has received less attention. In this study, relationships between forest floor properties and soil invertebrates were studied along black spruce–trembling aspen gradients in three stands of the eastern boreal forest of Canada. The forest floor layer of 36 plots differing in aspen basal area was sampled and analyzed to determine physical and chemical properties, the rates of decomposition of standard substrates, net N mineralization, as well as microbial basal respiration and metabolic quotient. Soil invertebrates were also collected using funnel-extraction and pitfall trapping methods. Based on redundancy analyses, we found that forest floor properties, the abundance and composition of soil invertebrates, and the rates of belowground processes changed along the spruce–aspen gradient. The increase in aspen basal area was associated with a reduction in forest floor thickness, moisture content and microbial biomass, and with an increase in the concentration of nutrients. It was also accompanied by changes in soil faunal communities, as soil invertebrates were associated with specific soil properties. In general, macroinvertebrates (i.e., Lumbricidae, Formicidae, Carabidae, Staphylinidae and Gastropoda) were related to the nutrient-rich forest floor associated with aspen, whereas microarthropods and Enchytraeidae tended to be negatively related to aspen basal area. According to mixed linear models, decomposition rates of standard substrates and net ammonification significantly increased along the spruce–aspen gradient. Given the functional significance of macroinvertebrates in soils, these results suggest that aspen favours the elaboration of a macrofaunal community, which in turn accelerates the rate of soil processes by having either direct or indirect influence on microbial activity. Moreover, this study shows that the changes in soil processes and in the biodiversity of soil organisms related to the presence of mixed stands can operate only in the immediate surroundings of a given tree species. Therefore, coarse-scale tree species mixing in a forest stand may have a different effect on soil biodiversity and soil processes than fine-scale mixing.     

Does forest continuity matter in conservation? - A study of epiphytic lichens and bryophytes in beech forests of southern Sweden

The effects of forest continuity at local scale for red-listed and indicator species of epiphytic lichens and bryophytes were investigated in 150 Fagus sylvatica stands in southern Sweden. Stands having forest continuity (n = 106) had continuous forest cover more than 350 years, whereas stands lacking continuity (n = 44) had forest cover less than 160 years. Forest continuity was identified by comparing a sequence of historical maps with a modern survey of beech forests. In the field woody beech substrates were searched for the epiphytes of interest. A number of environmental and spatial variables were inventoried and compiled for each stand. In all 64 species (51 lichens, 13 bryophytes) were found in the stands having continuity, and 21 (14 lichens, 7 bryophytes) in the stands lacking continuity. Controlling for the different number of surveyed stands, stands having continuity had significantly more species of lichens, but not of bryophytes. In the stands lacking continuity we did not find lichens associated with the very late succession stage. The quantity of substrates, stand age and forest continuity were the three most important factors explaining species richness as well as composition of studied epiphytes. The effect of continuity was probably due to a combination of a higher substrate quality, mainly old beeches, and a longer time available for colonization. Also, we found strong positive correlations between number of indicator and red-listed epiphyte species. In short-term conservation old stands having continuity, containing suitable substrates and indicator species are target areas.     

Aspen demographics in relation to spatial context and ungulate browsing: Implications for conservation and forest management

Aspen has a high conservation value and has been targeted as a priority species in sustainable forest management. However, sustained and intense browsing pressure by ungulates has raised concerns about the recruitment of aspen to mature growth stages. Here we investigate the influence of ungulate browsing on the demographic structure of European aspen populations in two managed boreal forest areas in Sweden with contrasting amounts of aspen and high densities of ungulates. The contrast in size distribution of ramets in the two areas was striking. The aspen-poor environment contained a much lower density of small-sized ramets than the aspen-rich environment. Ramet abundance was strongly associated with the abundance of ramets in the smaller size classes in both areas, reflecting demographic inertia in these populations. Ungulate use was most strongly associated with abundance of medium-sized aspen saplings growing in forest interiors. Otherwise, use of ungulates was only weakly correlated with aspen population parameters. The strong demographic momentum exhibited by these populations suggests that aspen conservation and management should pay more attention to additional factors than browsing to promote ramet establishment. Large variation in abundance of ramets in the smallest size-class shows that some aspen stands have a disproportionate effect on aspen recruitment, particularly in areas where aspen is less abundant. Identifying aspen stands with a large capacity for self regeneration may be the key to meet aspen conservation goals.     

