Effect of grass litter on seedling recruitment of the critically endangered Cistus heterophyllus in Spain

Cistus heterophyllus Desf. is an Iberian-north African endemic plant species with only one wild population left in Spain. A sowing experiment under controlled conditions was designed to assess both the seedling emergence rate and the growth fitness of C. heterophyllus on a litter layer of Brachypodium retusum (Pers.) Beau., which forms dense carpets around the remnant population of C. heterophyllus. In order to test the physical effects of B. retusum litter on Cistus seedling recruitment, six sowing treatments in pots were compared that combined three doses of litter (0, 0.6 and 1.1 kg m⁻²) and different physical locations of the seeds (above, below the litter layer or within a litter–peat mixture). B. retusum litter interfered with the seedling recruitment of C. heterophyllus, negatively affecting both seedling emergence and their early growth. Such an effect was more severe when the seeds were located above a 0.6 kg m⁻² litter layer, although seedlings from seeds sown under the litter showed a growth reduction ranging from 58% to 100% compared to the no-litter treatment. A significant negative effect of the litter–peat mixture on the number of leaves, leaf size and number of branches of seedlings was also inferred. These data provide experimental evidence on the serious threat that B. retusum litter represents for the conservation and regeneration of the last population of C. heterophyllus in Spain.     

Seed dispersal, plant recruitment and spatial distribution of Bactris acanthocarpa Martius (Arecaceae) in a remnant of Atlantic forest in northeast Brazil

Seed dispersal ecology of Bactris acanthocarpa Mart. (Arecaceae), an Atlantic forest understory palm, was investigated during two years as an attempt to test the following predictions: (i) seeds of Bactris are dispersed by mammals and large-gaped birds; (ii) Bactris benefits from seed dispersal in terms of seed predation avoidance, improvement of seed germination and seedling survival; and (iii) spatial distribution of adults is related to patterns of seed dispersal. The study was conducted at Dois Irmãos Reserve, a 387.4-ha reserve of Atlantic forest in northeastern Brazil (8º S–35º W). Black–rumped agoutis (Dasyprocta prymnolopha) and Guianan squirrels (Sciurus aestuans) were identified as the seed dispersers/predators, moving seeds short distances (< 4 m from parents) and at low rates (0.04-0.05 diaspore/palm/day). Pyrene burial prevented seed predation by vertebrates and reduced by half seed infestation by Scolytidae beetles. Only buried pyrenes germinated. Pyrene predation was not correlated with distance from conspecific adults. In contrast, early seedling mortality was higher near conspecific adults. Most adults (64%) had their nearest conspecific adult neighbour > 4 m away in contrast to 96% of seedlings that occurred concentrated within 4 m from adults (77% under the palm crowns). Here, we present evidence that spatial distribution of B. acanthocarpa is partly due to low rates of seed removal, short-distance seed dispersal by agoutis and squirrels, and early seedling mortality associated with presence of seedlings under palm crowns.     

Away‐field advantage: mangrove seedlings grow best in litter from other mangrove species

Plant community composition can impact ecosystem processes via litter feedbacks. Species variation in litter quality may generate different patterns of nutrient supply for plants that are dependent on litter inputs. However, it is not known whether plants grow faster in their own litter, litter from other species, or in litter mixtures from multiple species. To test whether litter identity and mixture status influenced mangrove seedling growth, biomass allocation, and stoichiometry, we performed mesocosm experiments. Two species of mangrove seedlings, Avicennia germinans, black mangrove and Rhizophora mangle, red mangrove, were exposed to all possible combinations of three mangrove litter types and were isolated from all other nutrient inputs. Litter treatments significantly altered seedling growth. Seedlings from both mangrove species grew most rapidly in litter from a different species rather than their own, irrespective of litter chemical quality, decomposition rate, and nitrogen release. Litter mixtures from white and black mangroves caused black mangroves to grow 65% more than expected. Litter treatments did not impact seedling root:shoot ratios or tissue C:N. Our finding that seedlings grow best in litter from other species may indicate a mechanism that helps sustain the coexistence of dominant species.     

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.     

