Seed germination of exotic and native winter annuals differentially responds to temperature and moisture,especially with climate change scenarios

Seeds of winter annuals require a summer after-ripening period for dormancy loss and low autumn temperatures for germination. With current and future changes in moisture and temperature, we tested the effects of warming along a relative humidity (RH) gradient on dormancy loss and effects of decreased diurnal temperature range (DTR) on germination. We further reasoned that the effects of changes in these variables would be disproportionate between the exotic and native winter annuals. Seeds of exotic species (Buglossoides arvensis, Lamium purpureum and Ranunculus parviflorus) and co-occurring native species (Galium aparine, Paysonia stonensis and Plantago virginica) were collected in middle Tennessee. After-ripening occurred over a 15–100% RH gradient at 25 and 30°C and germination was tested at 20/10 and 20/15°C. Niche breadth was calculated using Levins' B. Fresh Ranunculus seeds had high germination and those of other species did not. Germination for these species increased with after-ripening, mostly across the RH gradient irrespective of temperature. A decrease in DTR showed mixed results – the extreme being Ranunculus with no germination at 20/15°C. Most exotic species had wider germination niche breadths than native species. With climate change, we suggest that a decrease in DTR may have a larger effect on germination than increasing moisture or warming on dormancy break. Moreover, there is not a clear-cut winner with climate change when we compare exotic versus native species because the responses of our six species were species specific.     

The role of seed germination in the invasion process of Honey locust (Gleditsia triacanthos L., Fabaceae): comparison with a native confamilial

Identifying plant traits that promote invasiveness has been a major goal in invasion ecology. Germination plays a central role in the life cycle of plants and therefore could be a key trait in determining species invasiveness. In this study, seed germination of two confamilial, co‐occurring species that share ecological characteristics, the exotic invasive Gleditsia triacanthos L and the native Acacia aroma Gillies ex. Hook. & Arn., was compared. Seeds were obtained from individuals of three localities in the Chaco Serrano region of Córdoba, Argentina. Percent of seed germination and mean germination time were recorded in chemically and mechanically scarified seeds, and the former variable was also recorded in seeds subjected to: passage through the digestive tract of dispersers, fire simulations, fire simulation plus mechanical scarification, seed longevity, and dormancy break over time. In general, both species showed similar germination percentage. However, non‐scarified seeds of the exotic species lost physical dormancy when subjected to experiments of dormancy break over time, whereas, the native species had shorter mean germination time. The greater percentage of seed germination over time of the exotic species than of the native one might be triggering the spread of the former, whereas the shorter mean germination time might be hindering its expansion to more arid regions. The study of different mechanisms for achieving seed germination, particularly in hard seed species, could provide important information on the expansion of invasive species as well as useful knowledge for their management.     

Native Plant Regeneration on Abandoned Desert Farmland: Effects of Irrigation, Soil Preparation, and Amendments on Seedling Establishment

Direct seeding methods to revegetate abandoned farmland were tested at a desert site west of Phoenix, Arizona. Native seeds were broadcast onto plots prepared by mulching, imprinting, chiseling, and fertilizing with phosphorous in a split-plot design. Each main plot was split into subplots that were not irrigated, irrigated with saline (3.25 dS/m) well water, or irrigated and hand weeded of Salsola iberica . Native seeds germinated poorly on all treatments, and three annual exotic weeds ( Brassica nigra , S. iberica , and Schismus spp.) dominated the plots. None of the main plot treatments (mulching, imprinting, chiseling, or fertilizing) had a significant effect on seed germination or canopy cover. Irrigation increased plant cover on plots, but weeds dominated the cover (<4% native species, up to 50% weeds). Near the end of the second growing season a seed bank study was conducted in the greenhouse. Undisturbed desert soil had relatively few weed seeds and more native plant seeds than the disturbed agricultural soil samples, which had few viable native seeds and were dominated by Schismus spp., B. nigra , and S. iberica . The results illustrate the difficulty of establishing native plants on abandoned desert farmland due to the dominance of weedy species, the presence of salts in the soil, and the lack of adequate soil moisture in the treatments without supplemental irrigation.     

