Bård Pedersen

TESTING FOR POSITIVE DENSITY‐DEPENDENT PERFORMANCE IN FOUR BRYOPHYTE SPECIES

The impact of environmental conditions on the relative importance of competitive and facilitative interactions and their responses to shoot density was studied in four boreal forest bryophytes. Increased density is believed to have both a positive effect, by increasing water retention, and a negative effect, by reducing light availability. Mortality of shoot apices, offshoot production, and rates of biomass increase of both individual shoots and whole colonies were recorded in experimental monospecific, known-density colonies of Dicranum majus, Plagiochila asplenioides, Ptilium crista-castrensis, and Rhytidiadelphus loreus under controlled relative humidity (four levels) and irradiance (two levels). A phenomenological model was developed, and predictions were made about the responses to increased density under different humidity and resource (light) levels. Mortality was low, but offshoot production generally decreased with density. The effects of density on growth rates varied among species and environmental conditions. As predicted, in dry environments under the high light level, growth rates of D. majus and R. loreus peaked at an intermediate density at which the positive effects of a close packing of shoots balanced the increased competition for light. In humid or dark environments, the relationships between growth rates and density were mostly monotonic and negative. Growth rates of P. crista-castrensis decreased with density under most environmental conditions, whereas density was not shown to have an effect on growth rates of P. asplenioides. The results show that relatively high light levels, compared to those found in their natural spruce forest habitat, are necessary for aggregation to facilitate growth of these bryophytes. Thus, our results indicate that competition for light, rather than a low water availability, limits the performance of individual shoots in colonies of these bryophytes under a wide range of densities and humidity conditions.

Hierarchical selection and fitness in modular and clonal organisms

Modular growth generates at least three kinds of hierarchies: morphological, functional, and demographic. The morphological hierarchy corresponds to the phenotypic characters of both the units (modules) that are repeated by developmental processes, and the units (organisms, colonies, and clones) that develop by iteration, specialization and integration of modules. The functional hierarchy concerns the levels of interaction or, in evolutionary terms, the functional relationships between fitness and the phenotypic characters at different levels of modular organization. Finally, clonal growth and reproduction results in a nested hierarchy of demographic units that are replicated by asexual propagation. Each level of the demographic hierarchy that is characterized by specific birth and death rate, is a potential candidate for evaluating fitness. We propose a formal approach in order to analyze the hierarchical structure of phenotypic selection in modular organisms, and to evaluate the selective importance of various levels of modular organization. We derive a measure of selective importance from the sensitivity of fitness to a unit change in the characters of a given level: the sum of squared sensitivities associated with that level. We propose that clonal-level characters of disintegrated clones will make small contributions to the variation in fitness, while such characters will be more important if the clone is physically and physiologically integrated. Moreover, we present a decomposition of fitness variation in relation to the levels of trait variation. This decomposition demonstrates that the levels where variation in fitness is observed do not always correspond to the interaction-levels at which the causal agents of selection are acting on particular traits. Following the logic of phenotypic optimization models, we consider three examples of selection in order to examine whether the different demographic levels are equally suitable for evaluating fitness. In two examples of density-independent selection we show that the Malthusian parameter is identical at all levels in the hierarchy. However, the third example shows that this result is not valid in density-dependent selection models. The way density-dependent regulation is supposed to operate in the model system determines which of the demographic levels should be used to evaluate fitness. Consequently, there is no fundamental demographic level that should a priori be chosen when measuring fitness

Habitat-specific genetic effects on growth rate and morphology across pH and water-level gradients within a population of the moss Sphagnum angustifolium (Sphagnaceae).

To study genetic adaptations in bryophytes on small ecological and spatial scales and to assess the adaptive significance of morphological trait variation, genotypes of Sphagnum angustifolium originating from habitats characterized by different pH and height above water table were clonally propagated and grown along the same gradients that exist in the field. Clones from ombrotrophic habitats grew consistently better ombrotrophically than clones from minerotrophic habitats and vice versa, suggesting that the genotypes were adapted to different pH levels. Genetic variation was found in several morphological traits, but habitat-specific genetic effects were detected only in length of spreading branches. Covariation between morphology and growth was generally environmentally induced. Positive and negative cross-environment genetic correlations suggested the presence of constraints on adaptive reaction norm evolution. The indications of small-scale genetic adaptations suggest either selective establishment of genotypes adapted to specific habitats, strong selective forces operating at the later stages of the life cycle, restricted gene flow over short distances, or a combination of these. In contrast to prevailing views, these results indicate that bryophytes are likely to respond genetically to small-scale environmental gradients.

