Facilitation and interference at the intraspecific level: Recruitment of Kageneckia angustifolia D. Don (Rosaceae) in the montane sclerophyllous woodland of central Chile

Most of the studies that have evaluated the interplay between interference and facilitation have been done at the interspecific level, whereas studies at the intraspecific level are scarce. The montane sclerophyllous forests of central Chile are dominated by the tree Kageneckia angustifolia, a semi-deciduous species that lose part of its foliage during summer. It has been reported that during winter snow accumulates in lower amounts beneath the canopy of K. angustifolia favoring the recruitment of new individuals compared to open areas (i.e., facilitation effect). However, it has also been reported that the leaf litter accumulated beneath parental trees contains allelopathic compounds that decrease seed germination, suggesting that recruitment beneath parental plants can be disfavored (i.e., interference effect). Hence, this system seems appropriate to assess the net-outcome between facilitative and negative effects during the emergence and survival of seedlings during the first year. In this study, we asked (i) what is the net-outcome between facilitative and interfering effects for K. angustifolia? (ii) does this net-outcome varies with the distance to parental trees? (iii) are positive and negative effects consistent through the seedling emergence and first year seedling survival phases? (iv) what are the main mechanisms behind the observed net-outcome? and (v) which is the optimal microhabitat for successful recruitment of this species? In an experimental plot of 10,000 m², we selected ten K. angustifolia trees and evaluated the effect of leaf litter on the emergence and survival of seedlings produced by experimentally sown seed seeds in three different microhabitats: beneath adult trees, edge of canopy and in open areas. In addition, we sampled three K. angustifolia stands to evaluate the microhabitat where the natural recruitment of this species actually occurring. Results showed that (1) seedling emergence was greater beneath canopy, intermediate in canopy edge and low in open areas, (2) whilst leaf litter significantly reduced seed germination, the magnitude of this negative effect was lower than the positive effect of beneath canopy microhabitat, (3) seedling survival was affected by microhabitats but not by the presence of leaf litter, (4) that the main mechanisms behind the observed patterns are the lower and delayed emergence of seedlings in open areas due to the longer duration of snow cover, decreasing the time to growth before the onset of summer drought, and (5) the greatest natural recruitment of K. angustifolia seedlings occurs beneath parental plants. Therefore, our findings suggest that the net-outcome between facilitative and interfering effect during the first year is mostly facilitative, indicating that adult trees of K. angustifolia are exerting a conspecific nurse effect on the recruitment of new individuals, a form of parental care in plants.     

Effects of leaf litter and precipitation on germination and seedling survival of the endangered tree Beilschmiedia miersii

Question: What effects do leaf litter and rainfall regime have on seed germination (time and probability) and seedling survival of the endangered tree species Beilschmiedia miersii (Lauraceae)? Location: The species is a native tree from the mediterranean climate region of Chile. Seeds were collected from La Campana National Park (Chile). The study was carried out under controlled conditions at the Laboratory of Ecology, University of Chile, Santiago, Chile. Methods: During April 2001,200 seeds were assigned to four experimental treatments: high precipitation, with and without litter and low precipitation, with and without litter. Each treatment had 50 individual seeds, each seed in an individual pot. For statistical purposes, we considered each seed as one replicate. High and low values of artificial rainfall corresponded to mean dry and wet years, respectively, for the period 1958–1993 in the central zone of Chile. Results: Seeds germinated earlier, and in higher proportion, in the presence of leaf litter, but only under low rainfall. Seedling survival was insensitive to both litter and precipitation. Conclusions: We conclude that the presence of litter in native populations facilitates seed germination and recruitment of B. miersii, particularly during dry years. We suggest that the reduction of leaf litter due to extraction for gardens and horticultural activities might preclude regeneration of this endangered species.     

