Showing papers by "Bernhard Schmid published in 2000"

The effect of nutrient availability on biomass allocation patterns in 27 species of herbaceous plants

Abstract: We investigated allocation to roots, stems and leaves of 27 species of herbaceous clonal plants grown at two nutrient levels. Allocation was analyzed as biomass ratios and also allometrically. As in other studies, the fraction of biomass in stems and, to a lesser extent, in leaves, was usually higher in the high-nutrient treatment than in the low-nutrient treatment, and the fraction of biomass in roots was usually higher under low-nutrient conditions. The relationship between the biomass of plant structures fits the general allometric equation, with an exponent 1 in most of the species. The different biomass ratios under the two nutrient conditions represented points on simple allometric trajectories, indicating that natural selection has resulted in allometric strategies rather than plastic responses to nutrient level. In other words, in most of the species that changed allocation in response to the nutrient treatment, these changes were largely a consequence of plant size. Our data suggest that some allocation patterns that have been interpreted as plastic responses to different resource availabilities may be more parsimoniously explained as allometric strategies.

Plant diversity effects on soil heterotrophic activity in experimental grassland ecosystems

Abstract: The loss of plant species from terrestrial ecosystems may cause changes in soil decomposer communities and in decomposition of organic material with potential further consequences for other ecosystem processes. This was tested in experimental communities of 1, 2, 4, 8, 32 plant species and of 1, 2 or 3 functional groups (grasses, legumes and non-leguminous forbs). As plant species richness was reduced from the highest species richness to monocultures, mean aboveground plant biomass decreased by 150%, but microbial biomass (measured by substrate induced respiration) decreased by only 15% (P = 0.05). Irrespective of plant species richness, the absence of legumes (across diversity levels) caused microbial biomass to decrease by 15% (P = 0.02). No effect of plant species richness or composition was detected on the microbial metabolic quotient (qCO2) and no plant species richness effect was found on feeding activity of the mesofauna (assessed with a bait-lamina-test). Decomposition of cellulose and birchwood sticks was also not affected by plant species richness, but when legumes were absent, cellulose samples were decomposed more slowly (16% in 1996, 27% in 1997, P = 0.006). A significant decrease in earthworm population density of 63% and in total earthworm biomass by 84% was the single most prominent response to the reduction of plant species richness, largely due to a 50% reduction in biomass of the dominant `anecic' earthworms. Voles (Arvicola terrestris L.) also had a clear preference for high-diversity plots. Soil moisture during the growing season was unaffected by plant species richness or the number of functional groups present. In contrast, soil temperature was 2 K higher in monocultures compared with the most diverse mixtures on a bright day at peak season. We conclude that the lower abundance and activity of decomposers with reduced plant species richness was related to altered substrate quantity, a signal which is not reflected in rates of decomposition of standard test material. The presence of nitrogen fixers seemed to be the most important component of the plant diversity manipulation for soil heterotrophs. The reduction in plant biomass due to the simulated loss of plant species had more pronounced effects on voles and earthworms than on microbes, suggesting that higher trophic levels are more strongly affected than lower trophic levels.

Numerical responses of different trophic groups of invertebrates to manipulations of plant diversity in grasslands.

Abstract: We studied the effects of plant diversity on abundance of invertebrate herbivores, parasitoids and predators in two grassland communities (one in Switzerland and one in Sweden) in which plant species richness and functional diversity have been experimentally manipulated. Among herbivores, the abundance of only the most sessile and specialised groups (leafhoppers and wingless aphids) was affected by plant diversity. At both sites, numbers of leafhoppers in sweep net samples showed a linear, negative relationship with plant species number whereas numbers of wingless aphids in suction samples increased with the number of plant functional groups (grasses, legumes, and non-legume forbs) present in the plot. Activity of carabid beetles and spiders (as revealed by pitfall catches) and the total number of predators in pitfalls at the Swiss site decreased linearly with increases in the number of plant species and plant functional groups. Abundance of more specialised enemies, hymenopteran parasitoids, was not affected by the manipulations of plant diversity. Path analysis and analysis of covariance indicated that plant diversity effects on invertebrate abundance were mostly indirect and mediated by changes in plant biomass and cover. At both sites, plant species composition (i.e. the identity of plant species in a mixture) affected numbers of most of the examined groups of invertebrates and was, therefore, a more important determinant of invertebrate abundance in grasslands than plant species richness per se or the number of plant functional groups. The presence of legumes in a mixture was especially important and led to higher numbers of most invertebrate groups. The similarity of invertebrate responses to plant diversity at the two study sites indicates that general patterns in abundance of different trophic groups can be detected across plant diversity gradients under different environmental conditions.

