Wolfgang Schütz

The effect of cold stratification and light on the seed germination of temperate sedges (Carex) from various habitats and implications for regenerative strategies

The germination responses of 32 temperate Carex species were tested in light and darkness at five constant temperatures and under one fluctuating temperature regime, before and after cold-wet stratification. Using a linear logistic regression model, the probability of germination tested across all species was found to be significantly higher after stratification, in light and at the fluctuating temperature. In addition, the probability increased with temperature. Stratification increased germination in 28 species and had very little or no effect on four species. There was almost no germination in darkness prior to stratification, and the germination in light was considerably higher in all but two species compared with that in darkness. Thus, it can be concluded that the Carex species tested have broadly similar germination response patterns. The fact that Carices can be released from high levels of primary dormancy by low-temperature stratification implies that they are spring germinators. A light requirement after stratification in the major fraction of seeds and the capability of almost all investigated sedges to respond to fluctuating temperatures make it likely that persistent seed banks are formed. Additionally, sedges generally seem to have a high temperature requirement for germination which prevents them from emerging at the very beginning of the growing season. Regeneration by seed is probably largely restricted to gaps resulting from late spring disturbances where buried seeds have an opportunity to germinate and grow. Differences in germination were apparent between species occupying different habitats. Overall germination was significantly higher in wetland species than in dry-site species, probably owing to the greater capability of wetland species to respond to fluctuating temperatures. Differences in germination between forest and open-site species can be attributed to the higher capability of forest sedges to respond to low temperatures and temperature fluctuations. The influence of seed weight on germination was not significant in the 18 species adapted to wet, open habitats. There was, however, a tendency for the germination percentages to be low for large-seeded Carices. The interpretation of habitat differences is difficult due to a positive correlation between seed weight and dry habitats.

Ecology of seed dormancy and germination in sedges (Carex).

Abstract The genus Carex , with its wide distribution and large number of species yet with a rather uniform life history, is a very convenient group for comparative studies of germination ecology at the generic level. The combination of a strict or conditional primary dormancy, a light requirement for germination, low germination at constant temperatures, a positive response to diurnal temperature fluctuations and an induction of secondary dormancy in late spring by increasing environmental temperatures are attributes that were found to be characteristics shared by almost all the Carex species investigated, though there was variation between species in the degree to which these characters were expressed. In almost all species, dormancy was broken by stratification at low temperatures, though few species gained the ability to germinate at temperatures 0.9 mg), probably due to a thicker seed coat and hence a higher resistance to germination. Differences in primary dormancy between sedges of various habitats could not be established. However, there was a tendency for temperature limits to be low in forest sedges. Many species of wetlands and open sites showed a greater capability to respond to fluctuating temperatures than species of dry sites. These dormancy and germination traits not only enable the accumulation of seeds in the soil, but also constitute seasonal seed regeneration strategies that rely on the high longevity of seeds and the formation of persistent seed banks. Temperate Carices are mainly adapted to exploit the temporally and spatially infrequent occurrence of canopy gaps that become available only in late spring or early summer, whereas the colonization of gaps at the beginning of the vegetation period is largely prevented by a high temperature requirement for germination. Many of the dormancy and germination characteristics of Carices are important in Cyperaceae generally. A greater diversity of germination responses, however, can be found in the related families, Juncaceae and Poaceae. Our present knowledge is not sufficient to determine unequivocally whether a phylogenetic component contributes significantly to the germination behaviour of the genus Carex , but certain tendencies are clearly indicated.

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.