The interaction between propagule pressure,habitat suitability and density‐dependent reproduction in species invasion

Seedling recruitment limitations create a demographic bottleneck that largely determines the viability and structure of plant populations and communities, and pose a core restriction on the colonization of novel habitat. We use a shade‐tolerant, invasive grass, Microstegium vimineum, to examine the interplay between seed and establishment limitations – phenomena that together determine recruitment success but usually are investigated individually. We add increasing amounts of seed to microhabitats containing variable levels of leaf litter thickness – with reduced leaf litter simulating disturbance – to investigate whether reduced seed limitation overcomes the establishment limitation posed by litter cover. We do this across gradients in understory light, moisture and temperature, and quantify germination, survival, and then per capita adult biomass and reproduction in order to understand the implications for invasion across the landscape. We find that the combined effects of seed and establishment limitation influence recruitment; however, propagule pressure overwhelms the inhibitory effects of leaf litter thickness. Leaf litter reduces germination by 22–57% and seedling survival by 13–15% from that observed on bare soil. However, density‐dependent reproduction compensates as 1–3 plants can produce far more seeds (approx. 525) than are required for persistence. As such, just a few plants may establish in understory forest habitat and subsequently overwhelm establishment barriers with copious propagule production. These results, for a widespread, invasive plant, are consistent with the emerging perspective for native plants that seed and establishment limitation jointly influence recruitment. The ability for an exotic plant species to compensate for low population densities with high per capita seed production, that then overrides establishment limitations, makes its invasive potential daunting. Further work is required to test if this is a common mechanism underlying plant invasions.     

Differential effects of several “litter” types on the germination of dry grassland species

Question: Accumulation of litter can have serious implications on the recruitment of plant species, by modifying the physical, biological and chemical features of the microenvironment or acting as a mechanical barrier for seeds and seedlings. Isolating these different effects has rarely been achieved experimentally. Location: Transylvanian Lowland, Romania. Methods: We tested the effects of different “litter” types on the germination of dry grassland species using a controlled pot experiment with three natural litter types, differing in decay state and composition (Stipa pulcherrima fresh leaves, partly decomposed leaves and mixed and partly decomposed plant material) and an artificial plastic litter, with two levels of water addition. As a complementary field study, seed sowing was conducted in grassland plots with litter removal and plastic litter application. Results: Litter effects were mainly positive (intermittent watering) or neutral (frequent watering) under controlled experimental conditions, and mostly negative in the field. Seed size and environmental conditions were the major determinants of litter effects on germination. Significant differences were found in the effect of litter type on germination, much of which could be explained by chemical factors determined by the decay state, as we confirmed a higher concentration of allelopathic compounds in fresh S. pulcherrima litter than in the senescing leaf litter. Conclusions: The effects of litter on seed germination are strongly context dependent, and it is hard to define common rules that apply consistently under various environmental conditions. “Litter” identity and quality matter, i.e. the litter composition and decay state, and influence seed germination.     

Leaf litter and irrigation can increase seed germination and early seedling survival of the recalcitrant‐seeded tree Beilschmiedia miersii

Although regeneration of recalcitrant‐seeded tree species can be affected by prolonged drought, especially in Mediterranean regions, little is known about the response of such species to varying site conditions. A field experiment was performed to determine the effect of irrigation and leaf litter cover on seed germination and early seedling survival of the vulnerable recalcitrant‐seeded tree Beilschmiedia miersii (Lauraceae). Two levels of irrigation (non‐irrigated and irrigated units) and three levels of leaf litter depth (0, 5 and 12 cm) were applied to 72 groups of 30 seeds along a wet ravine of the Coastal Range of Central Chile, equally distributed across sites with different levels of canopy cover. Seed germination was significantly increased by irrigation only under closed‐canopy cover, and by leaf litter cover (>5 cm) under all canopy conditions. The effect of leaf litter on germination increased with canopy openness, while the effect of irrigation did not show any tendency. Meanwhile, early seedling survival was significantly increased by irrigation under intermediate canopy cover, and by leaf litter (>5 cm) under closed‐canopy cover. As a result of its overall positive effect on germination, leaf litter should be maintained within B. miersii communities, particularly under intermediate to closed‐canopy conditions, where it can also increase early seedling survival, and both seed germination and early seedling survival might be increased through additional water inputs. The presence of leaf litter might help retaining such inputs, prolonging their effect on regeneration of B. miersii communities. We see this as a baseline assessment of regeneration and persistence that needs further testing on species with similar traits, given the expected increase in the frequency and length of dry periods into the future.     