The role of moose Alces alces L. in boreal forest – the effect on ground beetles (Coleoptera, Carabidae) abundance and diversity

The indirect effect of moose Alces alces browsing on ground beetles abundance and diversity was investigated by pitfall trapping in a mixed coniferous forest in Vestfold County (59°19′ N, 9°50′ E, Norway), during the summer of 2002. Three areas with different browsing pressures, ranging from non- to medium- and heavily browsed were chosen and dry weight of bilberry Vaccinium myrtillus and humidity at ground level were measured in the three locations. As predicted, the gradient analyses showed that browsing by moose influenced the composition of carabid fauna, and that browsing intensity and humidity covaried with the most important gradient in carabid species composition found across the three locations. Species that live in light stands with rather dry soil, were more often captured in the browsed areas, whereas shade tolerant and hygrophilous species were more abundant in the non-browsed area. The carabid abundance increased significantly with increasing browsing pressure. According to our predictions, the diversity at trap level (α-diversity) was higher in the highly browsed area. Conversely, species turnover (β-diversity) decreased with browsing intensity. On the other hand, the rarefaction analysis showed that the regional species richness (γ-diversity) was considerably higher in the medium browsed area than in the heavily browsed one, which is consistent with the intermediate disturbance hypothesis. This study indicates that moose, by reducing the bilberry that constitutes the field layer in summer, affect carabid species composition and might be capable to reshape the whole ecosystem in our study area by a cascade effect.

Studies of fertility of Dicranum majus in two populations with contrasted sporophyte production

Abstract Some populations of the dioicous moss Dicranum majus Sm. in southern Norway do not or only rarely produce sporophytes, while most populations in central Norway produce sporophytes frequently. Two populations of D. majus in spruce forest, one from southern Norway (without sporophytes) and one from central Norway (with many sporophytes) were investigated to find differences in sexual reproduction between them. The population in southern Norway had only a few perichaetia and neither dwarf males nor sporophytes, while perichaetia, males and sporophytes were abundant in the population from central Norway. Shoots with dwarf males formed perichaetia more often than shoots without males, and the proportion of fertilized archegonia increased with increasing numbers of males on the shoots. Genetic differences or some earlier environmental ‘disaster’ related to extreme drought or heavy pollution loads are discussed as possible explanations for the sparse sporophyte production in southern Norway. In order for sporophytes to be produced in the population in southern Norway, new males must be established from spores from outside.

Seasonal and interannual dietary variation during winter in female Svalbard reindeer (Rangifer tarandus platyrhynchus).

Abstract We investigated dietary variation during three winters in female Svalbard reindeer (Rangifer tarandus platyrhynchus) on the basis of rumen samples from animals culled in early and late winter during 2000–2002. The winter diet was compared to a late summer diet on the basis of samples from 1999, with variations in dietary composition in early and late winter among years also investigated. We conducted our assessment by identifying plant fragments in rumen samples using a point identification method. The effects of season and year on dietary composition of females were tested using non-parametric multifactorial MANOVA. During early winter, the diet seemed to be a function of both plant availability and plant quality, with the latter also highly important for the summer diet. During late winter, restricted forage availability due to snow cover seemed to have influenced dietary composition. We also found dietary variation among years both in early and late winter. We hypothesize that climatic effects previously found on the growth rate of Svalbard reindeer populations may be mediated through interannual variation in their diet where increased summer precipitation and winter rain may affect the dietary composition through decreased availability of forage biomass.

The impact of watering regime and ambient relative humidity on the effect of density on growth in two boreal forest mosses, Dicranum majus and Rhytidiadelphus loreus

Abstract The impact of watering regime, ambient relative humidity and shoot density on production of offshoots, biomass increase and elongation growth in two boreal forest mosses, Dicranum majus and Rhytidiadelphus loreus, was studied under controlled conditions. Experimental colonies of the mosses at three shoot densities were subjected to humidity conditions differing with respect to desiccation rate, desiccation severity and length of the moist period, by varying the ambient relative humidity (RH), and the frequency and continuity of days when the shoots were watered. Overall, increased density had a negative effect on relative growth rate (RGR) and green biomass in both species. However, under several watering regimes, optima for both RGR and green biomass were found at intermediate densities, especially at low RH. Production of offshoots decreased with density in D. majus. Length growth rate (LGR) increased with RH in both species, and LGR also increased with density in D. majus. The differences in response to watering regimes were large, but few consistent patterns emerged with respect to the impact of the length of wet and dry periods, watering frequency, and the desiccation rate and intensity. Nevertheless, RGR increased with the length of the wet-dry cycle when the total number of wet and dry days remained equal.

