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Seed weight, seed germination, seedling survival, and juvenile/adult fitness in chasmogamously (CH) and cleistogamously (CL) derived offspring of Oxalis acetosella were compared during three growing seasons, to test hypotheses of fitness differences between the offspring types accounting for the maintenance of cleistogamy. In plots at three field sites, CH and CL seeds originating from all sites were sown to compare the performance of offspring growing in their habitat of origin and offspring growing in new habitats. Seeds were also sown in pots in a common garden, to test for effects of sibling competition. CL seeds had significantly lower germination than CH seeds in the field, possibly because of lower mean seed weight due to later flowering. Since the outcrossing rate in the CH flowers of O. acetosella is not known, it is uncertain whether the lower CL germination is a consequence of inbreeding depression. CH seeds had higher germination if sown at their home sites than at new sites, while for CL seeds this made no difference; this contradicts the local adaptation hypothesis for cleistogamy. No other fitness differences were found between the offspring types, and the findings did not support the sibling competition or local adaptation hypotheses. We suggest that the maintenance of the dimorphic reproductive system in O. acetosella is not explained by offspring characteristics, but rather by the two flowering phases complementing each other in maximizing annual seed production in the face of environmental variability. It is, therefore, important to include temporal and spatial variation in studies of reproductive strategies.

Explosive seed dispersal in the cleistogamous perennial forest herb Oxalis acetosella was studied during two growing seasons, to determine whether seeds derived from chasmogamous (CH) and cleistogamous (CL) flowers differ in dispersal distance. Seed dispersal distance, seed weight, and height of fruits were measured for both flower types, and the effects of phenology and year were also taken into account. The dispersal experiment was performed indoors, using plants transplanted from natural populations to pots. CL seeds were thrown significantly further than were CH seeds, though there was a considerable overlap in dispersal distances. There was also a significant positive relationship between seed weight and dispersal distance. No relationship was found between fruit height and dispersal distance. The results of this study contradict the common view that CL progeny should always be dispersed closer to the mother plant than CH progeny. The ecological implications of the dispersal difference are unclear, especially since it is uncertain whether CH seeds are generally outcrossed or not. Variation in dispersal distance in O. acetosella seems to be mainly dependent on a combination of reproductive mode and variation in seed weight.

Benthic diatoms are valuable tools for biological monitoring and paleo-ecological reconstruction of past environmental conditions. This thesis aims at describing size-related properties of benthic diatoms and suggests that data assessment for community analysis can be improved by considering the importance of scale. It investigates which environmental factors structure epilithic diatom communities on the coast of the Gulf of Bothnia and identifies environmental factors correlated with phytobenthic biomass. It also contributes to the floristic knowledge of the Baltic Sea.

Diatom species show large variation in size. The responses of large species (≥1000 μm3) in diatom communities to environmental factors are underestimated if solely measured as relative abundance, as is the common practice. However, relative abundance gives the best gradient resolution, as compared to surface area and biovolume, if species are counted separately in two biovolume classes. Small and large species in the same community may respond differently to the same environmental factors.

To assess the principal environmental factors structuring diatom communities in the Gulf of Bothnia, 270 quantitative samples were collected from submerged stones. Sampling was carried out in spring in four areas of the Bothnian Bay, characterized by a stable north-south salinity gradient (0.4-3.3 psu), and in three areas of the Bothnian Sea which has a rather uniform salinity of ca. 5 psu. Canonical correspondence analysis (CCA) identified salinity and exposure to wave action as the principal factors structuring the diatom communities of the Bothnian Bay, whereas exposure to wave action was the principal factor in the Bothnian Sea. Measurements of relative ignition loss suggested that the cover of macroalgae, and thereby the higher abundance of epiphytic diatoms in the epilithic samples, was positively correlated with salinity in the Bothnian Bay and with water movement in the Bothnian Sea.

Two new brackish water species are described, Navicula sjoersii S. Busse & Snoeijs and N. bossvikensis S. Busse & Snoeijs. The new species are compared with N. perminuta Grunow, a common brackish-water species.