The effectiveness of active and passive restoration on recovery of indigenous vegetation in riparian zones in the Western Cape,South Africa: A preliminary assessment

Riparian ecosystems in South Africa's fynbos biome are heavily invaded by alien woody plants. Although large-scale clearing of these species is underway, the assumption that native vegetation will self-repair after clearing has not been thoroughly tested. Understanding the processes that mediate the recruitment of native species following clearing of invasive species is crucial for optimising restoration techniques.This study aimed to determine native species recovery patterns following implementation of different management interventions. We tested the influence of two clearing treatments (“fell & remove” and “fell & stack burn”) on the outcomes of passive restoration (natural recovery of native riparian species) and active restoration (seed sowing and planting of cuttings) along the Berg River in the Western Cape. Under greenhouse conditions we investigated seed viability and germination pre-treatments of selected native species.There was no recruitment of native species in sites that were not seeded (passive restoration sites), possibly because of the dominance of alien herbaceous species and graminoids or the lack of native species in the soil-stored seed bank. Germination of our targeted native species in the field was low in both “fell & remove” and “fell & stack burn” treatments. However, “fell & stack burn” gave better germination for the species Searsia angustifolia, Leonotis leonurus and Melianthus major. Seedling survival in the field was significantly reduced in summer, with drought stress being the main cause for seedling mortality. Germination rates in the greenhouse were high, an indication that harvested seeds were viable. Most seeds germinated without germination pre-treatments.We conclude that failure of native seeds to germinate under field conditions, secondary invasion of alien herbs and graminoids, the lack of native species in the soil-stored seed bank, and dry summer conditions hamper seedling establishment and recovery on sites cleared of dense stands of alien trees. For active restoration to achieve its goals, effective recruitment and propagation strategies need to be established.     

Edge Effects and Seedling Bank Depletion: The Role Played by the Early Successional Palm Attalea oleifera (Arecaceae) in the Atlantic Forest

In this study, we examined the impacts of Attalea oleifera on the structure of seedling bank and discuss potential mechanisms of palm influence. Seed rain, seedling bank, and palm leaf fall were assessed beneath the canopy and in the vicinity of 16 adult palms across the edges of a large fragment (3500 ha) of the Atlantic forest. Moreover, we examined A. oleifera impacts on seed germination and seedling mortality by experimentally submitting seeds and seedlings to prolonged palm-leaf covering. As expected, seedling bank beneath the adults exhibited reduced abundance and species richness at local and habitat scale. Small to large seeds (3.1–30 mm) were underrepresented in the seed rain below adults palms, while experimental leaf covering drastically reduced both seed germination and seedling survivorship. A. oleifera leaf fall occurred over the whole year (3.6±2.7 leaves/individual/yr), which resulted in deep leaf litter mounds (10.7±9.2 cm). Finally, adult palm density (21.6±11.9 individuals/ha) correlated negatively with seedling density across Attalea clusters. Our results suggest that A. oleifera exerts negative effects on the seedling bank by reducing seedling abundance and richness as a consequence of two complementary mechanisms: impoverished and size-biased seed rain plus reduced seed germination and increased seedling mortality due to prolonged covering by fallen leaves.     

The coupling of recruitment and disturbance by fire in two resprouting Proteaceae species

Recruitment in plant populations is often tightly coupled to major disturbances such as fires. For species with persistent seed banks, fire-related cues may allow or enhance germination. The litter layer influences germination and may modify the impact of seed predators on seeds and seedlings. The litter layer is obviously affected by fire, providing one mechanism by which disturbance can determine recruitment. We tested the role of litter in the disturbance–recruitment coupling of two species with contrasting seed release timing after fire—Banksia serrata (canopy seed bank) and Telopea speciosissima (transient seed bank) by planting their seeds both early and late in the post-fire recruitment period (PRP) and manipulating litter density in orthogonal treatments. Vertebrate seed predators were excluded. Both species established more seedlings late in the PRP, although results were strongly influenced by very poor establishment at one site. Invertebrate seed predators consumed more T. speciosissima seeds in sites early (69.5%) than late in the PRP (51.2%), while consumption of B. serrata seeds was lower overall and comparable across sites (average 47.3%). Surprisingly, litter had very little effect on establishment and none on invertebrate seed predation, suggesting that other factors are more important. Recruitment was only loosely coupled to disturbance for the canopy seed bank species; for the transient seed bank species, the coupling was tighter but separated in time from the disturbance. Understanding both the strength and temporal aspects of the disturbance–recruitment coupling is necessary for appropriate management of plant functional diversity in fire-prone habitats.     