Seed dispersal and germination patternsin a rare Mediterranean island endemic (Anchusa crispa Viv., Boraginaceae)

We analyse and discuss patterns of seed dispersal and germination of a rare endemic plant species, Anchusa crispa Viv. (Boraginaceae) from Corsica and Sardinia. This coastal plant inhabits dunes and back-dunes, and currently numbers only a few thousand individuals which generally occur in isolated populations. This study included experiments conducted in the field in Corsica and also under controlled conditions in the laboratory. Short-distance dispersal of A. crispa is performed by ants, both by myrmecochory and dyszoochory. The invasion of an exotic species of ant, Linepithema humile, could locally modify the dispersal system and possibly the population dynamics of A. crispa. Long-distance dispersal may occur by water transport since seeds can germinate after at least 1 week of immersion in seawater and readily float on the surface. Burial of seeds is favourable for germination, percentage germination being maximised at a depth of 1–2 cm below the soil surface. A. crispa has a seed bank of about ten viable seeds per m², which may contribute to the survival of this species which exists in small populations with a short life span. Due to its seed dispersal and germination patterns, the conservation of this species will necessitate that human disturbance, which can destabilise the surface of the sand is prevented and that new populations are introduced to favourable sites.     

The incidence of exotic species following clearfelling of Eucalyptus regnans forest in the Central Highlands,Victoria

Abstract Invasion by exotic species following clearfelling of Eucalyptus regnans F. Muell. (Mountain Ash) forest was examined in the Toolangi State Forest in the Central Highlands of Victoria. Coupes ranging in age from < 1- to 10-years-old and the spar-stage forests (1939 bushfire regrowth) adjacent to each of these coupes and a mature, 250-year-old forest were surveyed. The dispersal and establishment of weeds was facilitated by clearfelling. An influx of seeds of exotic species was detected in recently felled coupes but not in the adjacent, unlogged forests. Vehicles and frequently disturbed areas, such as roadside verges, are likely sources of the seeds of exotic species. The soil seed bank of younger coupes had a greater number and percentage of seeds of exotics than the 10-year-old coupes and the spar-stage and mature forests. Exotic species were a minor component (< 1% vegetation cover) in the more recently logged coupes and were not present in 10-year-old coupes and the spar-stage and mature forests. These particular exotic species did not persist in the dense regeneration nor exist in the older forests because the weeds were ruderal species (light-demanding, short-lived and short-statured plants). The degree of influence that these particular exotic species have on the regeneration and survival of native species in E. regnans forests is almost negligible. However, the current management practices may need to be addressed to prevent a more threatening exotic species from establishing in these coupes and forests.     

Ingestion by an endemic frugivore enhances seed germination of endemic plant species but decreases seedling survival of exotics

Aim To test whether ingestion by endemic frugivores differentially affects the seed germination time, germination percentage and seedling survival of endemic, native and exotic fleshy fruited plant species, and to identify the principal processes and attributes driving such effects. Location Round Island, Mauritius. Methods We conducted a germination and seedling survival experiment for 3 months to test whether ingestion (gut passage and deposition in faeces) by the endemic Telfair’s skink (Leiolopisma telfairii) had a differential effect on the germination time, germination percentage and seedling survival of two endemic, four native and two exotic fleshy fruited plant species. To assess the importance of factors involved in the ingestion process, we used a factorial design with gut passage (gut‐passed vs. not gut‐passed), depulping (whole fruit vs. manually depulped seed) and the presence of faecal material (faeces vs. without faeces). In addition, the roles of species‐specific traits, seed size and deposition density (average number of seeds per faeces) were examined. Results Exotic species had a higher germination percentage than indigenous (native and endemic) species when not ingested. Following skink ingestion, there was no longer a difference, as ingestion enhanced germination percentage most in endemic species. The exotic species still germinated faster overall than the indigenous species, despite ingestion accelerating the germination time of endemics. However, ingestion strongly reduced seedling survival of the exotic species, while having no negative effect on the survival of indigenous seedlings. Overall, ingested indigenous seeds were more likely to germinate and the seedlings more likely to survive than ingested exotic seeds and seedlings. Seed size, deposition density and the removal of fruit pulp by either manual depulping or gut passage were important predictors of germination time, germination percentage and seedling survival. Main conclusions These endemic frugivores can enhance the competitiveness of endemic compared with exotic fleshy fruited plants at the critical germination and seedling establishment stage. Consequently, conservation and restoration of mutualistic endemic plant–animal interactions may be vital to mitigating the degradation of habitats invaded by exotic plants, which is of particular relevance for island ecosystems in which large numbers of endemics are threatened by exotic invaders.     

