Recruitment Of Other Species Research Articles

Ecosystem engineering kelp limits recruitment of mussels and microphytobenthic algae

Ecosystem engineers often exert strong effects on the recruitment of other species through modification of the local abiotic and biotic environment. In 2015, artificial reefs in eastern Tasmania (− 42.64693, 148.01481) spanning seven different patch sizes (0.12–7.68 m2) and supporting four densities of transplanted kelp (Ecklonia radiata at 0, 4.1, 8.2 and 16.4 kelp m−2) were used to determine how the patch size and density of this ecosystem engineer influenced the recruitment of microphytobenthic (MPB) algae, and a secondary ecosystem engineer, the mussel Mytilus galloprovincialis. Increasing kelp density and patch size inhibited the establishment of subcanopy MPB algae on settlement slides and reduced the recruitment of mussels in standardised rope fibre habitats (RFHs). The productivity:biomass ratio (P:B) of mussel recruits tended to be lower on small reefs and reefs without kelp, relative to larger reefs with high densities of kelp. Canopy shading and reduced cover of turf algae appeared to negatively impact the recruitment of MPB algae and mussels, whilst reduced sediment accumulation on the reefs due to the kelp was also negatively associated with mussel recruitment. These findings highlight the role of ecosystem engineering by kelp in inhibiting the establishment of other species which may additionally impact community dynamics and primary and secondary productivity. The limited capacity of small kelp patches to inhibit the recruitment of other organisms supports the notion that fragmented patches of ecosystem engineers could be more suspectable to adverse outcomes from species interactions making them less resistant to shifts towards an alternative ecosystem state.

Thinning and plantation of resprouting species redirect overstocked pine stands towards more functional communities in the Mediterranean basin

Post-fire regeneration in Pinus halepensis' forests, one of the most abundant vegetation types in the Mediterranean basin, often generates overstocked and vulnerable stands. They accumulate a high fuel load, increasing the risk of further fires, and present high levels of vulnerability due to their reduced seed production. In addition, these dense stands substantially reduce the availability of light and nutrients, which may hinder the recruitment of other species, often generating mono-specific and homogeneous stands, which potentially supply fewer ecosystem services than mixed forests with more heterogeneous structures. In these dense pine stands, management is of high priority to reduce fire hazards and promote their functionality. In overstocked pine stands (>75,000 trees·ha−1), we assessed the long-term effects (10 years) of two thinning levels (600 and 1200 trees·ha−1), in combination with the plantation of Quercus faginea (a resprouter species typical of advanced successional stages in our study area) on 28 above and belowground ecosystem attributes, including fire hazard. After ten years, thinning and plantation interacted to enhance ecosystem attributes associated with disturbance regulation and biodiversity conservation (up to 200%) and food production (up to 90%), while no effects were observed on those attributes related to carbon sequestration and supporting services. These effects were mainly driven by aboveground attributes, as they responded more strongly to our treatments than those belowground. Our results are relevant for the restoration of Mediterranean degraded ecosystems, and show that tree thinning in overstocked pine stands, combined with the plantation of resprouter species, may not only reduce fire risks and accelerate post-fire succession but also enhance the supply of multiple ecosystem services in the long run.

Plant–soil feedbacks and the introduction of Castanea (chestnut) hybrids to eastern North American forests

The reintroduction of disease‐resistant hybrids is a commonly proposed solution to the introduction of pathogens and pests that weaken or eliminate native plant species. Plant interactions with soil biota result in plant–soil feedbacks (PSFs), which have consequences for individual plant growth and survival as well as broader community‐level processes, such as species diversity and coexistence. Because of their importance, species reintroduction should consider these interactions, yet little work has integrated this perspective. Here, we investigate the effects of hybrid Castanea (chestnut) reintroduction on PSFs and how these mechanisms may influence the recruitment of other species in contemporary forests. We also examine how blight‐resistant Castanea hybrids perform in the soil conditions of contemporary forests and we compare their belowground interactions with those of Castanea dentata. We conducted a reciprocal greenhouse experiment testing the effect of species‐specific soil inoculum on the growth and survival of C. dentata, Castanea hybrids, and other forest dominants. Our results suggest that C. dentata and hybrids had similar belowground interactions and were regulated by negative PSFs, meaning soil microbial communities reduced conspecific growth and survival. These negative PSFs may involve the presence of the non‐native pathogen Phytophthera cinnamomi. Soil inoculum of C. dentata and Castanea hybrids had similar effects on heterospecific growth, suggesting Castanea restoration will have neutral effects on natural regeneration in restoration plantings. We conclude that Castanea hybrids may fill a similar belowground niche to their parent species, and that site selection, screening for soil pathogens, and site planting density will be important to restoration.

