Effectiveness of seed sowing techniques for sloped restoration sites

Practitioners are challenged with choosing among many potentially effective methods for sowing seed in ecological restoration projects to achieve sufficient native plant establishment. We tested the effectiveness of seed sowing techniques on moderate and steep slopes in a Mediterranean climate by measuring native seedling density immediately following germination, as well as plant density, recruitment success, and soil movement through the second growing season. We calculated cost effectiveness of different methods as the native plant density per dollar spent sowing seed. While all sowing techniques resulted in significant native establishment compared with unseeded controls, hydro seeding on moderate slopes was the most cost effective (native seedlings established per dollar spent). Although all steep‐sloped seeding techniques resulted in high densities of native species, all methods also resulted in significant soil loss. Shrubs preferred hand seeding followed by jute netting on steep slopes, while forbs reached greatest densities with hydro seeding on moderate slopes. Seedlings of species with heavy seeds were present in greater densities than species with lighter seeds in imprint sowing treatments. The “best” seed sowing technique varied depending on slope and metric of success (native density, species richness, shrub density, or forb density). Different combinations of slope, technique, and success metric resulted in significantly different project costs, which implies opportunities for savings given careful decision‐making relative to mitigation needs on heterogeneous landscapes. Evaluations of techniques for restoring slopes are limited, yet critical for expanding the area capable of being restored and the application of limited conservation funding.     

Establishment of tree seedlings in the understory of restoration plantations: natural regeneration and enrichment plantings

Little is known about the potential of restoration plantations to provide appropriate understory conditions to support the establishment of seeds arriving from neighboring native forests. In this article, we investigated how seedling establishment is affected in the understory of restoration sites of different ages and assessed some of the potential environmental factors controlling this ecological process. We first compared the density and richness of native tree seedlings among 10‐, 22‐, and 55‐year‐old restoration plantations within the Atlantic Forest region of southeastern Brazil. Then, we undertook a seed addition experiment in each study site, during the wet season, and compared seedling emergence, survival, and biomass on local versus old‐growth forest soil (transferred from a reference ecosystem), in order to test whether local substrate could hamper seedling establishment. As expected, the oldest restoration site had higher density and richness of spontaneously regenerating seedlings. However, seedling establishment was less successful both in the oldest restoration planting and using substrate transferred from a reference ecosystem, where emergence and survival were lower, but surviving seedlings grew better. We attribute these results to lower light availability for seedlings in the understory of the oldest site and speculate that higher incidence of pathogens on old‐growth forest soil may have increased seedling mortality. We conclude that the understory of young restoration plantations provides suitable microsite conditions at the early establishment phases for the spontaneous regeneration or enrichment planting of native trees.     

Endozoochory of a dry‐fruited tree aids quarry passive restoration and seed soaking further increases seedling emergence

As practitioners promote passive restoration as a complementary approach to technical reclamation, it is imperative to know its drivers. Although the consequences of endozoochory can be crucial to passive restoration success, few experimental studies assess the use of heavily disturbed sites by seed dispersers such as carnivores and how the seeds they bring in emerge and survive. Using an indoor sowing experiment conducted in a quarry located within a natural park in Portugal, we examined for the first time how carnivore endozoochorous seeds collected in the quarry potentially influence its passive restoration, through effects on plant emergence and survival. Also, we tested whether sowing date and water soaking, relevant factors when sowings are to be carried out, would affect seedling emergence and mortality rates when compared with the effect of endozoochory. Our target species were included in the revegetation plan of the quarry, of which endozoochorous seeds of Carob tree (Ceratonia siliqua) were collected in sufficient number for analysis. Irrespective of the carnivore species, endozoochorous carob seeds performed similarly to untreated seeds regarding emergence rates. Endozoochorous carob seedlings showed greater mortality rates but the net result for the plant can still be the colonization of recently vacant habitats by a large proportion of viable seeds. Concerning sowing date, the later carob seeds were sown over the fruit‐ripening season the faster seedlings emerged. Water soaking increased emergence rate by 6.5 times. Broadly, sowings with previous soaking and carnivore‐mediated seed dispersal of this dry‐fruited tree can jointly enhance quarry restoration.     

