Seed bank versus seed rain in the regeneration of a tropical pioneer tree

Summary We used the tropical pioneer tree, Cecropia obtusifolia to evaluate the relative importance of different sources of seeds in the regeneration of species that depend on ephemeral sites. We studied seed production in a population established in a 5 ha plot, and dispersal, dormancy and seed predation in two recent treefall gaps (<1 year-old), two building or successional forest patches (10–15 since disturbed), and two mature forest patches (>35 years since disturbed) for a one year period at Los Tuxtlas (Mexico). Flowers and fruits were counted at monthly intervals. Annual fecundity per tree ranged from 1.4×10⁴ to 1.4×10⁷ seeds. Seeds were continuously available on the trees and on the ground. Average annual seed rain per m² (as measured by 0.5×0.5 m seed traps) varied from 184 to 1925 among the six sites. Distance to nearest seed source and patch type explained more than 60% of the seed rain variation among sites. Soil seed density, estimated by counting seeds from ten samples (78.5 cm²×10 cm deep) collected from each site in October and January, ranged among the six sites from 269 to 4485 seeds per m² in January and from 204 to 5073 in October. Soil seed viabilities were much lower (17.1% in October and 5.1% in January) than those of rain seeds (48.26%). Annual survivorships of 2.2% were estimated for seeds artificially sown on the soil surface of a gap and a mature patch, and 3.75% in a building patch. In two other experiments seed removal rates ranged from 27% to 98% in 4 days. Removal rates were significantly higher in gap and mature patches than in building patches. Ants (Paratrechina vividula) and grasshopper nymphs (Hygronemobius. sp.) were the main predators. We draw three main conclusions from our data: (1) Pathogens and predators determine low survivorship of C. obtusifolia's seeds in the soil and a rapid turnover rate (1.07 to 1.02 years) of its seed bank; (2) a continuous and copious seed production and an abundant and extensive seed rain replenish the soil seed pool in patches with different disturbance ages at least up to 86 m from nearest source; (3) more than 90% of the seeds contributing to C. obtusifolia seedling recruitment in gaps are less than one year-old. We discuss our results in the context of previous similar studies for tropical forests.     

Ecology of seed germination of eight non-pioneer tree species from a tropical seasonal rain forest in southwest China

We compared various aspects of the seed biology of eight non-pioneer tree species from a tropical seasonal rain forest in Xishuangbanna, SW China, that differ in time of dispersal, size and fresh seed moisture content (MC). Seeds were tested for germination under laboratory conditions after dehydration to different moisture levels and under 3.5, 10 and 30% solar irradiances in neutral-shade houses. For six species, germination was also compared in forest understory (3.5% light) and center of a forest gap (32.5% light). Under continuous dehydration over activated silica gel, 100% of seeds of four species had lost the ability to germinate after 48 h, and those of all species except Castanopsis hystrix (decreased from >90 to 30% germination) had lost the ability to germinate after 120 h. Four species did not differ in final germination percentages at the three irradiances (i.e. uniform germination). However, final germination percentages of Horsfieldia pandurifolia and Litsea pierrei var. szemaois were significantly lower in 30% than in 10 or 3.5% light, and seeds of Antiaris toxicaria and C. hystrix germinated to higher percentages in 30 and 10% than in 3.5% light. Mean time to germination (MTG) of the eight species (forest and shade house data combined) ranged from 5–5 days for Pometia tomentosa to 72–207days for L. pierrei; MTG for four species was ≤21 days. There was no obvious relationship between relative desiccation resistance and either time of dispersal, MTG or uniformity of germination at the three light levels, or between seed size and MC or MTG. However, the relationship between seed MC at maturity (25–60% fresh mass basis) and MC at 50% loss of seed viability (12.4–42.5%) was significant. Seven of the species fit Garwood’s (Ecol Monogr 53:159–181, 1983) rapid-rainy germination syndrome and one, L. pierrei, either her delayed-rainy or intermediate-dry germination syndrome. However, fresh, non-dehydrated seeds of all eight species germinated in ≤30 days at constant 30°C in light.     

