Boreal Spruce Forest Research Articles

The effects of windthrow on forests at different spatial scales: a review

This paper aims at reviewing the ecological effects of windthrow and the processes by which the structure and composition of the boreal forest are affected. The windthrow problem has been investigated at different levels: landscape; forest community; and fallen tree ecosystem. All original data are from the natural, protected, uneven-aged boreal spruce forests of the central Russian Plain. Mapping of vegetation was used to detect windthrow processes in primary forest communities. The scales of windthrow gaps in space and time determine the patch structure of the forest ecosystems. The main result of this phenomenon is the occurrence of gap-phase dynamics in forest communities. The development of gaps is very important for the survival of small- and broad-leaved trees in boreal coniferous forests. In the case study of the scales of natural windthrow disturbances it was found that in uneven-aged spruce forests, 7–12% of the surface was covered by pit-and-mound topography. After widespread windthrow, this figure rised to 15–25%. In this context, a procedure for dating windthrow topography was developed, along with methods of diagnosing uprooting disturbances of the soil. Much attention was also paid to the structure and properties of soil where uprooting had occurred. It was found that the microsites which are found in pit-and-mound topography differ pedogenically. Furthermore, the nature and rate of recovery of the background soil profile after uprooting varied a lot, i.e. under the influence of shallow uprooting, the reconstruction of the background soil combination and processes was completed in 100–200 years. Where as in cases of major uprootings, it took more than a 200–300 year cycle. The role of tree uprooting in maintaining stable tree and grass population structures in forest communities was found very important. The spatial distribution of trees was associated with pit-and-mound topography in all types of forests. Furthermore, spruce regenerated better on mounds and fallen trees than on undisturbed surfaces. Multi-aged tree regrowth was therefore being constantly formed in virgin forests. There was also increase in the community floristic richness due to the establishment of certain pioneer species from other communities in microsites disturbed by uprooting. The gene pool of a population was rejuvenated as the plants appeared in these microsites from seeds and spores, while propagation in undisturbed areas was predominantly vegetative. As a result, the structure of forest plant populations was found to be in a stable multi-aged state.

Population biology of the clonal moss Hylocomium splendens in Norwegian boreal spruce forests. 5. Vertical dynamics of individual shoot segments

Demographic information was obtained for 12 528 mature segments (for which dry weight was estimated and vertical position in the bryophyte carpet recorded) and 3109 regenerated growing points for the perennial clonal moss Hylocomium splendens, recorded in Norwegian boreal spruce forests during a 6‐year period. Branching frequency varied with vertical position in the bryophyte carpet. Termination risk (probability of producing no offspring) was highest (44%) for buried segments, lowest (12%) for segments at intermediate vertical positions, and also high (26%) for emergent segments (due to increasing exposure to external mortality agents). Segment size increased from low levels in the bryophyte carpet to a maximum ca 2–10 mm below the top of the bryophyte carpet. This intermediate level was interpreted as the optimal compromise between incoming radiation (attenuating downwards) and microclimatic moisture conditions (improving downwards). Size‐corrected fitness, the number of offspring emerging from a mature segment within one year after maturation after allowance for differences in size, was lower for buried and emergent segments than for segments at intermediate positions. Small emergent segments were apparently liable to suffer from vitality reductions due to desiccation. The vertical position of a daughter segment depended on that of its parent segment, but also showed considerable stochastic variation. Burial acted as a strong sink for small segments regardless of vertical position. No evidence was found for species‐specific differences in the way pleurocarpous bryophytes interact, but reduced vertical mobility of H. splendens when growing among acrocarps indicated that growth‐form is an important determinant of bryophyte interactions. Evidence was found for vertical layering of the bryophyte carpet according to dominant type of interactions among individuals: none (environmental stress) above and at top, facilitation [a (+, +) interaction] at intermediate levels because of favourable water relationships in closed stands, and amensalism [a (0, −) interaction] from higher‐situated segments that deprive lower‐situated segments access to light at lower relative levels. The intensity of amensalism increased downwards in the bryophyte carpet as indicated by a reinforced size hierarchy. The tendency for small H. splendens segments to become buried and lost from the population by amensalism is likely to represent a general mechanism for interactions between bryophyte species and succession in bryophyte‐dominated stands. Population effects of climatic and local environmental factors (favourability vs stress), disturbance and apparently random events are discussed with reference to their impact on the relative sizes of subpopulations acting as sources (due to facilitation) and sinks (due to amensalism).