Seed banks in cattle dung, soil under cattle dung and soil under vegetation and growth response of plant species to the changes in soil nitrogen availability were studied in an alvar limestone grassland on Öland, Sweden, in order to analyse the impact of dung deposition and decomposition on the formation of patches of plant species. Results suggest that patches of four plant species could result from cattle dung deposition and decomposition. Impact of dung could proceed in three ways: (1) by changing the relative abundance of species in the soil seed bank under dung, and/or (2) by influencing the deposition of seeds in the dung, and/or (3) by intensifying the growth of some species through nutrient release. Species patches could result from one or more of these aspects. For instance, patches of Arenaria serpyllifolia may be induced by dung deposition because of the dominance of its seeds in dung, while the pattern of Cerastium semidecandrum and Festuca ovina may be due to the abundance of their seeds in the soil seed bank under dung and their positive growth response to increased nitrogen availability.

Plant strategies under succession were evaluated for communities on rising seashores of the northern Gulf of Bothnia, Sweden, representing sites of primary succession. The succession could not be explained by Grime's CSR model. Early successional stages were neither characterized by high incidence of ruderals as proposed for secondary successions, nor by the dominance of stress tolerators, as proposed for primary successions. Short-lived species were almost totally absent. Instead, the shore habitat was characterized by species with an ability to tolerate and vegetatively recover from disturbance. The way in which different species experience one and the same form of stress or disturbance is an important reason why the classification on the basis of stress and disturbance seems to be insufficient to explain the course of this succession. Dominants of early and late successional stages differed with respect to root system, breeding system, leaf longevity and growth form (graminioid, herb).

In this thesis I examine and evaluate the effects of a documented large-scale eutrophication on macroalgal vegetation on the Swedish coast. During the past century the load of nutrients has increased manifold in the Baltic Sea area, increasing primary production and organic sedimentation significantly. By re-investigating a unique reference material of macroalgal vegetation from the 1940-60s and by conducting new diving studies at the same sites, I showed that long-term trends in macroalgal community composition and species’ distributions are consistent with changes expected from an increased nutrient availability. In the Öregrund archipelago (northern Baltic Sea), I documented a declined depth distribution of the dominant canopy forming alga, Fucus vesiculosus, and an increased abundance of opportunistic ephemeral algae compared to 52-53 years ago. In the Gullmar Fjord area (Skagerrak), trends included increased abundances of functional groups with opportunistic algae, decreased abundances of large perennial algae and a general decline in the depth distribution of the vegetation compared to 36-57 years ago. Sediment removal experiments in the northern Baltic Sea confirmed the hypothesis that sedimentation influences macroalgal community composition. Species depending on short periods of reproduction were clearly favoured by sediment removal, especially F. vesiculosus that seemed limited in depth by the local sediment load. Species with long continuous periods of dispersal by spores and/or fragments (for example the ephemerals Cladophora glomerata, Ceramium tenuicorne and Enteromorpha spp.) were more tolerant to the natural sediment load. In general, sediment removal favoured macroalgal establishment and development, indicating that variation in the natural sediment load is an important constraint for sublittoral rocky-shore macroalgal community development. I conclude by suggesting that the documented long-term changes in macroalgal vegetation on the Swedish coast partly are explained by an increased organic sedimentation in these areas.

We investigated the degree to which the spatial distribution of oaks (Quercus robur L.) was related to habitat conditions, as reflected by vegetation type and structural features presumed to attract animal dispersers (trails, community borders). We hypothesized that the distribution pattern of oaks, with their potential to establish in many habitats, depends on the behaviour of the dispersing animals to a greater extent than micro-habitat conditions. One 100 m x 100 m plot was surveyed in each of two coniferous forests in east-central Sweden. No adult oak trees grew in the forests; all oaks were considered as dispersed into the plots by animals. We tested whether oak distribution was clumped with spatial autocorrelation analyses and whether oak distribution was related to vegetation type, species composition, tree cover, distance to nearest fertile oak tree, or distance to animal trails. Our study showed that oak trees were also spatially aggregated in a small-scale context. The spatial distribution of seedlings and older trees were associated with species richness and tree cover but not with any specific vegetation type, even though fewer oaks than expected grew in spruce forest habitats. Furthermore, we found that oak trees were associated with trails. There were differences in oak distribution between the two study sites in total number of oaks, the number of first-year seedlings, caches, and oak occurrence in relation to species richness and distance to nearest fertile oak. Seed-dispersing animals seem to be of importance for oak distribution even though animal activities seem to differ between sites.

