Knut Rydgren

Disturbance severity and community resilience in a boreal forest

We studied the resilience of southeastern Norwegian old-growth Picea abies forest floor vegetation to experimental disturbance. Five treatments, differing in depth of removal of vegetation and soil layers and making up a gradient in disturbance severity, and three controls, were replicated 10 times. The experiment was analyzed with respect to the full species composition before and for seven years after treatment. The soil-buried propagule bank and local environmental conditions were recorded before treatment. Total cover of vascular plants and bryophytes and lichens increased slowly after treatment and was still below pre-disturbance levels after seven years. The rate of succession, measured as change in floristic dissimilarity between recordings made in successive years, declined with time for all treatments. The magnitude of vegetation change was strongly influenced by disturbance severity. DCA ordination revealed a main gradient in species composition from undisturbed forest floor to severely disturbed vegetation three years after disturbance, while in the fourth year, the direction of vegetation change turned in the direction of pre- disturbance positions. The turning point represented the maximum abundance of pioneer species (e.g., Luzula pilosa and Pohlia nutans) relative to dominant species before the disturbance (e.g., Dicranum majus and Hylocomium splendens). The return to pre-distur- bance positions from the fourth year was, however, slow and will probably take 5-25 more years to be completed if current trends continue. DCA ordination revealed two additional, interpretable, gradients in vegetation; one related to pulses of regeneration from the soil- buried propagule bank; the other represented a gradient in pre-disturbance environmental conditions. We demonstrate that one vegetation gradient related to time after disturbance is insufficient to account for the full complexity of revegetation processes following dis- turbance.

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 points 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.

Scale-dependence of vegetation-environment relationships in semi-natural grasslands

Abstract Questions: Which environmental and management factors determine plant species composition in semi-natural grasslands within a local study area? Are vegetation and explanatory factors scale-dependent? Location: Semi-natural grasslands in Lærdal, Sogn og Fjordane County, western Norway. Methods: We recorded plant species composition and explanatory variables in six grassland sites using a hierarchically nested sampling design with three levels: plots randomly placed within blocks selected within sites. We evaluated vegetation-environment relationships at all three levels by means of DCA ordination and split-plot GLM analyses. Results: The most important complex gradient determining variation in grassland species composition showed a broad-scale relationship with management. Soil moisture conditions were related to vegetation variation on block scale, whereas element concentrations in the soil were significantly related to variation in species composition on all spatial scales. Our results show that vegetation-environment relationships are dependent on the scale of observation. We suggest that scale-related (and therefore methodological) issues may explain the wide range of vegetation-environment relationships reported in the literature, for semi-natural grassland in particular but also for other ecosystems. Conclusions: Interpretation of the variation in species composition of semi-natural grasslands requires consideration of the spatial scales on which important environmental variables vary. Nomenclature: Lid & Lid (1994) except for Betula pubescens agg. (which includes B. pubescens and B. verrucosa) and Hieracium spp. (which includes all Hieracium species except H.umbellatum).

Land-cover and structural changes in a western Norwegian cultural landscape since 1865, based on an old cadastral map and a field survey

Many studies of land-cover and structural changes in cultural landscapes have used historical maps as a source for information about past land-cover. All transformations of historical maps onto modern coordinate systems are however burdened with difficulties when it comes to accuracy. We show that a detailed land survey of the present landscape may enable transformation of an old cadastral map directly onto the present terrain with very high accuracy. The detailed resulting map enabled us to locate remnants of semi-natural grasslands and man-made structures with continuity from 1865 and to test hypotheses about relationships between landscape changes and landscape characteristics. The main land-cover change 1865–2002 was decrease of arable fields, and addition of three new land-cover classes: horticultural, orchard and abandoned areas. Of the 330 man-made structures present in 1865, only 58 remained in 2002, while 63 new structures had been built after 1865. We found that semi-natural grasslands with continuity since 1865 were situated on ground with significantly lower production capacity than mean 1865 production capacity. The man-made structures with continuity since 1865 were also associated with areas with significantly lower production capacity than the 1865 mean, situated in significantly steeper terrain but not further from the hamlet. Our study illustrates the potential of digitised and accurately transformed historical cadastral maps combined with detailed field surveys for analysis of land-cover and structural changes in the cultural landscape.

PLANT SPECIES COMPOSITION OF BOREAL SPRUCE SWAMP FORESTS: CLOSED DOORS AND WINDOWS OF OPPORTUNITY

Previous studies point to biogeographic (i.e., evolutionary and demographic) and ecological (i.e., habitat differentiation and disturbance) processes as the most important causes of spatial variation in species richness and species composition (occurrence and abundance). We examined patterns of variation in vascular plant and bryophyte species composition among 150 1-m2 plots distributed semi-randomly over 11 Norwegian boreal swamp-forest localities. Swamp forests are species-rich islands in an otherwise species-poor forest landscape. For each plot, 53 environmental variables were recorded. By using Canonical Correspondence Analysis (CCA), we found that ∼20% of the explainable variation in species composition was due to swamp-forest affiliation, in addition to the ∼35% that was due to environmental differences between swamp-forest localities. The uniqueness of the species composition of each swamp forest was also emphasized by analyses of compositional dissimilarity. Plots were significantly more dissimilar if situated in different swamp forests than if situated in the same swamp forest, after environmental differences had been corrected for. The lack of any significant relationship between compositional dissimilarity and geographical distance or swamp-forest area indicated that this pattern was not mainly due to recent successful dispersal and establishment events. We argue that the distinctness of swamp forests, in particular, those richer in species and soil nutrients, is due to a combination of factors among which randomness in establishment in gaps (“windows of opportunity”) and persistence of established clonal species are important. Furthermore, we argue that the probability for successful recruitment may have been higher in previous time periods than it is today. The unique combination of important determinants of the species composition in boreal swamp forests supports the view that there exists a diversity of explanations for diversity, and that these, to a large extent, are system and/or area specific.