Seed survival of the tropical tree Cryptocarya aschersoniana (Lauraceae): Consequences of habitat disturbance

Seed survival in soil could be strongly influenced by habitat characteristics, but little is known about the behaviour of seeds sensitive to desiccation in seed banks installed in natural or disturbed habitats. Cryptocarya aschersoniana seeds disperse at the end of the rainy season but do not germinate immediately; thus, they may form seed banks in soil. This study evaluated the behaviour of C. aschersoniana seed banks induced in the natural environment of the species and in a disturbed area. Recently harvested C. aschersoniana germination units were characterized according to their water content, germination and viability. In 2011 and 2012, seed banks were established by burying samples of seeds in the understory of a semi‐deciduous forest. In 2012, samples were also buried in a disturbed area. The seed banks were sampled at certain time intervals, and the samples were characterized as described above. Precipitation and air temperature data were collected. As a result, seeds in the seed bank established in the natural environment form a transient seed bank and showed the same behaviour in both years studied. A germination peak was observed starting 210 days after burial (coinciding with the onset of the rainy season) and reached germination percentages higher than 80% at the end of the experiment for both years. Seed mortality did not exceeded 28% in the natural environment. However, in the disturbed environment, the seeds lost their viability more rapidly, with 90% of the seeds becoming unviable 240 days after burial. Germinated seeds in the disturbed environment (maximum 21%) were not able to establish seedlings. These results underscore the importance of maintaining a natural, undisturbed forest for the conservation of this species.     

Distribution patterns of tree species in a Malaysian tropical rain forest

Abstract. Spatial patterns of tree species were studied in a 50-ha tropical rain forest plot in the Pasoh forest, Malaysia. This forest is characterized by a high diversity and very high number of rare species. Out of the 745 species occurring with > five individuals, 80.4 % had an aggregated distribution, 19.5% were randomly distributed and one species had a regular distribution. The spatial patterns of rare vs. common species, juvenile vs. adult trees, and coarse vs. fine scales were compared. Rare species are generally less aggregated than common ones and most of the randomly distributed species are rare. Spatial patterns shift from high clumping to looser intensity or random distribution when moving from juveniles to adults for the same species. No adult tree species display a regular pattern, however. Regular distributions were rarely found; this is probably due to intraspecific competition at a local scale. There is a negative correlation between per capita death rate and population density. This study suggests that the Pasoh forest and its high diversity are subjected to multiple controlling factors, e.g., topography, spacing effect, density-dependent processes and species rarity. The importance of any factor changes across spatial and temporal scales.     

Adaptation to habitat in Aquilegia species endemic to Sardinia (Italy): Seed dispersal,germination and persistence in the soil

Abstract

The autecology of the Sardinian endemics Aquilegia barbaricina Arrigoni et Nardi and A. nugorensis Arrigoni et Nardi were investigated. Peaks of anthesis and seed dispersal were recorded for five populations occurring in two distinct habitats, one riparian and one rupicolous. Germination tests were carried out on seed lots belonging to each population by sowing seeds at 10, 15, 20, 25 and 25/15°C. In addition, seeds were incubated for 2 months at either 25°C (summer), 5°C (winter) or 25°C for 2 months plus 2 months at 5°C (summer followed by winter–SW), and then moved to the germination temperatures. Embryo measurements were taken during pre-treatments and germination. Experimental seed burials were carried out for two populations of each species. Both species dispersed in summer. The population of A. nugorensis occurring on rocky outcrops differed in phenology from both the other A. nugorensis population from riparian vegetation and from A. barbaricina. Both species showed morphophysiological seed dormancy, with <50% germination under laboratory conditions. All riparian populations germinated only after the SW pre-treatment, while the rupicolous population germinated at 25°C, without any pre-treatment. Low germination percentages were observed in the experimental seed burials, suggesting the ability for both species to form a persistent soil seed bank.     