Plant functional traits involved in the assembly of canopy–recruit interactions

AbstractQuestionsThe assembly of plant communities depends strongly on mechanisms that determine the recruitment of different plant species. Studying recruitment using a trait‐based approach may help in the search for general or dominant mechanisms involved in this process. Here, we try to disentangle what traits of saplings and established (canopy) plants can be considered as functional for recruitment, and whether the complementarity of these functional traits may be a driver of the plant community assembly.LocationRegional. Two pine–oak forest communities in Sierra Sur de Jaén and one in Sierra de Segura, southeast of the Iberian peninsula.MethodsIn each forest community, we established three 50 m × 50 m plots, where we identified the saplings of each species recruiting under the canopy of each species or in open interspaces. The study focuses on 37 woody species. Measures of 18 traits were conducted for 30 saplings and 10 adults of each species.ResultsSeed mass and the ratio height:basal diameter of saplings were positively associated with species abundance in the sapling bank under vegetation, while abundance in the sapling bank in open interspaces was only related to the type of mycorrhizal association. On the other hand, some traits of adult plants (leaf mass per unit area [LMA], seed dispersal mechanism, leaf habit and branch density) favour the recruitment of other species in their close proximity. However, we found only circumstantial evidence of canopy/recruit trait complementarity affecting recruitment.ConclusionsOur results suggest that the assembly of canopy–recruit interactions is primarily driven by the filtering effect of canopy species on recruitment, through traits that affect both seed arrival and soil properties. Species abundance in the sapling bank was related to resource acquisition traits. We did not find any clear evidence supporting that trait complementarity is involved in the interaction between canopy and recruit species.

Integrated Pest Management for Fouling Organisms on Boat Hulls

AbstractBoating is a major vector for aquatic invasive species that cause significant economic and ecological impacts, necessitating biofouling control that goes beyond simply maintaining boat operations. However, new regulations restricting the use of antifouling paints—a common control tactic along with hull cleaning—have not considered the consequences to invasive species management. As a result, there is a critical need for a biofouling control strategy that both protects water quality and minimizes invasive species transport. We compared recruitment of fouling organisms to experimental plates (1) treated with hull coatings after 1 month and, for copper‐based paint, after 1‐, 3‐, 6‐, and 12‐month submersion times; (2) after application of California's in‐water hull cleaning practices; and (3) among locations within and between geographically separated harbors. Copper‐based paint was initially effective at reducing fouling but lost effectiveness over time and was fouled heavily within 12 months. On plates with copper‐based paint, nonnative species typically recruited first and facilitated the recruitment of other species. Nontoxic coatings were readily fouled, and invasive species (Watersipora subatra and Hydroides spp.) settled more often on ceramic epoxy and/or siliconized epoxy (“slick”) coatings. Recruitment was higher in the harbor in the warmer water region. Depending on the harbor, W. subatra, Ciona spp., and Filograna implexa recruitment was correlated with water flow, the presence of conspecifics on the docks, or both factors. Strong seasonal recruitment was evident for Ciona spp., F. implexa, and Bugula neritina. Algae dominated the light‐exposed surfaces of plates, and invertebrates dominated the shaded surfaces. California's hull cleaning practices did not stimulate fouling, which contradicted previous findings. Our findings informed the development of an integrated pest management framework for biofouling control on boat hulls that is adaptable to different regions and boater needs. This novel approach balances effective boat operations and protection of ecosystem health while simultaneously addressing water quality and invasive species transport.