CO 2 exchange at the floor of a boreal forest

Net CO 2 exchange at the forest floor in a mixed boreal spruce and pine forest in central Sweden was studied during 1996. Forest floor CO 2 efflux was measured continuously by means of a ventilated open soil chamber, covering a surface area of 0.6 m 2. The chamber was transparent and thus measured soil respiration by night, and soil respiration reduced by photosynthetic uptake by forest floor vegetation by day. Maximum nocturnal efflux rates were 0.2–0.7 and daytime rates were 0.05–0.2 mg m −2 s −1. Measured efflux rates were higher than reported in other studies, but have earlier been found to agree with nocturnal CO 2 exchange of the forest ecosystem measured by eddy-covariance technique. Soil temperature at 5 cm explained 49% of the variation in nocturnal soil respiration, while moss and air temperature explained 29 and 17% of the variation, respectively. For the relationship with soil temperature at 5 cm, base respiration rate and effective Q 10, derived from data over the seasonal course, were 0.04 mg m −2 s −1 and 4.75, respectively. Corresponding figures for the relationship with air temperature were 0.11 mg m −2 s −1 and 1.89. Over the 6 months (May–October) covered by measurements, August had the largest CO 2 efflux, 0.89 kg m −2 and May the smallest efflux, 0.20 kg m −2. During daytime photosynthetic uptake by forest floor vegetation reduced potential soil efflux through respiration by ca. 20%. On an annual basis total forest floor respiration was estimated to be 4.5 kg CO 2 m −2 and gross photosynthesis to be 0.7 kg CO 2 m −2, resulting in a net efflux of 3.8 kg CO 2 m −2.

A landscape perspective of bird nest predation in a managed boreal black spruce forest

Several landscape level studies have reported that bird nest predation increases as forest cover decreases. These studies have mainly been conducted in agricultural or urban regions. However, few studies have explored relationships between forest cover and nest predation in boreal forests managed for timber harvesting. In 1997 and 1998, we evaluated bird nest predation in a mosaic of clearcuts and forest remnants dominated by black spruce (Picea mariana [Mill.] B.S.P.) and located north of Lake Saint-Jean, Québec. We used a 7 km × 9 km grid of sampling points to determine nest predation at four landscape scales (local vegetation, and 250 m, 500 m, and 1000 m radii around sampling points). Artificial nests (ground and arboreal) containing a common quail (Coturnix coturnix L.) egg and a plasticine egg were used to calculate predation pressure and to identify nest predators. Nest predation was high over the entire study area. Dominant predators were the gray jay (Perisoreus canadensis L.) and the red squirrel (Tamiasciurus hudsonicus Erxleben). Depredation by squirrels was influenced by local variables in 1997 and by landscape variables in 1998. In the latter case, depredation by squirrels increased as spruce cover increased. Depredation by gray jays was positively related to water body area and jack pine (Pinus banksiana Lamb.) cover. Squirrels preyed more on ground nests than on arboreal nests, while gray jays preyed almost exclusively on arboreal nests. We conclude that these predators probably impose different threats to different songbird species in boreal black spruce forests. Our results show that, in the short term, timber harvesting did not seem to increase predation in a boreal black spruce forest.

Single-Tree Influence on Understorey Vegetation in a Norwegian Boreal Spruce Forest