Effect of Sowing Season,Plant Cover,and Climatic Variability on Seedling Emergence and Survival in Burned Austrocedrus chilensis Forests

Fire is the most important disturbance factor in Cypress (Austrocedrus chilensis) forests in Patagonia, Argentina. This ecosystem recovers poorly after fire, and direct sowing could be a potentially useful restoration practice. To evaluate the effect of season of sowing, post‐fire plant cover (PC), and climatic variability on seedling emergence and survival, three direct sowing studies were established in two burned cypress stands: Trevelin (xeric conditions) and El Bolsón (mesic conditions). Two studies were conducted in winter (2000 and 2001) and one in spring (2001). Precipitation was higher than the mean during the 2000–2001 growing season and lower during 2001–2002. At both sites, emergence and survival were much higher for winter‐ than for spring‐sown seedlings. In the xeric stand, emergence and survival of winter‐sown seedlings increased with medium and high PC values, after the humid and dry summers, respectively. However, most spring‐sown seedlings did not emerge, and those that did were short‐lived. Because of the more favorable growing conditions in the mesic stand, PC had no effect on emergence and only favored first year survival of winter‐sown seedlings after the dry summer. Spring‐sown seedlings showed no association with PC in the mesic site, probably because the first summer was exceptionally humid. We speculate that shading plants exert a positive effect on cypress seedling establishment, likely by reducing the stress from high temperatures and low water availability. Sowing of small patches under the protection of understory vegetation could be useful in restoring burned cypress stands.     

Testing Broadcast Seeding Methods to Restore Urban Forests in the Presence of Seed Predators

Forest restoration in urban areas often occurs in isolation from remnant forest, limiting the chances for recolonization by native species. Plants with bird‐dispersed seeds can be particularly vulnerable to dispersal limitation and regeneration can be further impeded by non‐native seed predators. We used a factorial experiment to investigate broadcast seeding as a method to reintroduce trees with large seeds and fleshy fruits into early successional forests. We assessed rates of seed and fruit loss, germination and seedling establishment in three seed treatments: (1) caging to exclude introduced mammalian seed predators; (2) removal of fleshy fruit pericarp; and (3) placing seeds in nutritionally enriched clay balls. Across all species (Beilschmiedia tawa, Elaeocarpus dentatus, and Litsea calicaris) seeds and fruits accessible to mammalian predators suffered significantly greater loss (58%) than those protected by cages (4%). However, seed and fruit loss in the presence of predators was reduced to only 35% across all species by the treatment combining the removal of fruit flesh and clay ball application to seeds. Establishment of B. tawa seedlings after 1 year was significantly enhanced by the clay ball treatment (12% of seeds sown vs. 6% without clay balls). Very low establishment rates were recorded for E. dentatus and L. calicaris. Broadcast seeding was found to be a viable method of improving regeneration of large‐seeded late successional trees and may be a cost‐effective alternative to planting saplings. Seedling establishment can be improved with fruit flesh removal and clay ball treatments, especially in the presence of mammalian seed predators.     

Seed Removal and Predation as Factors Affecting Seed Availability of Tree Species in Degraded Habitats and Restoration Plantings in Rainforest Areas of Queensland,Australia

The post‐dispersal removal or predation of seeds of native tree species was investigated in Queensland, Australia, at degraded habitats and rainforest restoration sites where direct seeding might be used to facilitate tree regeneration (old fields or open habitats, lantana thicket, rainforest edge, and 5‐ and 10‐year‐old restoration plantings). Seed removal/predation was assessed in relation to tree seed weight and canopy density of the habitats during the wet season period. Results indicated that seed removal/predation imposes limitations on seed availability, particularly for small seeded species. In most situations, larger seeds were less removed/predated, most likely due to the more limited range of large seed consumers. The use of large, hard‐coated seeds may potentially reduce seed loss in open situations (from both seed removal and desiccation), unless large seed consumers frequent the site. Canopy cover exerted an influence on seed removal/predation, though trends varied in relation to site and the time of season. Broadcast sowing of seed under planted tree canopies at the more advanced stages of closure may in some areas result in higher seed removal/predation. Likewise, seeding in areas dominated by woody weeds may result in high seed losses to consumers such as rodents. Results suggested that undertaking direct seeding to coincide with the maximal period of fruit production may in some situations be beneficial to minimize seed loss. Overall, site context, canopy cover, and species selection appear to be important considerations when aiming to reduce loss of seeds to animal seed consumers in restoration work.     