The potential exceptional role of a small Baltic boulder reef as a solitary habitat in a sea of mud

Abstract In this study, non‐destructive seafloor imaging techniques were employed to assess the benthic community structure of a recently discovered boulder reef within the central Arkona Basin of the Baltic Sea. Data indicate that geographical isolation, water column stratification and temporary oxygen deficiency create an exceptional reef habitat in a sea of mud. The reef forms a saline refuge for the deepest and easternmost known population of the plumose sea anemones (Metridium dianthus Ellis, 1768) in the Baltic Sea. Limited connectivity to other reefs and dominance of the aggressively colonizing and planktivorous M. dianthus hamper the recruitment of other species and lead to an overall low invertebrate richness. Large boulders attract ecologically and economically important fish like Baltic cod (Gadus morhua Linnaeus, 1758) that find shelter in the otherwise heavily fished Arkona Basin. Despite this very first exploration being only a snapshot of the observed community, findings imply a potential conservation importance of this Germany's deepest known natural habitat type 1170 (‘reefs’).

A dominance shift in arid savanna: An herbaceous legume outcompetes local C4 grasses.

The characteristic vegetation structure of arid savannas with a dominant layer of perennial grass is maintained by the putative competitive superiority of the C4 grasses. When this competitive balance is disturbed by weakening the grasses or favoring the recruitment of other species, trees, shrubs, single grass, or forb species can increase and initiate sudden dominance shifts. Such shifts involving woody species, often termed “shrub encroachment”, or the mass spreading of so‐called increaser species have been extensively researched, but studies on similar processes without obvious preceding disturbance are rare. In Namibia, the native herbaceous legume Crotalaria podocarpa has recently encroached parts of the escarpment region, seriously affecting the productivity of local fodder grasses. Here, we studied the interaction between seedlings of the legume and the dominant local fodder grass (Stipagrostis ciliata). We used a pot experiment to test seedling survival and to investigate the growth of Crotalaria in competition with Stipagrostis. Additional field observations were conducted to quantify the interactive effect. We found germination and growth of the legume seedlings to be facilitated by inactive (dead or dormant) grass tussocks and unhindered by active ones. Seedling survival was three times higher in inactive tussocks and Crotalaria grew taller. In the field, high densities of the legume had a clear negative effect on productivity of the grass. The C4 grass was unable to limit the recruitment and spread of the legume, and Crotalaria did outcompete the putative more competitive grass. Hence, the legume is able to spread and establish itself in large numbers and initiate a dominance shift in savannas, similar to shrub encroachment.

Grupos funcionais em áreas com histórico de queimadas em Cerrado sentido restrito no Distrito Federal

O estudo teve como objetivo avaliar a formação de grupos funcionais em duas áreas de Cerrado sentido restrito com diferentes históricos de fogo, em Planaltina, DF, em 1994 e em 2012. Na Área 1, foram aplicadas queimadas bienais, em agosto de 1988, 1990 e 1992; na Área 2, houve proteção do fogo até julho de 1994. Em agosto de 1994 um fogo acidental atingiu as duas áreas, mas de setembro de 1994 até 2012 estas áreas foram protegidas. Nas análises, foram incluídos os indivíduos com abundância ≥ 5 utilizando-se o método de agrupamento de ligação média não ponderada (UPGMA). Os grupos foram definidos de acordo com os atributos: síndromes de dispersão e de polinização, fenologia foliar, forma de vida, tipo de casca e capacidade de rebrotar após queimadas. Houve diferenças na formação dos grupos funcionais nas duas áreas e nas duas épocas. A maior frequência de queimadas bienais, na Área 1 em 1994, reuniu espécies funcionalmente semelhantes, com menor diversidade de atributos. A exclusão do fogo favoreceu a entrada de espécies e a maior diversidade de atributos funcionais em 2012.

Recruitment Dynamics of the Relict Palm, Jubaea chilensis: Intricate and Pervasive Effects of Invasive Herbivores and Nurse Shrubs in Central Chile.

