Origins and Relationships of the Mixed Mesophytic Forest of Oregon-Idaho, China, and Kentucky: Review and Synthesis

The Arcto-Tertiary Geoflora concept of Ralph Chaney, that the Mixed Mesophytic Forest of eastern Asia and eastern North America are relicts of a Northern Hemisphere high-latitude circumglobal deciduous forest of the Late Cretaceous–Early Tertiary that migrated south to the temperate zone as an intact unit, was shown by Wolfe and others to be invalid. To explain the origin and development of these disjunct forests, Wolfe and Tiffney developed the boreotropical hypothesis. Accordingly, a paratropical (near-tropical) rainforest flora containing a mixture of tropical, paratropical, and temperate genera developed at several places in the middle latitudes of the Northern Hemisphere in the Eocene and spread around the globe via the Bering and North Atlantic land bridges and shores of the Tethys Seaway. Further, the Mixed Mesophytic Forest of eastern Asia and eastern North America developed independently after disruption of the boreotropical flora by subsequent changes in climate and geography, thus accounting for differences in the flora and physiognomy of the present-day Mixed Mesophytic Forest in the two areas. The fruit and seed flora of the Middle Eocene Clarno Nut Beds of Oregon are representative of the boreotropical forest. In response to climatic cooling during the Eocene–Oligocene transition, this broad-leaved evergreen rainforest was replaced by a temperate broad-leaved deciduous (Mixed Mesophytic) forest, which remained present in the Pacific Northwest through most of the Miocene. The Early Oligocene Bridge Creek flora of Oregon, the Middle Miocene Succor Creek flora of eastern Oregon and adjacent Idaho, and the Middle Miocene Clarkia and Musselshell Creek floras of northern Idaho are good examples of the Mixed Mesophytic Forest. These Oligocene–Miocene fossil floras include important genera in the present-day Mixed Mesophytic Forest of eastern Asia and eastern North America, as well as those that today occur only in eastern Asia or only in eastern North America. Using Graham as the primary source of, and guide to, information on microfossil and megafossil plant paleoassemblages and paleoclimates in eastern North America, we chart the Late Cretaceous–Tertiary sequence of vegetation and climate for Kentucky. Further, we briefly review the palynofloral provinces in which Kentucky was situated during the Middle and Early Cretaceous. In contrast to the Mixed Mesophytic Forest flora (a component of the boreotropical forest) of the Middle Eocene Clarno Nut Beds, the Middle Eocene Claiborne flora of Tennessee and Kentucky represents a semideciduous tropical dry forest dominated by Leguminosae taxa that have strong phylogenetic and biogeographical relationships with the Old World and tropical South America. Apparently, this dry forest developed from a Paleocene–Early Eocene tropical rainforest following a decrease in amount and an increase in seasonality of rainfall. The Mixed Mesophytic Forest developed from this seasonally dry forest following the Eocene as a result of an increase in the amount of rainfall and a decrease in its seasonality. The hypothesis that closely related disjunct taxa between eastern Asia and eastern and western North America represent relicts of a circumglobal Mixed Mesophytic Forest in the Miocene is supported by fossil and molecular phylogenetic data

Effects of Germination Season on Life History Traits and on Transgenerational Plasticity in Seed Dormancy in a Cold Desert Annual

The maternal environment can influence the intensity of seed dormancy and thus seasonal germination timing and post-germination life history traits. We tested the hypotheses that germination season influences phenotypic expression of post-germination life history traits in the cold desert annual Isatis violascens and that plants from autumn- and spring-germinating seeds produce different proportions of seeds with nondeep and intermediate physiological dormancy (PD). Seeds were sown in summer and flexibility in various life history traits determined for plants that germinated in autumn and in spring. A higher percentage of spring- than of autumn-germinating plants survived the seedling stage, and all surviving plants reproduced. Number of silicles increased with plant size (autumn- \u3e spring-germinating plants), whereas percent dry mass allocated to reproduction was higher in spring- than in autumn-germinating plants. Autumn-germinating plants produced proportionally more seeds with intermediate PD than spring-germinating plants, while spring-germinating plants produced proportionally more seeds with nondeep PD than autumn-germinating plants. Flexibility throughout the life history and transgenerational plasticity in seed dormancy are adaptations of I. violascens to its desert habitat. Our study is the first to demonstrate that autumn- and spring-germinating plants in a species population differ in proportion of seeds produced with different levels of PD