Promotion of exotic weed establishment by endangered giant kangaroo rats (Dipodomys ingens) in a California grassland

Giant kangaroo rats (Dipodomys ingens) continually modify their burrow precincts by digging tunnels, clipping plants, and other activities. In the valley grasslands of the Carrizo Plain Natural Area (San Luis Obispo County, California), this chronic disturbance to soil and vegetation promoted the establishment of exotic ruderal and early successional plant species. Erodium cicutarium, Bromus madritensis ssp. rubens, and other Mediterranean annuals were found to constitute a very large proportion of the vegetation on giant kangaroo rat precincts. When vegetation on precincts was compared with the vegetation in less disturbed intermediate areas located between precincts, species richness, cover and frequency of exotic plants were significantly greater on precincts. The reverse was found for native species. In addition, exotic species encountered in this study had significantly larger seeds than did native species, suggesting that these granivorous kangaroo rats preferentially cache large weed seeds on their precincts. Since the kangaroo rats depend on exotic plants for food and the exotic plants depend upon the kangaroo rats to disturb their habitat continually, the weed-kangaroo rat relationship is mutualistic. This strong relationship may also inhibit population growth of native grassland plants which occupy disturbed habitats but have difficulty competing with exotic weeds for resources. From a conservation perspective, this mutualism presents an intractable management dilemma. Restoration of valley grasslands where endangered giant kangaroo rats occur, to conditions where native species dominate, may be impossible.     

Mortality of exotic and native seeds in invaded and uninvaded habitats

We examined seed survival in exotic- and native-dominated grasslands by placing seeds of a once-pervasive native grass species, Nassella pulchra, and two of the most common, widespread exotic grass species, Avena fatua and Bromus hordeaceus, in mesh bags in the field for 3 months. Compared to germination of unexposed seeds not placed in the field, exotic species experienced an approximately 40% reduction in viability, whereas the mortality experienced by the native species was <20%. Despite these differences, germination rates of exposed seeds were similar between native and exotic species because native N. pulchra seeds had lower initial viability prior to entering the seed bank. Seed mortality did not differ based on whether seeds were placed in habitats dominated by exotic or native grasses. Rather, our results suggest that re-establishment of native N. pulchra must focus on maximizing seed viability and survival, and that A. fatua and B. hordeaceus overcome relatively higher losses of viable seeds in the seed bank, potentially by producing large numbers of highly viable seeds.     

