Diethart Matthies

RAPD variation in relation to population size and plant fitness in the rare Gentianella germanica (Gentianaceae)

We investigated the distribution of genetic variation and the relationship between population size and genetic variation in the rare plant Gentianella germanica using RAPD (random amplified polymorphic DNA) profiles. Plants for the analysis were grown from seeds sampled from 72 parent plants in 11 G. germanica populations of different size (40-5000 fruiting individuals). In large populations, seeds were sampled from parents in two spatially distinct subpopulations comparable in area to the total area covered by small populations. Analysis of molecular variance revealed significant genetic variation among populations (P <0.001), while genetic variation among subpopulations was marginally significant (P <0.06). Average molecular variance within subpopulations in large populations did not differ significantly from whole-population values. There was a positive correlation between genetic variation and population size (P <0.01). Genetic variation was also positively correlated with the number of seeds per plant in the field (P <0.02) and the number of flowers per planted seed in a common garden experiment (P <0.051). We conclude that gene flow among natural populations is very limited and that reduced plant fitness in small populations of G. germanica most likely has genetic causes. Management should aim to increase the size of small populations to minimize further loss of genetic variation. Because a large proportion of genetic variation is among populations, even small populations are worth preserving.

Mating structure and inbreeding and outbreeding depression in the rare plant Gentianella germanica (Gentianaceae).

Isolation and small size of populations as a result of habitat destruction and fragmentation may negatively affect plant fitness through pollinator limitation and increased levels of inbreeding. To increase genetic variation in small populations of rare plants artificial gene flow has been suggested as a management tool. We investigated whether pollinator limitation and inbreeding depression could reduce fitness in Gentianella germanica, an endangered biennial of increasingly fragmented calcareous grasslands in Central Europe. We experimentally excluded pollinators and generated progenies by hand-pollinating flowers with pollen from different distances. G. germanica was highly selfing. Pollinator exclusion strongly reduced seed set, indicating that pollinator limitation could potentially reduce plant fitness. Germination rate as well as number of leaves and rosette size of progeny from 10-m crosses was higher than that of progeny from open pollinations, self-, 1-m, and interpopulation crosses. After 6 mo of growth differences in the number of surviving plants persisted, whereas differences in plant size did not. The results suggest that inbreeding depression may reduce plant performance in G. germanica. Outbreeding depression in the performance of progeny from interpopulation crosses indicates that caution is necessary in using artificial interpopulation gene flow as a management tool.

Impact of Plant Functional Group, Plant Species, and Sampling Time on the Composition of nirK-Type Denitrifier Communities in Soil

ABSTRACT We studied the influence of eight nonleguminous grassland plant species belonging to two functional groups (grasses and forbs) on the composition of soil denitrifier communities in experimental microcosms over two consecutive years. Denitrifier community composition was analyzed by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. The impact of experimental factors (plant functional group, plant species, sampling time, and interactions between them) on the structure of soil denitrifier communities (i.e., T-RFLP patterns) was analyzed by canonical correspondence analysis. While the functional group of a plant did not affect nirK-type denitrifier communities, plant species identity did influence their composition. This effect changed with sampling time, indicating community changes due to seasonal conditions and a development of the plants in the microcosms. Differences in total soil nitrogen and carbon, soil pH, and root biomass were observed at the end of the experiment. However, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, e.g., through root exudates. Assignment of abundant T-RFs to cloned nirK sequences from the soil and subsequent phylogenetic analysis indicated a dominance of yet-unknown nirK genotypes and of genes related to nirK from denitrifiers of the order Rhizobiales. In conclusion, individual species of nonleguminous plants directly influenced the composition of denitrifier communities in soil, but environmental conditions had additional significant effects.