Shrubs can have a net positive effect on the recruitment of other species, especially relict species in dry-stressful conditions. We tested the effects of nurse shrubs and herbivory defoliation on performance (survival and growth) of nursery-grown seedlings of the largest living palm, the relict wine palm Jubaea chilensis. During an 18-month period, a total of more than 300 seedlings were exposed to of four possible scenarios produced by independently weakening the effects of nurse shrubs and browsers. The experiment followed a two-way fully factorial design. We found consistent differences in survival between protected and unprotected seedlings (27.5% and 0.7%, respectively), and herbivory had a dramatic and overwhelmingly negative effect on seedling survival. The invasive rabbit (Oryctolagus cuniculus) is clearly creating a critical bottleneck in the regeneration process and might, therefore, partially explain the general lack of natural regeneration of wine palms under natural conditions. Apparently biotic filters mediated by ecological interactions are more relevant in the early stages of recruitment than abiotic, at least in invaded sites of central Chile. Our data reveal that plant-plant facilitation relationship may be modulated by plant-animal interactions, specifically by herbivory, a common and widespread ecological interaction in arid and semi-arid environments whose role has been frequently neglected. Treatments that protect young wine palm seedlings are mandatory to enable the seedlings to attain a height at which shoots are no longer vulnerable to browsing. Such protection is an essential first step toward the conservation and reintroduction of this emblematic and threatened species.

The effects of hemiparasitic plant removal on community structure and seedling establishment in semi‐natural grasslands

AbstractQuestionHemiparasitic plants can profoundly affect the structure of the community in which they occur, mainly due to parasitic suppression of hosts. As a consequence, non‐host species have the opportunity to colonize resulting gaps. In contrast to most grassland species, hemiparasites are generally short‐lived and can reach high densities; as a consequence, vegetation gaps are left after their death. These gaps form microsites more suitable for seed germination and therefore might increase recruitment of other species. Which species suffer or profit from parasitism and is there a positive effect on seedling establishment?LocationSemi‐natural grasslands in northern Belgium.MethodsWe selected two hemiparasitic plant species from contrasting vegetation types: Rhinanthus angustifolius growing in mesotrophic grassland and Pedicularis sylvatica growing in oligotrophic heath‐grassland. A weeding experiment was set up at six sites in which the hemiparasite was repeatedly removed in half of the plots during three growing seasons. The abundance change of individual species was compared between weeded and control plots. After the second growing season, seeds of up to ten species were added. The number of seedlings in the third year was then compared between weeded and control plots.ResultsRhinanthus removal significantly affected the abundance of species relative to control plots, both positively and negatively, and decreased the species evenness. Pedicularis removal only increased the abundance of some species. Only Juncaceae (but no other graminoid families) increased after Rhinanthus and Pedicularis weeding, and there was considerable variation within functional groups. Moreover, based on our observations, we propose as a new hypothesis that species with persistent clonal spread are more vulnerable to parasitism. Once attached, resources are potentially drawn from a whole network of interconnected ramets. Finally, only half of the sown species successfully established seedlings; hemiparasite removal had a significantly negative effect on seedling number for two of these species.ConclusionsEffects of hemiparasites on species differ considerably, also within functional groups. Persistent clonal spread emerges as an important plant trait determining vulnerability to hemiparasites. Finally, our results suggest that hemiparasitic plants might have a limited positive effect on seedling establishment in these semi‐natural grasslands where chances for successful establishment were shown to be low.

Vegetation dynamics across a chronosequence of created wetland sites in Virginia, USA

Plant species composition, dominance, richness, and diversity were measured across a 15-year chronosequence of created wetland sites in Virginia, USA. Using an age-class categorization (1–2 years, 3–5 years, 6–10 years, or 11–15 years), all classes had a predominance of herbaceous species with perennial life history strategy, and perennials contributed 68.6 % to the overall dominance measure (importance value; IV) averaged across all sites. There was no significant difference in species richness or diversity among age classes. Analysis of Similarity (ANOSIM) indicated that herbaceous species composition was similar between the youngest and oldest age classes, but not the intermediate classes. For woody shrubs and saplings, planted species were more prevalent in the youngest age classes, and volunteer species predominated in the oldest age classes. These results suggest that perennial herbaceous species are important in early plant development on created wetland sites, and may be influential in observed patterns of species composition over time. In the context of plant development in newly created wetlands, dominance shifts from planted to volunteer woody species suggest that planting early successional species, or species with reproductive strategies attuned to created wetland site management, may favor survivorship and recruitment of other species over time.