Seed predation limits post-fire recruitment in the waratah (<Emphasis Type="Italic">Telopea speciosissima</Emphasis>)

Seed predation may reduce recruitment in populations that are limited by the availability of seeds rather than microsites. Fires increase the availability of both seeds and microsites, but in plants that lack a soil- or canopy-stored seed bank, post-fire recruitment is often delayed compared to the majority of species. Pyrogenic flowering species, such as Telopea speciosissima, release their non-dormant seeds more than 1 year after fire, by which time seed predation and the availability of microsites may differ from that experienced by plants recruiting soon after fire. I assessed the role of post-dispersal seed predation in limiting seedling establishment after fire in T. speciosissima, in southeastern Australia. Using a seed-planting experiment, I manipulated vertebrate access to seeds and the combined cover of litter and vegetation within experimental microsites in the 2 years of natural seed fall after a fire. Losses to vertebrate and invertebrate seed predators were rapid and substantial, with 50% of seeds consumed after 2 months in exposed locations and after 5 months when vertebrates were excluded. After 7 months, only 6% of seeds or seedlings survived, even where vertebrates were excluded. Removing litter and vegetation increased the likelihood of seed predation by vertebrates, but had little influence on losses due to invertebrates. Microsites with high-density vegetation and litter cover were more likely to have seed survival or germination than microsites with low-density cover. Recruitment in pyrogenic flowering species may depend upon the release of seeds into locations where dense cover may allow them to escape from vertebrate predators. Even here, conditions suitable for germination must occur soon after seed release for seeds to escape from invertebrate predators. Seed production will also affect recruitment after any one fire, while the ability of some juvenile and most adult plants to resprout after fire buffers populations against rapid declines when there is little successful recruitment.     

Fire and flood: Soil‐stored seed bank and germination ecology in the endangered Carrington Falls Grevillea (Grevillea rivularis,Proteaceae)

Abstract Seed germination is dependent on the interaction between the dormancy state of a seed and the presence of favourable environmental conditions. Thus, the spectacular pulse of seedling recruitment in many Australian vegetation communities following disturbances such as fire can be attributed to changes in microsite conditions and/or the dormancy‐breaking effect of the disturbance on accumulated seed banks. Grevillea rivularis is a threatened species endemic to the area immediately above Carrington Falls in the NSW Southern Highlands. Most of the population is confined to the riparian vegetation zone in woodland and heath, and is therefore subject to periodic disturbance from fire and flood. For this species, a pulse of seedling recruitment has been recorded after fire, flood and mechanical soil disturbance. The aims of this study were to examine the density and vertical distribution of the soil‐stored seed bank and to investigate the role of heat and scarification as cues for germination of fresh and soil‐stored seed. There was a large seed bank under the canopies of established individuals (194 ± 73 seeds m^?2) and most seeds were found in the 0–2 cm and leaf‐litter layers of the soil profile. The germination response of soil‐stored and fresh seed was examined using a hierarchical series of laboratory experiments. Seeds of G. rivularis showed marked dormancy polymorphism. Thirty‐six percent of soil‐stored seed germinated without treatment, whereas no untreated fresh seeds germinated. Scarification or heating caused significant germination of dormant soil‐stored seed, but only scarification resulted in germination of dormant fresh seeds. These results highlight important differences in the dormancy state of soil‐stored and fresh seed. Thus, being a riparian species in a fire‐prone environment, the dormancy mechanisms in seeds of G. rivularis suit this species to disturbance by both fire and flood.     

Seed-to-seed potential allelopathic effects between <Emphasis Type="Italic">Ligularia virgaurea</Emphasis> and native grass species of Tibetan alpine grasslands

Allelopathy is an important process in plant communities, but the role of seed allelopathy in natural ecosystems remains poorly understood. In the present study, we examined the potential allelopathic effects of Ligularia virgaurea (a dominant species in degraded Tibetan grasslands) seeds on the germination of four native grass species (Festuca sinensis, Agrostis gigantean, Bromus inermis, and Elymus nutans). The results showed that L. virgaurea seeds can have potential allelopathic effects on seed germination, mean time to germination and root growth rates of native grass species. We further demonstrate that these effects are driven by a water-soluble seed leachate. Species with smaller seeds were generally more sensitive than larger seeded species. The results suggest that seed-to-seed allelopathic potential may be an important mechanism driving the dominance of L. virgaurea in degraded alpine grasslands on the Tibetan Plateau. Further studies are required to demonstrate effects of seed-to-seed potential allelopathy in a field setting.     