A model for tree influence on understorey vegetation in boreal spruce forests is developed according to the principles of ecological field theory. Single-tree influence at a point in space was modelled as the product of two factors: (1) the size (diameter at breast height) of the tree relative to that of the largest tree encountered in the area; a parameter specifies the weight given to large trees, and (2) the point's distance from the centre of the stem (one parameter specifies the zone of influence of a tree in crown radius units, another parameter specifies the relative weight given to closeness to, versus distance from, the tree stem). The total tree influence at the point is calculated from single-tree influence by a multiplicative model. Optimal choice of model parameters was found by maximizing the eigenvalue of a constrained ordination (CO) axis, obtained by use of the tree influence index as the only constraining variable. Two CO methods. Redundancy Analysis (RDA) and Canonical Correspondence Analysis (CCA), were applied to one vegetation data set with two subsets from a boreal spruce forest in SE Norway. The eigenvalue of CO axes constrained by the optimized tree influence index differed between the two CO methods, amounting to 9-10% of total inertia with RDA and 3% with CCA. The higher eigenvalue-to-total-inertia ratio with RDA was interpreted as due mainly to the low species turnover (low β-diversity) along the tree influence gradient, as demonstrated by the monotonic responses of a majority of species to tree influence. The linear species response model in RDA was therefore more appropriate in this case than the unimodal model of CCA. Vascular plants and cryptogamic species differed with respect to optimal parameters in the tree influence model. These differences were in accordance with throughfall precipitation and time in hydrated state as the most important determinants of cryptogamic species performance, and soil moisture and incoming radiation as the most important factors for vascular plants. Some applications of the tree influence model are proposed.

Emission of isoprene and carbonyl compounds from a boreal forest and wetland in Sweden

Emission measurements of light hydrocarbons, light carbonyl compounds, and monoterpenes have been made with canister, DNPH, and Tenax samples, respectively, on Norway spruce (Picea abies), Scots pine (Pinus sylvestris), the floor of a mixed pine and spruce forest, and a Sphagnum fen at the NOPEX site (Northern hemisphere climate Processes and land-surface Experiment) in the southern boreal zone of Sweden, in 1995. The branch and ground measurements were made with the flow-through enclosure technique and static chamber technique, respectively. Norway spruce was found to emit significant amounts of isoprene, the normalised emission rate (30°C, 1000μmolm−2s−1) averaging 24±18nmolgdw−1h−1 (1.4μgCgdw−1h−1), and of carbonyls with normalised rates ranging from 10 to 150nmolgdw−1h−1 (0.3–4.6μgCgdw−1h−1). Acetone/acrolein and acetaldehyde dominated the carbonyl flux with 61 and 27%, respectively. The normalised monoterpene emission rate (30°C) varied from 17 to 60nmolgdw−1h−1 (2–7μgCgdw−1h−1), with α-pinene accounting for 34% of the flux. The emission from Scots pine included only traces of isoprene, while the emission rate of acetone/acrolein was comparable to that of the monoterpenes. The BVOC flux from the forest floor made up only a few percent of the total forest flux and included ethene and propane at several tens of nmolm−2h−1, and the monoterpenes at rates reaching 380nmolm−2h−1 (50μgCm−2h−1), dominated by α-pinene. A Sphagnum fen emitted isoprene at rates fully comparable to the areal flux of isoprene from the boreal spruce forest. Highest emission rates were observed from the low and wet micro-sites, as compared to the higher and drier hummocks. The average flux in June was 912±750nmolm−2h−1 (55±45μgCm−2h−1) and in August 6800±4000nmolm−2h−1 (408±240μgCm−2h−1). Monoterpene fluxes were 160±80nmolm−2h−1 (19±9μgCm−2h−1) in June and 760±480nmolm−2h−1 (90±60μgCm−2h−1) in August. Isoprene from Norway spruce and Sphagnum wetlands, as well as acetone/acrolein from Norway spruce and Scots pine, are shown here to be important components of the boreal emission of BVOC. More diurnal and seasonal data is needed to correctly evaluate the seasonal flux.

Fine woody debris is important for species richness on logs in managed boreal spruce forests of northern Sweden

Species richness of wood-inhabiting cryptogams was compared on fine woody debris (FWD; diameter 5-9 cm) and coarse woody debris (CWD; diameter [Formula: see text]10 cm) logs of Picea abies (L.) Karst. Data were collected from managed boreal forests of northern Sweden. Species richness was higher on CWD than FWD when equal numbers of logs were compared. When equal surface areas were compared, species richness on FWD and CWD did not differ. When equal volumes of CWD and FWD were compared, FWD had more species. This was an effect of surface area and log quantity, as a certain volume of FWD was composed of more logs and had greater surface area than the same volume of CWD. We varied woody debris volume and proportion of FWD in a simulation model of species richness on logs. At low volumes of woody debris, species richness increased with the proportion of FWD. At higher log volumes, species richness increased with the proportion of CWD. In managed forests where there are small amounts of woody debris, FWD is important for the total species richness on logs and should not be neglected despite the fact that species richness is generally positively correlated with log diameter.