Assessing occurrence, specificity, and mechanisms of plant facilitation in terrestrial ecosystems

Plants alter environmental conditions enhancing the recruitment of other species. In spite of prior reviews of facilitation, the variability of its occurrence, mechanisms, and specificity across terrestrial ecosystems has not yet been assessed. In this article, we analyze facilitative mechanisms and the distribution of specific traits, such as nitrogen fixation and the presence of fleshy fruits, across ecosystems. A comprehensive database including 2,080 cases of facilitation among higher plants from 539 articles was analyzed with descriptive statistics for occurrences of positive interactions and underlying mechanisms in different terrestrial ecosystems. Positive interactions by plant-induced environmental changes are widespread in a range of ecosystems and not limited to conditions of chronic abiotic stress such as semiarid, alpine, and wetland ecosystems. The capability to act as nurse largely varied among different growth forms, and was observed more frequently for woody than for herbaceous plants. Nitrogen fixers occur much more frequently as nurse plants than as beneficiary plants in facilitation cases due to increasing soil fertility. As known for Mediterranean ecosystems, fleshy-fruited species appear more dependent on facilitative interactions than other plants, being more frequent among beneficiaries than among nurses. The pattern can be extended worldwide being consistent in wetland, temperate, and alpine ecosystems as well. Our description of the relationship between distribution, mechanisms, and specificity of facilitation in terrestrial ecosystems has implications for the understanding of plant community organization considering that plant nursing capacity is affected by their size, architecture, and life span.

The oral metagenome in health and disease

The oral cavity of humans is inhabited by hundreds of bacterial species and some of them have a key role in the development of oral diseases, mainly dental caries and periodontitis. We describe for the first time the metagenome of the human oral cavity under health and diseased conditions, with a focus on supragingival dental plaque and cavities. Direct pyrosequencing of eight samples with different oral-health status produced 1 Gbp of sequence without the biases imposed by PCR or cloning. These data show that cavities are not dominated by Streptococcus mutans (the species originally identified as the ethiological agent of dental caries) but are in fact a complex community formed by tens of bacterial species, in agreement with the view that caries is a polymicrobial disease. The analysis of the reads indicated that the oral cavity is functionally a different environment from the gut, with many functional categories enriched in one of the two environments and depleted in the other. Individuals who had never suffered from dental caries showed an over-representation of several functional categories, like genes for antimicrobial peptides and quorum sensing. In addition, they did not have mutans streptococci but displayed high recruitment of other species. Several isolates belonging to these dominant bacteria in healthy individuals were cultured and shown to inhibit the growth of cariogenic bacteria, suggesting the use of these commensal bacterial strains as probiotics to promote oral health and prevent dental caries.

Population changes of early successional forest species after shifting cultivation in Northwestern Vietnam

The population changes of early successional forest species, Wendlandia paniculata, Schima wallichii, Camellia tsaii, and Lithocarpus ducampii, were described and analyzed in a chronosequence of 1–30 year-old secondary forests representing regrowth after shifting cultivation in northwestern Vietnam, utilizing 51 temporary plots for stem census. Another five temporary plots were used for stem census in surrounding old-growth forest for comparison. In the first year after land abandonment, seedling stem density (H < 2 m) was 65,800/ha, increasing to 161,200/ha by third year, then sharply decreasing to 2,500/ha in 21–30 year-old forests. The sapling stratum (H ≥ 2 m and DBH < 5 cm) started to be recruited in the fifth year, increasing to reach a peak density of 4,530 stems/ha at year ten, then decreased to 580 stems/ha at year 21–30. The tree stratum (DBH ≥ 5 cm) achieved the maximum density of 600 stems/ha at year ten. Meanwhile, the density of all strata in the old-growth forest was 2,980 stems/ha. Sprouts played an important role, accounting for 34% at the first year then increased gradually to 73% at year 21–30. Total basal area attained a peak at 5.43 m2/ha at year ten; it was 4.9 m2/ha in the old-growth forest. These four species played an important role in providing ecological services for recruitment of other species during the first 10 years of fallow stand development, which increased to 35 species in 21–30 year-old secondary forest.