Influence of heat on seed germination of seven Mediterranean Leguminosae species

The influence of high temperatures (dry heat and hot water) on germination of seven Mediterranean Leguminosae species typical of fire-prone ecosystems in southern Spain is analyzed, in order to know the response of seeds to wildfires and the possible implications in their regeneration after this disturbance. Seeds were heated to a range of temperatures (50 °–150 °C) and exposure times (1–60 min) similar to those registered in the upper soil layers during wildfires. Germination tests were carried out in plastic Petri dishes over 60 days. In general, the degree of seed germination promotion by dry heat treatments showed a wide interspecific variation, although the final germination level was increased in all the studied species except for Scorpiurus muricatus. The thermal pretreatment of 50 °C, however, was not effective for germination in any species, and rising the temperature to 70 °C only slightly enhanced the germination in Cytisus patens. The preheatings of 90 °C (5 and 10 min), 120 °C (5 and 10 min), and 150 °C (1 min) were the most effective in promoting seed germination. Hot water (100 °C) scarification also increased the final germination level in all cases, with the exception of C. patens. The germination rates after preheating were much lower than in mechanically scarified seeds and closely resembled those of the untreated seeds, except for C. reverchonii, whose seed germination rate decreased with heat. The response of species to heat shock had no clear relationship with life trait or with the specific post-fire regeneration strategy (obligate seeder or facultative resprouter). Those species coexisting in the same habitats had different heat optimal requirements for seed germination, an strategy suggested by some authors as minimizing interspecific competition in the secondary succession started after fire.     

A plant‐traits approach to assessing the success of alien weed species in irrigated Mediterranean orchards

In addition to their impact on natural habitats, invasive alien plants can have a significant negative effect on agricultural systems and cause economic losses. Flood‐irrigated orchards in the Mediterranean Basin are vulnerable to the invasion of alien weeds, primarily because of the traditional management practices used in the orchards, which are characterized by high soil moisture during the dry summer period, nutrient availability and high levels of disturbance. This study sought to determine whether their biological traits can explain the success of alien weed species. To answer this question, 408 floristic relevés were conducted in 136 flood‐irrigated orchards on the Plains of Lleida (Catalonia, NE of Spain). Richness and cover of native and alien weeds were compared. Furthermore, a set of biological traits were compared between successful and non‐successful weeds for the whole data and separately between native and alien weeds using logistic regression and classification trees. In flood‐irrigated orchards, alien species covered most of their area, even though the richness of alien species was lower than that of the native species. The most important species were C4 species with seeds dispersed by water, and on the other hand, rosulate and caespitose‐reptant hemicryptophytes with long flowering period. Most of these traits fitted with those of the invasive alien weeds, which were mostly C4 species with seeds dispersed by water. Perennial life form characterized successful native weeds. In this study, we discuss how the traditional management of flood irrigation in fruit‐tree orchards favours invasive alien weeds that have specific traits, acting as a reservoir for the spread of alien weeds into other crops and surrounding riparian habitats. We also propose changing management practices in order to avoid the selection of alien weeds and to promote native species.     

Inhibitory effects of leachate from Eupatorium adenophorum on germination and growth of Amaranthus retroflexus and Chenopodium glaucum

Amaranthus retroflexus L. and Chenopodium glaucum L. are two widely distributed destructive weeds. Their strong adaptability and massive seed production make them the hardest weeds to deal with. This present study intended to investigate the effect of leachate from Eupatorium adenophorum on the growth of these weeds and explore the potential to develop an environmental friendly strategy to use the leachate to control the weeds. Seeds of A. retroflexus L. and C. glaucum L. were soaked in solutions containing 0%, 0.6%, 1.25%, 2.5%, and 5% leachate from E. adenophorum leaves. A. retroflexus and C. glaucum seedlings grown in pots were sprayed with leachate solutions in the same concentration range. The effects of these leachate solutions on membrane permeability and germination of seeds, and growth and physiological characteristics of the seedlings were investigated. The highest concentration of leachate (5%) caused significant damage to the cell membrane of seeds of both weed species, whereas lower concentrations (0.6%) promoted repair of the membrane system, as reflected by higher and lower than control in relative conductivity (RC), respectively. Different concentrations of leachate showed distinct allelopathic inhibitory effects on the two weed species; lower concentrations showed weak inhibitory or even positive effects, whereas higher concentrations showed stronger inhibitory effects. Higher concentrations of leachate (2.5% and 5%) delayed germination and significantly decreased the emergence rate of the seeds, survival rate, and dry matter accumulation of the seedlings. When treated by 5% leachate, the emergence date of A. retroflexus was delayed by 3.6 d, emergence rate of the seeds and survival rate was 69.1% and 70.6% of the control, respectively, seedling dry matter was 48.6% less than the control; In the case of C. glaucum, the emergence date was delayed by 2.7 d, emergence rate of the seeds and survival rate was 45.1% and 58.6% of the control, respectively, seedling dry matter was 44.7% less than the control. There were significant interactions among the different concentrations of leachate and the length of treatment period with respect to activities of antioxidant enzymes, malondialdehyde (MDA) contents, and chlorophyll contents. Seedlings treated with 0.6%, 1.25%, or 2.5% leachate solution for 24–72 h showed increased superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities. When seedlings were treated with leachate solutions for 96 h, antioxidant enzyme activities and chlorophyll content decreased in A. retroflexus, but only CAT activity decreased in C. glaucum. When seedlings of the two weed species were treated with 5% leachate solution, CAT activity and chlorophyll content decreased and MDA content gradually increased with longer treatment times (from 24 to 96 h). The two weed species showed different allelopathic responses to E. adenophorum; A. retroflexus was more sensitive than C. glaucum. Based on the investigation, it could be speculated that the delayed germination and low germination rate of the weeds after treatment by leachate could be due to the fact that leachate damaged the membrane system of the seeds. By delaying germination, lowering the germination rate of the weeds and inhibiting seedling growth, leachate from E. adenophorum could provide an effective way of controlling the weeds.     