Testing Germination of Species for Hydroseeding Degraded Mediterranean Areas

The germination performance of native species and their suitability for a rapid erosion control are uncertain. Together with their relatively low commercial availability and high costs, these are still strong reasons preventing their common use in hydroseeding for restoration of Mediterranean degraded slopes, despite the increasing number of studies recommending it. In this study, 14 non‐native (commercial) and native herb and woody species were tested. Their germination performance was evaluated under laboratory (Petri dishes) and greenhouse conditions (seeds sown in target substrate). The results obtained were compared with the seedling densities in a Mediterranean quarry slope hydroseeded with the same species. In the laboratory, commercial species had a better germination performance than most native species, but this trend was not maintained in the greenhouse. Greenhouse tests were extended beyond spring and showed that many native species germinated better, or exclusively in autumn. Germination performance and success decreased, from laboratory to greenhouse and field conditions, for many species, but not for all. Relative to field performance, the predictive value of laboratory and greenhouse tests was poor, yet sowing on the target substrate under greenhouse conditions may be a better approach for certain native species. The main drawbacks revealed by native species in the present study included: (1) relatively slow germination; (2) seasonality; and (3) seed dormancy‐breaking requirements. The results suggest that these problems may be overcome through species selection, seed pre‐treatments, hydroseeding scheduling, and/or manipulation of seeding density and relative species proportion.     

Direct seeding of indigenous tree and shrub species into New Zealand hill country pasture

Summary Our study aimed to examine the applicability of a native plant restoration technique involving close grazing, blanket herbicide spraying and mob stocking (to trample the seed into the soil) for establishing New Zealand native tree and shrub species into exotic hill country pasture. This approach represents an alternative to the usual spaced-planting of seedlings in the context of an indigenous vegetation restoration programme. Field-collected seed of Cabbage Tree ( Cordyline australis ), Kahikatea ( Dacrycarpus dacrydioides ), Kanuka ( Kunzea ericoides ), Karamu ( Coprosma robusta ), Kohuhu ( Pittosporum tenuifolium ), Koromiko ( Hebe stricta ), and Manuka ( Leptospermum scoparium ) were germination tested in the laboratory, and broadcast sown in two field experiments in spring 2001 and autumn 2002. The spring-sown experiment tested the effect of mob stocking and the autumn-sown experiment tested the effect of mob stocking and sowing rate. Laboratory germination varied widely among species (0–88%). In the field, the two shrubs, Koromiko and Karamu, established densities of up to 5 and 1.2 plants/m², respectively, after 2.5 years. Autumn emergence for these two species strongly related to sowing rate. Three tree species, Cabbage Tree, Kahikatea and Manuka, emerged in the field but did not survive beyond 6 months. Mob stocking was effective in spring but not autumn sowing, implicating soil moisture content as an important factor in germination success. The study demonstrated that the direct seeding technique was successful for two shrub species, highlighting the potential for development of a cost-effective native plant restoration approach.     

Rehabilitation of Degraded Areas of Central Amazonia Using Direct Sowing of Forest Tree Seeds

Deforestation in the Amazon Basin is still increasing, and the rehabilitation of these lands continues to be a challenge. Autoecological studies of most Amazonian species are rare, and efficient techniques for restoration of forested habitats have yet to be developed. The aim of this study was to test direct sowing as a rehabilitation technique for sites with different degrees of disturbance: bare soil, pasture, and secondary and mature forests in Central Amazonia, Brazil. At each site, we sowed seeds of 11 native tree species. Throughout the following year we evaluated germination and seedling survival. The germination differed according to the study site and species. Seedling survival in degraded sites was higher than in other areas. After 1 year in the bare soil site, 33% of the sown seeds of eight species developed seedlings; in the pasture the establishment was 23%, in secondary forest 15%, and in mature forest 12% of only four species. The only widespread survivor with more than 45% emergence in all perturbed sites was Caryocar villosum. No pioneer seedlings remained after 1 year. There was a positive correlation between seed size and survival. Large‐seeded non‐pioneer species seem to be more suitable for direct sowing than small‐seeded species. We recommend a combination of direct sowing and planting of seedlings as an appropriate means to accelerate the rehabilitation of degraded areas in Central Amazonia.     