Plant diversity affects culturable soil bacteria in experimental grassland communities

Abstract: Summary 1 Utilization of carbon sources by culturable soil bacteria can be assessed with BIOLOG microtiter plates (contain 31 C sources). We used this technique to investigate bacterial community structure at various levels of plant diversity. Plant diversity levels were replicated and we investigated the influence of three plant functional groups, grasses, legumes and non-leguminous herbs, as well as the influence of individual plant species. 2 Catabolic activity and catabolic diversity of culturable soil bacteria were used to estimate their density (abundance) and functional diversity, respectively. Both increased linearly with the logarithm of plant species number and with the number of plant functional groups in experimental grassland ecosystems. These effects may have been caused by an increased diversity and quantity of material and energy flows to the soil. They may also have been mediated by increased diversity of soil microhabitats via a stimulation of the soil fauna. 3 The presence of particular plant species or functional groups in the different experimental communities stimulated the activity and functional diversity of the culturable soil bacteria in addition to their contribution via plant diversity. The legume Trifolium repens had the strongest effect and may be regarded as a keystone species with regard to plant–microbial interactions in the systems studied.

Above-ground resource use increases with plant species richness in experimental grassland ecosystems

Abstract: 1. Diversity effects on canopy structure and the use of space and light were investigated in experimental grassland communities of one, two, four, eight or 32 species and of one, two or three functional groups (grasses, legumes and non-leguminous forbs) in north-western Switzerland. 2. Plant species and functional group richness improved the establishment of grassland assemblages and increased mean vegetation cover from 64% in monocultures to 100% in 32-species mixtures. 3. Above-ground biomass and leaf area index increased linearly with the logarithm of plant species number in all 3 years of the experiment. The most species-rich communities produced 143% more biomass than the mean of all monocultures and 25% more biomass than the most productive monoculture in the third year of the experiment. 4. Plant light absorbance per unit ground area increased by 44% with diversity (32-species mixtures compared with monocultures) for two reasons. (i) The lack of species complementarity in terms of success of seedling establishment and plant size in vegetation with low species diversity increased the gap area in the canopy, and consequently more light reached bare ground. (ii) Diverse ecosystems absorbed more light within their closed canopies (+20%) than did less diverse ecosystems, because of better three-dimensional space filling and greater biomass density due to complementarity in plant architecture, particularly between grasses and dicots. 5. The overall canopy height was increased by 46% and the centre of gravity of the canopy was raised by up to 10 cm over the range of experimental diversity treatments from one to 32 species. 6. These results cannot be explained solely by a higher likelihood of the inclusion of more productive dominant species in the more diverse communities (sampling effect), which suggests that biomass production may be determined by the most productive species present in a community. For all important canopy characteristics of multispecies communities, we show that with increasing diversity, mixtures perform better than the best monoculture. This is a strong indication that part of the positive effect of diversity on biomass production is driven by complementary resource use.

RAPD variation among and within small and large populations of the rare clonal plant Ranunculus reptans (Ranunculaceae).

Abstract: In the pre-alpine region of Europe numbers and sizes of populations of the clonal lake shore plant Ranunculus reptanshave declined because of the regulation of lake water levels. We investigated genetic variation among and within 17 populations of different size (cover 1‐10 000 m2 )i nR. reptans with RAPD (random amplified polymorphic DNA) profiles. We sampled 127 rosettes in 14 populations at Lake Constance and three populations at or near Lake Como. There was significant genetic variation between plants from the two lake regions (5.9%, analysis of molecular variance [AMOVA], P , 0.001), among populations within lake regions (20.4%, P , 0.001), and within populations (73.7%, P , 0.001). Under the assumptions of Wright’s island model the variation among populations corresponds to a gene flow of Nem 5 0.70. Within the 14 Lake Constance populations we detected significant genetic variation among subpopulations separated by only a few metres (4.0% of the within-population variation; P , 0.05). Molecular variance was 24% smaller in small populations covering ,100 m2 area than in larger ones (P , 0.03), indicating that samples from large populations were genetically more variable than samples representing comparable areas of smaller populations. We conclude that gene flow among populations is very limited and that genetic drift has caused reduced genetic variability of smaller populations. Conservation of genetic variability in R. reptans requires persistence of large and also of small populations (because of population differentiation), and it could be enhanced by increasing the size of small populations (to counter genetic drift).