The Effect of Chilling and Moisture Status on the Germination, Desiccation Tolerance and Longevity ofAesculus hippocastanumL. Seed

Effects of 2 °C chilling on the threshold moisture contentsand water potentials for various physiological processes wereestimated forAesculus hippocastanumL. seed. Seed harvested atthe time of maximum seed fall exhibited a dual response to drying:partial drying from near 50% to 32–40% moisture contentprogressively increased germination percentage (at 16 °C)up to various peak values; further desiccation was detrimental,confirming that the seeds are ‘recalcitrant’. Themoisture content for optimum germination was increased by atleast 10% as the chilling period was raised from 0 to 9 weeks.A negative linear relationship was found between log₁₀mean timeto germinate and probit final germination, regardless of pre-treatment,indicating that partial desiccation and chilling are interchangeablein promoting germination of hydrated seed. For nearly fullyhydrated seeds, increasing the chilling period from 6 to 26weeks increased the viability-loss onset point for desiccationinjury from near 40% to about 48% moisture content without alteringthe drying rates of seed tissues. Extending moist chilling invarious seed lots from 0 to 26 weeks decreased subsequent longevityat 16 °C. For 26-week-chilled seeds longevity (the periodto lose one probit of germination) differed above and belowa threshold moisture content of 48%. It remained constant inthe moisture-content range 48–38%, but increased progressivelyas moisture content was raised above 48%. This threshold moisturecontent coincided with the value above which chilled seed pre-germinatedin storage. The results indicate that post-harvest desiccationand chilling alter the water relations of various physiologicalprocesses and a schematic summary is presented which relatesthe results to an axis water sorption isotherm.Copyright 1998Annals of Botany Company Aesculus hippocastanumL., horse chestnut, chilling, moisture content, water potential, desiccation tolerance, longevity, recalcitrant seed, embryo axis, maturation, germination.     

Hydrothermal thresholds for seed germination in winter annual forbs from old‐field Mediterranean landscapes

• Under Mediterranean climates with dry‐hot summers and cool‐wet winters, many forbs with potential for habitat restoration are winter annuals, but there is little information about their germination.
• We performed laboratory germination experiments on 13 ruderal dicots native to Andalusia (southern Spain). We measured the germination of recently harvested seeds from natural populations across nine temperature treatments (from 5 to 35 °C, constant and alternate); two storage periods; and eight water stress treatments (from 0 to ?1.0 MPa). We then calculated the hydrothermal thresholds for seed germination.
• Final germination ranged from 0–100% and results were mixed in response to temperature. Base temperature was below 6 °C, optimal temperature was around 14 °C and the ceiling temperature around 23 °C. For five species, 10 months of storage improved total germination, indicating a dormancy‐breaking effect, but the other species did not respond or had their germination reduced. All species were relatively tolerant to water stress, with base water potential ranging from ?0.8 to ?1.8 MPa.
• Our results suggest that hydrothermal germination thresholds, rather than physiological dormancy, are the main drivers of germination phenology in annual forbs from Mediterranean semi‐dry environments. The variation in germination responses of these forb species differs from winter annual grasses, but their seeds are all suitable for being stored before restoration.

