Question: What is the relationship between species richness of vascular plants, bryophytes and macrolichens, and two important gradients in the alpine environment, altitude and local topography?Location: Northernmost Fennoscandia, 250-152 m a.s.l. corresponding to the range between timberline and mountain top.Methods: The vegetation was sampled in six mountain areas. For each 25 vertical metres, the local topographic gradient from wind-blown ridge to snowbed was sampled in quadrats of 0.8 m x 0.8 m. Patterns in species richness were explored using Poisson regression (Generalized Linear Models). Functional groups of species, i.e. evergreen and deciduous dwarf-shrubs, forbs, graminoids, mosses, hepatics and lichens were investigated separately.Results: Functional groups showed markedly different patterns with respect to both altitude and topography. Species richness of all vascular plants showed a unimodal relationship with altitude. The same was true for graminoids, forbs and lichens analysed separately, but forb richness peaked at Much higher altitudes than total richness. The richness of dwarf-shrubs decreased monotonically with altitude, whereas richness of mosses and liverworts showed an increasing trend. Significant interactions between altitude and local topography were present for several groups. The unimodal pattern for total plant species richness was interpreted in terms of local productivity, physical disturbance, trophic interactions, and in terms of species pool effects.Conclusions: Patterns in local species richness result from the action of two opposing forces: declining species pool and decreasing intensity of competition with altitude.

The arctic fox Alopex lugopus excavates its dens in gravely ridges and hillocks, and creates a local environment quite distinct from the surrounding tundra or heath landscape. In northern Sweden, the vegetation of 18 dens of the arctic fox was investigated, as well as reference areas off the dens but in geologically and topographically similar locations. The species composition showed considerable differences between den and reference areas, with grasses and forbs occurring more abundantly on the dens, and evergreen dwarf-shrubs occurring more in reference areas. The effect of the foxes' activities is thought to be either through mechanical soil disturbance, or through nutrient enrichment via scats, urine, and carcasses. This was expected to result in differences in plant traits with key functional roles in resource acquisition and regeneration, when comparing dens with reference areas. We hypothesised that the community mean of specific leaf area (SLA) would differ if nutrient enrichment was the more important effect, and that seed weight, inversely proportional to seed number per ramet and hence dispersal ability, would differ if soil disturbance was the more important effect. Specific leaf area showed a significant difference, indicating nutrient enrichment to be the most important effect of the arctic fox on the vegetation on its dens. Arctic foxes act as ecosystems engineers on a small scale, maintaining niches for relatively short-lived nutrient demanding species on their dens in spite of the dominance of long-lived ericaceous dwarf-shrubs in the landscape matrix. Thus, foxes contribute to the maintenance of species richness on the landscape level.

Climate change may strongly influence species distribution and, thus, the structure and function of ecosystems. This paper describes simulated changes in the position of the upper treeline in the Swedish mountains in response to predicted climate change. Data on predicted summer temperature changes, the current position of the treeline, and a digital elevation model were used to predict the position of the treeline over a 100-year timeframe. The results show the treeline advancing upward by 233-667 m, depending on the climate scenario used and location within the mountain chain. Such changes hypothetically caused a 75-85% reduction in treeless alpine heaths, with 60-93% of the remaining areas being scree slopes and boulder fields. For this change to occur, the migration rate of the trees would be in the order of 23-221 m yr-1, which is well within published migration rates for wind-dispersed deciduous trees. The remaining alpine areas would be strongly fragmented. These drastic changes would influence all aspects of mountain ecosystems, including biodiversity conservation and human land-use patterns.