Parasitoids (Hymenoptera, Ichneumonoidea) of Saproxylic beetles are affected by forest successional stage and dead wood characteristics in boreal spruce forest

The habitat requirements and effects of forest management on insects belonging to higher trophic levels are relatively unknown in forest ecosystems. We tested the effect of forest successional stage and dead wood characteristics on the saproxylic parasitoid (Hymenoptera, Ichneumonoidea) assemblage in boreal spruce-dominated forests in northern Sweden. Within each of nine areas, we selected three sites with different management histories: (1) a clear-cut (2) a mature managed forest and (3) an old-growth forest. Parasitoids were collected in 2003 using eclector traps mounted on fresh logs, which were either untreated (control), burned, inoculated with fungi, or naturally shaded, and on artificially-created snags.Both forest type and dead wood characteristics had a significant effect on parasitoid assemblages. Grouped idiobionts and some species, such as Bracon obscurator and Ontsira antica, preferred clear-cuts, while others, such as Cosmophorus regius (Hym., Braconidae) and other koinobionts, were associated with older successional forest stages. No single dead wood substrate was sufficient to support the entire community of parasitoids in any forest type, even when the regular host was present. In particular, snags hosted a different assemblage of species from other types of dead wood, with parasitoids of Tetropium spp. such as Rhimphoctona spp. (Hym., Ichneumonidae) and Helconidea dentator (Hym., Braconidae) being abundant. These results indicate that a diversity of dead wood habitats is necessary to support complete assemblages of beetle-associated parasitoids from early successional stages of dead wood and that parasitoids may be more sensitive to habitat change than their hosts.     

The role of remnant trees in seed dispersal through the matrix: Being alone is not always so sad

Remnant trees within the non-forest matrix are common structures of forest landscapes which have high conservation value due to their supposed roles as biological legacies and stepping stones in fragmented forest scenarios. Fleshy-fruited remnant trees are dispersal foci for many forest plants, as seeds accumulate under their canopies after visitation by forest frugivores. Despite this recognized effect, little is known about the relative role of remnant trees in maintaining their seed dispersal function after forest fragmentation. In this work, conducted in the Cantabrian Range (Northern Spain), seed deposition by frugivorous birds and post-dispersal seed predation by rodents were compared beneath the canopies of hawthorn (Crataegus monogyna) trees scattered in the non-forested matrix, to those embedded in forest patches. We studied two years which had strong differences in community-wide fruit abundance (high in 2004, and low in 2005). Hawthorn and holly (Ilex aquifolium) seeds dominated the seed rain. The density of dispersed seeds differed between years and tree type, with higher values under patch trees relative to remnant ones. However, the effect of tree type depended on both the year and the seed species, as larger differences were found in the year of high fruit availability, and with holly seeds. Higher levels of post-dispersal seed predation on holly seeds also contributed to offset the differences between tree types. Our results suggest that remnant trees, by functioning as dispersal foci, facilitate the dispersal of the two most abundant plant species for forest succession through the matrix. More importantly, we demonstrated that in years of low fruit availability in which forest frugivores are forced to exploit scattered fruit resources, the role of remnant trees may even be equivalent to that played by forest trees.     

Moisture effects on microbial communities in boreal forest floors are stand-dependent

Landscape level factors such as overstory canopy composition can have a profound effect on the ecology of microbial communities in boreal forest floors. However, factors influencing community composition at the microsite scale are still poorly described and understood. Here we explored moisture effects on microbial communities in forest floor derived from undisturbed trembling aspen and white spruce stands, two of the dominant trees in the boreal forest of western Canada. Forest floor samples were incubated in a laboratory experiment for a period of one month under a moisture regime ranging from moist to dry (field capacity, 60% of field capacity and wilting point). As in previous studies we found that the origin of the forest floor material had a strong effect on the microbial community. Additionally, we found that moisture manipulation did not alter the microbial communities of the white spruce forest floor. On the other hand, the moisture had a profound effect on the aspen forest floor, and resulted in structurally and functionally distinct microbial communities. This different response to moisture could be linked to the adaptation of microbial groups to the physical environment inherent to the aspen and spruce forest floors and provides an avenue to further work into microbial mediated biogeochemical processes in the boreal forest.     