Does forest restoration using taungya foster tree species diversity? The case of Afram Headwaters Forest Reserve in Ghana

The taungya agro‐forestry system is an under‐researched means of forest restoration that may result in high tree diversity. Within a forest reserve in Ghana, the forest core and its surrounding Teak‐ and Cedrela‐taungya on logged, cropped and burned land were mapped with ALOS satellite imagery. Native trees, seedlings and saplings were enumerated in 70 random, nested plots, equally divided between forest and taungya. The native tree regeneration was assessed by species richness (SR), Shannon‐Wiener Index (SWI), Shannon Evenness Index (SEI) and species density (SeD) for seedlings, saplings and trees separately and combined and subsequently correlated with canopy covers (CC) in taungya. As anticipated, the taungya diversity was lower than the forest diversity but higher than reported from nontaungya exotic plantations. In the forest, the diversity of native trees increased from seedlings through saplings to trees. The reverse was found in the taungya. Taungya seedling diversity was not significantly different from the forest, while the sapling and tree diversity were significantly lower. Weak correlations of CC with SR, SWI, SEI and SeD were found. Our results suggest the need for treatment to maintain the tree diversity beyond the seedling stage in the taungya.     

Germinative behaviour of ten tree species in white-water floodplain forests in central Amazonia

Amazonian floodplain forests (known as várzea) are classified into high or low várzea depending on the spatial position on the plains. This topographic feature exposes the terrain over different time periods of inundation, causing a major limiting factor for tree seedling establishment. We hypothesize that, strategically, most of the seeds produced by trees in low várzea forests germinate faster and in synchrony (temporally concentrated germination), and that their seedlings tend to have cotyledons without reserve or foliaceous cotyledons (PEF). By contrast, seeds produced by high-várzea specialist trees exhibit slower and temporally scattered germination, and their seedlings tend to have reserve storage cotyledons (CHR). Generalist species may show no clear pattern or may be related to high-várzea species. To test this hypothesis, diaspores of 10 tree species were collected: five of low-várzea specialist trees, three of high-várzea specialist trees and two of generalist species. Seedling emergence and morphology were monitored daily in a nursery for a period of 150 days of being subjected to non-flooded (sown directly in várzea soil) and flooded conditions (15 days in water before sowing in the same soil). The seedling emergence of low-várzea species showed an increase of 37% in germinability whereas high-várzea and generalist species exhibited a decrease of 38% and 35% of germinability, respectively. Foliaceous cotyledons were preferentially found in seedlings of low-várzea species, and storage cotyledons were more common in those of high-várzea species, indicating how cotyledon morphology may determine the amount and use of resources available to a seedling during the first stages of establishment and growth. Conservation plans aiming for the maintenance of ecosystem services must consider these strategies.     

Is the pollen‐limited mistletoe Peraxilla tetrapetala (Loranthaceae) also seed limited?

Abstract Seed production in the endemic New Zealand mistletoe Peraxilla tetrapetala (Loranthaceae) has been shown to be consistently pollen limited, but to date there has been no test of whether the species is also seed limited. If it is not seed limited, then pollen limitation may have no effect on population size. We tested for seed limitation by sowing seeds onto host trees at high and low densities at two sites, and following survival for up to 69 months. Our rationale was that unless there is strong density‐dependent seedling mortality sufficient to negate increases in seed supply, the species would be seed limited. We simultaneously measured pollen limitation at both sites using pollen augmentation. Peraxilla tetrapetala was strongly pollen limited at both sites over 7 years at Ohau (mean Pollen Limitation Index 0.62), and 12 years at Craigieburn (mean PLI 0.44), before and during the seed‐limitation study. There was no significant overall negative effect of density on survival in sown P. tetrapetala seeds over 42–69 months. There was a significant positive effect: seeds were more likely to adhere to branches when sown at high densities. This initial advantage to high‐density seeds was gradually eroded by slightly (but non‐significantly) lower survival rates of adhered seeds at high density. By the end of the study there was no significant difference in overall survival in high and low density plantings. This means that P. tetrapetala was apparently both pollen limited and seed limited at both our sites. Hence, reduced densities of native bird pollinators caused by introduced mammalian carnivores are likely to reduce the density of adult mistletoes in the next generation. However, the generality of this result may be affected by the fact that mistletoes do not have a dense ‘seedling shadow’ under the parent mistletoe.     

