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.     

Effect of competition intensity on recruitment of palatable and unpalatable grasses

In this study, we made an attempt to reveal how competition intensity from established plants impacts on palatable and unpalatable grass seedlings recruitment, in a natural mesic grassland of central Argentina. Our objective was to assess the seedling recruitment of a palatable species (Chascolytrum subaristatum) and an unpalatable species (Nassella trichotoma) in microsites differing in competition intensity from established plants. Identity (C. subaristatum and N. trichotoma) and defoliation severity were used as surrogate for competition intensity. In March 2017, we permanently marked established individuals of N. trichotoma and C. subaristatum and placed two circular plots adjacent to each individual. In one plot we added seeds of N. trichotoma and in the other seeds of C. subaristatum. After seeding, established plants were randomly assigned to one of three level of defoliation: without defoliation, low defoliation severity and high defoliation severity. From April to November 2017 (i.e. over a complete annual growing cycle), we measured seedling density, recruitment and growth. Our results supported the hypothesis that seedlings of palatable grasses are more competitive than seedlings of unpalatable grasses. Seedling of the palatable grass C. subaristatum recruited successfully regardless the intensity of competition from established plants, whereas seedlings of the unpalatable grass N. trichotoma recruited better under low competitive pressure from established plants. Our results suggest that the availability of microsites with low competitive pressure from the established vegetation, created by selective grazing of palatable grasses, promotes the recruitment of unpalatable grass seedlings. This mechanism may contribute to the species replacement process commonly observed in heavy grazed grasslands.     

Experimental demography of the rare Gentianella germanica: seed bank formation and microsite effects on seedling establishment

The persistence of populations of short-lived species requires regular reproduction and seedling establishment. A persistent seed bank can buffer populations against extinction in unfavourable years. We experimentally investigated seed fate in Gentianella germanica, an endangered biennial species characteristic for species-rich nutrient-poor calcareous grasslands in central Europe. We studied the effect of experimental gaps on seedling establishment from sown seeds and the fate of seeds buried in bags over two years. In December 1993 experiments were established at seven calcareous grassland sites in the Swiss Jura mountains. In spring 1994 seedlings emerged in all plots where seeds had been sown, including previously unoccupied patches. This suggests that limited dispersal within sites contributes to small population sizes. Significantly more seedlings emerged at sites with current populations of G. germanica than at unoccupied sites (5.95% vs 3.40%). Because this difference was not explained by germinations from the natural seed bank it indicates differences in habitat quality. Clipping of the vegetation and disturbance of the soil reduced vegetation cover in the following spring and enhanced seedling emergence. In undisturbed plots 4.5% of seeds sown produced a seedling in spring 1994, whereas in plots with clipped vegetation 9.9% and in disturbed plots 12.7% produced seedlings (p>0.01). This suggests that management measures which create gaps in the vegetation (e.g. grazing) could positively influence population size and persistence of G. geymanica. On average, we recovered 7.55% viable seeds after one year of burial in bags, and 4.05% after two years, indicating that G. geymanica has a persistent seed bank. The demographic data indicate that the number of viable seeds in the seed bank exceeds the number of established plants in a population at least by a factor of 20. Restoration of extinct populations of the species from the seed bank may thus be possible if appropriate management measures are taken within a few years.     

Requirement of vegetation gaps for seedling establishment of two unpalatable grasses in a native grassland of central Argentina

Abstract The objective was to determine the effects of root and shoot competition on seedling establishment of the unpalatable grasses Stipa gynerioides and S. tenuissima in a native grassland of central Argentina dominated by the palatable grass S. clarazii. Seeds of the two unpalatable species were sown in natural occurring microsites with shoot and root competition from the palatable species, and in artificially created microsites without either shoot competition or shoot and root competition. In addition, fresh seeds of the unpalatable species were subjected to daily alternating temperatures under laboratory and field conditions to determine the effect on seed dormancy and germination. Seedling establishment of S. gynerioides and S. tenuissima occurred only in microsites without shoot and root competition. Also, the fluctuation of temperature near the soil surface in these microsites reduced dormancy and promoted rapid germination in both species. Our results support the hypothesis that, in swards dominated by palatable grasses, vegetation gaps of low competitive pressure favour seedling establishment of unpalatable grasses. It is suggested that the creation of these gaps by overgrazing can be an important mechanism in the process of species replacement in native grasslands.     