Seed germination traits of Ailanthus altissima,Phytolacca americana and Robinia pseudoacacia in response to different thermal and light requirements

Invasion of alien plant species (IAS) represents a serious environmental problem, particularly in Europe, where it mainly pertains to urban areas. Seed germination traits contribute to clarification of invasion dynamics. The objective of this research was to analyze how different light conditions (i.e., 12-hr light/12-hr darkness and continuous darkness) and temperature regimes (i.e., 15/6°C, 20/10°C and 30/20°C) trigger seed germination of Ailanthus altissima (AA), Phytolacca americana (PA) and Robinia pseudoacacia (RP). The relationship between seed germination and seed morphometric traits was also analyzed. Our findings highlight that temperature rather than light was the main environmental factor affecting germination. RP germinated at all tested temperatures, whereas at 15/6°C seeds of AA and PA showed physiological dormancy. RP had a higher germination capacity at a lower temperature, unlike AA and PA, which performed better at the highest temperatures. Light had a minor role in seed germination of the three species. Light promoted germination only for seeds of PA, and final germination percentage was 1.5-fold higher in light than in continuous darkness. Seed morphometric traits (thickness [T], area [A] and volume [V]) had a significant role in explaining germination trait variations. The results highlight the importance of increasing our knowledge on seed germination requirements to predict future invasiveness trends. The increase in global temperature could further advantage AA and PA in terms of germinated seeds, as well as RP by enhancing the germination velocity, therefore compensating for a lower germination percentage of this species at the highest temperatures.     

Seed reserve composition in 19 tree species of a tropical deciduous forest in Mexico and its relationship to seed germination and seedling growth

Background and Aims

The size and composition of seed reserves may reflect the ecological strategy and evolutionary history of a species and also temporal variation in resource availability. The seed mass and composition of seed reserves of 19 co-existing tree species were studied, and we examined how they varied among species in relation to germination and seedling growth rates, as well as between two years with contrasting precipitation (652 and 384 mm).

Methods

Seeds were collected from a tropical deciduous forest in the northwest of Mexico (Chamela Biological Station). The seed dry mass, with and without the seed coat, and the concentrations of lipids, nitrogen and non-structural carbohydrates for the seed minus seed coat were determined. The anatomical localization of these reserves was examined using histochemical analysis. The germination capacity, rate and lag time were determined. The correlations among these variables, and their relationship to previously reported seedling relative growth rates, were evaluated with and without phylogenetic consideration.

Key Results

There were interannual differences in seed mass and reserve composition. Seed was significantly heavier after the drier year in five species. Nitrogen concentration was positively correlated with seed coat fraction, and was significantly higher after the drier year in 12 species. The rate and lag time of germination were negatively correlated with each other. These trait correlations were also supported for phylogenetic independent contrasts. Principal component analysis supported these correlations, and indicated a negative association of seedling relative growth rate with seed size, and a positive association of germination rate with nitrogen and lipid concentrations.

Conclusions

Nitrogen concentration tended to be higher after the drier year and, while interannual variations in seed size and reserve composition were not sufficient to affect interspecific correlations among seed and seedling traits, some of the reserves were related to germination variables and seedling relative growth rate.     

Seed dormancy and control of germination in Sisymbrella dentata (L.) O.E. Schulz (Brassicaceae)

• Seed germination responsiveness to environmental cues is crucial for plant species living in changeable habitats and can vary among populations within the same species as a result of adaptation or modulation to local climates. Here, we investigate the germination response to environmental cues of Sisymbrella dentata (L.) O.E. Schulz, an annual endemic to Sicily living in Mediterranean Temporary Ponds (MTP), a vulnerable ecosystem.
• Germination of the only two known populations, Gurrida and Pantano, was assessed over a broad range of conditions to understand the role of temperatures, nitrate, hormones (abscisic acid – ABA and gibberellins – GA) and after‐ripening in dormancy release in this species.
• Seed germination responsiveness varied between the two populations, with seeds from Gurrida germinating under a narrower range of conditions. Overall, this process in S. dentata consisted of testa and endosperm rupture as two sequential events, influenced by ABA and GA biosynthesis. Nitrate addition caused an earlier testa rupture, after‐ripening broadened the thermal conditions that allow germination, and alternating temperatures significantly promoted germination of non‐after‐ripened seeds.
• Primary dormancy in S. dentata seeds likely allows this plant to form a persistent seed bank that is responsive to specific environmental cues characteristic of MTP habitats.