Determining the Optimal Sowing Density for a Mixture of Native Plants Used to Revegetate Degraded Ecosystems

No standardized, objective methodology exists for optimizing seeding rates when establishing herbaceous plant cover for pastures, hay fields, ecological restoration, or other revegetation activities. Seeding densities, fertilizer use, season of seeding, and the interaction of these treatments were tested using native plants on degraded sites in northern British Columbia, Canada. A mixture of 20% Achillea millefolium, 20% Carex aenea, 20% Elymus glaucus, 20% Festuca occidentalis, 16% Geum macrophyllum, and 4% Lupinus polyphyllus seed was applied at 0, 375, 750, 1,500, 3,000, and 6,000 pure live seed (PLS) per m² in 2.5 × 2.5–m rototilled test plots, established in the fall and spring, with and without fertilizer. There was no significant difference in plant cover of sown species between fall seeding and spring seeding, and few treatment interactions were identified in the first 2 years after sowing. There was no significant difference in cover between seed densities of 3,000 and 6,000 PLS/m² in the first year, nor among 1,500, 3,000, and 6,000 PLS/m² treatments in the second year. Seed densities as low as 375 PLS/m² produced year 2 plant cover equivalent to that observed at 3,000 PLS/m² in year 1. Plots sown to seed densities less than or equal to 750 PLS/m² generally exhibited an increase (infilling) in plant density from year 1 to year 2, whereas plots sown to seed densities greater than or equal to 1,500 PLS/m² generally exhibited a decrease (density‐dependent mortality) in plant density. These results imply a most efficient sowing density between 750 and 1,500 PLS/m² (corresponding to 190–301 established plants^.m^?2 after two growing seasons). It is suggested that net changes in plant populations observed over a range of sowing densities are a robust and objective means of determining optimal sowing densities for the establishment of herbaceous perennials.     

Forb Species Establishment Increases with Decreased Grass Seeding Density and with Increased Forb Seeding Density in a Northeast Kansas, U.S.A., Experimental Prairie Restoration

Most prairie restorations fail to produce the diversity of species found in unplowed remnants. This lack of restored diversity is hypothesized to be partly due to the inhibition of forb species by high seeding densities of dominant grasses and partly due to the low seeding densities of forbs used in many restorations. We tested this hypothesis by sowing various densities of forb and warm-season grass seeds into a restoration begun on bare soil. This is the first replicated restoration experiment we are aware of that varies grass seeding densities to examine the effects on forbs. Four years after seeding, we found that higher densities of grass seeds decreased forb cover, biomass, and richness, and higher densities of forb seeds increased forb richness. These results suggest that dominant grasses compete strongly with native forb species and that many forb species thrive when they are spatially separated from dominant grasses. The results also suggest that seed availability limits the establishment of some forbs. Forb diversity can therefore be increased by decreasing grass seeding density, by increasing forb seeding density, or both. However, forb seeds are generally expensive, and increasing forb seeding density across the entire area of a restoration may be prohibitively expensive. We therefore recommend a low seeding density of dominant grasses, and we recommend spatially separating forbs from dominant grasses by adding most forb seeds to areas with little to no dominant grasses and by adding the rest of the forb seeds to areas with a low density of dominant grasses.     

Testing for enemy‐mediated density‐dependence in the mortality of seedlings: field experiments with five Neotropical tree species

The coexistence of plant species in species‐rich tropical forests can be promoted by specialised enemies acting in a negatively density‐dependent manner. While survival of tropical tree seedlings is often negatively density‐dependent, the causes have rarely been identified. We tested whether insects and plant pathogens cause density‐dependent seedling recruitment and survival in five forest tree species in Belize, Central America. We manipulated densities of seeds or newly germinated seedlings in small (1 m² or 0.25 m²) plots close to fruiting conspecific trees. Using a factorial design, we excluded enemies from subsets of the plots with fungicides and insecticides. Seed germination (for two species) and early seedling survival (for all species) were monitored at approximately weekly intervals for up to eight weeks, during the period when plants are likely to be most susceptible to natural enemies. In Terminalia amazonia, seed germination was negatively density‐dependent and the proportion of seeds germinating increased when insects were excluded. However, the magnitude of the insecticide effect was independent of density. The only significant density effect for survival of young seedlings was in Acacia polyphylla; counter to expectation, seedling survival was higher at high densities. In a few cases pesticide application had a significant effect on seedling survival, but in only one case (Terminalia amazonia) was a significant pesticide × density interaction detected. Our results caution against generalising from studies conducted on a single species at a single time and place and illustrate the challenges of experimentally testing for enemy‐mediated negative density‐dependence. Experimental outcomes are likely to depend on the spatial scale at which the principal enemies disperse and respond to plant density, and the timescales over which they act. Gathering information on these variables will improve our understanding of the natural histories of tropical forest species and help inform the design of future experiments.     