Genetic differentiation of life‐history traits within populations of the clonal plant Ranunculus reptans

Abstract: We tested the hypothesis that environmental heterogeneity can maintain genetic variation in life-history traits within populations of the clonal plant Ranunculus reptans. This may be important for the further evolution and potential adaptation to environmental change. Moreover, we asked to what extent environmental heterogeneity can also reveal trade-offs among fitness components, in particular between sexual reproduction and clonal growth. In the natural habitat of R. reptans two distinct types of environment can often be found within 10 m distance: vegetation-sparse zones close to the water (“lake”) and zones of competition with grasses (mostly Agrostis stolonifera) more inland (“land”). We grew vegetative offspring of 16 land genotypes and 16 lake genotypes from four populations in a plant room. Cuttings from each genotype were grown with and without competition by the grass A. stolonifera and on two different substrates, sand and gravel. We found considerable environmental (E) and genetic (G) variation in life-history traits. Competition strongly reduced growth, branching, flowering, and final biomass. Lake genotypes invested more into sexual reproduction (measured as proportion of ramets that flowered), whereas land genotypes invested more into vegetative reproduction (proportion of rooted ramets). Significant G×E interactions were consistent with a home-away effect: land-genotypes performed better if grown under competition, lake-genotypes without competition. These results indicate that genetic variation and phenotypic plasticity in life-history traits of R. reptans are maintained in a heterogeneous environment, thereby supporting the main hypothesis stated above. They further demonstrate that genetic differentiation occurs even over very short distances (10 m). We also found a significant genetic trade-off between sexual and vegetative reproduction (negative genetic correlation between the proportion of flowering ramets and the proportion of rooted ramets). In response to the supplementary question posed above, this suggests that the environmental heterogeneity finds its correspondence in the “genetic integration” of life-history traits within the phenotype of R. reptans.

Isolation and characterization of microsatellite loci in the bearded vulture (Gypaetus barbatus) and cross-amplification in three old world vulture species.

Abstract: Keywords: Aegypius monachus; conservation genetics; cross-species amplification; Gypaetus barbatus; Gyps fulvus; Neophron percnopterus

Root hemiparasites and plant diversity in experimental grassland communities.

Abstract: Summary 1 We studied the relationship between the diversity of grassland communities and the effects of the generalist hemiparasitic plant Rhinanthus alectorolophus. We compared resistance against biomass loss as a consequence of infection, performance of the parasite and resistance of the parasitized communities to invasion by other plant species. Seeds of the parasite were sown into experimental plots containing 1, 2, 4, 8 or 32 plant species belonging to one or more of three functional groups (grasses, legumes and non-leguminous herbs). 2 We predicted that infection will reduce host biomass, total community biomass and resistance to invasion, particularly in host communities with low diversity, but that the performance of the parasite will be at its lower level in such communities. 3 The presence of the parasite caused an overall reduction in host biomass per plot, which was mainly due to a strong reduction in the biomass of grasses. As predicted, the effect was smaller in communities with greater functional diversity. However, total community biomass (including the parasite biomass) was increased by more than a third in infected communities of one or two host species, while the parasite had no effect on total biomass of species-rich communities. 4 Germination of the parasite was hardly influenced by the diversity of its host community, but early survival decreased with increasing number of functional groups and was lower in plots with legumes than without. However, our hypothesis that the performance of the surviving parasites would benefit from a high functional diversity of hosts was supported. Parasite biomass per individual and per m2 increased with the number of functional groups in the host community, as did reproductive potential. 5 Death of the parasite led to a higher proportion of bare ground in communities when the previously infected communities had low functional diversity, thus enabling subsequent colonization by weeds.