     

西双版纳热带季节雨林四种珍稀濒危树种种子萌发对脱水和光的响应

绒毛番龙眼(Pometia tomentosa)、琴叶风吹楠(Horsfieldia pandurifolia)、箭毒木(Antiaris toxicaria)和思茅木姜子(Litseapierrei var. szemois)是西双版纳热带季节雨林乔木层的优势树种,也属于珍稀濒危物种。实验研究了4个树种的种子在经过不同脱水水平、人工遮荫条件下的不同光照水平以及热带季节雨林之林下和林窗的萌发特性。结果显示:脱水对于种子萌发特征具有显著影响,绒毛番龙眼和琴叶风吹楠经48h的连续脱水后丧失了萌发能力;箭毒木和思茅木姜子具较强的脱水耐性,经96h的连续脱水后仍具有较高的种子含水量,且种子萌发率分别为15%和33%。干旱季散布的种子比雨季散布的种子具有更强的脱水耐性。遮荫及森林种子萌发试验中,除绒毛番龙眼外,其它3个树种的种子萌发率以及平均萌发周期在不同光水平或生境下差异显著。琴叶风吹楠和思茅木姜子在30%的光照以及林窗中心的萌发率要显著低于低光处理(10%和3.5%光照)和林下,而箭毒木在30%和10%光水平以及林窗中心获得较高萌发率。雨季传播的具有干燥敏感性的种子,可以减少种子由于干燥而引起的死亡。试验结果可以为4种珍稀濒危植物的保护和人工繁育提供一定理论依据。     

Seed dispersal by red howling monkeys (Alouatta seniculus) in the tropical rain forest of French Guiana

A 2-year field study of the frugivorous diet of a howling monkey troop, in a tropical rain forest in French Guiana, shows that they disperse by endozoochory ≥95% of plant species from which they eat ripe fruit. Passage through the digestive tract of howlers does not significantly modify the germination success of most plant species samples. Their low digestion rate (X = 20 hr 40 min) is the ultimate cause of a bimodal defecation rhythm that results in the concentration of 60% of defecations being deposited under sleeping sites. The distance of seed dispersal can reach more than 550 m from parent trees,with a mean of 260 m. Although howling monkeys consume fruits differing in morphological characteristics, they are particularly able to disseminate seeds of species whose fruits have a hard and indehiscent external coat or large seeds or both. In French Guiana, they may be especially important dispersers of the Sapotaceae with fruits that simultaneously present both characteristics.     

上海崇明东滩互花米草种子产量及其萌发对温度的响应

外来物种互花米草通过快速的无性和有性繁殖,已在崇明东滩形成了大面积的单优势种群。研究了崇明东滩互花米草入侵区种子产量及温度对其萌发的影响。结果表明,互花米草在潮间带的中潮间带种子产量最高,为44523粒/m²,高潮间带次之,为31150粒/m²,低潮间带最低,为17250粒/m²。各潮间带的种子在10 30℃范围内均能萌发,而且随着温度的升高,萌发率和萌发速度总体呈现先增大后减小的趋势。互花米草种子在25℃恒温与20/30℃变温处理下的萌发率最高和萌发速度最快,而且互花米草种子在变温环境下的萌发率高于恒温处理。研究表明,崇明东滩中潮间带互花米草的种子产量和活性最高,是提供有性扩散种源的主要区域,适合种子萌发温度的4-5月是互花米草进行有性扩散的主要季节。研究结果对了解互花米草有性繁殖扩散的时空格局和入侵生态学过程,以及有效控制和管理入侵物种互花米草提供了科学依据。     