Effects of burial in sand and water supply regime on seedling emergence of six species

BACKGROUND AND AIMS: Air seeding has long been regarded as a quick and successful measure for vegetation rehabilitation in China. However, seedling emergence of often-used species including Agriophyllum squarrosum, Artemisia sphaerocephala, Artemisia ordosica, Hedysarum fruticosum, Caragana korshinskii and Medicago sativa is low. Experiments were conducted under controlled conditions to study the effects of sowing depth and water supply on seedling emergence, in order to understand the requirements for increasing seedling emergence. METHODS: Seeds were exposed to different environments of burial and water supply regimes in PVC pots (7 cm in diameter and 11 cm in height) under the same light intensity and alternating temperature regimes in a growth chamber. KEY RESULTS: Seedlings of three species (Agriophyllum squarrosum, Artemisia sphaerocephala, Artemisia ordosica) with relatively light seeds emerged well at a 0.5 cm sowing depth under a 7.5 and 10 mm water supply regime. However, few seedlings of these species emerged when the sowing depth was over 1 cm or when water supply was 5 mm. Seedlings of Caragana korshinskii, Hedysarum fruticosum and Medicago sativa emerged from sowing depths of 0.5-4 cm, 0.5-3 cm, and 0.5-4 cm, respectively, under both 7.5 and 10 mm water supply regimes. Under a 5 mm water supply regime, seedlings of these species also emerged at over 1 cm sowing depth. Seeds of all six species sown on the surface of sand did not germinate, and seedlings did not emerge when they were sown at depths greater than 6 cm. CONCLUSIONS: Based on these experiments, a 0.5 cm sowing depth resulted in the highest seedling emergence and it is concluded that this is the optimal sowing depth for seedling emergence of all six species.     

Tree–Shrub Interactions During Early Secondary Forest Succession in Uganda

Abstract One of the greatest challenges for ecologists this century will be restoring forests on degraded tropical lands. This restoration will require understanding complex processes that shape successional pathways, including interactions between trees and other plants. Shrub species often quickly invade disturbed tropical lands, yet little is known about whether they facilitate or inhibit subsequent tree recruitment and growth. We examined how shrubs and other vegetation (e.g., vines, grasses, herbs) affect tree recruitment, survival, and growth during the first 6 years of forest succession in Kibale National Park, Uganda. The study was undertaken in two recently logged exotic softwood plantations. We studied the successional trajectories in two recently logged areas that varied in their initial densities of trees and shrubs. Analyses suggested tree seedling presence and density were not strongly related to shrub density or height during succession. Tree sapling presence and density were positively significantly related to shrub density and height. We found little response in the tree community to experimental shrub removal, and although removal of all nontree vegetation temporarily enhanced tree growth, the effect disappeared after 2 years. Some early‐successional trees benefited from reduced competition, whereas some mid‐successional trees benefited from the presence of other vegetation. Some specific tree species responded strongly to vegetation removal. We interpret our findings in light of designing manipulations promoting forest restoration for biodiversity conservation and conclude with four tentative guidelines: (1) manage at the species level, not the community level; (2) increase facilitation for seedlings, reduce competition for saplings; (3) be cautious of assumptions about plant interactions; and (4) be adaptable and creative with new strategies when manipulations fail.     