Seedling recruitment of Succisella inflexa in fen meadows: Importance of seed and microsite availability

Abstract. The effects of different forms of land use on germination and establishment of the rare fen species Succisella inflexa were investigated in seed introduction experiments in a mown and an abandoned fen meadow in SE Germany. Treatments included abandonment, mowing in fall and mowing with creation of gaps in the moss and litter layer. Floating capacity of seeds was tested in order to estimate potential dispersal by water. On the mown meadow, gaps had a slightly positive effect on germination rates, while greatly increasing seedling survival until the next spring. At the abandoned site, litter inhibited germination, whereas mosses had a negative effect on germination and a positive effect on survival rates during the first year after germination. Both germination and seedling establishment were negatively affected by the presence of slug herbivores. On the abandoned site, no seedlings at all survived until the next spring. Even though seeds of Succisella inflexa were capable to float for several weeks and to germinate thereafter, the situation at the field sites indicates that longdistance dispersal is highly unlikely. Our results showed that not only direct effects of abandonment, such as accumulation of litter, may have led to poor germination and poor seedling establishment of the species. Additionally, indirect consequences of changes in land use, such as higher seedling herbivory by slugs and successional vegetation changes due to abandonment, were important in determining habitat quality and availability of microsites for seedling recruitment. Furthermore, early mowing imposed seed limitation on plant populations.     

Seed germination in Chrysophyllum sp. nov., a large-seeded rainforest species in north Queensland: effects of seed size,litter depth and seed position

Leaf litter affects seed germination in many ways and past studies have shown greater impacts on relatively small seeds, both within and among species. In this shade-house experiment I examined the impact of forest litter on seed germination in Chrysophyllum sp. nov. (Sapotaceae), a large-seeded (2.4 g) rainforest tree from north Queensland. Seed mass varies more than 30-fold in this species, making it useful for studying the role of litter as a possible selective pressure in the evolution of seed size in large-seeded species. Seeds of varying size (small, medium, large) were sown in planting boxes containing one of three litter levels (low, medium, high) and placed either below or on top of the litter. Seed size and litter biomass had no significant impact on the number of germinating seeds or the time to germination but seeds placed below the litter germinated around twice as frequently, and 20% sooner, than seeds placed on top of the litter. There were no significant interactions between any of the three factors. This shade-house experiment suggests that leaf litter is not an important selective pressure in the evolution of seed size in this species. However, if litter disturbance under field conditions differentially affects the probability of seed germination in relation to seed size and/or litter biomass, then litter could still act as a selective pressure in the evolution of seed size in Chrysophyllum and other large-seeded species.     

Effect of post-irradiation ageing on onion seeds

Seeds of onion (Allium cepa) cv. Ailsa Craig were exposed to various doses of gamma radiation (0, 10, 20, 40, 80 and 100 krad) and subjected to accelerated ageing (RH 100%, 42°C) for 12 h. Radiation doses significantly affected the percentage of normal seedlings, abnormality types (%) and seedling growth. Seed viability, conductivities of seed leachates, final germination (%), germination speed and time to reach 50% germination (T ₅₀) were not affected by the seed irradiation. Accelerated ageing after irradiation had significant influence on seed viability, conductivities of seed leachates, final germination (%) and percentage of normal seedlings. Germination speed, T ₅₀ and seedling growth were not affected by the accelerated ageing. However, all the parameters studied were significantly influenced by the interaction of radiation doses and accelerated ageing. Accelerated ageing generally enhanced the damaging effects of irradiation on seeds. Therefore, it was concluded that onion seeds should not be exposed to adverse environmental conditions after irradiation.     

Inhibitory effects of Eucalyptus globulus on understorey plant growth and species richness are greater in non‐native regions

Aim

We studied the novel weapons hypothesis in the context of the broadly distributed tree species Eucalyptus globulus. We evaluated the hypothesis that this Australian species would produce stronger inhibitory effects on species from its non‐native range than on species from its native range.