Seed germination in temperate rain forest species of southern Chile: chilling and gap-dependency germination

Canopy gap and chilling requirements for seed germination were assessed in 61 and 44 species, respectively, in the temperate rain forest of southern Chile. Germination assays within canopy gap and understorey were carried out under natural conditions. Germination tests in cold stratified and non-stratified seeds were performed under laboratory conditions. Seeds were collected of common trees, shrubs, vines and perennial herbs of forests in southern Chile. Final percent germination was significantly enhanced under canopy gap conditions in 19 species, and significantly reduced in 11 species. Germination proved indifferent under gap vs. understorey conditions in half the species tested. Cold stratification affected germination in 11 species: significantly increasing final germination of four species, and significantly decreasing final percent germination of seven species. A Principal Components Analysis (PCA) was applied with the purpose of identifying groups of species with similar germination strategies. Four attributes were included in the PCA: final germination percentage in canopy gap, germination rate in the laboratory, and gap-and chilling-dependency indices (EGAP and STRAT, respectively). The first axis separated species mainly on EGAP variation while the second axis separated them mainly according to STRAT variation; the two axes together explaining 73% of the among-species variation. A small group of trees and vines germinating best in the understorey and neutral to chilling could clearly be distinguished from the remaining species analyzed. Multifactorial ANOVAs were used for evaluating the combined effect of successional status, seed mass, dispersal period, life form, phylogenetic categories, and dispersal syndrome on EGAP and STRAT variation. The EGAP value of secondary successional species proved significantly greater than that of primary successional species and species with endozoochorous seed dispersal were significantly less dependent on chilling (according to STRAT values) compared to species with mainly wind-dispersed seeds. The possible implications of these results for seedling establishment are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light and temperature action in germination of seeds of the empress tree (Paulownia tomentosa)

Seeds of the empress tree ( Paulownia tomentosa Steud.) were imbibed for two weeks in darkness at constant temperatures (18, 23 or 28°C), and then irradiated with red light for 5 min. Germination was poor if it took place at the same temperature as imbibition, but a high percentage was achieved if the seeds were exposed to higher or lower temperatures before they were irradiated. Maximum germination was obtained when the difference between pretreatment and imbibition was about 10°C. The effect increased with the duration of the pretreatment and was optimal at 24 h. The effect decreased as the time lapse between temperature pretreatment and red light irradiation increased, and it was lost after two days. If pretreatment was shorter than 24 h (12 h). a high percent of germination was obtained by alternating pretreatment and imbibition temperatures. The germination of seeds imbibed in 40% heavy water was also stimulated by temperature pretreatments. Light and temperature also exhibited an interactive effect in the germination of seeds that were imbibed in darkness for only 3 days. For each of the germination phases there was a temperature at which the time needed for 50% germination was the shortest, namely 35°C during imbibition, 37.5°C in the period of P_fr activity. and 32.5°C during radicle protrusion. The data obtained are shortly discussed in relation to the domestication of empress tree in Southern Europe.     

Dormancy cycles in buried seeds of three perennial Xyris (Xyridaceae) species from the Brazilian campo rupestre

• Dormancy cycles are an important mechanism for avoiding seed germination under unfavourable periods for seedling establishment. This mechanism has been scarcely studied in tropical species. Here, we studied three tropical and perennial species of Xyris, X. asperula, X. subsetigera and X. trachyphylla, to investigate in situ longevity and the existence of seasonal seed dormancy cycles.
• Seeds of three species of Xyris were buried in their natural habitat, with samples exhumed bimonthly for 18 months. Germination of exhumed seeds was assessed under a 12‐h photoperiod over a broad range of temperatures. Seeds of X. trachyphylla were also subjected to treatments to overcome secondary dormancy.
• Seeds of all species are able to form a persistent seed bank and exhibit seasonal changes in germinability. Secondary dormancy was acquired during the rainy summer and was overcome during the subsequent dry season (autumn/winter). Desiccation partially overcomes secondary dormancy in X. trachyphylla seeds.
• Soil seed bank persistence and synchronisation of seed germination under favourable conditions for seedling establishment contribute to the persistence and regeneration of X. asperula, X. subsetigera and X. trachyphylla in their natural environment.

