Per Milberg

Large-seeded species are less dependent on light for germination than small-seeded ones

Germination in light and darkness was compared after cold stratification of seeds of 54 species known or suspected to accumulate persistent seed banks. Germination became less dependent on light with increasing seed mass. This pattern was clear in a direct correlation of individual species data (P <0.0001) as well as when considering phylogenetically independent contrasts (P <0.001). The latter analysis suggests that light response and seed mass coevolved.

Variation in seed dormancy among mother plants, populations and years of seed collection

Variation in dormancy level was tested in seeds of four species, each collected from three populations in 1994 and 1995 (experiment 1). Germination was tested in light and darkness on recently-harvested seeds and on those after-ripened in dry storage for one year. In addition, seeds from each of eight individual plants within each of eight populations were tested for germination when recently harvested and after warm stratification or cold stratification followed by a drying period (experiment 2). Seeds from the two years differed in dormancy level in Silene noctiflora, Sinapis arvensis and Spergula arvensis . Germination percentage differed significantly among populations in Sinapis arvensis and Spergula arvensis in both experiments and in Thlaspi arvense in experiment 2. Furthermore, dormancy level in seeds from different mother plants also varied in the three species tested in experiment 2. Variations at the three levels tested (year, population and mother plant) indicate that these species have a random pattern of variation in dormancy level. It is concluded that variation in seed dormancy among mother plants, populations and years must be taken into account when testing the germination characteristics of a species and also when attempting to model weed seed bank dynamics.

Soil seed bank after eighteen years of succession from grassland to forest

I studied the vegetation and seed bank in an 18-yr-old, replicated experiment with grazed and ungrazed plots in a semi-natural, perennial grassland in southern Sweden. In the ungrazed plots, a tall (16-20 m) and dense tree layer had developed. There were fewer plant species growing in ungrazed plots than in grazed plots, but the difference was not significant. However, the number of species per square metre was significantly lower in ungrazed plots. Hence, on a smaller scale, the ground vegetation had become less diverse, but on a larger scale few species had been lost. In the seed bank, a few species lost from the vegetation were still present as seeds in the soil, but in most cases species lost were not recorded in the seed bank. Seeds of species that had colonized plots over the 18 yr were evenly distributed in the soil between the upper (0-4 cm) and lower (4-8 cm) sampling depths. Most of the seeds of species that had disappeared from plots were deep in the soil. Hypotheses about changes in the seed bank during secondary succession, predicting decrease in species richness and seed density, were not confirmed

The impact of logging intensity on field-layer vegetation in Swedish boreal forests

The relationship between logging intensity and changes in ground cover vegetation was studied in 16 species and groups of species recorded at 10- or 11-year intervals in mature conifer-dominated forests. The 789 plots located in northern and central Sweden had been surveyed by the National Forest Inventory and the National Survey of Forest Soil and Vegetation. Thirty-seven percent of the plots had been subjected to a thinning or clear-cutting between the inventories. A principal components analysis showed that, of the variables considered, logging intensity had the highest explanatory power regarding change in ground cover vegetation between the inventories (the other variables were sum of temperatures, age of stand, timber volume, percentage Pinus sylvestris and site productivity). A multivariate direct gradient analysis technique (Redundancy analysis) showed that the logging intensity significantly affected the change in cover. This analysis also ranked the species in their responsiveness to logging. Epilobium angustifolium, narrow-leaved grasses and broad-leaved grasses, increased most with logging intensity. The response was not linear and only detectable at high logging intensities (>80%). In contrast, Vaccinium myrtillus seemed to decrease linearly with increased logging intensity. There was several years time-lag in the response to logging of E. angustifolium, V. myrtillus and narrow-leaved grasses. Several species and groups of species seemed unaffected by the logging. In sample plots unaffected by logging the cover of most species decreased.

Soil seed bank and species turnover in a limestone grassland

. The vegetation and soil seed bank in a limestone grassland in southern Sweden were studied in permanent 1-m2 subplots which had been either grazed or not grazed for 17 yr. Of the 92 species recorded, 18 were present only in the seed bank, 28 were more frequent and 24 were less frequent in the seed bank than in the vegetation and 22 were not detected in the seed bank. Among the species present in the seed bank, therophytes were over-represented. Species turnover in the vegetation was estimated from presence/absence data collected in 1980, 1986 and 1990. Turnover was high, but there were no differences between grazed and ungrazed subplots. The turnover for individual species was also high in many cases. There was no clear relationship between the turnover of a species and its presence in the persistent seed bank.