Aim This paper seeks to investigate whether alpine floras on isolated mountains in boreal forest show nestedness, and, if that is the case, to determine whether selective extinction or colonization is the likely cause of the observed patterns.Location Isolated mountains in the boreal coniferous forests of northern Sweden (province of Norrbotten, c . 66degreesN; 18degreesE). The timberline in the region probably has been 300-400 m above the present some thousands of years before present, potentially covering these mountains.Methods A data matrix of twenty-seven alpine plant species on twenty-seven isolated mountains was subjected to nested subsets analysis. Extinction probability was assumed to increase with decreasing area, and colonization probability was assumed to decrease with increasing isolation. By sorting the data matrix by these factors and sequentially computing the degree of nestedness, we were able to determine whether the alpine floras were structured mainly by selective extinction or mainly by differential colonization.Results When ordered by decreasing area the data matrix was significantly more nested than random, but that was not the case when ordered by decreasing isolation. Ordering by maximum altitude also produced significant nestedness.Main conclusions Contrary to the conventional view that isolated mountains were completely covered with boreal forest some thousands of years ago, the nestedness patterns of alpine plants indicate that many of them survived the forest period on the isolated mountains, probably on cliffs and slopes too steep for the formation of closed forest.

Abstract: In studies of edge effects, it is difficult to separate edge from size effects because size and edge are strongly correlated in most cases. In our study we separated these two effects by examining small forest patches of two different size classes ( <1 ha and 4–6 ha ), with patches chosen so that area and perimeter were not correlated within each class. We conducted our study in a mosaic of old‐growth Picea abies forest and wetland, consisting of forested moraine hills ( “islands” ) in a Sphagnum‐bog matrix. On each island, we demarcated a 0.1‐ha sample plot and measured site and forest characteristics. In each plot, we examined all fallen logs for the occurrence of epixylic hepatics ( liverworts ), and all standing trees for the occurrence of calicioid lichens. We examined correlations between species and area, shape, and distance to nearest edge. The effect of shape on forest‐interior conditions was analyzed with a core‐area model. Site and forest characteristics were similar between the island size groups. The majority of hepatics and lichens occurred more frequently on larger islands, and the cover of hepatics tended to be higher on larger islands. A rough estimation of depth‐of‐edge influence was made at 50 m, but the two species groups did not respond similarly to edge effects. The lichens showed several important correlations with island shape on small islands, suggesting that circular small islands may have a forest structure approaching interior conditions. Hepatics tended to respond to small islands as edge environments. We conclude that the response of species to edges is strongly species‐specific and context‐dependent. This points to the limitations of extrapolating results from studies on depth‐of‐edge influence to forest‐management situations.

Mathematical models of interacting populations have a prominent position in population and community ecology, but are often criticized for not being testable. The authors reviewed tests of a particular model, the exploitation ecosystem hypothesis as it was formulated in Oksanen et al. (1981), in order to study problems that may be encountered when testing models. A general problem is how to determine if an experimental system should be regarded as within the model's theoretical domain or not. The theoretical domain defines the type of system the model is meant to apply to. It is noted that both liberal and strict domain definitions can be problematic. Most important is that a too liberal domain definition can result in false understanding (i.e. that it is falsely concluded that the processes included in the model are controlling the study system). Other problems encountered were more system-specific. Equilibrium predictions were tested in experiments that were too short to reach steady state and in several studies ambiguous definitions and measurements of model variables were found such as productivity, biomass and the number trophic levels. It is concluded that a major obstacle when performing tests is the conceptual and methodological problems encountered when translating model abstractions into an empirical reality.

The competitive abilities of three montane indigenous New Zealand plant species (Acaena buchananii, Festuca novae-zelandiae, and Raoulia australis) when growing with the locally invasive, introduced Hieracium pilosella were compared in an outdoor pot experiment. Competitive ability was divided into the competitive effect, or the ability to deplete resources, and the competitive response, or the ability to tolerate low resource levels. The plants were grown in pots with or without Hieracium, in shade or full sunlight, and with high or low soil fertility. The competitive response rankings showed consistent hierarchies in the different treatments with Festuca being less suppressed than Acaena and Raoulia. Festuca performed especially well in low soil fertility and in shaded treatments, while the other two species were strongly suppressed by Hieracium even in those conditions. However, all three species did relatively better (less badly) in the low fertility and shaded treatments than in the more resource-rich treatments when interacting with Hieracium. The effect on Hieracium biomass of the indigenous species was generally small and the rankings of competitive effect showed no agreement between the species in the different environmental treatments. We suggest that competitive rankings based on the competitive response component is likely to be a more sensitive measure of competitive ability for these indigenous, slow-growing plants. The results also indicates that these two components of a plant's competitive ability were negatively correlated and thus reflect trade-offs in dealing with competitive situations. Finally, in competition with Hieracium, low-growing indigenous species are likely to perform best when nutrients and light are maintained at low levels.