Decomposition of aspen leaf litter results in unique metabolomes when decomposed under different tree species

We examined whether the decomposition rate of trembling aspen (Populus tremuloides) leaf litter differed when decomposed for one year in litter bags placed within adjacent monotypic stands of trembling aspen, Engelmann spruce, and lodgepole pine trees in four replicate blocks in the San Juan mountains of Colorado, and whether they were metabolized into different metabolic byproducts. Mass loss was 6-8% lower in pine stands than in spruce or aspen stands, but this trend was not significant (p = 0.27). Water-soluble leaf litter metabolites were characterized using ultra-performance liquid chromatography coupled to a quadrapole time-of-flight mass spectrometer (UPLC-MS). Aspen leaf litter metabolomes were highly chemically complex; thousands of unique molecular features were identified in each sample. Although many of the molecular features were common to litter decomposed in all three forest types, we identified a subset of features that differed in abundance among the forest types. Our results suggest that the decomposer communities associated with each forest type not only affected the overall decomposition rate, but also produced many compounds in the diverse suite of metabolic byproducts at different rates, which could be an important control on the long-term sequestration of C in soil organic matter.     

Arthropod responses to harvesting and wildfire: Implications for emulation of natural disturbance in forest management

Although natural disturbance has been widely adopted as a template for forest management that protects biodiversity, this hypothesis has not been adequately tested. We compared litter-dwelling arthropod assemblages (Coleoptera: Carabidae and Staphylinidae; Araneae) in aspen-dominated stands originating as clear-cuts or wildfires across three age classes (1-2, 14-15, and 28-29 years old) to test whether the post-harvest and post-fire assemblages converged following disturbances, and to compare faunal succession. These findings were compared to data about epigaeic arthropods in old and mature pyrogenic aspen stands (>70 years old) to determine whether diversity and community composition of arthropods from the younger age-classes approached what may have been typical pre-disturbance conditions. The resulting data-set of almost 27,000 arthropods and 230 species showed convergence in most taxa, and some general similarities between 28- and 29-year-old stands and old and mature stands. However, not all taxa responded similarly, and faunal succession following clear-cutting appeared to progress more rapidly than following wildfire. Rarefaction-estimated diversity was elevated in 1-2-year-old stands, compared to unharvested stands, reflecting a mix of closed-canopy and open-habitat species. Non-metric multi-dimensional scaling ordinations showed that samples from young wildfire disturbed stands (1-2 years old) included more variable assemblages than all other study sites, and contained species that may depend on unique post-fire habitat characteristics. The fauna of old and mature stands exhibited low diversity, but contained species with limited dispersal abilities, and species tied to old-growth habitats such as dead wood. Harvesting systems that do not allow adequate recovery following a first harvesting pass, or do not maintain microhabitat features associated with older fire-origin forests, may threaten persistence of some elements of boreal arthropod faunas.     

Predicted response of the lichen epiphyte Lecanora populicola to climate change scenarios in a clean-air region of Northern Britain

Studies in the response of vegetation to predicted future climate change have focussed on vascular plants and are therefore largely unrepresentative of wider botanical diversity (i.e. comprising cryptogams; algae, mosses, liverworts and fungi including lichens). This paper presents a study to predict the response of a cryptogam species, the epiphytic lichen Lecanora populicola, to climate change scenarios. L. populicola is an easily dispersed species that occurs predictably in a widespread habitat, i.e. aspen stands. The study area was geographically constrained to a clean-air region of northern Britain. Thus, using the popular bioclimatic envelope approach, the projected climatic response of L. populicola is not expected to be confounded by air-borne pollution effects, or dispersal and habitat limitation. Non-parametric multiplicative regression was used to describe the response of L. populicola to seven climate variables, and an optimum model projected using UKCIP02 scenarios, comprising two time-frames (2020 s and 2050 s) and two greenhouse gas emission levels (low and high). Model predictions suggest an overall increase in the potential range of L. populicola, and, by association, several other ‘Boreal’ lichen epiphytes. Projected increases in the occurrence of L. populicola are associated with predicted summer drying, and indicate a putative threat to negatively associated ‘oceanic’ lichens.     

Small isolated aspen stands enrich bird communities in southwestern ponderosa pine forests

Small aspen stands are disappearing from the landscape in the Southwest, so it is important to understand their contribution to the avian community. We sampled birds in 53 small, isolated aspen stands and 53 paired plots within the ponderosa pine forest in northern Arizona, during the 1996 and 1997 breeding seasons. Bird species richness and abundance were higher in aspen than in pine. However, bird species richness and abundance did not vary with size of the aspen patch or isolation index. In addition, direct ordination of species distributions with habitat factors suggested no distinct avian communities. This suggests that aspen stands do not harbor separate populations, but rather are locations where the regional avifauna reaches high local density and richness and may be crucial to birds in years of resource scarcity. Thus it is important for avian conservation to maintain many aspen stands across the landscape, encompassing a diversity of vegetation structure and composition.     