The role of colonial ascidians in altering biodiversity in marine fouling communities

A suite of invasive species, each with their own unique morphologies, have recently becomemembers of Gulf of Maine, ME USA subtidal communities (Harris and Tyrrell, 2001; Pederson et al., 2005). Three of these species, Botrylloides violaceus, Diplosoma listerianum and Didemnum sp., are colonial ascidians and have become major space occupants in New England fouling communities (Berman et al., 1992; Osman and Whitlatch, 1995, 1999; Whitlatch et al., 1995; Bullard et al., 2007-this issue). They are warm water, two-dimensional species that dominate during seasons of greatest fouling species recruitment (Stachowicz et al., 2002). Therefore, the presence of these species in the Gulf of Maine may significantly influence the recruitment of other species to fouling communities. The goal of this study was to examine the role of the three colonial ascidians in the development of fouling communities by comparing the recruitment of species onto panels containing organisms with different functional morphologies. We conducted field studies at two sites with similar species composition in Portsmouth Harbor, Newcastle, New Hampshire, USA. The communities occupying the floating docks at these sites included blue mussels (Mytilus edulis), the sea anemone Mitridium senile, encrusting

APPLYING PLANT FACILITATION TO FOREST RESTORATION: A META‐ANALYSIS OF THE USE OF SHRUBS AS NURSE PLANTS

After a millenarian history of overexploitation, most forests in the Mediterranean Basin have disappeared, leaving many degraded landscapes that have been recolonized by early successional shrub‐dominated communities. Common reforestation techniques treat these shrubs as competitors against newly planted tree seedlings; thus shrubs are cleared before tree plantation. However, empirical studies and theory governing plant– plant interactions suggest that, in stress‐prone Mediterranean environments, shrubs can have a net positive effect on recruitment of other species. Between 1997 and 2001, we carried out experimental reforestations in the Sierra Nevada Protected Area (southeast Spain) with the aim of comparing the survival and growth of seedlings planted in open areas (the current reforestation technique) with seedlings planted under the canopy of preexisting shrub species. Over 18 000 seedlings of 11 woody species were planted under 16 different nurse shrubs throughout a broad geographical area. We sought to explore variation in the sign and magnitude of interactions along spatial gradients defined by altitude and aspect. In the present work, we report the results of a meta‐analysis conducted with seedling survival and growth data for the first summer following planting, the most critical period for reforestation success in Mediterranean areas. The facilitative effect was consistent in all environmental situations explored (grand mean effect sized+= 0.89 for survival and 0.27 for growth). However, there were differences in the magnitude of the interaction, depending on the seedling species planted as well as the nurse shrub species involved. Additionally, nurse shrubs had a stronger facilitative effect on seedling survival and growth at low altitudes and sunny, drier slopes than at high altitudes or shady, wetter slopes. Facilitation in the dry years proved higher than in the one wet year. Our results show that pioneer shrubs facilitate the establishment of woody, late‐successional Mediterranean species and thus can positively affect reforestation success in many different ecological settings.