The Effects of Habitat on Dispersal Patterns of an Invasive Thistle, Cynara Cardunculus

Dispersal of offspring is a critical step in the spread of invasive species, yet dispersal patterns are seldom well studied, inhibiting effective management and ecological understanding of invasions. Dispersal patterns can be affected by characteristics of the parent plant and by climatic or site characteristics, including community vegetation structure, but these factors have not been studied in the context of plant invasion processes. Cynara cardunculus (L.) is a polycarpic perennial thistle invasive in coastal grasslands in California that produces large numbers of wind-dispersed seeds. This study quantified and compared C. cardunculus dispersal patterns in an exotic grassland (vegetated site) and an agricultural field (non-vegetated site). Seed size variation in C. cardunculus was also quantified within inflorescences, between inflorescences, and between years of production, and compared across dispersal distances. Results indicate that dispersal distance increased dramatically from less than 20 m in the vegetated site to more than 40 m in the non-vegetated site. Plants producing fewer seeds produced heavier seeds and dispersal distance decreased over time, but seed size was not related to dispersal distance, which may serve to spread the risk to seeds or seedlings across the environment. C. cardunculus has great potential for dispersal in open areas such as agricultural fields or disturbed sites, but may be limited in highly structured natural communities. Management of C. cardunculus and other wind-dispersed plant invasions may be improved by prioritizing populations with open or disturbed areas downwind and minimizing the removal of vegetation during dispersal.     

The influence of spatial heterogeneity on regeneration by seed in a limestone grassland

Abstract. The spatial distribution of seedlings in fertilized and non-fertilized grassland patches (1 m x 2 m) was examined in alvar vegetation on the island of Öland, Sweden. We established whether variation in seedling distribution in grassland patches reflects the distribution of conspecific adults. We also established the significance of microsite turf attributes for prediction of seedling distribution at a small scale (10 cm x 10 cm), and the significance of conspecific adults abundance, assemblage of adult species and cover of lichens, mosses, litter and bare ground. Our results indicate that fertilization reduces the importance of regeneration by seed of perennial species and enhances seedling recruitment of winter annuals and biennials. At the patch scale, enrichment does not affect species richness. Community patchiness contributes to the maintenance of diversity by providing a variety of environments with differential regeneration of the component species. Cover of lichens, mosses, litter and bare ground, accounts, in part, for the spatial variation in seedling numbers of certain species. The strong association of seedlings of some species with conspecific adults indicates that limitations in dispersal and/or the occurrence of a more suitable environment for emergence and/or establishment close to conspecific adult plants are likely to constrain the spatial distribution of new individuals in many cases. For other species, the lack of association of seedling densities with the cover of their conspecific adults, suggests that long-distance dispersal or a long-lived seed bank can play a role in counteracting the effect of short-distance dispersal.     

Reinvasion of a Riparian Forest Community by an Animal-dispersed Tree Weed Following Control Measures

Patterns of seed dispersal and the effects of mulching upon Celtis sinensis Pers. seedling establishment were investigated following the removal of this tree weed from a riparian forest community. At the commencement of the study there was virtually no representation of C. sinensis in the soil seed bank. However, subsequent rates of seed immigration were high since mature individuals of C. sinensis remained on the boundary of the site. Fruit bats (Pteropus spp.) were the principal dispersal vectors. Seed rain density of C. sinensis was best fitted by an inverse power distribution, with seed densities in excess of 20 m⁻² detected at 70 m from the seed source. Extrapolation from this relationship suggested that a site would have to be more than 350 m from a seed source to reduce the C. sinensis seed rain to less than 1 m⁻². More than 98% of the seed rain occurred below the canopies of the native tree species that remained following the removal of C. sinensis. For these trees, subarboreal C. sinensis seed distributions were not homogeneous, with peak seed densities occurring at different distances from tree trunks in each of the two years that seed distributions were assessed. Mulching with compacted sugar cane trash, corresponding to litter loadings of 6–12 kg m⁻², was imposed early in the study, some weeks before the C. sinensis seed rain commenced. These treatments had no measurable effect upon C. sinensis germination, but substantially reduced seedling survival and had variable effects upon the early growth of seedlings. The potential roles of seed limitation vs establishment limitation are discussed in relation to the management of animal-dispersed invasive species. It is argued that an understanding of the likely levels and patterns of invasion is essential for the formulation of management strategies that can effectively reduce the invasion and impacts of these plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mycorrhizal networks and distance from mature trees alter patterns of competition and facilitation in dry Douglas-fir forests