     

Rainfall seasonality predicts the germination behavior of a tropical dry‐forest vine

Seed dormancy is considered to be an adaptive strategy in seasonal and/or unpredictable environments because it prevents germination during climatically favorable periods that are too short for seedling establishment. Tropical dry forests are seasonal environments where seed dormancy may play an important role in plant resilience and resistance to changing precipitation patterns. We studied the germination behavior of seeds from six populations of the Neotropical vine Dalechampia scandens (Euphorbiaceae) originating from environments of contrasting rainfall seasonality. Seeds produced by second greenhouse‐generation plants were measured and exposed to a favorable wet environment at different time intervals after capsule dehiscence and seed dispersal. We recorded the success and the timing of germination. All populations produced at least some dormant seeds, but seeds of populations originating from more seasonal environments required longer periods of after‐ripening before germinating. Within populations, larger seeds tended to require longer after‐ripening periods than did smaller seeds. These results indicate among‐population genetic differences in germination behavior and suggest that these populations are adapted to local environmental conditions. They also suggest that seed size may influence germination timing within populations. Ongoing changes in seasonality patterns in tropical dry forests may impose strong selection on these traits.     

Species diversity,susceptibility to disturbance and tree population dynamics in tropical rain forest

Abstract. In 1964 a census of all trees > 9.7 cm diameter at breast height (DBH) was conducted on 22 plots totalling 13.2 ha in lowland tropical evergreen rain forest on Kolombangara, Solomon Islands. Over the following 30 yr (1964–1994), populations of all individuals > 4.85 cm DBH of the 12 most common tree species and amounts of disturbance have been monitored on a declining number of these plots (in 1994, nine plots totalling 5.4 ha were still being recorded). Between November 1967 and April 1970, Kolombangara was struck by four cyclones, although only two of these caused substantial amounts of damage to the canopy structure. Multivariate analysis has identified six forest types on Kolombangara (Greig-Smith et al. 1967). The species richness and diversity of trees in the 1964 census, turnover rates of the populations monitored over 1964–1975, and the amount of disturbance sustained during a cyclone in 1970, were all positively correlated across five of the forest types. The sixth forest type was a consistent outlier in these analyses and is believed to have been seriously disturbed by humans about a century ago. The floristics, turnover and disturbance data support Connell's intermediate disturbance hypothesis. The most species-rich forest types contained a higher proportion of fast-growing individuals and species that are early-successional and which have low density timber. Properties of these species rendered them more susceptible to damage when struck by the 1970 cyclone. They showed higher turnover rates because disturbance-dependent species are also characterised by higher mortality and recruitment rates. Thus, periodic cyclones appear to favour the maintenance of differences in species diversity and composition between forest types.     

Variation in the population structure between a natural and a human‐modified forest for a pioneer tropical tree species not restricted to large gaps

The distribution of tree species in tropical forests is generally related to the occurrence of disturbances and shifts in the local environmental conditions such as light, temperature, and biotic factors. Thus, the distribution of pioneer tree species is expected to vary according to the gap characteristics and with human disturbances. We asked whether there was variation in the distribution of a pioneer species under different environmental conditions generated by natural disturbances, and between two forests with contrasting levels of human disturbance. To answer this question, we studied the distribution patterns and population persistence of the pioneer tree species Croton floribundus in the size and age gap range of a primary Brazilian forest. Additionally, we compared the plant density of two size‐classes between a primary and an early successional human‐disturbed forest. Croton floribundus was found to be widespread and equally distributed along the gap‐size gradient in the primary forest. Overall density did not vary with gap size or age (F‐ratio = 0.062, P = 0.941), and while juveniles were found to have a higher density in the early successional forest (P = 0.021), tree density was found to be similar between forests (P = 0.058). Our results indicate that the population structure of a pioneer tree species with long life span and a broad gap‐size niche preference varied between natural and human‐disturbed forests, but not with the level of natural disturbance. We believe this can be explained by the extreme environmental changes that occur after human disturbance. The ecological processes that affect the distribution of pioneer species in natural and human‐modified forests may be similar, but our results suggest they act differently under the contrasting environmental conditions generated by natural and human disturbances.     