Establishment and spread of founding populations of an invasive thistle: the role of competition and seed limitation

Successful plant invasions require both the founding and local spread of new populations. High plant densities occur only when founding plants are able to disperse their seeds well locally to quickly colonize and fill the new patch. We test this ability in a 7-year field experiment with Carduus acanthoides, an invasive weed in several North American ecosystems. Founder plants were planted in the center of 64 m² plots and we monitored the recruitment, distribution pattern, mortality, and seed production of the seedlings that originated from these founding plants. Competing vegetation was clipped not at all, once, or twice each year to evaluate the importance of interspecific competition. More seedlings recruited in the intermediate once-clipped plots, and these seedlings also survived better. The control plots had fewer microsites for seedling recruitment; clipping a second time in September stimulated grasses to fill up the gaps. The number of C. acanthoides recruits and their median distances from the founder plants were also explained by the initial seed production of the founding plants. Overall, the experiment shows that the success of founder plants can fluctuate strongly, as 55% of the plots were empty by the sixth year. Our study suggests that the local invasion speed following initial establishment depends strongly on both the propagule pressure and availability of suitable microsites for seedling recruitment and growth.     

Competitive effects of herbaceous vegetation on tree seedling emergence,growth and survival: Does gap size matter?

Question: What is the effect of gap size on the seedling emergence, growth and survival of four common tree species in wooded pastures? Location: A pasture in the Jura mountains, Switzerland. Methods: Seeds were sown in a complete three‐way factorial design with eight blocks in May 2003. Each block consisted of a competition treatment (four gap sizes including zero) and a mowing treatment (mown and unmown). Emergence, survival and total biomass of tree seedlings of three species (Picea abies, Acer pseudoplatanus and Fagus sylvatica) were measured. A fourth species (Abies alba) failed to germinate. Results: Gaps had a positive influence on the early stages of tree development for all species. Larger gaps favoured growth and survival more than small gaps. Seedling growth was higher when vegetation around the openings was mown. Mowing the vegetation at gap size zero enhanced both growth and survival compared to unmown vegetation. Mown gaps larger than zero had increased seedling desiccation but decreased seedling predation. Species showed similar trends in their emergence and growth responses to gap size and mowing treatments but for Picea emergence rate was higher and survivorship was lower than for Acer and Fagus. Conclusions: Gap size does matter for tree seedling success but even in more favourable large gaps only a small percentage of seedlings emerged and survived. The effects of gaps on tree seedling establishment are complex as a result of interactions between biotic and abiotic changes caused by gaps.     

Microsites for seedling establishment of subalpine conifers in a forest with moss-type undergrowth on Mt. Fuji, central Honshu, Japan

We examined microsites for the seedling establishment of Tsuga diversifolia, Abies veitchii and Abies mariesii in a subalpine coniferous forest with moss-type undergrowth dominated by Hylocomium splendens and Pleurozium schreberi on Mt. Fuji, a less-snowy mountain located on the Pacific Ocean side of the central part of Honshu Island, Japan, and compared these microsites with cases in other regions of Japan with different undergrowth vegetation (herb type, dwarf-bamboo type) under snowier conditions. L-shaped size structures with abundant seedlings and small trees were identified for all three species. Established seedlings of T. diversifolia showed remarkable preference for elevated microsites such as fallen logs and convex ground (i.e. ground raised around trunk bases), while A. veitchii and A. mariesii indicated no significant bias toward such elevated microsites in the large seedling stage. However, the relative frequency of T. diversifolia seedlings established on flat ground reached high values of 54–77%, since the relative occupation area of the flat ground was large (85–89%). Such a large share suggests that flat ground should be regarded as the most important microsite for the seedling establishment of T. diversifolia in forests with moss-type undergrowth. This preference trait for T. diversifolia observed with moss-type undergrowth has not been reported in previous studies examining forests with herb- and dwarf bamboo-type undergrowth, where T. diversifolia seedlings are scarce on the ground and restricted to elevated microsites. We conclude that the major microsite for T. diversifolia seedling establishment changes according to the undergrowth vegetation type: a large ground share for moss-type undergrowth and a small ground share for herb- and dwarf bamboo-type undergrowth.     

