Patricia A. Werner

Population Growth Rates and Age Versus Stage‐Distribution Models for Teasel (Dipsacus Sylvestris Huds.)

Mathematical models are developed to examine the population—level response of an herbaceous plant species (teasel, Dipsacus sylvestris Huds.) which was experimentally introduced into several habitats and monitored for 5 yr. Models based on morphological stages (size) rather than chronological age give more satisfactory results. Populations growth rates (λm) range from 0.63 to 2.60, which are likely typical for fugitive plants. Values are interpreted as responses to both external and internal factors. Grass litter, and the presence of other dicotyledonous species, and the overall primary productivity of the rest of the community are important factors determining the success or failure of an attempted colonization by teasel. Individual plant and population—level growth rates seem to be determined independently.

Predictions of fate from rosette size in teasel (Dipsacus fullonum L.)

SummaryIn field populations of the biennial plant (Dipsacus fullonum L.) the probability that an individual will die, remain vegetative, or flower during a particular growing season is highly correlated with the size of its vegetative rosette at the end of the growing season of the preceding year. Further, a rosette forms a flowering stalk only after attaining a critical size. Correlations of various plant fates with rosette size are independent of rosette age; hence size provides better predictions of plant fate than age.

The effects of size of opening in vegetation and litter cover on seedling establishment of goldenrods (Solidago spp.)

We investigated the effects of size of opening in the vegetation and litter cover on seedling establishment of two species of goldenrods (Solidago spp.) in an abandoned field in southwestern Michigan, U.S.A. Seeds of S. canadensis and S. juncea were sown into clipped plots, ranging from 0 cm (control, unclipped) to 100 cm in diameter, with and without litter. Seedling emergence, survival and growth were followed for one year. Soil moisture was not significantly different among the opening sizes, but, within a size, tended to be lower when litter was removed. Light intensity at the soil surface was positively related to opening size early in the growing season, but later in the growing season reached a maximum in intermediate-sized openings and then leveled off.Litter strongly inhibited seedling emergence in both species. Emergence of S. canadensis seedlings was lower in 0 and 10 cm openings than in the larger openings, while emergence of S. juncea seedlings was lower in the largest openings (100 cm) than in all the smaller openings. In contrast, seedling growth and probability of survival increased with diameter of opening for both species. Some seedlings of S. juncea did survive in complete vegetation cover (controls, 0 cm openings) while seedlings of S. canadensis survived only in openings of at least 30 cm diameter. Thus, S. juncea had a smaller minimum opening size for seedling establishment than S. canadensis, although both species grew and survived best in the largest openings made in the experiment.

Competitive Effects and Responses Between Plant Species in a First-Year Old-Field Community

Competitive interactions involve two separate processes: the effect of a species on others in the community and the response of a species to all others. Five species from a 1 st-yr field were investigated to determine if there is any correlation between competitive effect and response and if the patterns of effect and response can explain the relative success of each species in the full community. Effect and response were measured by observing the growth of individuals in all possible monocultures and two-species mixtures, using ambient densities (Year 1) and a range of densities (Year 2) for each species. Both mono- cultures and two-species mixtures were obtained by removing unwanted individuals at emergence, leaving naturally emerging individuals of the desired species. Competitive effect and response were found to be inversely correlated. The inverse correlation lead to a hierarchy of competitive ability, with Ambrosia artemisiifolia being the competitive dominant, followed by Agropyron repens, Plantago lanceolata, and finally the competitive subordinates Chenopodium album, Lepidium campestre (used in Year 1), and Trifolium repens (used in Year 2). The interactions were generally asymmetric, e.g., Ambrosia artemisiifolia had a large suppressive effect on the other species and demonstrated no response to their presence. The hierarchy and a lack of specificity of the interactions suggest that all the species are limited by, and competing for, the same resource or resources. The hierarchy of competitive ability appears to be a major factor in determining the abundance of each species in the full community.

Regulation of seed yield within and among populations of Prunella vulgaris

Knowledge of how seed production is regulated by components of seed yield can provide insight into what determines fitness in natural populations. This study examines patterns and sources of variation and interaction among components of seed yield within and among 10 populations of a perennial weed. Much of the variation in total yield among individuals within populations could be explained by differences in plant size. In general, relationships between components of yield were weak, indicating that total yield was not tightly buffered by trade—offs among yield components. The pattern of relationships between yield components differed among populations, demonstrating that these relationships are not a constant characteristic of a species. The patterns of relationship between each component and an estimate of the number of successful seedlings produced also differed among populations, but seed number was generally more important than biomass per seed in determining plant fitness. Yield component means and ...

Juvenile tree growth and demography in response to feral water buffalo in savannas of northern Australia: an experimental field study in Kakadu National Park

Tree populations in the wooded savannas of northern Australia lack a well developed seed bank, but instead rely on a persistent ground layer of suppressed juvenile plants <1 m of indeterminate ages. The feral Asian water buffalo has been implicated as a factor in inhibiting the movement of juvenile trees to sapling stage. In a 6-year field study in Kakadu National Park, 656 juvenile plants were monitored for growth and survival, and in exploring the effects of buffalo, clipping surrounding vegetation and ambient fire. Buffalo were removed from one half of the sites across an environmental gradient in a landscape-scale experiment. Juvenile trees grew more slowly but fewer became dormant where buffalo were absent. Experimental clipping of surrounding vegetation produced significantly increased height at the buffalo-absent sites, but not at the buffalo-present sites, demonstrating that the positive effect of buffalo on growth was indirect, by reducing the competitive regime for juvenile trees. Within 3 years of removal of buffalo, ground-level green biomass increased 2–10 times and litter 1.3–2 times over a topographic gradient. Mortality of juvenile trees was not significantly different between buffalo-present and buffalo-absent sites when plots remained unburnt. By contrast, at buffalo-absent sites, three times more juveniles died after late dry-season fires than at unburnt sites, but there was no difference among buffalo-present sites. Overall, the total juvenile tree bank decreased by 7% where buffalo grazed and by 18% where buffalo had been removed. The study demonstrated a major mechanism(s) responsible for recorded changes in vegetation patterns of these savannas, whereby buffalo initiate a cascade of effects by changing ground-level biomass, which change competitive relationships and fuel loads, which then have an impact on tree growth and demography. The results are discussed with respect to dynamics of the juvenile tree bank and implications for long-term sustainability of these wooded savannas.

Water Use Physiologies of Co-occurring Goldenrods (Solidago juncea and S. canadensis): Implications for Natural Distributions

SummaryWater use patterns and the seasonal progression of functional leaf area were determined for Solidago canadensis L. var. scabra and S. juncea Ait., two species of cooccurring goldenrods which differ in their competitive ability and distribution along soil moisture gradients. Field measurements of diurnal trends in stomatal conductances and leaf water potentials indicate little difference between the species. Laboratory gas exchange measurements of assimilation rates (13.15–13.25 micromoles CO2 m-2 sec-1), stomatal conductances (31.53–38.44 centimoles H2O m-2 sec-1), water use efficiencies (8.10–9.66 mg CO2 g H2O-1) and stomatal response to low leaf water potentials (i.e. initiation of stomatal closure at -16 to -20 bars) were also similar for the two species. Differences in their maximum functional leaf areas (421 cm2 vs 209 cm2 for S. canadensis and S. juncea, respectively, at maturity), phenologies (S. juncea flowers about one month earlier than S. canadensis) and the presence of the non-reproductive rosette habit in the dry site species (S. juncea) are probably more important in explaining the differential distributions of these two species than differences in their water use patterns.