Genetic Allee effects on performance, plasticity and developmental stability in a clonal plant

Abstract: Negative effects of small population size on fitness, so-called Allee effects, may threaten population persistence even in intact habitat remnants. We studied genotypes of 14 isolated populations of the clonal plant Ranunculus reptans, for which molecular genetic (RAPD-) variability is higher for large than for small populations. In a competition-free greenhouse environment vegetative offspring of genotypes from large populations produced more rosettes and flowers, indicating higher fitness. Within-genotype coefficients of variation in performance traits, indicating developmental instability, were lower for genotypes from populations with higher RAPD-variability. In competition with a taller grass, we found relative reduction in leaf length less pronounced for plants from large populations, suggesting higher adaptive plasticity. Our experimental study of a plant with predominantly vegetative reproduction suggests, that negative genetic effects of recent habitat fragmentation, which so far rather were expected in plants with frequent sexual reproduction, are more severe and more common than previously acknowledged.

Costs of plasticity in foraging characteristics of the clonal plant ranunculus reptans

Abstract: In clonal plants, evolution of plastic foraging by increased lengths of leaves and internodes under unfavourable conditions may be constrained by costs and limits of plasticity. We studied costs and limits of plasticity in foraging characteristics in 102 genotypes of the stoloniferous herb Ranunculus reptans. We grew three replicates of each genotype with and three without competition by the naturally co-occuring grass Agrostis stolonifera. We used regression and correlation analyses to investigate potential costs of plasticity in lengths of leaves and stolon internodes, developmental instability costs of these traits, and a developmental range limit of these traits. We used randomization procedures to control for spurious correlations between parameters calculated from the same data. Under competition the number of rosettes, rooted rosettes, and flowers was 58%, 40%, and 61% lower, respectively, than in the absence of competition. Under competition lengths of leaves and stolon internodes were 14% and 6% smaller, respectively, than in the absence of competition. We detected significant costs of plasticity in stolon internode length in the presence of competition when fitness was measured in terms of the number of rosettes and the number of flowers (selection gradients against plasticity were 0.250 and 0.214, respectively). Within-environment variation (SD) in both foraging traits was not positively correlated with the corresponding plasticity, which indicates that there were no developmental instability costs. More plastic genotypes did not have less extreme trait values than less plastic genotypes for both foraging traits, which indicates that there was no developmental range limit. We conclude that in R. reptans costs of plasticity more strongly constrain evolution of foraging in the horizontal plane (i.e., stolon internode length) than in the vertical plane (i.e., leaf length).

Maternal and direct effects of elevated CO2 on seed provisioning, germination and seedling growth in Bromus erectus

Abstract: Elevated CO2 can affect plant fitness not only through its effects on seed production but also by altering the quality of seeds and therefore germination and seedling performance. We collected seeds from mother plants of Bromus erectus grown in field plots at ambient and elevated CO2 (m-CO2, maternal CO2) and germinated them in the greenhouse in a reciprocal design under ambient and elevated CO2 (o-CO2, offspring CO2). This design allowed us to examine both the direct effects of elevated CO2 on germination and seedling growth and the indirect (maternal) effects via altered seed quality. Elevated m-CO2 significantly increased seed mass and increased the C:N ratio of seeds from field-grown plants. Percentage and rate of germination were not affected by the m-CO2 or o-CO2 treatments. Similarly, elevated m-CO2 had no significant effect on seedling size as estimated by the total leaf length. When differences in seed mass were adjusted by using seed mass as a covariate in ANOVA, a negative effect of m-CO2 on seedling size appeared which increased with increasing seed mass (significant covariate×m-CO2 interaction). This may indicate that the advantage of increased seed mass at elevated m-CO2 was offset by the reduced concentration of nitrogen (and possibly other nutrients) in these seeds. In contrast to m-CO2, elevated o-CO2 greatly increased seedling size, and this stimulatory effect of elevated o-CO2 was found to increase with increasing seed mass (significant covariate×o-CO2 interaction). Taken together, these results suggest that in B. erectus transgenerational effects of elevated CO2 are relatively small. However, other factors (genetic and environmental) that contribute to variation in seed provisioning can critically influence the responsiveness of seedlings to elevated CO2.