The distribution of dry Douglas-fir forests in western North America is expected to shift northward with climate change and disappear from the grassland interface in the southern interior of British Columbia. This shift may be accentuated by clearcutting, a common harvesting practice that aims to reduce the competitive effects of residual mature trees on new regeneration, but in so doing, ignores their facilitative effects. In this study, we investigated the net effects of competition from and facilitation by mature trees retained on harvested sites on seedling establishment in the dry interface Douglas-fir forests. We demonstrate that access to a mycorrhizal network (MN) and proximity to trees have important influences on seedling performance. On six sites, we established trenched plots around 24 mature Pseudotsuga menziesii var. glauca (Douglas-fir) trees, then planted Douglas-fir seedlings into four mesh treatments that served to restrict MN access (i.e., planted into mesh bags with 0.5-, 35-, or 250-μm pores, or without mesh) or into impermeable bags (grown in isolation) at four distances (0.5, 1.0, 2.5, or 5.0 m). Seedling survival tended to be greater and water stress lower where seedlings had full access to the MN. Seedling height, shoot biomass, needle biomass, and nutrient uptake peaked at 2.5–5.0 m from mature trees. Seedlings 0.5 m from mature trees had lower CO₂ assimilation rates and wood δ¹³C compared to seedlings 5.0 m away. Competition for soil resources was highest near mature trees but facilitation was relatively greater at further distances, resulting in a zone of net benefit for seedlings. These results show that intraspecific tree-seedling interactions are both competitive and facilitative in dry Douglas-fir forests, and that they are spatially dependent. After disturbance, maintaining residual mature trees may be important for their beneficial regeneration zones. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Combined effects of folivory and neighbor plants on <Emphasis Type="Italic">Cirsium altissimum</Emphasis> (tall thistle) rosette performance

Predicting how herbivory and neighbor plant interactions combine to affect host plants is critical to explaining variation in herbivores’ impact on plant population dynamics. In a field experiment, we asked whether the combined effects of neighbor plants and folivores upon performance of tall thistle (Cirsium altissimum), a monocarpic perennial, can be predicted as the product of their individual effects (i.e., effects of neighbor plants and folivores act independently in suppressing tall thistle performance). Alternately, the combined effects of neighbor plants and folivores might be greater, indicating a synergistic interaction, or less, indicating an antagonistic interaction, than the product of their individual effects. Our experiment involved a neighbor plant clipping treatment and a folivory reduction treatment in a factorial design with manipulations applied to naturally occurring tall thistle rosettes in restored tallgrass prairie. Clipping neighbors at the soil surface within 40 cm of tall thistle rosettes increased light availability to rosettes, rosette growth, and the transition rate of 2007 rosettes to reproductive adults in 2008. Folivores’ and neighbor plants’ effects acted independently upon rosette growth. By contrast, folivory reduced the rate at which 2007 rosettes transitioned to reproductive adults in 2008 only where neighbor plants were unclipped, indicating a possible synergistic interaction of neighbor plants and folivores in suppressing tall thistle performance. Our results suggest that (1) promoting neighbor plant aboveground biomass should suppress rosette-forming weeds, and (2) folivory, which reduces light acquisition by rosettes, may generate synergistic herbivory × neighbor plant interaction effects on rosettes in grasslands, where light often limits rosettes.     

Experimental Evaluation of Herbivory on Live Plant Seedlings by the Earthworm Lumbricus terrestris L. in the Presence and Absence of Soil Surface Litter

Background

Recent studies suggested that the earthworm Lumbricus terrestris might act as a seedling predator by ingesting emerging seedlings, and individuals were observed damaging fresh leaves of various plant species in the field. To evaluate the significance of herbivore behavior of L. terrestris for plant and earthworm performance we exposed 23- to 33-days-old seedlings of six plant species to earthworms in two microcosm experiments. Plants belonged to the three functional groups grasses, non-leguminous herbs, and legumes. Leaf damage, leaf mortality, the number of leaves as well as mortality and growth of seedlings were followed over a period of up to 26 days. In a subset of replicates 0.1 g of soil surface litter of each of the six plant species was provided and consumption was estimated regularly to determine potential feeding preferences of earthworms.

Results

There was no difference in seedling growth, the number of live seedlings and dead leaves between treatments with or without worms. Fresh leaves were damaged eight times during the experiment, most likely by L. terrestris, with two direct observations of earthworms tearing off leaf parts. Another nine leaves were partly pulled into earthworm burrows. Lumbricus terrestris preferred to consume legume litter over litter of the other plant functional groups. Earthworms that consumed litter lost less weight than individuals that were provided with soil and live plants only, indicating that live plants are not a suitable substitute for litter in earthworm nutrition.