Dormancy characteristics and germination timing in two alpine Carex species

Summary The timing of seed germination is thought to play an important role for species abundance and community composition in alpine habitats. Seed dormancy and germination responses to temperature and light of two widespread alpine sedges were investigated in order to gain insight into mechanisms regulating germination in the field. Seeds of Carex ferruginea germinated at high temperatures when freshly matured and were thus conditionally dormant. Germination percentages increased markedly due to dry storage and cold stratification, accompanied by a decrease of the minimum temperature for germination. Freshly-matured seeds of C. frigida required cold stratification to release strict dormancy, but germination was restricted to high temperatures (> 15 °C). The existence of a carry-over mechanism, preventing the major fraction of current-year seeds to germinate even under optimum conditions in the next growing season was shown for this species. Seeds of both species showed very little germination in darkness and attained a germination peak in light after three month of stratification. The period of cold-stratification required to break dormancy was not related to winter duration as has been found in several species from high altitudes in America. Seeds buried outdoors underwent seasonal changes in dormancy, a trait which was hitherto not known in alpine plant species. Primary seed dormancy, a requirement for relatively high germination temperatures in stratified seeds and the induction of secondary dormancy in early summer by increasing temperatures restrict the ‘germination window’ to a short period after snowmelt in both species. Such a cautious type of seed regeneration may increase the chance of a seedling becoming established at the expense of the number of germinating seeds. This strategy is based on a light requirement for germination, enabling both species to built-up a large seed reservoir in the soil. Die Haufigkeit von Arten und die Zusammensetzung der Vegetation in alpinen Lebensraumen wird wahrscheinlich durch den richtigen Zeitpunkt fur die Keimung entscheidend mitbestimmt. Samendormanz und Keimungverhalten unter verschiedenen Licht- und Temperaturbedingungen wurden fur zwei weit verbreitete alpine Seggen untersucht, um Mechanismen zu identifizieren, die unter naturlichen Bedingungen die Keimung regulieren. Frisch gereifte Samen von Carex ferruginea keimten bei hohen Temperaturen und sind daher konditional dormant. Der Anteil gekeimter Samen wurde durch trockene Lagerung und kalte Stratifikation deutlich erhoht, bei gleichzeitiger Erniedrigung der Minimaltemperatur fur die Keimung. Frische gereifte Samen von C. frigida benotigten fur die teilweise Aufhebung einer absoluten Dormanz eine kalte Stratifikation, aber auch dann erfolgte eine Keimung nur bei recht hohen Temperaturen (> 15 °C). Fur diese Art wurde ein “carry-over”-Mechanismus nachgewiesen, der auch unter optimalen Bedingungen eine Keimung des groseren Teils der diesjahrigen Samen im folgenden Fruhjahr verhinderte. Samen beider Arten keimten kaum in Dunkelheit und erreichten die hochsten Werte nach einer dreimonatigen Stratifikationsperiode. Im Gegensatz zu einigen Arten hoherer Lagen in Amerika war die Dauer dieser Periode nicht mit der Dauer des Winters korreliert. Im Freiland vergrabene Samen zeigten saisonale nderungen des Dormanz-Niveaus, was bisher von alpinen Arten nicht bekannt war. Primare Samendormanz, relativ hohe Keimungstemperaturen auch nach Stratifikation und die Induktion einer sekundaren Dormanz im Fruhsommer durch steigende Temperaturen schranken das “Keimungsfenster” fur beide Arten auf eine kurze Periode nach der Schneeschmelze ein. Dieser durch zahlreiche Einschrankungen potentieller Keimungsmoglichkeiten gekennzeichnete Typ der “vorsichtigen” generativen Regeneration erhoht die Chance eines Keimlings sich zu etablieren, allerdings auf Kosten der Zahl auflaufender Samen. Ein wichtiger Bestandteil dieser Strategie ist die Notwendigkeit von Licht fur die Keimung, da sie die Voraussetzung fur den Aufbau eines umfangreichen Samenreservoirs im Boden bildet.

Seed dormancy cycles and germination phenologies in sedges (Carex) from various habitats

Seeds (nutlets) of four temperate-zone perennialCarex species from various habitats and with different growth forms were investigated to identify seasonal germination patterns. In the first experiment, freshly matured seeds were sown in summer 1994 on the soil surface and at a depth of 1 cm at a sunexposed site and under a leaf canopy in an experimental garden. In the second experiment, seeds were exhumed after various periods of burial (7-cm depth) for up to 30 months and tested for germination in both light and darkness at 15°, 25°, and 22/10°C. In the first experiment, freshly-matured seeds of the four sedges did not germinate during summer or autumn 1994. Almost all surface-sown seeds of the four species germinated from late spring to summer in 1995 at the sun-exposed site. Most buried seeds of three species germinated in the soil, but germinants died prior to emergence. InCarex flacca, however, about 50% of the buried seeds germinated and emerged until 1997, and the ungerminated seeds remained viable. At the shaded site, seeds ofC. acutiformis. andC. flacca did not germinate, whereas a small percentage of the buried and surface-sown seeds ofC. arenaria andC. extensa emerged in the second year. Seeds ofC. acutiformis andC. flacca were dormant, and those ofC. arenaria andC. extensa were conditionally dormant at the beginning of the second experiment. All four species had annual dormancy cycles at 22/10°C in light and in darkness and at 25° and 15° in light. Seeds came out of dormancy in late autumn or winter and re-entered dormancy or conditional dormancy in late spring or early summer. Very few seeds of any of the four species germinated at constant temperatures in darkness. No seeds germinated during burial in soil at a depth of 7 cm. These field germination patterns indicate that the four sedges have the potential to form a persistent seed bank but that losses due to germination in the seed bank are dependent on temperature amplitudes, mean temperatures, and red: far red ratios under a leaf canopy.

Seasonal dormancy patterns and stratification requirements in seeds of Verbena officinalis L.

