Brita M. Svensson

Clonal plants and environmental change : Introduction to the proceedings and summary

Clonal growth is an important attribute of many groups of plants and animals (White 1979, Harper et al. 1986). In the plant kingdom, phyla from all evolutionary levels show the capacity for clonal growth. The ability to proliferate vegetatively is found within 10 out of 11 classes of vascular plants and within all seven classes with extant representatives, although rare within the Gymnospermopsida (Mogie and Hutchings 1990), and 69 out of 163 families of vascular plants are able to proliferate vegetatively (Tiffney and Nicklas 1985). The importance of clonal growth in temperate and more northerly ecosystems is apparent when the ground area covered by clonal plants is estimated. In the Arctic, 0.9 x 106 km2 of land is covered by Eriophorum vaginatum (Miller 1982) which is a caespitose clonal plant, where recruitment from seedlings depends on small and large scale disturbances in the vegetation (Gartner et al. 1986). In Britain, the most productive natural/seminatural vegetation is dominated by clonal plants such as Phragmites communes and Pteridium aquilinum (Callaghan et al. 1981), while infertile areas are also dominated by clonal plants such as Calluna vulgaris and Nardus stricta (Bunce and Barr 1988). The area of Britain is 230000 km2 and 19% of this is covered by Britains ten most extensive species which are all clonal (Table 1; Bunce and Barr 1988). In temperate and boreal forests, clonally proliferating species such as Vaccinium and Empetrum species and Deschampsia flexuosa are also abundant.

Competition between Sphagnum fuscum and Drosera rotundifolia: a case of ecosystem engineering

In a three-year field experiment in a subarctic Swedish bog, the moss Sphagnum fuscum and the perennial vascular plant Drosera rotundifolia were selectively fed with a growth-limiting factor, nitrogen. When fertilised, S. fuscum responded by an increase in the height of its green parts. D. rotundifolia also responded to S. fuscum fertilisation with an increase in height of the vertical stem that connects the leaf rosettes of two successive years. Thus, D. rotundifolia avoided being overgrown. Fertilisation of D. rotundifolia led to increases in number of leaves and specific leaf mass. No leaf area effect was seen ; i.e., the leaves were smaller and thicker, implying that the increased nitrogen was allocated to resources that increased survival probabilities but did not shade the moss. It is concluded that competition for growth-limiting mineral nutrients in Sphagnum peatlands strongly influences community structure and species diversity and that the mechanism whereby this is brought about lies in the spatially differentiated uptake zones of the moss and the vascular plant. The moss acts as a small-scale autogenic ecological engineer by capturing mineral nutrients early in their circulation through the ecosystem and the moss retains and relocates them within itself, thereby counteracting the potential spread of vascular plants.

Apical dominance and the simulation of metapopulation dynamics in Lycopodium annotinum

Individual branched systems of the modular plant Lycopodium annotium L. were studied in Swedish Lapland. Survival and fecundity of horizontal modules varied with module age and the health of the apical growing points. The negative relationship found between survival and reproduction is explained as the effects of released dominance, since more daughter-modules are generally produced behind dead, or dying apices than behind living apices. Apical dominance also controlled the size of sub-dominant modules. Behind an unhealthy main dominant apex, the closest daughter-module to be produced was always horizontal. It is suggested that this increases the possibilities for the continuation of the life of the plant, since differentiation into only vertical modules would terminate the growth of the axis. When the main apex dies, opportunistic branching occurs and widens the potential zone of exploitation, thereby increasing the chance of survival of the plant. Simulations of age-class distributions of horizontal modules using transition probability matrices showed that the stable age class distribution coincides with that found in the field, except for the younger age classes. This may be due to the difficulties in identifying young horizontal modules in the field. The survivorship of horizontal modules shows a Deevey type II curve, characteristic of perennial plant species. An elasticity analysis indicated that survival is more important than fecundity in determining population growth rate.