Effect of Seed Position on Parental Plant on Proportion of Seeds Produced with Nondeep and Intermediate Physiological Dormancy

The position in which seeds develop on the parental plant can have an effect on dormancy-break and germination. We tested the hypothesis that the proportion of seeds with intermediate physiological dormancy (PD) produced in the proximal position on a raceme of Isatis violascens plants is higher than that produced in the distal position, and further that this difference is related to temperature during seed development. Plants were watered at 3-day intervals, and silicles and seeds from the proximal (early) and distal (late) positions of racemes on the same plants were collected separately and tested for germination. After 0 and 6 months dry storage at room temperature (afterripening), silicles and seeds were cold stratified for 0–16 weeks and tested for germination. Mean daily maximum and minimum temperatures during development/maturation of the two groups of seeds did not differ. A higher proportion of seeds with the intermediate level than with the nondeep level of PD was produced by silicles in the proximal position than by those in the distal position, while the proportion of seeds with nondeep PD was higher in the distal than in the proximal position of the raceme. The differences were not due only to seed mass. Since temperature and soil moisture conditions were the same during development of the seeds in the raceme, differences in proportion of seeds with intermediate and nondeep PD are attributed to position on parental plant. The ecological consequence of this phenomenon is that it ensures diversity in dormancy-breaking and germination characteristics within a seed cohort, a probable bet-hedging strategy. This is the first demonstration of position effects on level of PD in the offspring

Effects of Litter on Seedling Emergence and Seed Persistence of Three Common Species on the Loess Plateau in Northwestern China

Litter accumulation resulting from land use change (enclosure) is one of the key variables influencing seedling recruitment and consequently the regeneration of plant populations and seed persistence in the soil seed bank. A better understanding of the effects of litter on seed germination and seedling emergence is crucial for developing a new set of indicators for grassland ecosystem health and for grassland management policy. We investigated the effects of seed position in litter and amount of litter covering the seed on seedling emergence and seed persistence of three common species on the Loess Plateau in northwestern China. Seed position beneath the litter layer provided a suitable environment for seedling emergence of the three species. A moderate amount of litter (160 g/m2) was beneficial for seedling emergence of the small-seeded species Stipa bungeana and Lespedeza davurica from seeds from beneath the litter layer. The large-seeded species Setaria glauca was more tolerant of a high amount of litter (240 g/m2) than the two small-seeded species. Seed persistence in the soil differed among the three species and also was affected by seed position in litter and amount of litter cover. The proportion of viable seeds of Stipa bungeana and Setaria glauca on top of the litter layer increased with an increase in amount of litter. Seedling emergence and seed persistence varied significantly among species, amount of litter and seed position in litter. A moderate amount of litter and seeds positioned beneath the litter layer were better for seedling recruitment than for those on top of the litter layer. A high amount of litter was more favorable for persistence of seeds positioned on top of the litter than for those beneath the litter. Our study showed that maintaining litter amount between 80 and 160 g/m2 is optimal for S. bungeana dominated grassland on the Loess Plateau. We suggest that litter amount can serve as a guide for monitoring and managing grassland ecosystems, as it is an indicator of ecosystem processes that are essential for biodiversity conservation and restoration

Effect of Seed Morph and Light Level on Growth and Reproduction of the Amphicarpic Plant \u3cem\u3eAmphicarpaea edgeworthii\u3c/em\u3e (Fabaceae)

Amphicarpic plants produce aerial and subterranean fruits on an individual plant, and these heteromorphic diaspores give rise to plants that differ in growth and ecology. Amphicarpaea edgeworthii is a summer annual amphicarpic species that grows over a range of light levels. We aimed to compare the response to shading intensity of plants of A. edgeworthii grown throughout their life cycle from aerial seeds (ASP) and from subterranean seeds (SSP). We hypothesized that vegetative and reproductive growth of plants from ASP and SSP respond differently to light. Plants were grown from ASP and SSP under 0, 46, 71 and 90% shading intensities. With plant height as a covariate, vegetative biomass of ASP and SSP did not differ. Leaf area and seed production of SSP were greater and internode length less than they were for ASP in all shading intensities. Aerial and subterranean seed yield, seed mass and number for both ASP and SSP were highest in full light. Aerial seed yield was affected more than subterranean seed yield by shading intensity. The growth and reproductive responses of ASP and SSP of A. edgeworthii may be adaptive to the range of low to high light environments in which this species grows