Controls on Evaporation in a Boreal Spruce Forest

The surface energy balance over a boreal spruce forest is analyzed using 3 yr of 30-min-averaged data collected during the 1994–96 Boreal Ecosystem–Atmosphere Study experiment 40 km west of Thompson, Manitoba, to show the climatic controls on surface evapotranspiration. The seasonal variation of evaporation is shown: lowest in spring when the ground is frozen, highest in summer (although daytime evaporative fractions are only 0.4), and lower again in fall after frost. The surface sensible heat flux in contrast is high in spring, when evaporation is low. Evaporation is much higher when the surface, including the moss layer, is wet. At all temperatures (in summer), evaporative fraction falls with increasing light level, because of the high vegetative resistance of the forest system. Using a Monin–Obukhov formulation and a bulk vegetation model, the vegetative resistance for the boreal spruce forest system is calculated. This bulk vegetative resistance decreases with increasing photosynthetic radiation, decreases sharply with relative humidity, decreases with increasing surface water storage, and is lower on cloudy days than on sunny days with the same incoming photosynthetic radiation. Vegetative resistance at its midmorning minimum is lower by a factor of 4 when the moss surface is very wet. As over grassland sites, the lower surface resistance to evaporation directly influences the diurnal cycle of lifting condensation level and cloud-base height, which are much lower on days with a wet surface. The reduction of vegetative resistance under cloudy skies at the same incoming radiation level presumably reflects the more efficient use of diffuse radiation by the canopy for photosynthesis. Vegetative resistance is roughly doubled in spring, when the ground is frozen, and is higher in fall after frost. About 63% of the observed variance in vegetative resistance can be explained in terms of meteorological variables using multiple linear regression. Some measurement issues are addressed in an appendix. The residual in the energy balance falls with increasing wind speed, which may be due to a small (10%–15%) underestimation of the sensible and latent heat fluxes at low wind speeds. During spring melt, however, this residual has a high daytime value of 30% of net radiation. The residual is also much higher on sunny days than on cloudy days.

Seasonal dynamics of biomass increase and shoot elongation in five co-occurring boreal forest bryophytes

The seasonal patterns of length and biomass increase in five co-occurring bryophytes (Dicranum majus, Hylocomium splendens, Plagiochila asplenioides, Ptilium crista-castrensis and Rhytidiadelphus loreus), were studied throughout one growing season in a boreal spruce forest by growing shoots in experimental assemblages at two densities at consecutive four-week intervals. Seasonal trends in the mean relative growth rate (RGR) were found, with highest RGR in spring and early summer (0.0025-0.005 gg-1 day-1) and a low or negative RGR in autumn. This temporal variation could best be explained by the variation in precipitation, but interactions with other environmental factors were important, especially in spring and autumn. The five species responded differently to the environmental conditions, with the result that their relative rank in RGR changed from period to period. However, the effect of density on RGR varied between periods, indicating that the impact of macroenvironmental conditions depends on shoot density. Mean rate of length increase (LGR) peaked in early summer (0.12-0.20 mm day-1) with low length increases in spring and late in the season. LGR varied among species, showing differences in both maximum length growth rate and temporal patterns of growth. Length increase did not follow the temporal patterns of biomass increase, and an evident uncoupling of biomass and length increase was found in all species. The cumulative seasonal increase in length and biomass varied among species with a twofold and ninefold variation among species in cumulative length and biomass increase respectively. Further, an effect of shoot density on cumulative biomass increase was found, with the higher biomass increase at the lower shoot density.

Fine woody debris is important for species richness on logs in managed boreal spruce forests of northern Sweden

Species richness of wood-inhabiting cryptogams was compared on fine woody debris (FWD; diameter 5-9 cm) and coarse woody debris (CWD; diameter 10 cm) logs of Picea abies (L.) Karst. Data were colle...