Post‐fire shrub recruitment in a semi‐arid grassland: the role of microsites

Question: (1) Which factors regulate post‐fire recruitment and spread of the shrub Senecio bracteolatus in Patagonian grasslands? (2) What is the role of the grass Stipa speciosa on S. bracteolatus establishment in the post‐fire succession? Location: Northwest Patagonia, Argentina. Methods: We studied the effect of fire on S. bracteolatus recruitment and density by comparing these variables between burned and unburned grasslands. In burned areas, we compared abiotic characteristics and seedling establishment under the canopy of grasses (S. speciosa) and in gaps (inter‐tussock areas). Post‐fire interactions between S. bracteolatus seedlings and S. speciosa were studied using field and greenhouse experiments. Results: Density of S. bracteolatus was higher in burned than in unburned areas. In burned sites, seedlings were more abundant under tussock grasses, whereas juveniles were more abundant in gaps. Tussocks generated more attenuated micro‐environmental conditions than gaps during stressful summers. Gaps were more abundant in burned sites, while “under tussock” microsites were more frequent in unburned sites. In burned areas, tussocks allowed higher establishment of seedlings (facilitation), but gaps allowed more seedling growth and higher persistence of juveniles. Conclusions: Fire promoted S. bracteolatus recruitment in Patagonian grasslands by increasing the availability of favourable gap microsites. Grass protection for shrub seedlings became negative with time, probably due to competition with grasses. Gaps led to better performance and persistence of shrub plants. Six years after fire, higher shrub recruitment and adult density (observed as a trend) in burned grassland provides an opportunity for potential S. bracteolatus invasion.     

Limits to tree species invasion in pampean grassland and forest plant communities

Factors limiting tree invasion in the Inland Pampas of Argentina were studied by monitoring the establishment of four alien tree species in remnant grassland and cultivated forest stands. We tested whether disturbances facilitated tree seedling recruitment and survival once seeds of invaders were made available by hand sowing. Seed addition to grassland failed to produce seedlings of two study species, Ligustrum lucidum and Ulmus pumila, but did result in abundant recruitment of Gleditsia triacanthos and Prosopis caldenia. While emergence was sparse in intact grassland, seedling densities were significantly increased by canopy and soil disturbances. Longer-term surveys showed that only Gleditsia became successfully established in disturbed grassland. These results support the hypothesis that interference from herbaceous vegetation may play a significant role in slowing down tree invasion, whereas disturbances create microsites that can be exploited by invasive woody plants. Seed sowing in a Ligustrum forest promoted the emergence of all four study species in understorey and treefall gap conditions. Litter removal had species-specific effects on emergence and early seedling growth, but had little impact on survivorship. Seedlings emerging under the closed forest canopy died within a few months. In the treefall gap, recruits of Gleditsia and Prosopis survived the first year, but did not survive in the longer term after natural gap closure. The forest community thus appeared less susceptible to colonization by alien trees than the grassland. We conclude that tree invasion in this system is strongly limited by the availability of recruitment microsites and biotic interactions, as well as by dispersal from existing propagule sources.     

A vegetation switch as the cause of a forest/mire ecotone in New Zealand

Abstract. Vegetation switches are those processes in which there is positive-feedback between vegetation and environment, i.e. a vegetation state modifies its environment producing conditions more favourable to itself (Wilson & Agnew 1992). Switches can produce and maintain abrupt ecotones between plant communities. Such a sharp ecotone exists between beech-podocarp forest and mire vegetation, both on deep peat, in southwest New Zealand. One such site was examined. There was no apparent explanation for the ecotone in the present topography nor in the substrate. Levelled transects through the forest demonstrated that most seedling establishment occurred on dead fallen tree boles. These microsites were significantly richer in N, P and K than the wet sump microsites. We argue that this is a mechanism whereby trees can become established in the forest, but not in the open mire. In the forest, the presence of trees ensures the presence of dead-log microsites on the ground, permitting tree seedlings to grow. In the mire, there are no such micro-sites, and trees cannot establish. The ecotone may be sharpened because of the presence of an ecotonal band of the small tree Leptospermum sco-parium between forest and mire. This species can reproduce vegetatively by root suckers in the mire. Its boles are light, and even if they fall to the mire surface they are not thick enough to form a substrate for tree-seedling establishment. The larger tree species may be prevented from falling onto the mire by the wind-sheltering of the forest, and by the zone of Leptospermum. The postulated process would represent a new kind of water-/nutrient-mediated switch, of Type 1 (‘One-sided’). It may occur in many waterlogged forests worldwide.     