Invasion of a deciduous forest by earthworms: Changes in soil chemistry, microflora, microarthropods and vegetation

Ecosystems of northern North America existed without earthworm fauna until European settlers arrived and introduced European species. The current extent of invasion by some of these species, Lumbricus terrestris L., Octolasion tyrtaeum Savigny and Dendrobaena octaedra Savigny, into an aspen forest in the Canadian Rocky Mountains and the effects of the invasion on soil chemistry, microflora, soil microarthropods and vegetation were investigated. Densities of earthworm species, soil structure, plant coverage and abundance were determined along three transects starting at the edge of the forest. At locations with L. terrestris, litter was incorporated into the soil, and where O. tyrtaeum was present, organic layers were mixed with mineral soil layers. Organic layers disappeared almost entirely when both species occurred together. Carbon and nitrogen concentrations were reduced in organic layers in the presence of L. terrestris and O. tyrtaeum. Microbial biomass and basal respiration were reduced when L. terrestris and O. tyrtaeum were present, presumably due to resource competition and habitat destruction. Microarthropod densities and the number of microarthropod species were strongly reduced in the presence of O. tyrtaeum (−75% and −22%, respectively), probably through mechanical disturbances, increasing compactness of the soil and resource competition. The coverage of some plant species was correlated with earthworm abundance, but the coverage of others was not. Despite harsh climatic conditions, the invasion of boreal forest ecosystems by mineral soil dwelling earthworm species is proceeding and strongly impacts soil structure, soil chemistry, microorganisms, soil microarthropods and vegetation.     

Effects of habitat fragmentation on pollen flow and genetic diversity of the endangered tropical tree Swietenia humilis (Meliaceae)

Fragmentation of tropical forest represents a major threat to some tree populations by reducing local population size and gene flow from other populations. Both processes can decrease outcrossing rates and genetic variation in remnant stands. Despite these risks, some tree species have pollen vectors that mitigate these negative consequences for fragmented populations. In this paper, we assess both pollen flow and diversity of pollen sources in continuous forest and isolated stands of Swietenia humilis, a tropical tree species pollinated by small insects. Using seven nuclear microsatellite markers, we test the hypothesis that genetic diversity and the number of pollen donors are lower in remnant populations. Results show that allelic richness of seeds is lower in isolated populations (6.1 vs. 8.3 alleles per locus), even though adult populations do not show this difference.Pollen pool structure is greater in isolated patches (Φ_Iso = 0.26) than in continuous forest (Φ_For = 0.14), which yields estimates of the average effective number of pollen donors (N_ep) of 1.9 and 3.6 respectively. In addition, estimates of number of sires per mother indicate that isolated trees have half the number of pollen sources (4.98) than trees in the forest (9.8). Although extensive pollen movement (>2000 m) was recorded on both habitat conditions, indicating that fragmented patches are not isolated from pollen-mediated gene flow, this extensive pollen flow among trees in fragmented landscapes may not serve to counteract deleterious reproductive and genetic consequences of habitat fragmentation.     

Boreal forest futures: Modelling the controls on tree species range limits and transient responses to climate change

Range limits and broad-scale geographic variations in the productivity of boreal and northern deciduous tree species in Europe are simulated with a bioclimatic model (STASH). STASH is based on a small number of distinct mechanisms by which climate is thought to affect the survival, regeneration and growth of trees. Survival is limited by summer warmth requirements and winter cold tolerance; regeneration, additionally by winter chilling requirements; and growth rate by net assimilation, which in turn is related to photosynthetically active radiation, growing-season length, temperature (relative to species-specific optima) and soil moisture supply (relative to evaporative demand). These mechanisms are quantified either as thresholds (for survival and regeneration) or multipliers (for growth), based on bioclimatic variables computed from monthly climate normals interpolated three-dimensionally to a 10-minute grid. Growing-season and growing degree day calculations take into account the effects of chilling. The drought calculations also take into account the effects of soil-moisture storage by means of a physically based evapotranspiration calculation coupled to a one-layer soil hydrology model. STASH is used to examine changes in potential range limits under a 2xCO₂ climate-change scenario. Associated transient responses at selected sites in the boreal and boreo-nemoral zones of Sweden are also simulated, using the forest gap model FORSKA2. The species-specific survival and regeneration constraints and growth responses of STASH modify the growth, establishment and mortality of trees in FORSKA2. The results obtained in this way differ sharply from the results of conventional forest gap models, where growth rates are assumed to decline to zero at minimum and maximum growing degree day limits. For example, towards the southern limit ofPicea abies (Norway spruce), STASH correctly shows no decline in productivity, but rather an abrupt cut-off corresponding to a chilling requirement during regeneration that is not met further south or west. In transient warming scenarios, this mechanism has the effect that natural regeneration can be blocked due to the warm winters even as yield is increasing due to the longer and warmer summers. STASH predicts drastic changes in species distributions in response to the large climate changes (especially winter warming) expected for northern Europe. Some of the common boreal species (e.g.,Picea abies; Pinus sylvestris; Alnus incana) are unlikely to survive in much of their present range, withdrawing to the far north. Other species already widespread may be able to occupy some of the few sites that are today unavailable to them (e.g.,Betula spp.;Corylus avellana). Other temperate deciduous species such asFagus sylvatica could have dramatic range expansions, potentially occupying large tracts of the present boreal zone. FORSKA2 transient simulations illustrate some of the possible routes towards different types of forest in a changed climate. Some sites in the north show little change in species composition, but sites towards the southern boundary of the boreal zone could develop a new suite of dominants. The degree of sensitivity of a particular site depends both on the climate change prediction and on the transient dynamics of the forest community. Many types of transient behaviour are shown to be possible. Coupled with uncertainties about the future role of dispersal and changes in disturbance rate, the complexity and variety of these transient responses imply a highly uncertain future for the north European boreal forests.     