Dissolved Organic Matter (DOM) in Acid Forest Soils at Gådsjön (Sweden): Natural Variabilities and Effects of Increased Input of Nitrogen and of Reversal of Acidification

Effects of (1) the addition of 35 kg N ha-1yr-1(as NH4NO3) and (2) interception of throughfall and litterfall by means of a roof on concentrations, chemical characteristics and dynamics of dissolved organic matter (DOM) in acid forest soils (podzols) were studied at Gardsjon, Sweden. A non-manipulated catchment served as a reference. After 4 yr of treatment no significant effects of either manipulation were found on concentrations and characteristics of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). The variability of these parameters within catchments appeared to be of the same order of magnitude as the variability between catchments. At all sites, DON contributed more than 70% of the total dissolved nitrogen. In general, the proportion of hydrophilic DOC increased with depth. In contrast, to other studies the C/N ratio of DOM at Gardsjon did not show a clear pattern with depth. Other studies in SE U.S.A. have suggested that relatively low C/N ratios at depth are due to increased contribution of hydrophilic neutrals to DOC. However, this fraction appears to be small in temperate and boreal spruce forest soils, including Gardsjon.

Exploring potential biodiversity indicators in boreal forests

The present study evaluates indicators in Swedish spruce forests. We ask whether different species groups co-vary in their occurrence and to what extent species richness and composition is predictable from habitat structures. We studied 10 boreal spruce forest stands constituting a gradient in degree of selective logging. Occurrences of vascular plants, bryophytes, epiphytic lichens and wood-inhabiting fungi as well as habitat structures was inventoried. In addition, in five of the stands, beetles were sampled with windows traps. Total species richness was correlated with several habitat factors, mainly particular substrates and degree of forestry impact. However, the richness of a set of species regularly used as indicators did not correlate with habitat factors. Correlation in species richness among different organism groups were few and scale dependent. Only lichens and vascular plants formed nested subset patterns (i.e. species composition at poorer sites is subsets of the species present at richer sites) among the study sites. The study shows that in this forest type one cannot a priori assume that richness in one group of species correlated with richness in other, and measures of single habitat features may be relevant only to particular groups of species. Instead, monitoring and inventories should be based on a set of factors reflecting important aspects for different groups of organisms and if indicator species are to be used these should be chosen from several species groups.

Population Biology of the Clonal Moss Hylocomium splendens in Norwegian Boreal Spruce Forests. 4. Effects of Experimental Fine-Scale Disturbance

We studied demographic effects of density reduction by fine-scale disturhance on the perennial clonal moss species Hylocomium splendens in a field experiment in an SE Norwegian boreal forest. Half the bryophyte cover was removed from 10 plots in a fine-scaled pattern. Size of ca 800 mature segments (estimated from morphological field measurements) and demographic changes in this experimental population and an unmanipulated reference population were followed from 1993 to 1995. The mean size of mature segments decreased in both populations during the study, due to an extended drought period in 1994. The number of mature segments in the unmanipulated population remained almost unchanged. Branching rates and population growth rates were significantly higher in the experimental population. Rates of ramification were higher than in the unmanipulated population the first year after treatment, and rates of regeneration from fragments (in the gaps) and intact shoot chains were higher both in the first and second year after treatment. We consider enhanced light penetration deeper into the moss carpet as the main cause of the observed differences between the populations. The number of Hylocomium splendens growing points in the experimental population reached 68% of the pre-treatment number in two years. Mean segment size decreased more strongly in the experimental than in the unmanipulated population. Further size reductions are expected. because segments produced by regeneration are smaller than a population's average segment size. Our study demonstrates that the resilience of a population after disturbance depends on the population's current size structure, the frequency and severity of disturbance. and environmental conditions. Experimental disturbance also influenced sporophyte production: in 1995 the frequency of sporophytes was more than ten times as high in the experimental population than in the unmanipulated reference population. Stimulation of fertile branch production (and gametangia) by enhanced radiation, and higher probability of successful fertilization, e.g., due to amelioration of the microclimate, may be involved. Like ramification frequency, sporophyte production in Hylocomium splendens is positively dependent on the size of the source segment, and a well-defined reproductive threshold, a minimum resource (size) level for successful reproduction, seems to occur.