Experiments on the mechanism of tree and shrub establishment in temperate grassy woodlands: Seedling emergence

Field experiments were designed to examine tree and shrub seedling emergence in temperate grassy woodlands on the New England Tablelands. The effects of study sites, intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds and ant seed theft on seedling emergence were tested in two field experiments. Six tree and seven shrub species were used in the experiments and their cumulative emergence was compared with laboratory germination studies. All species used in field experiments had lower cumulative emergence than those in laboratory germination studies despite prolonged periods of above average rainfall before and after seeds were sown. Eucalypt species emerged faster in the field than the shrub species and generally attained higher cumulative emergence than the shrubs. Spatial effects of sites and patches within sites, and of previous grazing history did not strongly influence patterns of seedling emergence in most species. Ground and litter cover generally did not enhance or suppress the emergence of seedlings, although the removal of cover in recently grazed areas enhanced the emergence of some species. Burning enhanced the emergence of some tree and shrub species where plots had more fuel and intense fires, but this effect was not strong. Compared with other treatments, seedbed manipulations produced the strongest effects. In the absence of both invertebrate and vertebrate predators, seedling emergence was lower for surface‐sown seed, compared with seed sown on scarified soil surfaces. Higher seedling emergence of buried seeds in the presence of invertebrate predators probably resulted from the combined effects of predator escape and enhanced moisture status of the germination environment. Some promotion of emergence was achieved for all species in most sown treatments probably as a result of a prolonged above average rainfall. In contrast, the natural recruitment of trees and shrubs was negligible in experimental plots, highlighting the importance of seed supply and dispersal as ultimate determinants of recruitment.     

Positive and Negative Interactions among Individuals of a Root Hemiparasite

Abstract: I studied the effects of a wide range of densities on establishment, survival, growth and reproduction of the annual root hemiparasite Rhinanthus alectorolophus in a field experiment. Seeds of the parasite were sown with those of a mixture of grassland plants as potential hosts. In most plants, seedling survival is strongly reduced by self-thinning at high densities, but in R. alectorolophus the proportion of seeds producing a young plant increased linearly with sowing density, indicating positive interactions among seedlings. Because survival to maturity was not influenced by density, the number of flowering plants per seed sown also increased with density. In contrast, mean plant size and reproduction were strongly reduced at high densities. It is suggested that resource sharing among parasites connected by haustoria is the most likely mechanism responsible for the reduced mortality of seedlings at high densities. The results indicate that facilitation among cohorts of conspecific root hemiparasites can increase the recruitment of young plants. The number of seeds produced per seed sown (a multiplicative fitness measure) was, however, independent of density in Rhinanthus because the early positive effects of density on recruitment were compensated later by the negative effects of crowding on growth and reproduction. Increased survival of seedlings could, however, indirectly increase fitness because it will increase the genetic diversity of offspring and may thus, for instance, reduce the impact of pathogens.     

Seed Dynamics of Early and Late Successional Tree Species in Tropical Abandoned Pastures: Seed Burial as a Way of Evading Predation

We explored different treatments to enhance the probability of sowed seeds of two early successional (ES, Cecropia obtusifolia and Ochroma pyramidale) and two late successional (LS, Brosimum costaricanum and Dialium guianense) species to escape predation and germinate in abandoned cattle‐raising pasture fields in Southeastern Mexico. ES species were sown in groups of 50 seeds under three treatments: invertebrate exclusion, burial, and exposition to seedeaters. LS species were sown in groups of 10 seeds under three treatments: vertebrate exclusion, burial, and exposition to seedeaters. We registered seed predation and germination 2, 4, 8, 16, 32, and 64 days after the initial sowing. Overall, ES showed higher predation rates (mean ± SE = 0.45 ± 0.07 seed seed^?1 day^?1; n = 3) than LS species (0.09 ± 0.02 seed seed^?1 day^?1). Cecropia obtusifolia was completely predated in all treatments after 8 days. Burial and exclusion treatments reduced final predation in circa 6% for O. pyramidale, relative to that of exposed seeds (85% after 8 days); most germination occurred in buried seeds (3.7%). In B. costaricanum, burial enabled germination by 10%; exposed and excluded seeds were removed 100%. Dialium guianense showed 12% germination in buried seeds and circa 20% of the seeds were not removed after 64 days. Direct sowing would be a recommended rainforest restoration practice for species with relatively large seeds if deposited in groups and buried. Studies which address variation across numerous sites are necessary in order to generate more consistent seed predation patterns and rainforest restoration principles in tropical pastures.