Flooding tolerance of four tropical peatland tree species in a nursery trial.

In order to facilitate hydrological restoration, initiatives have been conducted to promote tree growth in degraded and rewetted peatlands in Indonesia. For these initiatives to be successful, tree seedlings need to be able to survive flooding episodes, with or without shade. We investigated the survival rates and the formation of adventitious roots in the case of four tree species exposed to combinations of different shading and water levels under controlled conditions in a nursery, with artificial rainwater and with peat soil as the medium. The research focused on the following questions (i) whether trees can grow on flooded peat soils; and (ii) which plant traits allow plants to cope with inundation, with or without shade. The four tree species compared (Shorea balangeran, Cratoxylum arborescens, Nephelium lappaceum and Durio zibethinus) include two natural pioneer and two farmer-preferred fruit trees. The experiment used a split-split plot design with 48 treatment combinations and at least 13 tree-level replicates. The study found that S. balangeran and C. arborescens had relatively high survival rates and tolerated saturated condition for 13 weeks, while N. lappaceum and D. zibethinus required non-saturated peat conditions. S. balangeran and C. arborescens developed adventitious roots to adapt to the inundated conditions. D. zibethinus, S. balangeran and N. lappaceum grew best under moderate (30%) shading levels, while C. arborescent grew best in full sunlight.

Seed germination and seedling development ecology in world-wide populations of a circumboreal Tertiary relict.

BACKGROUND AND AIMS: Temperate forests are disjunct in the Northern Hemisphere, having become fragmented from the earlier widespread (Tertiary) boreotropical forest. We asked 'What are the contemporary patterns of population variation in ecological traits of a Tertiary relict in a macroecological context?'. This issue underpins our understanding of variation in populations occurring in the same biome but on different continents. METHODOLOGY: We examined characters associated with root and shoot emergences among populations of Viburnum opulus in temperate forests of Asia, North America and Europe. This species has complex seedling emergence extending over several years and requiring various temperature cues. PRINCIPAL RESULTS: Populations varied in germination responses and clustered into groups that were only partly related to varietal status. Whereas roots (at warm temperatures) and shoots (following a cold period) simultaneously emerged from seeds of all populations when simulated dispersal occurred in winter, they were delayed in some populations when dispersal occurred in summer. CONCLUSIONS: Viburnum opulus populations, some separated by 10 300 km, showed high similarity in seedling development and in germination phenology, and we suggest that stabilizing selection has played a key role in maintaining similar dormancy mechanisms. Nevertheless, there was some degree of variation in other germination characters, suggesting local adaptation.

Sympatric species of Hibbertia (Dilleniaceae) vary in dormancy break and germination requirements: implications for classifying morphophysiological dormancy in Mediterranean biomes.

BACKGROUND AND AIMS: Several ecologically important plant families in Mediterranean biomes have seeds with morphophysiological dormancy (MPD) but have been poorly studied. The aim of this study was to understand the seed ecology of these species by focusing on the prominent, yet intractably dormant Australian genus Hibbertia. It was hypothesized that the slow germination in species of this genus is caused by a requirement for embryo growth inside the seed before germination, and that initiation of embryo growth is reliant upon a complex sequence of environmental cues including seasonal fluctuations in temperature and moisture, and an interplay with light and smoke. Using the results, the classification of the MPD level in species of Hibbertia is considered. METHODS: Four species of Hibbertia in winter rainfall south-western Australia were selected. These species, whilst differing in geographic distributions, are variously sympatric, and all are important understorey components of plant communities. The following aspects related to dormancy break, embryo growth and germination were investigated: temperature and moisture requirements; effects of karrikinolide, gibberellic acid and aerosol smoke; and phenology. KEY RESULTS: Following exposure to wet/dry cycles at low or high temperatures, embryo growth and germination occurred, albeit slowly in all species at low temperatures when moisture was unlimited, corresponding to winter in south-west Australia. Photo regime influenced germination only in H. racemosa. Aerosol smoke triggered substantial germination during the 1st germination season in H. huegelii and H. hypericoides. CONCLUSIONS: Although the study species are con-generic, sympatric and produce seeds of identical morphology, they possessed different dormancy-break and germination requirements. The physiological component of MPD was non-deep in H. racemosa but varied in the other three species where more deeply dormant seeds required >1 summer to overcome dormancy and, thus, germination was spread over time. Embryos grew during winter, but future studies need to resolve the role of cold versus warm stratification by using constant temperature regimes. To include Mediterranean species with MPD, some modifications to the current seed-dormancy classification system may need consideration: (a) wet/dry conditions for warm stratification and (b) a relatively long period for warm stratification. These outcomes have important implications for improving experimental approaches to resolve the effective use of broadcast seed for ecological restoration.