Species richness in boreal swamp forests of SE Norway: The role of surface microtopography

Abstract Question: What is the relative importance of environmental gradients and surface microtopography (variation in vertical level within sampling units) for fine-scale plant species richness in Picea abies swamp forests? Location: 11 swamp forests in SE Norway. Methods: We recorded species richness (number of species of vascular plants, mosses, Sphagnum and hepatics), depth to water table, soil base status and vertical range (microtopographic relief) in 2400 microplots, (each 1/16 m2), in 150 1-m2 plots in the 11 swamp forests. Relationships between species richness and environmental predictors were modelled by GLMM. Results: Moss and hepatic species richness increased with increasing microtopographic relief, most strongly for wet acid sites, in which similar trends were also found for Sphagnum. Relief was a poor predictor of vascular plant species richness. Conclusions: Microtopographic relief is a good predictor of local species richness in Picea abies swamp forests, partly because larger vertical variability means higher within-plot habitat diversity with respect to the wet-dry gradient, and partly because qualitatively new microhabitats associated with steep slopes are added in drier sites. The relationship between species richness and microtopographic relief is context dependent, differing in complex ways among species groups and among sites with different environmental conditions.

Road Verges: Potential Refuges for Declining Grassland Species Despite Remnant Vegetation Dynamics

Whether road verge vegetation can be manipulated to resemble traditionally managed grassland has been much debated. This short-term study compared management effects on road verge and pasture vegetation in western Norway. We quantified vegetation change and explored whether it occurred along underlying environmental gradients. We found management-related variation in species richness and vegetation physiognomy, but high resistance in species composition prevented directional changes in vegetation dynamics. Initial differences between the habitats indicated historical management effects on traditionally managed pastures and road verges. Given proper management, road verges may have a nature conservation potential. Moreover, their linear structure may enable fine-scale mosaic management that allows the coexistence of a wide range of grassland species.

Moss species benefits from breakdown of cyclic rodent dynamics in boreal forests

Bryophytes have increased in abundance in northern regions, and climate changes have been proposed to account for this change. However, changes in the population dynamics of microtine rodents may also contribute to changes in bryophyte abundance. New evidence indicates a tendency for microtine rodent population oscillations to change from periodicity of 3-5 years to become irregular or acyclic. The impact on ecosystem functioning is potentially great. We study the impact of variation in microtine rodent population characteristics, such as cycle length and amplitude, on the population dynamics of the boreal, clonal moss Hylocomium splendens. We use experimental and observational demographic data to construct 127 scenarios representing all combinations of disturbance type (gap formation and/or clipping), period (cyclic with 4, 6, 12, or 24 years between rodent peaks; or acyclic with constant or stochastically varying annual disturbance severity) and disturbance severity (fraction of individuals affected by disturbance in each year relative to the maximum disturbance carried out in the field experiment; seven levels). Population data collected in the field during 13 years were used as a baseline scenario. By subjecting all scenarios to stochastic matrix modeling, we demonstrate considerable impact of microtine rodent on the population dynamics of H. splendens, most notably when rodent populations fluctuate with short periods and high peak disturbance severities. Under the same average disturbance severity, H. splendens population growth rates are highest in acyclic scenarios and are progressively reduced with increasing peak disturbance severities (i.e., with increasing period). Stochastic elasticity analyses show that in less variable environments mature segment survival contributes more to the population growth rate, while in more variable environments the regeneration pathway (branching of older parts of the plant) plays a stronger role, inevitably leading to lower population fitness. Our results support the hypothesis that breakdown of cyclic rodent population dynamics accentuates increase in the abundance of H. splendens and other large bryophytes in boreal forests in Norway, observed empirically in recent years and primarily ascribed to climatic change.

Sporophyte production in the clonal moss Hylocomium splendens: the importance of shoot density

Abstract The importance of light intensity and shoot density for sporophyte production in the dioecious moss Hylocomium splendens was addressed by following populations subjected to two types of fine-scale disturbance treatments and an unmanipulated reference (“REF”) for 3 years. The two treatments were: reduction of the bryophyte layer to approximately 50% of its original density (“REMOVAL”) and clipping and removal of all H. splendens growing points (“CLIPPED”). Sporophyte production was low all 3 years in all populations except in the “REMOVAL” treatment the second year after disturbance, when 4.4% of the mature segments carried sporophytes (more than tenfold higher than in the reference or any other treatment or year). We interpret the high sporophyte production in the “REMOVAL” treatment as triggered by the enhanced radiation input brought about by the disturbance, conditional on two basic requirements being fulfilled: (1) that segments were large enough to produce sex organs and support sporophyte development, and (2) that the average distance between shoots of opposite sex was sufficiently low for transfer of spermatozoids from male plants to female plants to take place.

Management history affects grassland seed bank build-up

Disturbance in the form of different management regimes affects established vegetation, but how the same management affects the corresponding seed banks is poorly understood. We used the seedling emergence method to investigate how present and previous management intensity impacts the dynamics of established vegetation and corresponding seed bank in six semi-natural grasslands (three pastures and three road verges) in W Norway. The increased management intensity reduced seed bank species richness but increased seedling density and the fraction of species with a persistent seed bank. Higher intensity also increased the components’ floristic similarity, probably through formation of gaps where seeds may germinate. Moreover, the seed bank responded in parallel with the established vegetation to underlying environmental variables as well as to management intensity. Management intensity hence impacted directly on many aspects of seed bank—established vegetation relationships, and controlled established vegetation partly through seed bank dynamics involving both temporal and spatial dispersal.