Location

We worked in four countries where this species is exotic (U.S.A., Chile, India, Portugal) and one country where it is native (Australia).

Time period

2009–2012.

Major taxa studied

Plants.

Methods

We compared species composition, richness and height of plant communities in 20 paired plots underneath E. globulus individuals and open areas in two sites within its native range and each non‐native region. We also compared effects of litter leachates of E. globulus on root growth of seedlings in species from Australia, Chile, the U.S.A. and India.

Results

In all sites and countries, the plant community under E. globulus canopies had lower species richness than did the plant community in open areas. However, the reduction was much greater in the non‐native ranges: species richness declined by an average of 51% in the eight non‐native sites versus 8% in the two native Australian sites. The root growth of 15 out of 21 species from the non‐native range were highly suppressed by E. globulus litter leachates, whereas the effect of litter leachate varied from facilitation to suppression for six species native to Australia. The mean reduction in root growth for Australian plants was significantly lower than for plants from the U.S.A., Chile and India.

Main conclusions

Our results show biogeographical differences in the impact of an exotic species on understorey plant communities. Consistent with the novel weapons hypothesis, our findings suggest that different adaptations of species from the native and non‐native ranges to biochemical compounds produced by an exotic species may play a role in these biogeographical differences.     

Inhibition of seedling survival under Rhododendron maximum (Ericaceae): could allelopathy be a cause?

In the southern Appalachian mountains a subcanopy species, Rhododendron maximum, inhibits the establishment and survival of canopy tree seedlings. One of the mechanisms by which seedlings could be inhibited is an allelopathic effect of decomposing litter or leachate from the canopy of R. maximum (R.m.) on seed germination, root elongation, or mycorrhizal colonization. The potential for allelopathy by R.m. was tested with two bioassay species (lettuce and cress), with seeds from four native tree species, and with three ectomycorrhizal fungi. Inhibitory influences of throughfall, fresh litter, and decomposed litter (organic layer) from forest with R.m. (+R.m. sites) were compared to similar extractions made from forest without R.m. (-R.m. sites). Throughfall and leachates of the organic layer from both +R.m. and -R.m. sites stimulated germination of the bioassay species above that of the distilled water control, to a similar extent. There was an inhibitory effect of leachates of litter from +R.m. sites on seed germination and root elongation rate of both bioassay species compared with that of litter from -R.m. sites. Native tree seed stratified in forest floor material from both forest types had a slightly higher seed germination rate compared with the control. A 2-yr study of seed germination and seedling mortality of two tree species, Quercus rubra and Prunus serotina, in field plots showed no significant influence of litter or organic layer from either forest type. Incorporating R.m. leaf material into the growth medium in vitro depressed growth of one ectomycorrhizal species but did not affect two other species. Leaf material from other deciduous tree species depressed ectomycorrhizal growth to a similar or greater extent as leaf material from R.m. In conclusion, R.m. litter can have an allelopathic effect on seed germination and root elongation of bioassay species as well as some ectomycorrhizal species. However, this allelopathic affect is not manifest in field sites and is not likely to be an important cause for the inhibition of seedling survival within thickets of R.m.     

Effects of litter on seedling establishment: an indoor experiment with short-lived Brassicaceae species

Recruitment by seeds is essential both in vegetation dynamics and in supporting biodiversity in grasslands. The recruitment by seeds is feasible in suitable vegetation gaps from the seed rain and/or by establishment from persistent soil seed banks. Cessation of grassland management results in litter accumulation, which leads to the decline of species diversity by the decreased availability of open patches. Low amounts of litter is often beneficial, while high amounts of litter is detrimental for seed germination and seedling establishment of short-lived species. In a designed indoor experiment, we explored the effect of litter on seedling establishment by germinating six short-lived Brassicaceae species with both increasing seed mass and litter cover. We found that both seed mass and litter had significant effect on germination and establishment of the sown species. Small-seeded species were significantly negatively affected by the 300 and/or 600 g/m² litter layers. No negative litter effect was detected for species with high seed masses (Lepidium spp.). No overall significant positive litter effect was found, although for most of the species cumulative seedling numbers were not the highest at the bare soil pots. Our results suggest that the negative effects of litter are less feasible on the large-seeded short-lived species than on that of small-seeded ones.     