Dispersal and persistence: Population processes and community dynamics

If we look at plant cover at a certain site, we can observe that it is made up of a number of species of different size and form, conferring a certain structure to the vegetation cover. If we compare two sites, we invariably see differences in structure and species composition. These differences are repeated across a hierarchy of scales: at the local scale, the regional scale and ultimately at the continental scale. By comparing structure and species compositions across a larger number of sites, a statistical pattern emerges from the data: some species are found together more frequently than expected and others seem to avoid each other. In fact, these positive and negative associations result in the observation that the species composition of a site is not just a random collection of species but is patterned, resulting in recognizable plant communities. In a spatial context we can fruitfully distinguish between within-site and among-site processes that influence the plant community. This distinction enables the description of species composition as a result of a dynamic process of within-community extinctions and colonizations from the outside. According to this view, community structure results from the differential performance of species as a function of biotic and abiotic environmental factors during the period between colonization and extinction. It now very much depends on the temporal scale of the dynamics of the system whether one puts emphasis on deterministic within-community processes, such as intraand interspecific competition, or on the stochastic processes of colonization and extinction. In fact there has been a long-standing debate about the relative importance of these two underlying assortative processes that are responsible for the perceived patterns in species composition (resulting in recognizable plant communities). This debate has its roots in the beginning of the last century and is conveniently summarized as the Clementsian versus the Gleasonian view on plant communities. The Clementsian view with its emphasis on the role of abiotic conditions and the regulative power of competitive processes has been dominant for a number of decades. In fact, a larger part of vegetation science is about finding the relationship between species composition and environmental factors. The fact that, given certain environmental conditions, we are able to

How can we protect rare hemiparasitic plants? Early-flowering taxa of Euphrasia and Rhinanthus on the Baltic island of Gotland

Euphrasia stricta var.suecica, E. stricta var.tenuis, andRhinanthus serotinus subsp.vernalis are three endangered, hemiparasitic annual herbs found in traditionally managed hay meadows on Gotland, Sweden. We have studied — experimentally and in the field — how some features in the present and traditional management cycle affect their fitness.We set up permanent 50 × 50-cm plots in a coastal hay meadow where bothE. stricta var.suecica andRhinanthus are still quite common. The survival, growth and fecundity of cohorts ofEuphrasia andRhinanthus were followed throughout the growing season for three years. The length of the growing season was vital forEuphrasia andRhinanthus performance. In cooler summers, the percentage ofEuphrasia individuals that produced mature seeds was decreased by 20% at the time of mowing. This indicates the need to individually adjust the time of mowing, as was formerly done, to the actual phenological development in order to maintain healthy populations. It is also important to consider the effect of time of mowing on total species richness, asEuphrasia more successfully established in 10 × 10-cm squares with high species richness.Also, the time in spring when hemiparasite growth started was crucial. Cohorts of bothEuphrasia stricta var.suecica andRhinanthus that connected to hosts and started growing early in the season had a strong advantage over later cohorts by having fitness values four to eight times higher. From this we conclude that the traditional practice of spring raking is important for the long-term persistence of these hemiparasites, since raking promotes an earlier onset of both host and hemiparasite growth.Another important issue is the traditional, but nowadays often neglected practice of letting the hay dry in the meadow after mowing. Hay that was left to dry in the meadow contributed significantly moreRhinanthus seed to the meadow than hay that was taken away immediately after mowing.We also found that seedling emergence and subsequent growth ofEuphrasia stricta var.suecica was greatly enhanced by a second hay cut in September. A second cutting mimics some of the positive effects of the traditional practice of aftermath grazing, which is nowadays often abandoned.

Fitness, population growth rate and flowering in Carex bigelowii, a clonal sedge

without daughter tillers, together with an increase in the proportion of tillers with more than one daughter tiller. At the third site the same phenomena were observed but, in addition, the proportion of tillers with one daughter tiller was slightly reduced as well. Tiller systems at this site also differed from the others by showing a positive correlation between fitness and variation in daughter tiller production. At no site was mean rhizome length of tiller systems correlated with fitness. At the population level, the tiller densities at the lower and intermediate sites appeared to have been stationary (no net trend) during the past 10-20 years. At the upper site, the number of tillers in the sampled quadrats had increased, either due to high mobility of the tiller systems or to an actual increase in overall population density. We also measured the flowering rates in the populations and found no major differences between the sites. No seedling establishment of C. bigelowii has been recorded in this area and the costs of sexual reproduction, in terms of decreased vegetative reproduction, are therefore expected to be low. This expectation was in accordance with an earlier study finding no such costs associated with flowering and seed set.