The aim of this study is to examine the information given by various indices of rhizome morphology that describe grazed and ungrazed rhizome systems of Acaena magellanica (Rosaceae). Internode lengths, branching probabilities, and branching angles were estimated from grazed and ungrazed rhizomes in the field. These parameter values were then used in computer simulations of rhizome growth, and the structural complexity of the simulated rhizomes were described using size, topology, and fractal dimensions. Grazed rhizomes had shorter internodes, higher probabilities of branching, and more open branching angles than ungrazed rhizomes. This resulted in a more directional growth (herring-bone pattern) in the simulated ungrazed rhizomes, whereas the grazed rhizomes had a more space-filling growth pattern. Most indices, even though they are based on different mathematical and theoretical backgrounds, were highly correlated and thus equally good at describing the structural complexity exhibited by the rhizomes. However, indices have different relationships to theories about function, and we suggest that any study of structural complexity of branching systems should use several different indices of shape depending on the questions asked.Key words: Acaena magellanica, fractal dimension, grazing, growth simulation, topology.

Present discussions on competitive interactions and the occurrence of predictable patterns in species composition including assembly rules - are likely to benefit from appropriate analyses of the spatial structure in plant communities. We suggest such an analysis when we specifically want to detect scale regions where fine-scale local processes may affect the spatial pattern of species composition. We combine indirect ordination in the form of Detrended Correspondence Analysis (DCA) and geostatistics in the form of variography. The species abundance data in the sampled quadrats are summarized as positions on the axes in the ordination. Each axis is used as a regionalized variable in the variography to obtain the spatial dependence of the quadrats. The spatial pattern found will suggest the relevant scale region in which to perform an analysis of species associations. A significant spatial dependence (the 'range' in geostatistical jargon) will define the size of a sampling plot that will minimize both the problem of being too small and thus having the risk of oversampling of e.g. clonal individuals and of being too large which will risk including individuals that do not interact. We also suggest that plots are spaced at least a 'range' apart to insure spatial and statistical independence. Comparisons of species compositions in such plots will reveal any positive or negative associations between species on a scale where these should reflect species-species interactions. To illustrate the method it is applied to three different data sets from two different plant communities.

FORUM is a lighter channel of communication between readers and contributors: it aims to stimulate discussion and debate, particularly by presenting new ideas and by suggesting alternative interpretations to the more formal research papers published in ECOGRAPHY and elsewhere. A lighter prose is encouraged and no summary is required. Contributions should be concise and to the point, with a relatively short bibliography. Formal research papers, however short, will not be considered.

Vegetation on Bear Island, Jan Mayen and Spitsbergen was investigated along altitudinal and topographic gradients in order to describe the main patterns in plant community distributions and compare them with those on the mainland. In a numerical classification the communities were distinctly differentiated; however, physiognomically similar Racomitrium and Sanionia communities dominated in most habitats on Bear Island and Jan Mayen. On Spitsbergen, moss-dominated communities prevailed in depression sites. High-altitude sites were occupied either by moss-dominated communities or by variable assemblages of fragmented moss cover and scattered vascular plants. Dwarf shrub and grass heaths that were common on the mainland did not occur on grazer-free Bear Island and Jan Mayen, and were confined to the lowest altitudes on Spitsbergen. The lack of grazers on Bear Island and Jan Mayen accounts in part for the differences in vegetation between the mainland and the islands.

Shoot survival of a toxic herb, Actaea spicata, and a herb with a digestibility-reducing defense, Geranium sylvaticum, was studied in an experimental set-up where microtine rodents were allowed to graze freely. It was hypothesized that the defense system of Actaea would work in a risk-reducing way, while the defense system of Geranium would have cost-reducing properties. If so, shoot survival of Actaea would be higher at all times than for Geranium, and shoot survival would also be higher for Actaea when grazed by a more generalized herbivore (Microtus agrestis) than by a more specialized one (Clethrionomys rufocanus). Using failure-time analyses, it was shown that Actaea indeed had a significantly higher shoot survival than Geranium in all treatments but one, and that survival was also higher when grazed by the generalist than by the specialist. It was concluded that the defense system of Actaea had stronger characteristics of a risk-reducing defense than that of Geranium.