Traditionally mire ecosystems (especially bogs) have been viewed as stable systems with slow changes in the vegetation over time. In this study the mire Akhultmyren, south-central Swedenw as re-investigatedin 1997 after 40 yr of continued natural development. The results show a high degree of dynamics in a Sphagnum dominated bog and fen. Altogether 97 vascular plant and bryophyte species were recorded in the two inventories of the bog and poor fen vegetation ,p H and electrical conductivity in the mire water were also surveyed. In 1997 we found 10 new species and that 8 species had disappeared since 1954 but the over-all mean number of species per plot (size 400 m2) had hardly changed. However, 21% of the species increased and 21% decreased significantly in frequency. Most of the species that decreased in frequency were low-grown vascular plants, most common in wet microhabitats. Vascular plant species that increased in frequency included trees (defined as > 1.3 m in height) and were generally taller than the unchanged or decreasing species. The frequency of dwarf shrubs and hummock bryophytes increased too. Areas with an initial pH of 4.5-5.0 showed the strongest decrease in pH, coinciding with an enlarged distribution of some Sphagnum species. The species diversity increased on the bog, but decreased in the wettest parts of the fen, where the pH also decreased. Species with unchanged or increasing frequency often showed high capacity to colonise new plots. On average the sum of gains and losses of species in the plots in 1997 was ca 50% of the species number in 1954. The vegetation changes indicate a drier mire surface and anincreaseda vailability of nitrogen. The increased tree cover may have triggered further changes in the plant cover.

The effects of increased nitrogen influx on Sphagnum growth and on interspecific competition between Sphagnum species were studied in a 3-yr experiment in mires situated in two areas with different rates of airborne N deposition. Sphagnum growth was recorded after various supplementary N influxes (0, 1, 3, 5 and 10 g m ⁻² yr⁻¹) in hummocks and lawn communities. Sphagnum biomass production decreased with increasing N influx in both areas. After the first season at the low-deposition site, Sphagnum showed an increased growth in length with the intermediate N treatment, but in the second and third seasons the control treatment had the highest growth in length. Capitulum dry mass increased with increasing N influx. Sphagnum N concentration and N/P quotient were higher at the high- than at the low-deposition site. The low quotient at the low-deposition site, together with the initial growth increase with intermediate N supplements, indicates that growth was N-limited at this site, but our lowest N supplement was sufficient to reduce growth. The N treatments had no effect on interspecific competition between the Sphagnum species. This indicates that the species have similar responses to N. The species studied all occur naturally on ombrotrophic, N-poor sites and show low tolerances to increased N influx. Reduced Sphagnum production may affect the carbon balance, changing the mires from C sinks to sources.

The Skattlosbergs Stormosse min was reinvestigated in 1995 after 50 years of natural development since the previous investigation. The undrained mire is situated in an area with low anthropogenic deposition. The distribution of 106 plant species was mapped in detail and pH was measured at 251 locations, providinga unique opportunity to quantify long-term mire dynamics. The resultss how decreased pH in the richer (high-pH) parts of the mire, but little or no change in the poor fen andombrotrophipca rts.1 4 species had disappeared while two news pecies were recorded. Most species had a more restricted distribution in the mire area in 1995 than in 1945. Species richness in lOm x 0Omp lots had decreased, especially in plots with higher pH. Most Sphagnum species had unchanged distributions over the mire, while 7 3 % of other bryophyt sepecies and 3 8% of vascular plant species had decreased by more than 20 % in plot frequency. There was a strong relationship between number of species and pH-value. The mean and standard eviation of pH in plots where the species occurred have both decreased since 1945. We interpret the changes in species richness in the richer fens to be mainly caused by acidification. This could partly be an autogenic succession, but may be enhanced by increased atmospheric deposition. The mire represents an almost untouched site which can act as a reference for mires in morepolluted areas.