Ramet dynamics in a centrifugally expanding clonal sedge: a matrix analysis

Carex humilis is a clonal sedge that can form distinct rings of densely aggregated ramets. We hypothesize that rings form because both production of new ramets and ramet dispersal are positively correlated to ramet size. This would lead to an overrepresentation of fast-moving and large ramets with high ramet production at the periphery, whereas slow-moving and small ramets with low ramet production would mainly be found in the interior of rings. We use matrix models to analyse how ramet populations both at the periphery and in the interior develop in the absence of ramet dispersal. We found that the stable size class distributions of ramets predicted by the models were not different from the distributions found in the field. Also, the asymptotic ramet population growth rates (λ1) were the same. Hence, we conclude that rings would form even in the absence of a link between ramet dispersal and ramet production. Further analysis of the matrix models showed that the ramet population increases at the periphery but decreases in the interior of rings because medium and large ramets produce fewer large ramets in the interior than at the periphery. We also found that the temporal variance in λ1 and transitions rates during the four study years was much higher at the periphery than in the interior. Our results suggest that rings may form because C. humilis ramets use below-ground resources from a much larger area than the one covered by the shoots. As the clone grows larger, the soil volume available to the ramets in the interior decreases because their access to soil outside the ring is cut-off by the ramets at the periphery. Ramet density in the interior is therefore decreasing.

Reliance on prey‐derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition

• Carnivory in plants is presumed to be an adaptation to a low-nutrient environment. Nitrogen (N) from carnivory is expected to become a less important component of the N budget as root N availability increases. • Here, we investigated the uptake of N via roots versus prey of the carnivorous plant Drosera rotundifolia growing in ombrotrophic bogs along a latitudinal N deposition gradient through Sweden, using a natural abundance stable isotope mass balance technique. • Drosera rotundifolia plants receiving the lowest level of N deposition obtained a greater proportion of N from prey (57%) than did plants on bogs with higher N deposition (22% at intermediate and 33% at the highest deposition). When adjusted for differences in plant mass, this pattern was also present when considering total prey N uptake (66, 26 and 26 μg prey N per plant at the low, intermediate and high N deposition sites, respectively). The pattern of mass-adjusted root N uptake was opposite to this (47, 75 and 86 μg N per plant). • Drosera rotundifolia plants in this study switched from reliance on prey N to reliance on root-derived N as a result of increasing N availability from atmospheric N deposition.

Changes versus homeostasis in alpine and sub-alpine vegetation over three decades in the sub-arctic.

Plant species distributions are expected to shift and diversity is expected to decline as a result of global climate change, particularly in the Arctic where climate warming is amplified. We have recorded the changes in richness and abundance of vascular plants at Abisko, sub-Arctic Sweden, by re-sampling five studies consisting of seven datasets; one in the mountain birch forest and six at open sites. The oldest study was initiated in 1977–1979 and the latest in 1992. Total species number increased at all sites except for the birch forest site where richness decreased. We found no general pattern in how composition of vascular plants has changed over time. Three species, Calamagrostis lapponica, Carex vaginata and Salix reticulata, showed an overall increase in cover/frequency, while two Equisetum taxa decreased. Instead, we showed that the magnitude and direction of changes in species richness and composition differ among sites.

Genetic structure in two meadow varieties of Euphrasia stricta on the Baltic island of Gotland (Sweden) and implications for conservation

The annual hemiparasiteEuphrasia stricta occurs on Gotland in two early-flowering meadow varieties.E. stricta var.suecica is on the Swedish red-list as endangered, occurring in Sweden only on the Baltic island of Gotland. It probably has near relatives east of the Baltic proper. The other variety,E. stricta var.tenuis, has a wider distribution occurring almost all over Sweden, but is declining in abundance. Both varieties have close morphological similarities and habitat preferences, raising questions about which level conservation efforts should be focused on, varieties or populations. In this study we describe the genetic structure between and within these two varieties using amplified fragment length polymorphism, AFLP.FST between varieties is 0.14, and between populations within the varietiesE. stricta var.suecica and var.tenuis FST is 0.60 and 0.26 respectively. The partitioning of gene diversity to different levels shows that 14% of the genetic diversity occurs between varieties, 42% between populations within varieties, and 44% within populations. Significant genetic differentiation was detected between varieties, populations and subpopulations within populations using a constrained principal coordinate analysis.We suggest that all of the existing populations of these two varieties on Gotland should be preserved, since they are few (6E. stricta var.suecica and 5E. stricta var.tenuis) and much of the genetic diversity is partitioned between populations.