Establishment of Fagus sylvatica and Fraxinus excelsior in an old-growth beech forest

Abstract. Distribution of tree seedlings, forest architecture, light conditions, ground vegetation and humus conditions were studied in a 45 m × 100 m area including multiple gaps in an old-growth beech forest. Gaps were created after some beech trees had been felled in severe storms in February 1990. A group of adult ash trees is found near the study site. The data were analyzed by Correspondence Analysis. Young seedlings (< 4 yr), of both Fraxinus (a sun species) and Fagus (a shade species), were most abundant under the crown of beech trees in semi-shade conditions, and where beech litter did not accumulate. Differences in the dissemination of Fraxinus and Fagus explained differences in the establishment of the two species. In contrast, older seedlings of beech established before the storms were more numerous in the gaps, suggesting a change in the ecological requirements of beech seedlings in the course of time.     

Interaction between large herbivore activities,vegetation structure,and flooding affects tree seedling emergence

Tree establishment in grazed vegetation mosaics involves a series of early bottlenecks, including seed dispersal, germination, seedling emergence, survival and growth. In a field experiment, we studied seedling emergence of two species with contrasting recruitment strategies, Fraxinus excelsior and Quercus robur, in five structurally different vegetations: grazed and ungrazed grassland, ruderal pioneer vegetation, soft rush tussocks, tall sedge mats and bramble scrub. In a simulation experiment, we studied the interaction effects of pre-emergence flooding (3 weeks of inundation), trampling and grazing (simulated by clipping) of grassland vegetation on the emergence and early growth of both tree species in grass swards. Seedling emergence was enhanced in low swards and sparse vegetation types. Despite different recruitment strategies, the interaction of flooding and trampling of swards enhanced seedling emergence of both species. Grazing of soft rush and tall sedges enhanced emergence of F. excelsior. Clipping grass swards increased early growth of emerging Q. robur. Our results support the hypothesis that natural disturbances of soil and vegetation create microsites for seedling emergence and reduce above-ground competition. In grazed systems however, these results suggest a discordant relationship between successful seedling emergence and subsequent seedling growth/survival during the establishment process in structurally different vegetations.     

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.     

Role of nurse plants in Araucaria Forest expansion over grassland in south Brazil

Abstract Araucaria Forest expansion over grassland takes place under wet climate conditions and low disturbance and it is hypothesized that isolated trees established on grassland facilitate the establishment of forest woody species beneath their canopies. Forest with Araucaria angustifolia is a particular type of Brazilian Atlantic Forest and the main forest type on the highland plateau in south Brazil, often forming mosaics with natural Campos grassland. The objectives of this paper were to evaluate the role of isolated shrubs and trees as colonization sites for seedlings of Araucaria Forest woody species on grassland, to determine which species function as preferential nurse plants in the process and the importance of vertebrate diaspore dispersal on the structure of seedling communities beneath nurse plants. The study was conducted in São Francisco de Paula, Rio Grande do Sul State, where we sampled isolated shrubs and trees in natural grassland near Araucaria Forest edges. Seedlings were counted and identified, and seedling diaspore dispersal syndromes, size and colour were registered. We detected 11 woody species with a potential role in nucleating grassland colonization by forest species. Beneath the canopies of nurse plants more forest species seedlings were found compared with open field grassland and the seedlings had diaspores mostly dispersed by vertebrates. Also, more seedlings were found under the canopy of A. angustifolia than beneath other nurse plant species. We conclude that A. angustifolia trees established on grassland act as nurse plants, by attracting disperser birds that promote colonization of the site by other forest species seedlings, and that under low level of grassland disturbance, conservation of frugivorous vertebrate assemblages may increase forest expansion over natural grassland and also facilitate the regeneration of degraded forest areas.     