Seed–litter–position drives seedling establishment in grassland species under recurrent drought

Changes in land use and climate interfere with grassland ecosystem processes. Here I experimentally investigated the combined effects of land‐use change related litter cover and contrasting water supply on seedling emergence. In this context, the role of the initial relative position of seeds, i.e. seeds on top of the litter versus seeds beneath the litter in interaction with water supply has not been investigated so far. I hypothesised that facilitative effects of litter on seedling emergence occur when seeds are covered by litter and deteriorate when litter covers the ground and seeds fall on it (seeds on top of the litter). Further, I hypothesised that the importance of seed position for seedling emergence will increase under conditions of recurrent drought. I performed a controlled pot experiment on seedling emergence of three common European grassland species (Pimpinella saxifraga, Leontodon autumnalis, Sanguisorba officinalis) by experimental manipulations of litter and water availability. Seedling emergence under moist conditions showed no significant differences between each litter position compared to the control across species. In contrast, under recurrent drought, seedling emergence was significantly higher below the litter compared to seeds on top of the litter and the control (i.e. no litter). In abandoned land, seedling emergence may be limited when seeds fall on ground‐covering litter. In contrast, in grasslands with regular low‐intensity land use, seedling emergence may be enhanced when a moderate level of litter covers seeds at the end of the growing season. Protective mechanisms that occur with seeds positioned beneath litter are particularly important under recurrent drought.     

Integration of invasive <Emphasis Type="Italic">Opuntia</Emphasis> spp. by native and alien seed dispersers in the Mediterranean area and the Canary Islands

The success of many alien plant species depends on mutualistic relationships with other species. We describe the assemblage of seed dispersers on three species of alien Opuntia invading Mediterranean and Macaronesian habitats, and examine the quality of such plant-animal interactions. We identified vertebrates consuming O. maxima, O. dillenii and O. stricta fruits by direct observation and collecting droppings and pellets. Phenology of the alien species, as well as that of coexisting native species, was monitored for an entire year. Germination tests of ingested and non-ingested seeds were performed both in the greenhouse and in the field. Seed coat thickness and viability were also measured for all treatments. A great variety of taxa, including reptiles, birds and mammals actively participate in the seed dispersal of Opuntia. Phenology of Opuntia fruits in Menorca and Tenerife overlaps with only a few native fleshy-fruited plants present in the study areas, which suggests an advantage for the invader. Most seeds germinated during the second year of the experiment, independently of the effect produced by the dispersers’ guts. We found great variation in the germination percentage of Opuntia after gut passage and in the effects of ingestion on seed coat thickness. Seed viability was somewhat reduced after gut passage compared to manually depulped seeds. Our results show how different Opuntia species are integrated into native communities by means of mutualistic interactions, with both native and alien dispersers. Although with heterogeneous effects, either type of disperser potentially contributes to the spread of these alien cacti in the recipient areas.     

Germination and early growth of <Emphasis Type="Italic">Ailanthus</Emphasis> and tulip poplar in three levels of forest disturbance

Increasing rates of forest disturbance may provide greater opportunity for invasion of nonnative species, thereby altering the successional trajectory of native plant communities. In the eastern U.S., invasive Ailanthus altissima and native Liriodendron tulipifera have similar life histories and niches and often co-occur. To examine how disturbance affects the establishment of these species, we performed field experiments to evaluate the response of sown seeds and transplanted seedlings to three levels of disturbance on north- and south-facing aspects. L.␣tulipifera germination was severely limited by low seed viability and had significantly lower germination than A. altissima in all sites. The effect of disturbance regime on A. altissima germination depended on aspect in the second growing season. In contrast, mean seedling survival, biomass, leaf area and leaf area ratio were greater for L. tulipifera in all field sites. Overall, the north-facing selective cut forest provided a disproportionately large number of suitable microsites for L. tulipifera establishment. Collectively, this study demonstrated that different timber harvest practices produce heterogeneous mosaics of suitable microsites for germination and establishment. Limited L. tulipifera germination may be a serious constraint to population establishment if seeds are deposited for the first time immediately after a disturbance event. However, if sufficient viable seeds of both species exist, L. tulipifera out-performs the invasive in the first two years following disturbance. This may explain why A. altissima has shown explosive population growth in a limited number of sites in the past century.