Conclusion

Our results demonstrate that L. terrestris damages live plants; however, this behavior occurs only rarely. Pulling live plants into earthworm burrows might induce microbial decomposition of leaves to make them suitable for later consumption. Herbivory on plants beyond the initial seedling stage may only play a minor role in earthworm nutrition and has limited potential to influence plant growth.     

Old-field seedling responses to insecticide,seed addition,and competition

The controls of seedling emergence and survival determine the potential distribution of adult plants and, thereby, plant community structure. Seed availability, competition from established neighbors, and seedling predation may all limit seedling recruitment. In this field experiment, we followed the emergence and survival of seedlings of three perennial forbs, Achillea millefolium, Hypericum perforatum, and Monarda fistulosa, in old-fields in southeastern Michigan, USA. As adults, all three have aromatic foliage that may deter herbivory, but seedlings may be more susceptible than adults. To establish the relative importance of potential controls on seedling numbers, we manipulated seed availability through seed additions, the influence of competitors by neighbor-removals, and the influence of insect herbivores with insecticide in a fully factorial field experiment. Seed addition and insecticide never affected seedling emergence for any species. Competition from established neighbors controlled seedling emergence for all three species and decreased Achillea survival. Insecticide significantly increased Monarda seedling survival in competition plots, significantly increased Hypericum survival in open plots, and had no effect on Achillea. Notably, insecticide increased survival of the native Monarda fistulosa more than the two introduced species. While neighbors strongly reduced emergence and survival of all three species, herbivores acted on a species-specific basis. These results suggest the differential effects of insects may contribute more to the seedling species composition and abundance patterns than the less-selective influence of competition.     

Allelopathic Effects,Physiological Responses and Phenolic Compounds in Litter Extracts of Juniperus rigidaSieb. et Zucc.

The allelopathic effects of Juniperus rigida litter aqueous extract (LE) on wheat and Pinus tabuliformis were studied, as well as the physiological responses to the extract. High concentration LE (0.10 g Dw/ml) significantly inhibited the seed germination and seedling growth in receptor plants. The chlorophyll content and root activity in the wheat seedlings were reduced significantly across all treatments; however, those were more prominently reduced at high concentration (0.10 g Dw/ml) but received little stimulation at low concentration (0.025 g Dw/ml) in P. tabuliformis. The content of malonaldehyde (MDA) increased with increasing concentrations of LE, except at 0.025 g Dw/ml. Activities of antioxidant enzymes (POD, CAT and SOD) in receptor plants were all significantly inhibited at high concentrations but stimulated at low concentrations. These results demonstrate that the aqueous extract from J. rigida litter has allelopathic potential. Various phenolic compounds were identified in litter aqueous extract and litter ethanol extract by HPLC. The phenolic compound content in the aqueous extract was significantly lower than that in the ethanol extract. Chlorogenic acid and podophyllotoxin were the predominant phenolic compounds in both types of litter extracts. These findings suggest that the seed germination and seedling growth of P. tabuliformis and wheat would be inhibited when planted near large amounts J. rigida litter.     

Interactions after death: plant litter controls priority effects in a successional plant community

We performed a field experiment to test whether the presence of litter produced by the dominant species in the first successional year affects the plant community structure in the following year. We removed the litter of Setaria faberii (the first-year dominant) in midfall, early spring, mid-spring, or late spring. Both the fall and early spring removal increased the biomass of Erigeron annuus, which became dominant, and reduced the biomass of S. faberii. In the fall-removal treatment more plants of E. annuus flowered, while early spring removal increased the biomass of rosettes (non-flowering individuals) at the end of the growing season. In the other treatments and in the control S. faberii retained dominance, but its biomass was the highest in mid-spring removal plots. The removal of litter of S. faberii in the fall and in early spring allowed E. annuus to pre-empt the site and dominate the community. When litter was not removed, it strongly hindered the growth of E. annuus, favoring S. faberii. These results highlight the importance of litter as a historical factor linking interactions across successive generations, and controlling the community structure.     