Age and size at maturity in mountain and lowland populations of the expanding moss Pogonatum dentatum

The moss Pogonatum dentatum has expanded its distribution in Fennoscandia from mountainous areas into the lowlands. This recent expansion appears to be associated with changes in important life-history parameters in female shoots. We examined shoot age and size at first production of sex organs and mature spores in P. dentatum to investigate this phenomenon. Female shoots produced mature spores for the first time in the lowlands in their second year but in their third year in the mountains. However, sex organs were produced by second year plants in both areas. There was no size difference between the mountain and lowland female shoots at the time of spore production. Among mountain females reproducing for the first time, 41% of the shoots branched, making them potentially ‘iteroparous’. Branching was not observed among lowland females. Male shoots showed no difference in production of sex organs, and were produced by second year shoots in both areas. Female shoots in the lowlands have earlier spore production and exhibit ‘semelparous’ behaviour by not producing branches. This suggests that the lowland phenotypes of P. dentatum are more ‘invasive’ than the mountain phenotypes. Earlier studies showing high rates of diaspore establishment in lowland areas also support this observation.

Flowering in the Rich Fen Species Eriophorum latifolium Depends on Climate and Reproduction in the Previous Year

In this long-term study, we identify the climate variables most important to flowering density in Eriophorum latifolium in boreal rich fen vegetation, and assess their relative importance. We analysed time-series data (1982–2008) of flowering density in 126 permanent plots in an oceanic and a continental area in Norway, and developed an autoregressive model where the density of flowering plants was determined by the cost of previous flowering, climate during the same year as flowering, and climate during the year prior to flowering. The cost of reproduction was the most influential of the factors affecting flowering. Our results suggest that dry conditions during the previous summer affected flowering negatively in the oceanic population, but had no effect in the continental population. We attribute this to differences in hydrology, with steeply sloping fens in the oceanic area, and gently sloping, spring-fed fens in the continental area. Furthermore, flowering increased with the length of the previous growing season (oceanic population), decreased with the amount of precipitation during the previous spring (continental population), and increased with temperature in spring the same year (both populations). We conclude that climate conditions during the previous year are more important than climate conditions during the year of flowering.

Long‐term effects of nutrient enrichment controlling plant species and functional composition in a boreal rich fen

Co-ordinating Editor: Sabine Güsewell Abstract Questions: How does longterm increase in nutrient availability affect species composition, species diversity and functional composition in boreal rich fens, and how does this differ from shortterm effects? What are the possible mechanisms behind the observed changes, and how does nutrient limitation influence species diversity in these communities? Location: Sølendet Nature Reserve, central Norway. Methods: A fullfactorial field experiment. Plots in two localities received one of following treatments (n = 3): no nutrient addition (control), N, P, K, NP, NK, PK and NPK addition. Cover of plant species was recorded before treatment, and after two and 15 years of treatment. Results: Two years of nutrient addition caused small changes in species composition, but addition of NP led to large increase in abundance of species with high ability to exploit the added nutrients—a direct result of the elimination of nutrient limitation in the communities. After 15 years of nutrient addition there were significant changes following three different pathways, one for each of N, P and NP addition. The addition of NP led to large community shifts, considerable species turnover and reduced species and functional richness, mainly caused by increase in cover of highly competitive and tussockforming grasses like Deschampsia cespitosa, Festuca ovina and Molinia caerulea, outcompeting other species, especially bryophytes. Addition of N led to smaller changes in species turnover, and without clear dominant species. Addition of P led to considerable species turnover, but no reduction in species or functional richness, and the bryophyte diversity increased. This is explained by the bryophytes’ association with Nfixing cyanobacteria, suggesting less N limitation and a greater ability to utilize the added P when vascular plants suffer from N shortage. In addition, bryophytes are more sensitive to low P availability, due to larger P requirements compared to vascular plants. There was no effect of K addition. Conclusions: Both N and P limitation is essential for the maintenance of high species diversity in boreal rich fens, and P limitation controls bryophyte diversity. From a management perspective, N and P limitation is vital in the conservation of boreal rich fens or when a functional fen system is reestablished through restoration measures.