Consequences of disperser behaviour for seedling establishment of a mistletoe species

The dispersal process in plants links adults and their offspring. For frugivore‐dispersed plants, animal behaviour can have a strong effect on plant fitness. Many mistletoes are totally dependent on animals that deposit seeds on suitable hosts and particular branch diameters. We characterised the seed dispersal and seedling establishment of the mistletoe Tristerix corymbosus, which at our study site, is exclusively dispersed by the marsupial Dromiciops gliroides. Mistletoes’ fruits have a viscous pulp that remains in the seed even after dispersal. This substance adheres the seed to the host branch. We estimated host branch availability in the forest and seed deposition (faeces) by the marsupial in the study area. Specifically, the branch suitability factors we assessed were host identity, branch status (alive or dead), branch diameter, height, and canopy cover. Lodged faeces were individually marked and the number of seed deposited within these droppings was counted, and we recorded the number of seedlings with true leaves that had established after 1 year to estimate the probability of seedling establishment. Branch diameter and canopy cover had a significant positive effect on seed deposition probability. Seedling establishment probability decreased with the number of seeds deposited per faeces and with canopy cover. In general, the marsupial deposited mistletoe seeds in microsites that increase the chance of seedling establishment. Thus, the movement behaviour of the marsupial has a positive effect on the regeneration process of this mistletoe species.     

Plant recruitment in the High Arctic: Seed bank and seedling emergence on Svalbard

Abstract. Composition and density of the soil seed banks, together with seedling emergence in the field, were examined on Svalbard. 1213 soil samples were collected from six drymesic habitats in three regions representing various stages of colonization from bare moraines to full vegetation cover and spanning a range of typical nutrient and thermal regimes. Of the 165 vascular plant species native to Svalbard, 72 were present as mature plants at the study sites and of these 70% germinated seed. Proglacial soil had 12 seedlings per m², disturbed Dryas heath 131, intact Dryas heath 91, polar heath 715, thermophilic heath 3113, and a bird cliff 10437 seedlings. Highest seed bank species richness was at the thermophilic heath (26 species). Seedlings of 27 species emerged in the field, with fewer seedlings in disturbed habitats (60 seedlings per m²) than in intact Dryas heath (142), suggesting that an absence of ‘safe sites’ limited seedling establishment in disturbed habitats. Measurement of seedling emergence in the field increased awareness of which species are able to germinate naturally. This may be underestimated by up to 31% if greenhouse trials alone are used, owing partly to unsuitability of greenhouse conditions for germination of some species and also to practical limitations of amount of soil sampled. Most thermophilic species failed to germinate and some species present at several sites only germinated from the thermophilic heath seed bank, suggesting that climate constrains recruitment from seeds in the High Arctic.     

Improving restoration success through microsite selection: an example with planting sagebrush seedlings after wildfire

Post‐fire restoration of foundation plant species, particularly non‐sprouting shrubs, is critically needed in arid and semi‐arid rangeland, but is hampered by low success. Expensive and labor‐intensive methods, including planting seedlings, can improve restoration success. Prioritizing where these more intensive methods are applied may improve restoration efficiency. Shrubs in arid and semi‐arid environments can create resource islands under their canopies that may remain after fire. Seedlings planted post‐fire in former canopy and between canopies (interspace) may have different survival and growth. We compared planting Wyoming big sagebrush (Artemisia tridentata Nutt. ssp. wyomingensis Beetle & Young) seedlings post‐fire in former sagebrush canopy and interspace microsites at five locations. Four growing seasons after planting, seedling survival was 46 and 7% in canopy and interspace microsites, respectively. Sagebrush cover was 5.8 times greater in canopy compared to interspace microsites. Sagebrush survival and cover were likely greater because of less competition from herbaceous vegetation as well as benefiting from resource island effects in canopy microsites. Initially, post‐fire abundance of exotic annual grasses was less in canopy microsites, but by the third year post‐fire it was substantially greater in canopy microsites, indicating that resource availability to seedlings was greater, at least initially, in canopy microsites. These results suggest microsites with greater likelihood of success should be identified and then utilized to improve restoration success and efficiency. This is important as the need for restoration greatly exceeds resources available for restoration.     

