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Germination of seeds of some summer annuals in Kentucky (eastern USA) in late-winter lead to the hypothesis that under present climate conditions the whole length of the winter cold stratification (CS) period is not required for dormancy-break of seeds of summer annuals with physiological dormancy (PD). We evaluated our data from germination phenology studies of 45 species (69 datasets) and buried-seed studies of 33 species (44 datasets). We determined time and temperature of germination after CS and percentage of the total number of hours of CS during winter (% of winter CS) seeds received prior to start of germination. In the phenology studies, mean temperature during the week of first germination for C3 and C4 species was 11.1 and 14.4°C, respectively, and % of winter CS was 80.8 and 87.4, respectively. In the buried-seed studies, % of CS for C3 and C4 species was 40.8 and 48.1, respectively, when they germinated to 25% at 20/10°C. For 32 of 33 species in the buried-seed studies, the minimum temperature at which seeds germinated decreased with increased CS; thus, seeds had Type 2 non-deep PD. The time of germination is controlled by a number of hours of CS, a decrease in minimum temperature at which seeds can germinate and a temperature increase in early spring. Seeds can germinate at relatively high temperatures as early as December and January, but they continue to be CS until spring. Temperature increases in eastern North America due to global warming are not likely to inhibit the germination of summer annuals with PD in spring. [ABSTRACT FROM AUTHOR]

To persist (without immigration) in habitats with unpredictable environmental conditions, annuals must produce seeds each year or have a seed bank. Thus, we predicted that compared to perennials, annuals have a wider germination temperature range (GTR, the difference in temperature between the week with the highest and the week with the lowest germination during the natural germination season). We determined the GTR via germination phenology data for 350 herbaceous species in 59 families from the eastern USA: summer annuals (SA), 63; winter annuals (WA), 83; monocarpic perennials (MP), 28; and polycarpic perennials (PP), 176. There was no significant phylogenetic signal for the GTR. The width of the GTR during the first spring germination season was 9.6, 8.7 and 8.8°C for MP, PP and SA, respectively, and during the first autumn germination season 12.8, 11.8 and 12.4°C for MP, PP and WA, respectively. Annuals did not have a wider GTR than perennials in either the spring or the autumn germination season. Our data suggest that selection for early germination in either spring or autumn has resulted in only small differences in the GTR. We predict that global warming will have little or no effect on reshaping the germination phenology of herbaceous species of temperate eastern North America. [ABSTRACT FROM AUTHOR]

To increase our knowledge of the seed dormancy breaking and germination requirements of native and non-native winter annual Apiaceae in temperate eastern North America, a study was conducted on seeds of Cyclospermum (Apium) leptophyllum, which is native to South America, but naturalized in North America and elsewhere. A high percentage of fresh seeds germinated over a narrow range of temperatures, but the range increased when seeds were incubated continuously in light or buried in soil at high summer temperatures, indicating the presence of conditional physiological dormancy (PD). Embryos grew from approximately 0.3 to 1.25 mm (316% increase in length) inside the seeds before the radicle emerged; thus, seeds had morphological dormancy (MD) as well as PD, that is, morphophysiological dormancy (MPD). Embryo growth did not occur during summer in seeds exposed to natural temperatures, but embryos did grow in autumn. For seeds sown in the field in June, both embryo growth and germination occurred in autumn. Thus, seeds have non-deep simple MPD, with PD being broken in summer and MD in autumn. A comparison of C. leptophyllum with native and non-native winter annual Apiaceae in the region reveals that all species have MD, but the proportion of fresh seeds that also has PD (thus, MPD) varies between species. A combination of factors, including PD, an inability to germinate at high summer temperatures, a relatively low temperature requirement for embryo growth and a lack of dispersal until autumn (in two non-natives), delays germination of both natives and non-natives until autumn, when conditions are favorable for both germination and seedling establishment. [ABSTRACT FROM AUTHOR]

The term ‘cedar glades’ has been applied to several different types of plant communities that occur on rocky calcareous soils in eastern North America. A previous paper by the first two authors reviewed in considerable detail the use of this term with regard to type of vegetation on rocky limestone soils in the Nashville (Central) Basin of Tennessee. The present paper reviews use of ‘cedar glades’ as a descriptive term for vegetation in other physiographic regions of eastern North America. In the Nashville Basin, and in some other physiographic regions, the term has been applied to true open cedar glades (‘glades’) plus the surrounding redcedar/redcedar-hardwood forest, or (more recently) to the natural rocky treeless openings only. However, outside the Nashville Basin, it has also been used to describe several other disparate vegetation types, such as xeric limestone prairies (limestone glades, prairie barrens), redcedar-little bluestem savanna, dense redcedar forest, redcedar-hardwood forest, and even an open stand of redcedar-pine-sweetgum with warm-season perennial prairie grasses in the understory. In the Great Lakes region of North America and in the Baltic region of Europe, some atvars are similar to cedar glades sensu stricto of southeastern United States. Since many of the rocky calcareous openings in eastern North America can be classified as either cedar glades or xeric limestone prairies, a suite of abiotic, biotic, and anthropogenic-related factors is presented for use in distinguishing between these two vegetation types. [ABSTRACT FROM AUTHOR]

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The seed germination of Polymnia canadensis was studied in relation to its wide variation in life history and its ecology as a mostly facultative biennial, a life cycle type that is rare in stable forest habitats in eastern North America. The seed dormancy and germination characteristics of P. canadensis were like some other herbaceous woodland species of diverse life cycle types. That is, its seeds have physiological dormancy, and they (1) germinated to higher percentages after cold stratification, (2) germinated better in light than in darkness and (3) formed a persistent soil seed bank. Since its seed dormancy and germination characteristics are not uncommon and are not particularly associated with any one life cycle type in woodlands, those properties do not contribute to the rarity of its life cycle type in deciduous forests. These properties are also found in many facultative biennials of open, disturbed habitats and in other Asteraceae of various life cycle types. In addition, the germination phenology of P. canadensis differed between mesic and dry field sites in central Kentucky. At the dry sites, most of the yearly germination occurred in fall, while at the mesic sites most germination occurred in the spring due to being inhibited during fall. A reciprocal seed transplant experiment conducted in two different years demonstrated that the cause of this difference was environmental and that there was no source x site interaction; i.e., no difference between sources in phenotypic plasticity for germination phenology. Absence of genetic effects was also shown by no consistent pattern in the few differences between seed sources in seed incubation laboratory experiments and in a germination phenology experiment in glasshouse flats. Exposure of seeds to a high ratio of far-red/red light prior to or during incubation did not inhibit germination. Neither did changes in leaf litter thickness qualitatively alter germination phenology in field quadrats or in glasshouse flats. Thus, germination in fall at mesic sites was inhibited by other unknown factor(s), perhaps interacting with the effects of closed tree canopy and/or thick leaf litter. [ABSTRACT FROM AUTHOR]