Importance of molehill disturbances for invasion by Bunias orientalis in meadows and pastures

Small-scale soil disturbances by fossorial animals can change physical and biotic conditions in disturbed patches and influence spatial and temporal dynamics, and the composition of plant communities. They create regeneration niches and colonization openings for native plants and, according to the intermediate disturbance hypothesis, they are expected to increase plant community diversity. However, it also has been reported that increased disturbance resource availability and decreased competition with native species may result in the invasion of communities by alien plant species, as predicted by the fluctuating resources theory of invasibility. In this study, we investigated the importance of European mole disturbances for the invasion of semi-natural fresh meadows and pastures by the alien plant, Bunias orientalis, which has mainly spread throughout Central Europe on anthropogenically disturbed sites. We hypothesized that the invader, being particularly well adapted to anthropogenic disturbances, enters into dense vegetation of meadows and pastures mainly on mole mounds. To assess the seedling recruitment of B. orientalis in relation to disturbance, we counted the number of seedlings that emerged on molehills and control plots in meadows and pastures. The establishment of juvenile (0–1 year) rosette plants on and off molehills was surveyed on 5 × 5 m plots. In accordance with our hypothesis, mole disturbances were found to serve as a gateway for B. orientalis by which the invader may colonize semi-natural grasslands. The seedlings of the species emerged almost solely on molehills and the young rosettes were established predominantly on mole mounds. Although the seedling density did not differ significantly between the meadows and pastures, the number of established plants in the pastures was considerably higher. We suggest that the invasion by B. orientalis in pastures may be facilitated by vegetative regeneration following root fragmentation by sheep pasturing.     

Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition

Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100 m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabs m^–2) than in the inland zone (0.76±0.78 crabs m^–2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.     

Rosette Recruitment of a Rare Endemic Forb (Gaura neomexicana Subsp. coloradensis) with Canopy Removal of Associated Species

Gaura neomexicana subsp. coloradensis Munz (Onagraceae), (Colorado butterfly plant), a short‐lived perennial forb, became listed as a threatened species by the U.S. Fish and Wildlife Service in October 2000 because of its small global range and population size. This endemic subspecies consists of only 18 extant populations within southeastern Wyoming, southwestern Nebraska, and northeastern Colorado. Wyoming occurrences are on private land with the exception of three occurrences on F. E. Warren Air Force Base, near Cheyenne. Gaura neomexicana subsp. coloradensis may be displaced by many competitors, including Cirsium arvense (L.) Scop. (Canada thistle), an invasive, noxious weed. In June 1998, three G. neomexicana subsp. coloradensis populations were examined for rosette establishment in the presence of and after removal of associated species. The four study treatments removed: (1) Cirsium arvense, (2) grass and forb canopy and associated litter, (3) grass and forb canopy, litter, and C. arvense, and (4) no removal of associated plant species (control). Mature G. neomexicana subsp. coloradensis plants were evaluated for height, number of leaves, and capsule production. The mature plant characteristics monitored in 1998 were not good indicators of subsequent G. neomexicana subsp. coloradensis rosette densities in the following spring. Recruitment was assessed by counting new rosettes in the fall 1999. Although both G. neomexicana subsp. coloradensis and the exotic C. arvense have comparable habitat, removal of C. arvense did not increase G. neomexicana subsp. coloradensis vegetative growth, seed capsule production, or rosette density. Removal of other forbs, grass, and litter, however, increased G. neomexicana subsp. coloradensis rosette density in the following two years, indicating that canopy removal of associated species can enhance rosette establishment of this rare native. The accumulation of dense vegetative cover and litter associated with the absence of herbivory and fire may contribute to the decline of rare species in rich riparian habitats. Return of herbivory and fire in mesic sites to reduce standing biomass accumulations should be considered in restoring recruitment potential to rare monocarpic species.     

Effects of topographic position,leaf litter and seed size on seedling demography in a semi-deciduous tropical forest in Panamá

This study examined whether topography-induced gradients in water potential and leaf litter depth contribute to species coexistence in tropical forests through species-specific effects on seedling emergence and mortality. Seedling emergence and mortality were followed for a period of 12 months in 36 (1 × 2 m) plots on Barro Colorado Island (BCI), Panamá. Plots with and without litter were distributed on slope and plateau sites in three catchments. In the absence of manipulations, the lower litter depth on slopes resulted in approximately four times as many emergent seedlings than on plateau sites. However, litter depth had little effect on seedling community composition. By the end of the first dry-season, post-emergence, there were no significant differences in surviving seedling numbers between any treatments. There were differences in the emergent seedling community between slope and plateau sites within the same catchment as well as differences in composition between catchments, suggesting that both niche partitioning and dispersal limitation might play a role in structuring seedling community composition. During the wet-season seedling mortality was highest on slope sites although this pattern was reversed during the dry-season. In both seasons mortality was higher for small-seeded species. These results demonstrate that gradients in water potential related to topography impact on patterns of seedling emergence and mortality although processes in the first year after emergence may be insufficient to explain observed habitat preferences of adult plants.     