Activation of Methanogenesis in Arid Biological Soil Crusts Despite the Presence of Oxygen

Methanogenesis is traditionally thought to occur only in highly reduced, anoxic environments. Wetland and rice field soils are well known sources for atmospheric methane, while aerated soils are considered sinks. Although methanogens have been detected in low numbers in some aerated, and even in desert soils, it remains unclear whether they are active under natural oxic conditions, such as in biological soil crusts (BSCs) of arid regions. To answer this question we carried out a factorial experiment using microcosms under simulated natural conditions. The BSC on top of an arid soil was incubated under moist conditions in all possible combinations of flooding and drainage, light and dark, air and nitrogen headspace. In the light, oxygen was produced by photosynthesis. Methane production was detected in all microcosms, but rates were much lower when oxygen was present. In addition, the δ13C of the methane differed between the oxic/oxygenic and anoxic microcosms. While under anoxic conditions methane was mainly produced from acetate, it was almost entirely produced from H2/CO2 under oxic/oxygenic conditions. Only two genera of methanogens were identified in the BSC-Methanosarcina and Methanocella; their abundance and activity in transcribing the mcrA gene (coding for methyl-CoM reductase) was higher under anoxic than oxic/oxygenic conditions, respectively. Both methanogens also actively transcribed the oxygen detoxifying gene catalase. Since methanotrophs were not detectable in the BSC, all the methane produced was released into the atmosphere. Our findings point to a formerly unknown participation of desert soils in the global methane cycle.

Parasitic and Competitive Interactions between the Hemiparasites Rhinanthus Serotinus and Odontites Rubra and their Host Medicago Sativa

1 Host plants are for hemiparasites both their main source of water and nutrients and potential competitors for light. To investigate the balance between costs and benefits of host presence a greenhouse experiment was performed with the two facultative root hemiparasites Rhinanthus serotinus and Odontites rubra and their host Medicago sativa. Above-ground (light competition) and below-ground (parasitic) interactions were separated and the host plants were killed 4, 8 and 12 weeks after planting by cutting off the shoots. 2 R. serotinus remained smaller than 0. rubra when grown autotrophically and its growth was more stimulated by the presence of the host, indicating that R. serotinus has a higher degree of host dependence. The marked reduction in root allocation by both hemiparasites in the presence of the host was more pronounced in R. serotinus. 3 Above-ground competition by the host reduced the growth of both hemiparasites by more than 30%. However, the negative effect of removing the host after 4, 8 and 12 weeks of development indicated that the beneficial effects of the host predominated at all stages. 4 Both parasites had negative effects on host growth that were much stronger than that of another M. sativa individual. Above-ground separation of parasites and hosts had no effect on host biomass, indicating that the negative effect was due exclusively to parasitism. 5 Host root biomass was reduced more strongly than host shoot biomass by the parasites, resulting in a lower biomass allocation to roots. 6 The presence of the hemiparasites reduced the total productivity per pot. This suggests that the parasites have a lower efficiency of resource utilization than the hosts. 7 It is suggested that the balance between the effects of root parasitism on the host (e.g. via nutrient competition) and the effects of light competition by the host on the parasite restricts hemiparasites to relatively nutrient-poor habitats.

Interactions between the root hemiparasite Melampyrum arvense and mixtures of host plants: heterotrophic benefit and parasite- mediated competition

Root hemiparasitic plants frequently establish haustorial connections simultaneously to several host species, which may differ in the quality of their contribution to the nutrition of the parasite and in their sensitivity to parasitic attack. The obligate hemiparasite Melampyrum arvense was grown with three host species (Lolium perenne, Medicago sativa and Linum usitatissimum) and with all possible binary host mixtures to test whether (1) a mixed host supply (e.g. a legume and a grass) results in an increase in heterotrophic benefit to the parasite and (2) whether the hemiparasite may influence the competitive balance between hosts. Growth of M. arvense was much stronger with the legume (M. sativa) as host than with the two non-leguminous species. The biomass of parasites grown with binary mixtures of the host species was not higher than that of parasites grown with the three host species separately. Thus, the results do not support the hypothesis that hemiparasites benefit from a mixed host supply. The rank order of species in terms of host quality (M. sativa > L. usitatissimum > L. perenne) was the same as that in terms of damage suffered from the hemiparasite. Host and parasite biomass in individual pots were also negatively correlated. In two of the binary mixtures of host species M. arvense influenced the competitive balance between hosts, because the hemiparasite reduced the growth of the legume M. sativa much more strongly than that of the other species. Overall productivity per pot (hosts + hemiparasite) was reduced by M. arvense, because parasite biomass did not fully compensate for the reduction in host biomass caused by parasitism.