Gap phase regeneration of tree species of differing successional status in a humid tropical forest of Kerala,India

Germination, establishment and growth of seedlings of tree speciesPalaquium ellipticum (primary),Actinodaphne malabarica (late secondary) andMacaranga peltata (early secondary) were studied in a humid tropical forest at Nelliampathy, in the Western Ghats of Kerala. While the primary species completed its germination within a brief period of 1.5 months, at the other extreme, early secondary species showed slow germination extending for about 5 months, the late secondary species falling in between. Although, all the species studied showed higher establishment and growth under gaps, the early secondary species were more responsive compared to the primary species. Primary species showed better establishment in undisturbed sites and natural gaps than under selection felled gaps; the reverse was true for late and early secondary species. Survival of seedlings increased with gap size, but sharply declined with gap age. Shoot/root ratio was consistently higher in the early secondary species than in the primary species.     

Seed dormancy and germination in three Crocus ser. Verni species (Iridaceae): implications for evolution of dormancy within the genus

The aim of this work was to examine whether seed ecophysiological traits in three closely related Crocus species were associated with ecological niche differentiation and species divergence. Seeds of the temperate tetraploid cytotype of Crocus neapolitanus, the sub‐Mediterranean C. etruscus and the Mediterranean C. ilvensis were placed either on agar in the laboratory under different periods of simulated seasonal conditions or in nylon mesh bags buried outdoors to examine embryo growth, radicle and shoot emergence. In agreement with the phenology observed outdoors, in the laboratory embryos required a cool temperature (ca. 10 °C) to grow to full size (embryo length:seed length, E:S ratio ca. 0.75) but only after seeds received a warm stratification; radicle emergence then followed immediately (November). Shoot emergence is a temporally separated phase (March) that was promoted by cold stratification in C. neapolitanus while in the other two species this time lag was attributed to a slow continuous developmental process. These species have similar embryo growth and radicle phenology but differ in their degree of epicotyl dormancy, which is related to the length of local winter. Conclusions from laboratory experiments that only consider root emergence could be misleading; evaluating the phenology of both root and shoot emergence should be considered in order to demonstrate ecologically meaningful differences in germination behaviour and to develop effective propagation protocols. Although these taxa resulted from recent speciation processes, the outcomes suggest an early onset of adaptation to local ecological factors and that phylogeny may represent a significant constraint in the evolution and expression of seed traits in Crocus.     

The responses to shade of seedlings of very small-seeded tree and shrub species from tropical rain forest in Singapore

1. Newly germinated seedlings of six tree and shrub species with very small seeds (31–460 μg dry mass), one light-demanding and five shade-tolerant at the stage of establishment in the wild, were grown for 5 months in neutral shade houses with 0·5, 1, 3·5 and 7·5% daylight.
2. The ratio of yield in 7·5% to that in 1% was 8:1 for the light-demanding Melastoma malabathricum but only 2:1 for the confamilial shade-tolerator Pternandra echinata. The Pternandra, Urophyllum hirsutum, Ficus chartacea, Ficus grossularioides and Pellacalyx saccardianus showed a graded series of responses to irradiance, generally consistent with their apparent demands for light in the wild. In contrast, survival of very deep shade was not clearly related to light demand in nature.
3. The results support the conclusion drawn from observational studies that large seed size is not primarily adaptive in resisting shade but in resisting the associated risks of burial by litter, desiccation during dry spells, uprooting by birds and other kinds of damage by animals or falling debris.