Effects of leaf litter on woody seedlings in xeric successional communities

We investigated the effect of leaf litter on the establishment of Eucalyptus incrassata, a mallee eucalypt. It has been suggested that litter accumulation may hinder seedling establishment, and that the removal of litter may be one of the mechanisms through which fire enhances recruitment. We conducted factorial experiments testing the effects of three kinds of leaf litter on E. incrassata seeds and seedlings at three contiguous sites with different land use histories. One site was an uncleared E. incrassata open mallee woodland (Mallee site), one a cleared area that had been ungrazed for about five years (Pasture site) and the third an area of mallee rolled some 40 years ago and permitted to regenerate (Regrowth site). Litter had no effect on emergence of planted E. incrassata seeds, but emergence differed between sites. Overall, the percentage of seeds that germinated and emerged was substantial (mean 35.2% ± 25.9%). Seedling shoot biomass did not differ between sites or litter treatments. Although seedlings grown in Pasture litter suffered higher mortality rates, overall mortality rates were low (mean 13.2% ± 15.5%), suggesting that leaf litter has little effect on recruitment rates during winter and spring. We conclude that leaf litter does not affect emergence or growth in young E. incrassata seedlings during winter and spring, when most establishment occurs. Our results emphasize the difficulty in predicting litter effects on recruitment.     

The relationship between bark peeling rate and the distribution and mortality of two epiphyte species

Tree bark characteristics influence epiphyte establishment and survival and consequently the way in which epiphytes are distributed on trees. Tree species with peeling bark have been reported as poor epiphyte hosts. We analyzed the distribution and seedling mortality of two Tillandsia species (Bromeliaceae) in relation to rate of bark peeling of Bursera fagaroides (Burseraceae). The highest peeling rate (0.12% per day) took place on the trunk and the lowest rate on twigs (0.04% per day; branches ≤2 cm in diameter). The highest proportion of Tillandsia plants appeared on twigs. The distributions of juvenile and adult plants on twigs were higher than those expected based on the distribution of first-year seedlings, suggesting that on twigs, survival could be greater than on trunks and branches, canopy areas where peeling is faster. On the trunk and branches, in contrast, the proportion of juveniles and adults were similar to or less than that expected for first-year seedlings. The main cause of mortality was peeling and the area of minor overall mortality was the trunk, suggesting that this area should be favored as the main distribution area for the Tillandsia species but is not. Our results show that the peeling rate of B. fagaroides depends on branch size and suggest that the Tillandsia distribution depends not only on peeling rate but also on seed dispersion. We suggest that to colonize B. fagaroides epiphytes would either have adaptations to counteract the peeling rate or should occur in the areas of lowest peeling rate located in the exterior crown of trees.     

Germination and population structure of spear thistle Cirsium vulgare in relation to experimentally controlled sheep grazing

Summary The size and number of rosettes of Cirsium vulgare were censused in a 4 ha sheep grazing trial on lowland calcareous grassland in paddocks receiving controlled winter, spring and summer grazing treatments. Spring grazing significantly increased thistle rosette numbers, and there were always fewer rosettes in size classes 250mm–300 mm under the heavier grazing treatments. The emergence of seeds of Cirsium vulgare sown by hand into each of the grazing treatments was monitored and showed a positive effect of spring grazing. When the percent emergence of sown seeds was used as a covariate in the analysis of rosette numbers in the experimental paddocks, it was found to account for 77% of the variance in thistle numbers. It was concluded that the main effect of grazing on thistle rosette numbers is an indirect one exercised through the effects of grazing on germination conditions. Seeds of Cirsium vulgare were sown into artificial gaps at another grassland site, to determine the effect of gap-size upon seedling emergence. Germination was poor, but significantly more seedlings emerged in gaps 10–20 cm diameter, than in 5 cm gaps or in control plots without a gap. A computer simulation model was used to explore the relationship between gap density and thistle population dynamics. A threshold density of gaps was found to exist, below which thistle populations went extinct, and above which the thistle population grew geometrically. The degree of aggregation of dispersed seeds did not alter the threshold gap-density for plant extinction, but did affect the rate of increase of the thistle population when the threshold gap-density was exceeded. It is concluded that strategies for controlling Cirsium vulgare populations by grazing manangement will be most effective if aimed at reducing suitable sites for establishment in spring. The success of such attempts will depend upon the soil fertility of the site, and sudden outbreaks of Cirsium vulgare infestation can be expected if a sward gradually deteriorates through over-grazing.