Demographic stochasticity in population fragments of the declining distylous perennial Primula veris (Primulaceae)

Abstract We studied ecological consequences of distyly for the declining perennial plant Primula veris in the Swiss Jura. Distyly favours cross-fertilization and avoids inbreeding, but may lead to pollen limitation and reduced reproduction if morph frequencies deviate from 50%. Disassortative mating is promoted by the reciprocal position of stigmas and anthers in the two morphs (pin and thrum) and by intramorph incompatibility and should result in equal frequencies of morphs at equilibrium. However, deviations could arise because of demographic stochasticity, the lower intra-morph incompatibility of the pin morph, and niche differentiation between morphs. Demographic stochasticity should result in symmetric deviations from an even morph frequency among populations and in increased deviations with decreasing population size. If crosses between pins occurred, these would only generate pins, and this could result in a pin-bias of morph frequencies in general and in small populations in particular. If the morphs have different niches, morph frequencies should be related to environmental factors, morphs might be spatially segregated, and morphological differences between morphs would be expected. We tested these hypotheses in the declining distylous P. veris. We studied morph frequencies in relation to environmental conditions and population size, spatial segregation in field populations, morphological differences between morphs, and growth responses to nutrient addition. Morph frequencies in 76 populations with 1–80000 flowering plants fluctuated symmetrically about 50%. Deviations from 50% were much larger in small populations, and six of the smallest populations had lost one morph altogether. In contrast, morph frequencies were neither related to population size nor to 17 measures of environmental conditions. We found no spatial segregation or morphological differences in the field or in the common garden. The results suggest that demographic stochasticity caused deviations of the morph ratio from unity in small populations. Demographic stochasticity was probably caused by the random elimination of plants during the fragmentation of formerly large continuous populations. Biased morph frequencies may be one of the reasons for the strongly reduced reproduction in small populations of P. veris. Wir untersuchten die Folgen der Distylie fur die seltene, ausdauernde Pflanzenart Primula veris im Schweizer Jura. Distylie begunstigt Fremdbestaubung und vermeidet Inzucht, konnte aber, falls die Haufigkeit der beiden Morphen unterschiedlich ist, zu Pollenlimitierung und verringerter Reproduktion fuhren. Bei distylen Pflanzen werden dissortative Paarungen durch die unterschiedliche Position von Narben und Antheren bei den beiden Morphen (lang- und kurzgriffelig) sowie durch weitgehende Selbstinkompatibilitat der Morphen begunstigt, was ein ausgeglichenes Morphenverhaltnis zur Folge haben sollte. In kleinen Populationen konnte jedoch demographische Stochastizitat zu einem unausgeglichenen Morphenverhaltnis fuhren. Bei P. veris konnten zudem Kreuzungen zwischen gleichen Morphen auf Grund der groseren Selbstkompatibilitat der langgriffeligen Pflanzen diese haufiger werden lassen. Auch Nischendifferenzierung zwischen den Morphen konnte das Morphenverhaltnis beeinflussen. In diesem Fall sollten die Morphen raumlich segregiert sein, morphologische Unterschiede aufweisen und das Morphenverhaltnis sollte durch Umweltfaktoren beeinflusst werden. In den untersuchten 76 Populationen von P. veris mit 1–80000 bluhenden Pflanzen variierte die relative Haufigkeit der Morphen symmetrisch um 50%. Abweichungen von 50% waren in kleinen Populationen viel groser und sechs der kleinsten Populationen hatten eine Morphe vollig verloren. Im Gegensatz dazu wurde die Haufigkeit einer Morphe nicht durch die Populationsgrose oder durch 17 untersuchte Umweltfaktoren beeinflusst. Wir fanden keine raumliche Segregation zwischen den Morphen und weder in der Natur noch im Versuchsgarten Unterschiede in vegetativen oder reproduktiven Merkmalen. Die Ergebnisse deuten darauf hin, dass demographische Stochastizitat als Folge der Fragmentierung ehemals groser Populationen fur die ungleiche Haufigkeit der Morphen in kleinen Populationen verantwortlich ist. Das unausgeglichene Morphenverhaltnis konnte eine der Ursachen fur die stark reduzierte Reproduktion der Pflanzen in kleinen Populationen sein.