Biomass allocation and growth by the clonal plant Acaena magellanica were characterized for three populations grazed by introduced reindeer on the subantarctic island of South Georgia. Annual growth markers (internode lengths) were used to divide each rhizome into current year's shoots, one-year-old and two-year-old rhizome segments. Total dry weights were significantly smaller in grazed than in ungrazed populations. Leaf biomass of current year's shoots was very much lower in grazed shoots. Rhizome length and number of leaves were less affected than dry weight by grazing, and the reindeer grazing thus seems to mainly influence biomass accumulation rather than morphology in Acaena. Interactions with Festuca contracta in both grazed and ungrazed areas were also studied in a two-year competition experiment. No apparent release of soil resources (as measured by an increase in plant growth) was apparent in plots where Festuca was removed, but the current year's shoots of Acaena were smaller and more numerous in these plots than in controls, especially in the ungrazed area.

Positive interactions within species aggregations have been suggested as particularly important for increasing germination and survival probabilities of plants in the adversive environment of high alpine block fields. One often-cited example is the elevated temperature and moisture levels in cushion plants, which increases the probability of germination as compared to open ground. However, it is not clear how this applies to plant communities with more erect species. Seeds of Oxyria digyna were sown in close proximity to resident Ranunculus glacialis individuals, and in cleared microsites to test whether positive or negative interactions occur in this kind of high alpine block field plant community. Seeds sown in 1988 showed a higher germination in cleared than in occupied microsites the year after, while no differences were seen for seeds sown in 1989. Ground surface temperature measured in 1990 showed a lower temperature under the resident individuals, which is proposed as a mechanism for the difference in germination found for seeds sown in 1988. Air temperature and precipitation data suggested that 1990 was a warmer and sunnier year than 1989, which could explain why the results from the 2 yr differed. The data show that interactions in this plant community vary from neutral to negative, whereas no evidence for positive interactions was obtained.

In many productive terrestrial biomes there seems to be an abundance of forage which is not exploited by herbivores. This observation has generated several hypotheses. One set of hypotheses suggests that the herbivores are either regulated by predators, social interactions or pathogens, and that the abundant plant biomass is an effect of relatively low herbivore densities and a low grazing pressure. The other set claims that the quality of the bulk of the vegetation is persistently too low to give herbivores positive energy or nutrient balance. The herbivores should be critically dependent on high-quality plants or plant organs, but the bulk of the vegetation will remain untouched. The herbivores should thus live in a «green desert». The first set assumes that all plants are edible and of sufficiently high quality, while the second set assumes that plants are generally toxic or of low quality (...)

The impact of a lemming population on a snowbed in northern Fennoscandia was examined during a population peak. Twelve exclosures and 12 open plots were established on a moderately early snowbed. The plots were photographed in autumn 1988, spring 1989, autumn 1989, and autumn 1990, and the cover of graminoids, woody plants, lichens, litter, lemming feces, and mosses were measured. The changes in cover were analyzed with repeated measures ANOVAs. The lemmings significantly decreased cover of graminoids by 33% and mosses by 66% during the peak winter. Our results show that grazing needs to be considered when discussing the development of snowbed vegetation.

Standard exploiter-victim models assume an instantaneous connection between foraging and population dynamics. However, in nature this connection is inevitably time-delayed: it takes time to convert food into offspring. R. Arditi and L. R. Ginzburg proposed that this time delay implies ratio-dependent exploitation. Models with ratio-dependent exploitation predict that primary productivity has no impact on food chain length and that the standing crop of all trophic levels increases linearly with increasing primary productivity. Conversely, the traditional victim-dependent models imply that food chain length increases with increasing primary productivity and that only top trophic levels and trophic levels that are an even number of links below the top respond positively to increasing primary productivity. We study the impact of time-delayed numerical responses on predator isoclines, and we do not find support of the theoretical arguments of Arditi and Ginzburg. We also review data on biomass patterns in plants and herbivores, which seem to support the predictions of the victim-dependent model.