Movement Patterns and Home Range Of The Bongo (Tragelaphus Eurycerus) In The Rain Forest Of The Dzanga National Park, Central African Republic

Abstract: Bongos (Tragelaphus eurycerus Ogilby) were studied for 8 months in the Dzanga National Park, Central African Republic. Tracks were followed and mapped with a compass and a pedometer to study movement patterns and home range. Natural licks were shown to be central points in the home range of the bongos: they visited the licks recurrently to consume soil, but also to forage on grass and herb species, and for social reasons. Forest areas far from licks were used much less than forest areas close to licks. When a lick was visited, distances between two resting sites were longer than in the forest without lick visits, caused by a direct and straight movement from the denser forest areas toward a lick. The study area of about 150 km2 was presumably occupied by two groups of bongos. One of them seemed to split temporarily into two subgroups. Groups were not larger than 10–20 individuals. Estimated home ranges were at least 49 km2 and 19 km2 for the two groups, respectively. Estimated density in the Dzanga National Park was 0.25 bongos per km2. This study shows the importance of natural licks for the largest social forest antelope, the bongo, and provides information which is important for its future conservation.

Seasonal dynamics of biomass and nitrogen in canopies of Solidago altissima and effects of a yearly mowing treatment

Abstract: Seasonal dynamics of biomass and nitrogen allocation in ramets of the clonal perennial Solidago altissima in response to yearly mowing were assessed in a field experiment. Final biomass of all component organs of the ramets was lower in mown than in unmown plots. Except for a negative effect on rhizome nitrogen concentration, mowing did not influence tissue nitrogen concentrations but, as a consequence of the reduced biomass accumulation, pools of standing-crop nitrogen were reduced in all organs. At all times during the annual stand development, the nitrogen concentration in the leaf canopies declined exponentially from the top downwards. The gradient of this decline was most marked at the beginning of the season and then became less apparent. In the mature stands, the area-based nitrogen contents of `upper-canopy' leaves (receiving > 50 % of incoming light), for a given light availability, on average were greater in mown than in unmown plots. About half of the nitrogen still remaining in pre-senescent leaves sampled in July was re-absorbed before leaf death. Nevertheless, the decreases in the absolute leaf- and stem-nitrogen pools per ramet during reproductive growth were much lower than the concomitant nitrogen gains of the developing inflorescences, implying that more than half of the nitrogen allocated to reproductive structures was taken up from the soil. On the other hand, there was evidence for nitrogen re-allocation from old to new leaves during vegetative growth which apparently enabled individual ramets to maintain an exponential nitrogen profile even during the phase of rapid leaf production in spring, thus using nitrogen efficiently.

RANUNCULUS REPTANS (RANUNCULACEAE)l

Abstract: In the pre-alpine region of Europe numbers and sizes of populations of the clonal lake shore plant Ranunculus r-eptans have declined because of the regulation of lake water levels. We investigated genetic variation among and within 17 populations of different size (cover 1-10000 m2) in R. r-eptanis with RAPD (random amplified polymorphic DNA) profiles. We sampled 127 rosettes in 14 populations at Lake Constance and three populations at or near Lake Como. There was significant genetic variation between plants from the two lake regions (5.9%, analysis of molecular variance [AMOVA], P < 0.001), among populations within lake regions (20.4%, P < 0.001), and within populations (73.7%, P < 0.001). Under the assumptions of Wright's island model the variation among populations corresponds to a gene flow of Nin = 0.70. Within the 14 Lake Constance populations we detected significant genetic variation among subpopulations separated by only a few metres (4.0% of the within-population variation; P < 0.05). Molecular variance was 24% smaller in small populations covering <100 m2 area than in larger ones (P < 0.03), indicating that samples from large populations were genetically more variable than samples representing comparable areas of smaller populations. We conclude that gene flow among populations is very limited and that genetic drift has caused reduced genetic variability of smaller populations. Conservation of genetic variability in R. reptans requires persistence of large and also of small populations (because of population differentiation), and it could be enhanced by increasing the size of small populations (to counter genetic drift).