Simulation of mixedwood management of aspen and white spruce in northeastern British Columbia

FORECAST, an ecosystem simulation model, was calibrated for aspen (Populus tremuloides Michx) and white spruce (Picea glauca (Moench) Voss) stands using data collected in the Boreal White and Black Spruce biogeoclimatic zone in northeastern British Columbia and published data. Simulations were undertaken to examine the effects of initial density of aspen on yield of white spruce in an aspen and spruce mixedwood stand, and to compare the predicted stemwood biomass yields of aspen, white spruce and mixedwood stands. Results of the simulations suggest that mixedwood management regimes on the same medium quality site should have higher stemwood yield compared to pure white spruce stand. Simulated stemwood biomass yield of pure aspen stands over 240 years on medium site varied from 682.5 Mg ha^?1 to 239.1 Mg ha^?1 for different rotation lengths (30 to 120 years). Repeated rotations of monoculture white spruce produced much less stemwood biomass, simulated yields over 240 years ranging from 877.3 Mg ha^?1to 248.4 Mg ha^?1 for rotation lengths of 60 to 240 years. Simulated aspen and white spruce mixedwood stands produced higher stemwood biomass yields than the pure white spruce stands, but less than the pure aspen stands; from 217.4 Mg ha^?1 to 292.8 Mg ha^?1 over 240 years. Variations in initial densities of aspen did not affect spruce stemwood biomass yield over the simulation period. This model shows potential for comparing the relative effects of different management strategies on harvestable volume and variety of other ecosystem variables. A calibrated version of the model should be useful as both a management simulator and a research tool. However, shortcomings in the representation of the canopy architecture of mixed species stands suggested the need to develop an individual tree version of this ecosystem management model for application to mixed species stands.     

Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction

The 1995/1996 reintroduction of gray wolves (Canis lupus) into Yellowstone National Park after a 70 year absence has allowed for studies of tri-trophic cascades involving wolves, elk (Cervus elaphus), and plant species such as aspen (Populus tremuloides), cottonwoods (Populus spp.), and willows (Salix spp.). To investigate the status of this cascade, in September of 2010 we repeated an earlier survey of aspen and measured browsing and heights of young aspen in 97 stands along four streams in the Lamar River catchment of the park’s northern winter range. We found that browsing on the five tallest young aspen in each stand decreased from 100% of all measured leaders in 1998 to means of <25% in the uplands and <20% in riparian areas by 2010. Correspondingly, aspen recruitment (i.e., growth of seedlings/sprouts above the browse level of ungulates) increased as browsing decreased over time in these same stands. We repeated earlier inventories of cottonwoods and found that recruitment had also increased in recent years. We also synthesized studies on trophic cascades published during the first 15 years after wolf reintroduction. Synthesis results generally indicate that the reintroduction of wolves restored a trophic cascade with woody browse species growing taller and canopy cover increasing in some, but not all places. After wolf reintroduction, elk populations decreased, but both beaver (Caster canadensis) and bison (Bison bison) numbers increased, possibly due to the increase in available woody plants and herbaceous forage resulting from less competition with elk. Trophic cascades research during the first 15 years after wolf reintroduction indicated substantial initial effects on both plants and animals, but northern Yellowstone still appears to be in the early stages of ecosystem recovery. In ecosystems where wolves have been displaced or locally extirpated, their reintroduction may represent a particularly effective approach for passive restoration.