Fire enhances weed invasion of roadside vegetation in southwestern Australia

We documented the invasion of native vegetation by exotic species after fire. The study was conducted in linear remnants of sclerophyll woodland along a highway in southwestern Australia. Weeds spread into the remnants mainly from the roadside, while their expansion was less marked from the opposite direction (fenceside). The number of weed species, and their frequency and cover increased after the fires, while the abundance of native species often decreased. The impact of the fires was still evident 7 years later, while the unburnt controls remained almost unchanged. The most abundant weeds were the perennial grasses Eragrostis curvula and Ehrharta calycina. Since grasses are an insignificatn component of sclerophyll vegetation, their abundance is likely to increase the fire proneness of the vegetation. We advocate restrictions on the frequency of burning and on further narrowing of these corridors.

Seed dormancy in Carex canescens: regional differences and ecological consequences

We compared the germination of seeds of Carex canescens from populations in four regions: northern and southern Sweden, and northern and southern Germany. Germination behaviour was expected to differ between regions owing to adaptations to the local climates. We compared three or four populations from each region in 31 germination experiments (different pre-treatments, different temperature regimes, light vs darkness). In a parallel experiment, we recorded seedling emergence under outdoor conditions from seeds sown in the late summer in northern Germany. Although there were differences in the level of dormancy between the 13 populations and four regions, we were unable to detect geographic patterns which could be explained in terms of differential adaptation to the local climates. Our data did not support a proposed hypothesis that populations experiencing more severe winter conditions would require a longer cold-stratification period. We compared the results from the outdoor experiment with germination data from the laboratory experiments. The latter could not be used to predict differences in the timing of seedling emergence, the time-span during which seedlings emerged, or the number of seedlings emerging. We conclude that the relevance of detected differences in dormancy level among species or populations must be interpreted with great care. First, the detection of geographical patterns requires extensive sampling, second, the ecological consequences of such differences should be established.

A Survey of Weeds in Organic Farming in Sweden

ABSTRACT The weed flora on 57 arable fields on 17 farms employing organic farming was recorded. With the ordination technique pCCA (partial Canonical Correspondence Analysis) the relative importance of some farming practices was evaluated (crop, preceding crop, ± undersown ley, ± weed harrowing, ± weed hoeing, ploughing regime, ± compost, ± animal husbandry, ± biodynamic agriculture). The variables that explained most of the variation in the weed flora in fields (along with typical weed species) were the crops ‘beans and peas’ (analysed together; Centaurea cyanus), ‘ley as preceding crop’ (Cerastium fontanum), ‘weed hoeing’, ‘fallowing in the preceding year’ (Lamium spp.) and ‘animal husbandry’ (Sinapis arvensis). A number of species considered endangered, rare or decreasing in Sweden were recorded on these farms, suggesting that organic farming can contribute to maintaining biodiversity in an agricultural landscape. The weed flora was also compared with that of unsprayed control plots in herbicide trials. Ranking of weeds according to either frequency or a pCCA both showed clear differences, with organic fields having much more weeds. Species typical of organic farming were Centaurea cyanus and Vicia hirsuta. There was a tendency for weed species that dominate in conventional farming to be more nitrophilous than those species characteristic in organic farming.

The effect of light and number of diurnal temperature fluctuations on germination of Phragmites australis

The effect of light and number of diurnal temperature fluctuations on germination of Phragmites australis

Does cold stratification level out differences in seed germinability between populations

Populations of seeds can vary greatly in their dormancy-breaking and germination characteristics. The purpose of this study was to determine if such dormancy differences are levelled out by cold stratification. Seeds of 33 annual weed species, each represented by three populations, were tested in light and darkness 7 weeks after harvest and after two stratification treatments: 18 weeks at 3 °C in the laboratory and 19 weeks outdoors in soil during winter. Cold stratification removed population differences in some species, but in several species such differences became apparent only after stratification. This happened either because dormancy became stronger in weakly dormant seeds (winter annuals) or weaker in strongly dormant seeds (summer annuals). In several species, the light requirement for germination increased after stratification. These results clearly indicate that germination tests performed on fresh seeds from a single population may not adequately predict germination percentages in the field.