Abstract Dormancy patterns and germination responses to incremental stratification were investigated in Verbena officinalis L. (Verbenaceae), a widely-distributed ruderal, but considered as an originally mediterranean plant. In an initial experiment, samples of seeds buried in the field were exhumed at monthly intervals over a period of 38 months and tested for germination in light and darkness at 25/15 °C and 15/5 °C. Seeds were strongly dormant when fresh and mature and came out of dormancy during winter. Seeds entered conditional dormancy in spring, while main dormancy relief occurred in winter. There was no seed mortality, and no germination occurred in the soil during burial. The results imply, that V. officinalis germinates preferably in late spring and has the potential to accumulate a persistent seed bank. The effects of stratification temperature and duration on dormancy were investigated in a second experiment. Dry-stored seeds were stratified wet at a range of temperatures from 3–18 °C in darkness for periods of up to 28 weeks, and tested over a range of constant temperatures (3–25 °C) and at 15/5 °C in light, and at 25 °C and 15/5 °C in the dark. Seeds came out of primary dormancy when kept at temperatures between 3–12 °C, while higher temperatures were ineffective in breaking dormancy. There was no induction of secondary dormancy during the stratification period of 28 weeks. We conclude that a gradual increase of the ambient temperatures and/or an increase in amplitude size is required for dormancy induction. Weakly dormant seeds required either high constant mean (>15 °C), or fluctuating temperatures for germination. Only a small fraction of the seeds acquired the ability to germinate in darkness with incremental stratification. There is no indication of germination and dormancy traits that can be interpreted as relict adaptations to mediterranean conditions. Der zeitliche Verlauf der Samendormanz und die Veranderung des Keimungsverhaltens von Verbena officinalis L. (Verbenaceae) unter verschiedenen Stratifikationsbedingungen wurden untersucht. Verbena officinalis ist eine weit verbreitete Art ruderaler Standorte, die wahrscheinlich aus dem mediterranen Raum stammt. In einem ersten Experiment wurde die Keimung von Samen, die in einem Versuchsgarten vergraben wurden, in monatlichen Abstanden uber einem Zeitraum von 38 Monaten, bei 25/15 °C und 15/5 °C in Licht und Dunkelheit getestet. Frisch gereifte Samen wiesen eine starke Dormanz auf, die sie wahrend des Winters verloren. Vom Fruhling bis zum Sommer wurde eine sekundare Dormanz induziert, deren Niveau starken Schwankungen unterlag und die erst im Winter wieder abgebaut wurde. Bis zum Ende des Versuchs konnte weder Samenmortalitat, noch eine Keimung im Boden festgestellt werden. Die Ergebnisse zeigen, dass V. officinalis bevorzugt im spaten Fruhjahr keimt und die Fahigkeit besitzt, eine dauerhafte Samenbank aufzubauen. Die Auswirkungen von Dauer und Temperatur einer Stratifikation auf die Samendormanz wurden in einem zweiten Experiment untersucht. Die Keimung trocken gelagerter und anschliesend bei Temperaturen zwischen 3 °C und 18 °C bis zu 28 Wochen feucht stratifizierter Samen wurde getestet. Hierzu wurden die Samen bei mehreren konstanten Temperaturen zwischen 3 °C und 25 °C, sowie bei einer Wechseltemperatur von 15/5 °C im Licht und zusatzlich bei 25 °C und 15/5 °C in Dunkelheit inkubiert. Die Samen verloren ihre Dormanz, wenn sie in einem Tempera-turbereich von 3 °C bis 12 °C stratifiziert wurden, wohingegen hohere Temperaturen keinen Einfluss hatten. Wahrend des gesamten Stratifikationszeitraums war eine Induktion sekundarer Dormanz nicht zu beobachten. Wir schliesen daraus, dass ein Temperaturanstieg und/oder ein Anstieg der Hohe der Tagestemperaturamplitude fur eine Induktion sekundarer Dormanz notwendig ist. Nicht dormante Samen benotigten eine hohe Durchschnittstemperatur (>15 °C) oder fluktuierende Temperaturen zur Keimung. Lediglich ein geringer Teil der Samen erlangte mit zunehmender Stratifikation die Fahigkeit in Dunkelheit zu keimen. Keine der Dormanz- oder Keimungseigenschaften von V. officinalis kann als reliktische Anpassung an mediterrane Bedingungen interpretiert werden.

Special feature: Plant population biology in a multidisciplinary context

Population biology is concerned with spatio-temporalvariation in numbers and sizes of individuals (Harper1977). Populations represent a level of biological inte-gration intermediate between the level of the individu-al and the genetic level on the one hand, and the levelsof the metapopulation and of the community on theother. By many interactions populations are linked tothese other levels. Reflecting such interactions, mod-ern population biology not only comprises demo-graphic studies, but increasingly integrates and takesadvantage of disciplines such as evolutionary ecology,population and conservation genetics, community andlandscape ecology, and systematics. Clearly, todaypopulation biology is best understood in a multidisci-plinary context. Since J.L. Harper’s book “Populationbiology of plants” (1977), which may be regarded asthe starting point of plant population biology, this hasalso become apparent in a widening of the scope ofpopulation biological publications.Plant population biology is the topic of a very activesection of the “Gesellschaft fur Okologie”, whose ac-tivities mirror the development and comprehensivescope of population biology (first documented inSchmid & Stocklin 1990). Hosted by the Institute ofBotany of the University of Agricultural Sciences, Vi-enna, this section held its 14