Seed dormancy in Trillium camschatcense (Melanthiaceae) and the possible roles of light and temperature requirements for seed germination in forests.

PREMISE OF THE STUDY: Seeds of some temperate plants require multiple treatments to break complex forms of dormancy, such as deep simple double morphophysiological dormancy (MPD), but embryo growth and radicle and cotyledon emergence have not been studied in depth for this level of MPD. Here we studied Trillium camschatcense, a species that is purported to have this MPD and that is sensitive to habitat fragmentation with decreased recruitment at forest edges. METHODS: Trillium camschatcense seeds were collected and experiments conducted in Hokkaido, Japan over 5 years. Growth of embryos and emergence of radicles and cotyledons were tracked in relation to field temperature to determine specific temperature and light requirements for these stages under laboratory conditions. KEY RESULTS: Low (winter) temperatures overcame dormancy in the underdeveloped embryos, and embryo growth and radicle emergence occurred between July and September, ca. 1 year after seed dispersal. Radicles emerged optimally over a narrow temperature range (20-25°C), in darkness, and at constant temperatures. Roots developed during the second autumn. Cotyledons needed a second low temperature (second winter) to emerge from seeds with roots, doing so in April, slightly over 1.5 years after dispersal. CONCLUSIONS: Seeds of T. camschatcense have deep simple double MPD and requirements for radicle emergence: darkness and constant temperatures. Ecologically, edges of forests may be deleterious for germination of the species due to increased light and to higher temperature fluctuations as compared to the interiors of forests. Thus, these specific requirements may play an important role in reducing seed germination of this plant at forest edges.

Understanding the germination of bulbils from an ecological perspective: a case study on Chinese yam (Dioscorea polystachya).

BACKGROUND AND AIMS: Bulbils serve as a means of vegetative reproduction and of dispersal for many plants; this latter aspect making them analogous to seeds. However, germination of bulbils may differ considerably from seeds due to dissimilar anatomical structures and perhaps environmental cue perception. The few laboratory studies done on bulbils suggest that their germination is similar to that of seeds in the same habitats and to vegetative buds of winter-dormant plants. The present study is the first to examine how bulbil germination is controlled in nature in relation to dispersal (before vs. after winter of the same cohort) and to ambient temperatures. METHODS: Under laboratory conditions, temperature and light requirements for root and shoot emergences from bulbils of Dioscorea polystachya collected in September, 2005, February, 2006 (produced in 2005) and July, 2006 were determined. Effects of cold stratification and dry storage for releasing dormancy were tested on September and July bulbils. The phenology of dormancy release and of root and shoot emergences and the persistence of bulbils in soil were followed over time under field conditions. KEY RESULTS: Although a low percentage of bulbils collected in July or in September produced roots, but no shoots, in the laboratory and field, these roots died within approx. 1 month. Regardless of collection date, cold stratification markedly increased root and shoot emergences. Bulbils sown outdoors in October produced roots and shoots the following March and April, respectively. The soil bulbil bank is short lived. CONCLUSIONS: Bulbils of D. polystachya are similar to seeds of many temperate plants being mostly dormant when dispersed in summer or autumn and overcoming dormancy with cold stratification during winter. Adaptively, bulbil germination primarily occurs in spring at the beginning of a favourable period for survivorship and growth.

Agroecosystem management for rare species of Paysonia (Brassicaceae): integrating their seed ecology and life cycle with cropping regimens in a changing climate.