Reproductive phenology of Dicranum majus in central Norway

Shoots of Dicranum majus were collected monthly during the snow-free period from permanent plots to study the reproductive phenology in a boreal spruce forest in central Norway. Numbers of sporophytes (current and previous years), gametangia and dwarf males on each shoot and the reproductive stage of gametangia and sporophytes were registered. Gametangia are formed either in late autumn or in early spring, fertilization occurs in June and July and spores are dispersed during August to October in the following year.Mature archegonia were present only during a restricted period (June-July) while mature antheridia were found throughout the summer and autumn.Dwarf males were found only in a region about one cm below the apex. There was no trace of old male gametophores below, indicating that gametophores are annual. However, from this study it is not possible to conclude whether the gametophyte is annual or new males are formed through, for instance, perennial protonema.Fertilization frequency in the samples was high (75%), but the fertilization frequency of the entire population is lower as the samples were taken from subpopulations that produce sporophytes frequently.

A four-scale bidirectional reflectance model based on canopy architecture

Open boreal forests present a challenge in understanding remote sensing signals acquired with various solar and view geometries. Much research is needed to improve our ability to model the bidirectional reflectance distribution (BRD) for retrieving the surface information using measurements at a few angles. The geometric-optical bidirectional reflectance model presented in this paper considers four scales of canopy architecture: tree groups, tree crowns, branches and shoots. It differs from the Li-Strahler's model in the following respects: 1) the assumption of random spatial distribution of trees is replated by the Neyman distribution which is able to model the patchiness or clumpiness of a forest stand; 2) the multiple mutual shadowing effect between tree crowns is considered using a negative binomial and the Neyman distribution theory; 3) the effect of the sunlit background is modeled using a canopy gap size distribution function that affects the magnitude and width of the hotspot; 4) the branch architecture affecting the directional reflectance is simulated using a simple angular radiation penetration function; and 5) the tree crown surface is treated as a complex surface with microscale structures which themselves generate mutual shadows and a hotspot. All these scales of canopy architecture are shown to have effects on the directional distribution of the reflected radiance from conifer forests. The model results compare well with a data set from a boreal spruce forest.

Do shrews have an impact on soil invertebrates in Eurasian forests?

:Abundance and food consumption of shrews in the European part of the southern taiga (Valdai district, Russia) were investigated. Average population densities of these animals were estimated 31/ha in dry boreal spruce forest, 179 in boreal Picea abies – Oxalis acetosella forest, and 224–258 in rich spruce forest. Food consumption in those habitats was estimated at 25 kg/ha, 152 kg/ha, and 212–244 kg/ha per year, respectively. These data are similar to those for western Siberian taiga (132–264 kg/ha), mixed forest in the Russian Far-East (177–351 kg/ha), and larch forests in north-east Siberia (265–330 kg/ha). These figures also correspond with the biomass of soil invertebrates in the above ecosystems. This provides evidence that shrews have a significant impact on litter decay processes. In particular, shrews diminish the population density of soil invertebrates thus increasing their productivity. Such a response to harvesting is well-known for many populations, from microorganisms to ungulates. Red-toothed shrews do not significantly change the density of soil invertebrates during summer periods, but they probably accelerate decomposition of forest litter and organic matter in the upper soil layers.

Effect of forestry on the abundance and diversity of arboreal spiders in the boreal spruce forest

Effect of forestry on the abundance and diversity of arboreal spiders in the boreal spruce forest

SPIROGRAPHA VINOSA, A NEW ODONTOTREMOID FUNGUS ON OCHROLECHIAAND PERTUSARIA

Spirographa vinosaHolien & Triebel is described from boreal spruce forest of Norway. It is lichenicolous on different species of Ochrolechiaand Pertusaria. Within the genus S. vinosais characterized by its 8-spored asci, up to 5-septate ascospores with obtuse ends and its apothecial pigments causing aeruginose and purplish reactions with KOH.