Seedling establishment of late colonizer is facilitated by seedling and overstory of early colonizer in a post-mined peatland

The seedling establishment of late colonizer may be promoted in the environments created by the early colonizer without interspecific seedling competition. To confirm the hypothesis, seed-sowing experiments were conducted in a post-mined peatland, northern Japan. Seeds on three grasses (Rhynchospora alba, Moliniopsis japonica and Phragmites australis) were sown in four vegetation sites: bareground, R. alba grassland, M. japonica grassland, and grassland mixed with three examined species. R. alba is the earliest colonizer, and M. japonica and P. australis are the later ones. Seedling emergence, survival, growth and resource allocation were monitored for two growing seasons. The seedling emergence of all the species was lowest on bareground. The emergence of R. alba and P. australis was high in R. alba and mixed grasslands, but was low in M. japonica grassland. M. japonica seedlings emerged more in vegetation with moderate plant cover and litter. R. alba seedlings yielded the highest biomass on bareground, and the biomass decreased with increasing shading. R. alba did not change resource allocation of height to above-ground biomass between vegetation types. The seedling biomass of M. japonica was affected little by shading, probably because M. japonica adjusted resource allocation with shading. In the two summers, R. alba and M. japonica seedlings survived more when the neighboring seedlings emerged more. P. australis seedlings failed in overwintering. The seedling characteristics of each species explained the species replacement, i.e., R. alba invaded bareground and facilitated the establishment of late colonizer, M. japonica, by the shelter effects of the overstory and seedling.     

Interspecific variation in tree seedling establishment in canopy gaps in relation to tree density

Abstract. We tested whether interspecific variation in tree seedling establishment in canopy gaps was significantly related to interspecific variation in tree density, for seven deciduous forest tree species (Quercus alba, Hamamelis virginiana, Acer rubrum, Sassafras albidum, Quercus rubra, Prunus serotina, Ostrya virginiana). For each species, seedling establishment was calculated as the difference in seedling density before experimental gap creation versus three years after gap creation. In each of the six experimentally-created gap types (33 % or 66 % removal of tree basal area from 0.01-ha, 0.05-ha or 0.20-ha patches), differences in seedling establishment among species were significantly related to differences in their density in the tree canopy. A regression model with log_e tree density as the independent variable accounted for between 93 % and 98 % of interspecific variation in seedling establishment. Our results provide empirical support for models of tree dynamics in gaps that assume seedling establishment depends on canopy tree density.     

Woody plant seed dispersal and gap formation in a North American subtropical savanna woodland: the role of domestic herbivores

The relationship between domestic cattle and vegetation change in a savanna woodland was evaluated with respect to dung deposition and the dispersal and establishment of mesquite (Prosopis glandulosa var. glandulosa, Mimosaceae), a cosmopolitan woody invader of grasslands in the southwestern USA. Dung deposited in autumn disintegrated rapidly, leaving patches of bare ground ranging from 50 to 900 cm². Herbaceous cover on gaps created by dung deposition recovered to levels comparable to neighboring vegetation by the end of the following growing season. Vegetation colonizing gaps consisted primarily of grasses not found in the surrounding vegetation. Dung deposition increased species diversity and spatial heterogeneity of the herbaceous vegetation and contributed to the development of a fine-grain mosaic of small patches of varying successional age-states.The role of cattle in facilitating the ingress and establishment of mesquite has broader implications with regard to the conversion of grasslands to woodlands. On the site with cattle, mesquite seedlings were found in 75% of dung pats surveyed in September (mean =4.2 seedlings per pat; maximum =50). Although seedling survival in dung (79%) was only 16% greater than that of mesquite emerging from seeds experimentally sown away from dung, no seedlings were found on areas without cattle. Mean (± SE) density of mesquite seedlings ranged from 12±2 to 15±2 m^-2 on the site with cattle. Seed densities away from parent plants averaged 10.7 m^-2 and 0.0 m^-2 on areas with and without cattle, respectively. Seed densities beneath adult plants were comparable between sites.The high density of seedlings on areas with cattle, in contrast to absence of seedlings on the area without cattle, suggests rates of invasion of grasslands by mesquite would have increased substantially in North America following the settlement and introduction of domestic ungulates. Prior to the introduction of livestock, poor seed dissemination and germination may have limited its Holocene spread.