Seed dormancy and germination of Asarum sieboldii, a disjunct relict species in East Asia.

Disjunct species in the same genus are a common feature of the flora in the temperate forests of East Asia and eastern North America. This study aimed to evaluate whether the kind of seed dormancy was maintained after species in the genus Asarum (Aristolochiaceae) were separated from their common ancestor. We classified the seed dormancy of Asarum sieboldii, an East Asian species, based on a phenology study and experiments in controlled temperature conditions, and then compared it to that of the previously studied A. canadense, an eastern North American species. The underdeveloped embryo of A. sieboldii grew and germinated (radicle emergence) in autumn but shoot emergence did not occur until the following spring. The seeds of A. sieboldii had deep simple epicotyl morphophysiological dormancy because the seeds with emerged radicle required a relatively long period of cold stratification to break epicotyl dormancy and produce a shoot. Although the seed of A. sieboldii had weaker radicle dormancy and stronger epicotyl dormancy compared to A. canadense, the kind of seed dormancy was the same for the two species. The trait of seed dormancy was inherited from a shared common ancestor and maintained in populations well after the two species (or their ancestors) separated. However, quantitative differences in temperature requirements for radicle and shoot emergence suggest the possibility of adaptation to the environment.

Germination responses to winter warm spells and warming vary widely among woody plants in a temperate forest.

Winter underpins key ecological processes, such as dormancy loss and seedling emergence. Enhanced warm spells, together with warming are occurring and will continue in the future. The consequences of these climate phenomena on germination were investigated among co-occurring woody plants, whose seeds are bird-dispersed in autumn and require cold stratification for spring emergence. Seeds from nine common southeastern USA plants were collected in autumn. We verified that seeds of the study species required cold stratification for dormancy loss. We then examined the following aspects in the laboratory or field: effect of warm spells during cold stratification on germination, effect of a warm spell during winter on seed survival and germination phenology, and effect of warming from autumn dispersal through winter dormancy loss on timing of germination. While no consistent effects of warm spells were found in the laboratory on quantity of germination, warm spells advanced spring field germination for several species. Some species germinated during cold stratification and during warm spells, especially extreme spells, in the laboratory. In the field, about half of Lonicera maackii seedlings that emerged with a warm spell died by late winter. With warming from autumn through spring, laboratory germination shifted from spring to predominately autumn for some species. With precocious germination during warm spells or germination phenology shifts, two scenarios are possible. Seedlings may die during winter, reducing the size of the soil seed bank and number of emergents, or they would survive in warmer winters, which would give them a competitive advantage over spring-emerging seedlings.

Germination niche breadth varies inconsistently among three Asclepias congeners along a latitudinal gradient.

Species responses to climate change will be primarily driven by their environmental tolerance range, or niche breadth, with the expectation that broad niches will increase resilience. Niche breadth is expected to be larger in more heterogeneous environments and moderated by life history. Niche breadth also varies across life stages. Therefore, the life stage with the narrowest niche may serve as the best predictor of climatic vulnerability. To investigate the relationship between niche breadth, climate and life stage we identify germination niche breadth for dormant and non-dormant seeds in multiple populations of three milkweed (Asclepias) species. Complementary trials evaluated germination under conditions simulating historic and predicted future climate by varying cold-moist stratification temperature, length and incubation temperature. Germination niche breadth was derived from germination evenness across treatments (Levins Bn ), with stratified seeds considered less dormant than non-stratified seeds. Germination response varies significantly among species, populations and treatments. Cold-moist stratification ≥4 weeks (1-3 °C) followed by incubation at 25/15 °C+ achieves peak germination for most populations. Germination niche breadth significantly expands following stratification and interacts significantly with latitude of origin. Interestingly, two species display a positive relationship between niche breadth and latitude, while the third presents a concave quadratic relationship. Germination niche breadth significantly varies by species, latitude and population, suggesting an interaction between source climate, life history and site-specific factors. Results contribute to our understanding of inter- and intraspecific variation in germination, underscore the role of dormancy in germination niche breadth, and have implications for prioritising and conserving species under climate change.

Relative competitive abilities and growth characteristics of a narrowly endemic and a geographically widespread Solidago species (Asteraceae).

Relative competitive ability and growth characteristics of the narrow endemic Solidago shortii were compared to those of the geographically widespread S. altissima. Competition and growth studies were conducted over the entire growing season in an ambient-temperature greenhouse, using a 3:1 (v/v) native limestone soil/river sand mixture. Results from a de Wit replacement series experiment (relative yield, relative yield total, plant height, aggressivity values) with S. shortii, S. altissima, and Festuca arundinacea (common competitor) suggested the following competitive hierarchy: S. altissima = F. arundinacea > S. shortii. Using classical growth analysis, we found that the competitive hierarchy was related closely to components of plant size (dry mass, height, leaf area, leaf area duration) and not to relative growth rate or any of its components (net assimilation rate, leaf area ratio, leaf weight ratio, specific leaf area). Solidago shortii allocated proportionately more dry mass to roots (but not to rhizomes) and had significantly greater root/shoot and (root + rhizome)/shoot ratios than did S. altissima. Thus, while the morphological traits of S. shortii enable it to tolerate drier habitats than S. altissima, in moist sites S. shortii easily would be overtopped and shaded out by S. altissima. Low competitive ability may be one of several factors contributing to the narrow endemism of S. shortii.