Long-term effects of short-term perturbation in a subalpine grassland.

Theoretical advances and short-term experimental studies have furthered our understanding of how ecosystems respond to perturbation. However, there are few well-replicated experimental studies that allow an assessment of long-term responses. Results from a controlled, large-scale field experiment in a subalpine grassland near Interlaken, Switzerland, show that 2-4 years of liming (Ca: 40 g x m(-2) x yr(-1)) still significantly affected the composition of the vegetation and the soil microbial community nearly 70 years after the treatments were imposed, whereas NPK fertilization (8 g x m(-2) x yr(-1)) only marginally affected vegetation composition. The exchangeable content of Ca ions and soil pH were higher in limed plots but were unaffected in fertilized plots. Plant species and PLFAs (phospholipid fatty acids) indicating low pH values were found in higher abundance in the unlimed plots, suggesting that the long-lasting effects of liming on the above- and belowground communities were mediated through changes in soil pH. The results of this long-term study indicate that the resilience of mountain ecosystems may be particularly low in response to perturbations that substantially alter soil pH or other key determinants of belowground processes.

Plant presence and species combination, but not diversity, influence denitrifier activity and the composition of nirK‐type denitrifier communities in grassland soil

To explore potential links between plant communities, soil denitrifiers and denitrifier function, the impact of presence, diversity (i.e. species richness) and plant combination on nirK-type denitrifier community composition and on denitrifier activity was studied in artificial grassland plant assemblages over two consecutive years. Mesocosms containing zero, four and eight species and different combinations of two species were set up. Differences in denitrifier community composition were analysed by canonical correspondence analyses following terminal restriction fragment length polymorphism analysis of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. As a measure of denitrifier function, denitrifier enzyme activity (DEA) was determined in the soil samples. The presence as well as the combination of plants and sampling time, but not plant diversity, affected the composition of the nirK-type denitrifier community and DEA. Denitrifier activity significantly increased in the presence of plants, especially when they were growing during summer and autumn. Overall, we found a strong and direct linkage of denitrifier community composition and functioning, but also that plants had additional effects on denitrifier function that could not be solely explained by their effects on nirK-type denitrifier community composition.

Plasticity of reproductive components at different stages of development in the annual plant Thlaspi arvense L.

SummaryThis study examines the effect of different densities and the removal of all neighbours at different stages of development on all components of reproduction in the inbreeding annual Thlaspi arvense L. A 64-fold increase in density significantly reduced all repooductive components. The number of flower buds per plant was decreased most strongly; the order of decreasing plasticity among the other components was number of capsules per flower, individual seed weight, ovule number per capsule, flowers per flower bud and seeds per ovule. Removing neighbours at all stages of development increased seed yield of plants in comparison to the control without density reduction, but patterns of plasticity depended strongly on time of treatment. The main effect of the removal of neighbours at the vegetative stage was to increase the number of flowers per plant, but the number of ovules per capsule and seed weight increased also, and abortion of capsules decreased. Removing neighbours at the onset of flowering initially failed to affect flower number per plant, instead it resulted in a strong reduction of capsule abortion and an increase in seed weight. However, several weeks after flowering had initially ceased, fresh lateral inflorescences were produced, resulting in a second flush of reproduction. Removing neighbours at the stage of fruit ripening resulted at first only in an increase in seed witht, but later a second reproductive phase occurred. Fresh lateral branches were produced, but the apical meristem was also reactivated. The overall pattern of plasticity among all reproductive components in response to a removal of neighbours was the same as in response to density. The position of a capsule along the inflorescence influenced its number of ovules, the rate of seed abortion and the mean weight of seeds, with the type of effect depending on the developmental stage at which neighbours were removed. Significant negative correlations were found between the mean weight of seeds and the number of seeds in a capsule under all treatments.