     

Seed traits and germination behaviour of four Sardinian populations of Helichrysum microphyllum subsp. tyrrhenicum (Asteraceae) along an altitudinal gradient

• Helichrysum microphyllum subsp. tyrrhenicum (Asteraceae) is an endemic taxon of Sardinia and Corsica, where it grows at different altitudes. The objective of this study was to investigate the seed traits and germination behaviour of four Sardinian populations of this taxon located at different altitudes.
• Seed traits were evaluated, and germination tests were carried out by incubating seeds at a range of constant (5–30 °C) and alternating (25/10 °C) temperatures. The dry after‐ripening (DAR) pre‐treatment was also applied by storing seed in dry conditions for 3 months at 25 °C. Seed traits and germination behaviour data were statistically analysed to identify if there was a correlation with altitude.
• Differences in seed size, area and mass among populations were recorded, however, no relationship was found with altitude. High germination percentages were obtained in all populations, both in untreated and DAR seeds, and were positively affected by alternating temperatures. The final germination percentage and time required to reach 50% final germination (T₅₀) showed no relationship with altitude.
• The differences in seed traits and germination detected among the studied populations of H. microphyllum subsp. tyrrhenicum were not correlated with altitude. This study provides new and important knowledge for this taxon. H. microphyllum subsp. tyrrhenicum is characterised by high germination percentages and low T₅₀ values and does not seem to require any dormancy‐breaking treatment. This species represents a high‐potential native plant species that should be considered within environmental management plans.

     

本地和外来草本物种对水分条件时间异质性的可塑响应

极端气候导致的干旱和水淹事件频发，影响了外来植物和本地植物的生长。为了解外来种和本地种植物对干旱和水淹事件发生顺序的响应，探讨草本植物适应水分时间异质性的策略，该文以美国蒙大拿州西部4种本地植物和4种外来植物为研究对象，将所有植物分别进行持续湿润(对照，CK)、水淹-干旱(I-D)和干旱-水淹(D-I)处理，并观测一系列形态和生物量特征的变化。结果表明：(1)与CK相比，D-I和I-D处理均显著降低了外来种的总生物量(P<0.05)。(2)D-I显著降低了本地种早期总生物量、后期地下生物量和根冠比，但显著提高了其后期的相对生长(P<0.05)。(3)D-I处理显著降低了所有植物的地下-地上生物量关系的异速指数，外来种异速指数显著高于本地种(P<0.05)。综上认为，极端事件(水淹和干旱)的发生顺序能改变外来植物和本地植物的生物量分配，早期干旱比后期干旱更容易减少植物生物量的积累，但能促进本地种后期的生长；本地种在环境胁迫下不被降低的总生物量表现说明维持表型稳定的能力较强；D-I处理下本地种和外来种地上和地下生物量关系的分配方式不同。     

The use of water and some inorganic salt solutions to advance sugar beet seed. I. Laboratory studies

The emergence of sugar beet seedlings is often slow or irregular and insufficient plants may be established for the crop to yield fully. Treating the seed prior to sowing, such that the subsequent germination percentage is not reduced but germination is more rapid and better synchronised should be beneficial. A series of laboratory experiments was made to investigate seed treatment procedures involving water and various inorganic salt solutions. Many treatment combinations were identified which gave faster germination without decreasing germination percentage and left the seed dry and intact and suitable for sowing with conventional precision drills. No treatment was found which ‘primed’ the sugar beet seed and the term seed ‘advancement’ describes, more accurately, the effects observed. There was little difference in the performance of seeds optimally treated with water or some salt solutions generating an osmotic potential of between about –10 and –20 bars. However, the use of salt solutions, although more complicated, was preferred to water as inadvertent germination during treatment was less likely. The optimum treatment for the one bulk of sugar beet seed studied was, firstly, to wash the seeds for 3.5 h with water, then after air-drying, to moisten them on filter paper dampened with –15 bar (0–34 M) sodium chloride solution for 6 days at 15^oC followed by a final wash and air-drying.     