Vegetation-environment relationships of boreal spruce forests in ten monitoring reference areas in Norway

AbstractVegetational and environmental monitoring of boreal spruce forest was initiated in 1988, as a part of the programme “Contrywide Monitoring of Forest Health” at the Norwegian Institute of Land Inventory (NIJOS). As a basis for monitoring, relationships between trees, understory vegetation and environmental conditions (vertical relationships) were analysed for each of ten reference areas. The reference areas were selected to span regional gradients, in climatic conditions and deposition of airborne pollutants, in old-growth, so-called “bilberry-dominated”, “small-fem” and “low-herb”, also paludified, spruce forests south of the Polar Circle. Fifty 1-m2meso sample plots, randomly chosen within ten 50-m2macro sample plots in each reference area, were subjected to vegetation analysis, using frequency in subplots as species abundance measure. Environmental (including soil chemical) and tree parameters were recorded for meso as well as macro sample plots.The main vegetational gradients were found by parallel use of DCA and LNMDS ordination methods and subjected to environmental interpretation, mainly by means of non-parametric correlation analyses. DCA and LNMDS in most cases revealed the same main gradients in vegetation, but outliers were more frequent in LNMDS ordinations, due to higher vulnerability of this method to plots with deviating number of species. A complex-gradient in nutrient conditions, with pH and the concentration of nitrogen as the most constantly contributing variables, but with considerable between-area variation with respect to important cations, was evident in nine reference areas. Soil moisture varied along the second vegetational gradient in most areas. In the three most humid reference areas, the Ca concentration was related to variation in soil moisture and gradients from below to between trees, while unrelated or inversely related to the same vegetational gradient as pH. Species abundances were plotted on plot positions in DCA ordinations in order to summarize the species· responses to environmental variation in each area.Variation in vegetation in the total data set (500 meso sample plots) was partitioned onto two sets of explantory variables (environmental and climatic/geographical) by use of CCA, in order to find the relative importance of environmental and climatic/geographical variation. The fraction of variation exclusively explained by environmental variables was about 1 7%, while only 5% of the variation was explained exclusively by climatic variables. The variation shared by both sets of variables was about 8%.The main vegetational gradients and environmental/climatic/geographical complex-gradients in the total data set were found by DCA and subsequent interpretation of axes. The main complex-gradients found by separate analyses of data from each reference area, were reflected along the DCA axes in total ordinations, but differences between areas with respect to positions along both environmental and climatic/geographical gradients were also evident.Meso plot occurrences of selected species were plotted in a DCA ordination of the total data set, with variation exclusively due to climatic/geographical variables removed, in order to express regional similarities and differences in the species· responses to the environment. The different patterns of species· distributions in the DCA ordination were discussed in the light of their use as indicators of specified environmental conditions.

Population Biology of the Clonal Moss Hylocomium Splendens in Norwegian Boreal Spruce Forests. II. Effects of Density

1 Hylocomium splendens is a perennial clonal moss in which new modules may emerge annually by ramification of the current mature segment or by regeneration, the time-delayed development from dormant buds. Demographic information on 14 776 growing points was obtained by following 119 Norwegian boreal spruce forest populations for 5 years. The dry weight of each mature segment was estimated from morphological field measurements. 2 The mean size of mature segments was positively correlated with the density (cover) of the bryophyte layer; a density increase from 0 to 100% was associated with a doubling in mean size. This relationship is attributed to the improved moisture conditions, and hence to prolonged periods with positive net photosynthesis, in dense stands. Mean size was not significantly related to the density of Hylocomium splendens growing points, suggesting that interactions in bryophyte stands are mainly with neighbours regardless of species. 3 The frequency of regeneration declined with increasing density, notably when the effects of segment size were corrected for. Negative density-dependence of regeneration is considered to be due to reduction of light-induced bud initiation in dense stands. 4 The size-corrected mortality (termination and loss) rate was not related to density. Danger of burial in the moss carpet, causing about one tenth of all terminations and losses, increased strongly with density and affected c. 5% of all growing points at high densities. 5 At high densities, Hylocomium splendens populations are not regulated by a negative relationship between size and density, but by a combination of reduced branching (ramification and regeneration) and increased danger of burial in the bryophyte carpet. However, such high densities are infrequent in the forest floor (affecting less than 10% of the area investigated). It is suggested that differences in history due to finescale disturbance and microenvironmental variation are reflected in size differences of individual Hylocomium splendens segments, and that this is the major factor that keeps boreal forest bryophyte populations at low densities.