Dormancy break and germination of seeds are governed by climatic cues, and predicted changes in climate may impact the ecology and conservation of species. Paysonia perforata and P. stonensis are rare brassicaceous winter annuals occurring primarily in fields on floodplains, where corn or soybeans are recommended for habitat maintenance. We tested the effects of precipitation, based on two predictions of changes in climate, on seed germination in these two species and placed the results into a management framework. Seeds of both species, collected during peak dispersal in late April/early May, were given various periods of light (or darkness) followed by darkness (or light) at summer temperatures before placement in darkness during late summer/early autumn in both laboratory and field. The light requirement was met earliest at 10 wk (mid-July) on alternating wet/dry substrate (simulating current climatic conditions). However, seeds of P. perforata and P. stonensis were photostimulated earliest at 2 wk (mid-May) and 6 wk (mid-June), respectively, on a continuously moist substrate (simulating predicted future conditions). The soil seed bank could be depleted if plowing coincides with photostimulation of seeds. Fields should be prepared after dispersal but before seeds are photostimulated and harvesting completed before seed germination in early September. Because seeds are highly photostimulated in late summer, disturbance from harvesting must be low to prevent burial. Cultivation of soybean, particularly for forage, is better matched to the seed biology and life cycle of Paysonia than that of corn under current and predicted climates.

Complex combination of seed dormancy and seedling development determine emergence of Viburnum tinus (Caprifoliaceae).

BACKGROUND AND AIMS: The shrub Viburnum tinus is widely distributed in mattoral vegetation of the Mediterranean basin. The purpose of the present study was to classify the seed dormancy type and examine the requirements for embryo growth, root protrusion and shoot emergence. METHODS: Overwintered fruits were collected in western Spain in April 2001 and prepared in three ways: entire pericarp was removed, exocarp and mesocarp were removed or fruits were left intact. Fruits treated in these three ways were subjected to artificial annual temperature cycles or to constant temperature regimes for 1.5 years. KEY RESULTS: Removal of exocarp and mesocarp was necessary for embryo growth and germination. High temperature favoured dormancy alleviation and embryo growth, intermediate to low temperatures favoured root protrusion, and intermediate temperature shoot emergence. There was substantial germination at constant temperature regimes, indicating an overlap between temperature intervals suitable for the different stages of embryo and seedling development. Functionally, V. tinus has the same root and shoot emergence pattern that is described for other Viburnum species considered to have epicotyl dormancy. However, the requirement for high and low temperatures for radicle protrusion and epicotyl emergence, respectively, was missing in V. tinus; these characters are the foundation for the epicotyl dormancy classification. CONCLUSIONS: It is concluded that V. tinus does not have epicotyl dormancy. Instead, there is a combination of a weak morphophysiological dormancy and a slow germination process, where different temperatures during an annual cycle favour different development stages. The present study suggests that the first complete seedlings would emerge in the field 1.5 years after fruit maturation in October, i.e. seed dispersal during winter, embryo growth during the first summer, root protrusion and establishment during the second autumn and winter, and cotyledon emergence during the second spring.

Seed germination ecophysiology of the Asian species Osmorhiza aristata (Apiaceae): comparison with its North American congeners and implications for evolution of types of dormancy.

Osmorhiza aristata is an herbaceous perennial that grows primarily in Japan, through southern China, to the Himalayas. It closely resembles the eastern North American species O. claytonii and O. longistylis, and, together, the three species are an example of the well-known North American-Asian pattern of disjunction. Requirements for dormancy break and embryo growth were determined for seeds of O. aristata collected in Japan during the summers of 1998-2000. Embryos in fresh seeds were ca. 0.5 mm long, and they had to grow to 9 mm before the radicle emerged from the mericarp. Embryo growth and germination occurred during cold stratification at 5°C, the optimum temperature for germination. Gibberellic acid did not substitute for cold stratification. Thus, O. aristata seeds have deep complex morphophysiological dormancy (MPD). The type of MPD in O. aristata is similar to that in two western North American congeners but different from that in eastern North American congeners (nondeep complex MPD). Mapping the types of MPD onto a phylogeny of the genus suggests that nondeep complex MPD is derived from deep complex MPD. Although eastern North American-Asian disjuncts often exhibit morphological stasis, the taxa may differ greatly in physiological traits, such as seed dormancy.