Associational resistance occurs when herbivore damage to a focal plant is reduced by the presence of other plant species. Neighboring plants can reduce herbivore damage (1) by their effects on the predator community, (2) by reducing the ability of herbivores to find their host plants, and (3) by reducing the time herbivores remain on their host plants. We examined how the presence of the aromatic low shrub Myrica gale and of predatory lady beetles affected herbivore damage and reproductive output in a population of the perennial herb Lythrum salicaria in northern Sweden. An observational study showed that L. salicaria growing in Myrica thickets were less damaged by herbivores, had a lower abundance of the monophagous, leaf-feeding, chrysomelid beetle Galerucella calmariensis, and had higher flower and seed production than L. salicaria outside Myrica thickets. To test whether these differences could be explained by (a) differences in some aspect of plant quality, or (b) differences in predator abundance, we placed potted L. salicaria within and outside Myrica thickets. To determine whether differences in the abundance of G. calmariensis were primarily the result of different rates of colonization or emigration, we marked adult beetles and placed them on a second set of potted plants in the two micro-habitats. The results show that differences in herbivore abundance, plant damage, and reproductive output between potted L. salicaria placed within and outside Myrica thickets were in the same direction and of the same magnitude as those observed for naturally occurring plants, indicating that the observed patterns were not an effect of differences in the chemical composition of the host plant. Moreover, we found no support for the hypothesis that a higher abundance of insect predators could explain the lower abundance of G. calmariensis on L. salicaria in Myrica thickets. On the contrary, lady beetles (Coccinella quinqempunctata and Coccinella septempunctata) were observed on a greater proportion of the plants placed outside the Myrica thickets. The monitoring of marked beetles indicated that differences in the abundance of G. calmariensis were the result of a markedly higher colonization rate and a somewhat lower emigration rate from L. salicaria outside Myrica thickets. Outside the Myrica thickets, the survival of G. calmariensis and the magnitude of herbivore damage were lower, and fruit and seed output were higher on plants with observations of lady beetles than on plants without lady beetles. The results indicate that the abundance of the specialist herbivore G. calmariensis, and the herbivore damage and reproductive output of its host plant, L. salicaria, are affected both by the presence of the nonhost Myrica and by predation from lady beetles. We suggest that the most likely mechanism causing decreased feeding on L. salicaria growing in Myrica thickets is that Myrica affects the ability of G. calmariensis to find its host, either through visual or olfactory interference.

We show how photosynthesis and UV sensitivity of algae are related to thallus morphology and depth distributions. This was studied for typical depth zonations of red and brown macroalgae in the Skagerrak (ca. 25 psu) and the Baltic Sea (6.5 psu). The algae were collected from the water surface down to 20.5 m of depth, whereby each species was sampled at its maximum abundance depth. Altogether, we measured photosynthetic and respiratory rates of 19 red and 13 brown algal species as O₂ evolution at different light intensities. Photosynthesis versus irradiance curves (PI curves) showed that light-saturated net photosynthetic rates (P_max), respiratory rates in darkness (R_d) and the initial slope (α) were strongly related to algal morphology with higher values for thinner species. The compensation irradiance (I_c) and saturating irradiance (I_k) were strongly related to water depth with lower values at greater depth. A novel approach to analyse PI data with principal component analysis (PCA) is presented. The method makes it possible to assign a quantitative morphological gradient to algal species based on photosynthetic properties. Such a gradient can be used in ecological studies as an alternative to more subjective discrete subdivisions into functional-form groups. Another type of PCA analysis, with the relative shapes of the PI curves as input data, summarises α and convexity but discards all interference of morphology. This results in a gradient of genuine physiological responses, which in our study was strongly correlated to maximum abundance depth. The UV sensitivity of the same 32 algal species was determined as the change in net O₂ evolution after exposure to UV light and the recovery after this treatment. Deeper-growing algae were more sensitive to UV and species with thinner thalli recovered better after UV treatment in the Skagerrak. No such trends were observed for the algae in the northern Baltic Sea, which suggests that no real deep-water species occur here. This is further supported by the lack of a clear pattern in I_c and I_k values with depth for the algae in the Baltic Sea. Our results advocate that the reduced species diversity of the Baltic Sea is also coupled to a loss of functional groups in the sense of general photosynthetic performance and not only in the sense of pure morphology (loss of canopy-forming species).

The aim of this thesis was to examine factors affecting the distribution of macroalgal species both vertically (depth zonation) and geographically along the Baltic Sea salinity gradient. Interactions between the physical environment and basic biological traits of the algae are emphasised.

Previously well-described macroalgal vegetation profiles were re-investigated. Long-term changes of the vegetation that can be coupled to an increased large-scale eutrophication were recorded in both the Baltic Sea and in the Skagerrak.

Photosynthetic properties of macroalgae with different morphologies from typical depth zonations in the Skagerrak and the Baltic Sea were assessed. A novel method that makes it possible to classify macroalgal species along a morphological gradient based on photosynthetic properties is presented. Such a gradient can be used as an alternative to more discrete subdivisions into functional-form groups.