Seed Germination Ecology of the Cold Desert Annual \u3cem\u3eIsatis violascens\u3c/em\u3e (Brassicaceae): Two Levels of Physiological Dormancy and Role of the Pericarp

The occurrence of various species of Brassicaceae with indehiscent fruits in the cold deserts of NW China suggests that there are adaptive advantages of this trait. We hypothesized that the pericarp of the single-seeded silicles of Isatis violascens restricts embryo expansion and thus prevents germination for 1 or more years. Thus, our aim was to investigate the role of the pericarp in seed dormancy and germination of this species. The effects of afterripening, treatment with gibberellic acid (GA3) and cold stratification on seed dormancy-break were tested using intact silicles and isolated seeds, and germination phenology was monitored in an experimental garden. The pericarp has a role in mechanically inhibiting germination of fresh seeds and promotes germination of nondormant seeds, but it does not facilitate formation of a persistent seed bank. Seeds in silicles in watered soil began to germinate earlier in autumn and germinated to higher percentages than isolated seeds. Sixty-two percent of seeds in the buried silicles germinated by the end of the first spring, and only 3% remained nongerminated and viable. Twenty to twenty-five percent of the seeds have nondeep physiological dormancy (PD) and 75-80% intermediate PD. Seeds with nondeep PD afterripen in summer and germinate inside the silicles in autumn if the soil is moist. Afterripening during summer significantly decreased the amount of cold stratification required to break intermediate PD. The presence of both nondeep and intermediate PD in the seed cohort may be a bet-hedging strategy

Divergence in Life History Traits Between Two Populations of a Seed-Dimorphic Halophyte in Response to Soil Salinity

Production of heteromorphic seeds is common in halophytes growing in arid environments with strong spatial and temporal heterogeneity. However, evidence for geographic variation (reflecting local adaptation) is almost nonexistent. Our primary aims were to compare the life history traits of two desert populations of this halophytic summer annual Suaeda corniculata subsp. mongolica and to investigate the phenotypic response of its plant and heteromorphic seeds to different levels of salt stress. Dimorphic seeds (F1) of the halophyte S. corniculata collected from two distant populations (F0) that differ in soil salinity were grown in a common environment under different levels of salinity to minimize the carryover effects from the field environment and tested for variation in plant (F1) and seed (F2) traits. Compared to F1 plants grown in low soil salinity, those grown in high salinity (\u3e0.2 mol⋅L-1) were smaller and produced fewer seeds but had a higher reproductive allocation and a higher non-dormant brown seed: dormant black seed ratio. High salinity during plant growth decreased germination percentage of F2 black seeds but had no effect on F2 brown seeds. Between population differences in life history traits in the common environment corresponded with those in the natural populations. Phenotypic differences between the two populations were retained in F1 plants and in F2 seeds in the common environment, which suggests that the traits are genetically based. Our results indicate that soil salinity plays an ecologically important role in population regeneration of S. corniculata by influencing heteromorphic seed production in the natural habitat

Changes in Oxidative Patterns During Dormancy Break by Warm and Cold Stratification in Seeds of an Edible Fruit Tree

The transition from seed dormancy to germination is triggered by environmental factors, and in pomegranate (Punica granatum) seeds higher germination percentages are achieved by warm + cold stratification rather than by cold stratification alone. Our objective was to define the pattern of internal oxidative changes in pomegranate seeds as dormancy was being broken by warm + cold stratification and by cold stratification alone. Embryos isolated from seeds after 1–42 days of warm stratification, after 56 days of warm stratification + 7, 28 or 56 days of cold stratification, and after 1–84 days of cold stratification alone, were used in biochemical tests. Hydrogen peroxide (H2O2), nitric oxide (NO), proline, lipid peroxidation, protein carbonylation, and activities of the scavenging enzymes superoxide dismutase (SOD), hydrogen peroxide enzyme and peroxidase in the embryos were assessed by colorimetric methods. Our results indicated that warm + cold stratification had a stronger dormancy-breaking effect than cold stratification (85% versus 50% germination), which may be attributed to a higher yield of H2O2, NO, lipid peroxidation and protein carbonylation in warm + cold stratification. Furthermore, warm + cold stratification-induced H2O2 change led to greater changes (elevation followed by attenuation) in activities of the scavenging enzymes than that induced by cold stratification alone. These results indicated that restriction of the level of reactive oxygen species change within a positive and safe range by such enzymes promoted seed germination. In addition, a relatively strong elevation of proline during warm + cold stratification also contributed to dormancy breakage and subsequent germination. In conclusion, the strong dormancy alleviating effect of warm + cold stratification on pomegranate seeds may be attributed to the corresponding active oxidative change via H2O2, NO, proline, malondialdehyde, protein carbonylation and scavenging enzymes