Seed flotation and germination of salt marsh plants: The effects of stratification, salinity, and/or inundation regime

We examined the effects of cold stratification and salinity on seed flotation of eight salt marsh species. Four of the eight species were tested for germination success under different stratification, salinity, and flooding conditions. Species were separated into two groups, four species received wet stratification and four dry stratification and fresh seeds of all species were tested for flotation and germination. Fresh seeds of seven out of eight species had flotation times independent of salinity, six of which had average flotation times of at least 50 d. Seeds of Spartina alterniflora and Spartina patens had the shortest flotation times, averaging 24 and 26 d, respectively. Following wet stratification, the flotation time of S. alterniflora seeds in higher salinity water (15 and 36 ppt) was reduced by over 75% and germination declined by more than 90%. Wet stratification reduced the flotation time of Distichlis spicata seeds in fresh water but increased seed germination from 2 to 16% in a fluctuating inundation regime. Fresh seeds of Iva frutescens and S. alternflora were capable of germination and therefore are non-dormant during dispersal. Fresh seeds of I. frutescens had similar germination to dry stratified seeds ranging 25-30%. Salinity reduced seed germination for all species except for S. alterniflora. A fluctuating inundation regime was important for seed germination of the low marsh species and for germination following cold stratification. The conditions that resulted in seeds sinking faster were similar to the conditions that resulted in higher germination for two of four species.     

Exotic species display greater germination plasticity and higher germination rates than native species across multiple cues

Rapid germination or flexible germination cues may be key traits that facilitate the invasion of exotic plant species in new environments. We investigated whether robustness or plasticity in response to environmental cues were more commonly exhibited by exotic than native species during germination, evidenced by (1) exhibiting consistently greater germination rate under a variety of conditions (robustness), or (2) increasing germination rate more strongly than native species in response to favorable conditions (plasticity). We conducted growth chamber germination trials of 12 native and 12 exotic species common to coastal sage scrub, a shrub-dominated Mediterranean-type ecosystem in California. Time to germination and percentage germination were recorded in response to variation in three environmental cues: temperature, day length, and soil moisture. Exotic species, especially annuals, displayed consistently higher germination percentages and more rapid germination than native species. Exotic germination percentages also responded more strongly when conditions were favorable (warm temperatures and high soil moisture), and germinated earlier than natives when conditions were indicative of typical growing season conditions in Mediterranean ecosystems (short day length and cool temperatures). Exotic species had more rapid and prolific germination across a variety of environmental cues and in response to increased resource availability compared with native species, indicating both germination plasticity and robustness. These traits may enable colonization of novel environments, particularly if they allow exotic species to establish earlier in the growing season than native species, setting the stage for seasonal priority effects.     

Effect of soil moisture and sowing density on population growth ofEupatorium adenophorum andE. riparium

Summary Efect of sowing density on germination, establishment and growth of two perennial weeds,Eupatorium adenophorum spreng. andE. riparium Regel, was studied by sowing varying number of seeds of each species in pots. At high sowing density, seedling emergence did not proportionately increase with seed input. The yield and seed output per unit area was independent of sowing density. Survival of established plants was independent of soil moisture stress. The dry matter yield of both species declined at low moisture regime, the reduction being more inE. riparium. E. adenophorum produced more seeds at low moisture level, whileE. riparium at high moisture level. The resource allocation to roots was reduced due to moisture stress, especially inE. andenophorum.     

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。