Seed Dormancy-Breaking in a Cold Desert Shrub in Relation to Sand Temperature and Moisture

Seasonal periodicity of seed germination and its relationship to seasonal changes in temperature and soil moisture have been well studied in seeds of species with physiological dormancy. However, relatively little information is available on the role of these environmental factors in controlling germination of seeds with physical dormancy (PY). Our primary aim was to determine if seeds of the cold desert sand dune semi-shrub Eremosparton songoricum exhibits seasonal periodicity of seed germination and the relationship between seed dormancy break and soil temperature and moisture. In the laboratory, seeds incubated on dry, wet, wet-dry and dry-wet sand were exposed to a 1-year sequence of temperature regimes simulating those in the field. In the field, seeds were buried at different depths on a sand dune, and germination of periodically exhumed seeds was tested at five temperature regimes during a 2-year period. In the one-year sequence of simulated natural temperature regimes, breaking of PY was more effective under constantly wet than under constantly dry conditions, and germination percentage was significantly higher under dry-wet than under wet-dry conditions. Seeds buried in the field exhibited a distinct peak of germination in spring and little or no germination in other seasons. The final (two-year) monthly cumulative germination percentage differed among burial depths and temperature, and it was highest (47%) in seeds buried at 3 cm and tested at 25/10 °C. A seed cohort of E. songoricum likely exhibits a long-term annual periodicity of spring germination in the field, and dormancy break appears to be driven by low (winter) temperatures and relatively high sand moisture content. To our knowledge this is the first study to document seasonal periodicity in seed germination in a cold desert species with PY and to identify the mechanism (at the whole-seed level) of its occurrence

Physiological Epicotyl Dormancy and Recalcitrant Storage Behaviour in Seeds of Two Tropical Fabaceae (Subfamily Caesalpinioideae) Species

BACKGROUND AND AIMS: Physiological epicotyl dormancy in which the epicotyl elongates inside the seed before the shoot emerges has been reported for only a few tropical rainforest species, all of which are trees that produce recalcitrant seeds. In studies on seeds of Fabaceae in Sri Lanka, we observed a considerable time delay in shoot emergence following root emergence in seeds of the introduced caesalpinioid legumes Brownea coccinea and Cynometra cauliflora. Thus, our aim was to determine if seeds of these two tropical rainforest trees have physiological epicotyl dormancy, and also if they are recalcitrant, i.e. desiccation sensitive. METHODOLOGY: Fresh seeds were (i) dried to various moisture levels, and (ii) stored at -1 and 5 °C to determine loss (or not) of viability and thus type of seed storage behaviour (orthodox, recalcitrant or intermediate). To identify the kind of dormancy, we tested the effect of scarification on imbibition and monitored radicle emergence and epicotyl growth (inside the seed) and emergence. PRINCIPAL RESULTS: FRESH SEEDS OF BOTH SPECIES HAD HIGH MOISTURE CONTENT (MC): 50 % for C. cauliflora and 30 % for B. coccinea. Further, all seeds of C. cauliflora and the majority of those of B. coccinea lost viability when dried to 15 % MC; most seeds of both species also lost viability during storage at -1 or 5 °C. Intact seeds of both species were water permeable, and radicles emerged in a high percentage of them inHowever, shoot emergence lagged behind root emergence by 77 ± 14 days in B. coccinea and by 38 ± 4 days in C. cauliflora. Further, plumule growth inside seeds of C. cauliflora began almost immediately after radicle emergence but not until ∼30-35 days in B. coccinea seeds. CONCLUSIONS: Seeds of both species are recalcitrant and have physiological epicotyl dormancy. The kind of physiological epicotyl dormancy in seeds of C. cauliflora has not been described previously; the formula is C(nd) (root)-[Formula: see text] (epicotyl)