Nurse effect of Bolax gummifera cushion plants in the alpine vegetation of the Chilean Patagonian Andes

Abstract. It has been proposed that in the harsh arctic and alpine climate zones, small microtopographic variations that can generate more benign conditions than in the surrounding environment could be perceived as safe sites for seedling recruitment. Cushion plants can modify wind pattern, temperature and water availability. Such modifications imply that cushion plants could act as ‘nurse plants’ facilitating the recruitment of other species in the community. This effect should be more evident under stressful conditions. We tested these hypotheses comparing the number of species that grow inside and outside Bolax gummifera cushions at two elevations (700 and 900 m a.s.l.) in the Patagonian Andes of Chile (50°S). At both elevations, and in equivalent areas, the number of species was registered within and outside cushions. A total of 36 and 27 plant species were recorded either within or outside B. gummifera cushions at 700 and 900 m a.s.l., respectively. At 700 m a.s.l., 33 species were recorded growing within cushions and 29 outside them, while at 900 m a.s.l. these numbers were 24 and 13 respectively. At both elevations there were significantly more species growing within than outside cushions, and the proportion of species growing within cushions increased with elevation. Thus there is a nurse effect of cushion plants and it is more evident at higher elevations. Shelter from wind and increased soil water availability seem to be the factors that increase plant recruitment within cushions.

Nurse effect of Bolax gummifera cushion plants in the alpine vegetation of the Chilean Patagonian Andes

It has been proposed that in the harsh arctic and alpine climate zones, small microtopographic variations that can generate more benign conditions than in the surrounding environment could be perceived as safe sites for seedling recruitment. Cushion plants can modify wind pattern, tem- perature and water availability. Such modifications imply that cushion plants could act as 'nurse plants' facilitating the recruitment of other species in the community. This effect should be more evident under stressful conditions. We tested these hypotheses comparing the number of species that grow inside and outside Bolax gummifera cushions at two eleva- tions (700 and 900 m a.s.l.) in the Patagonian Andes of Chile (50 ∞S). At both elevations, and in equivalent areas, the number of species was registered within and outside cushions. A total of 36 and 27 plant species were recorded either within or outside B. gummifera cushions at 700 and 900 m a.s.l., respec- tively. At 700 m a.s.l., 33 species were recorded growing within cushions and 29 outside them, while at 900 m a.s.l. these numbers were 24 and 13 respectively. At both elevations there were significantly more species growing within than outside cushions, and the proportion of species growing within cushions increased with elevation. Thus there is a nurse effect of cushion plants and it is more evident at higher elevations. Shelter from wind and increased soil water availability seem to be the factors that increase plant recruitment within cushions.

Soil properties and seedling establishment in soils from monodominant and high‐diversity stands of the tropical deciduous forests of México

Abstract. The hypothesis that soils from mature monodominant forests are unfavourable for establishment of tree species other than the dominant was tested for a lowland tropical deciduous forest with stands dominated by Celaenodendron mexicanum Standl. (CS). This species of Euphorbiaceae occurs almost exclusively in monodominant stands in which recruitment of other species appears to be poor. Soil properties were examined and experiments were conducted on germination and establishment of Celaenodendron mexicanum and three other species common in adjacent high‐diversity mixed stands (MS): Recchia mexicana Moc. &amp; Sessé, Caesalpinia eriostachys Benth., and Cordia alliodora (Ruiz &amp; Pav.) Oken. Soil moisture regimes are affected by topographic position in the study area, but Celaenodendron mexicanum was found occupying hillsides as well as flatlands, and slope gradients of its stands were typical of the region. The microsites occupied in relation to soil moisture and light availability also appear to be undistinctive. There were no significant differences between stand types in any soil property (pH, O.C., total N, total P, avail. P, K, Ca, Mg, Na, Mn, Fe, Cu, Zn concentrations). A stand of intermediate dominance of Celaenodendron (ICS) did not differ from its adjacent mixed stand (MS) in soil texture or most of the nutrients tested; however, seasonally restricted differences in the contents of Fe and Mn (ICS&lt;MS) were detected. In fact, the contents of most nutrients in all the stands changed significantly between seasons: mostly, they were lower in the wet season.Germination percentages and seedling survival to 4 months did not differ between CS and MS soils in greenhouse tests, and in field tests Caesalpinia eriostachys established in both stand types.The results suggest that the distinction between monodominance and high diversity is not maintained by some simple physiological restriction on the establishment of new seedlings.

Trophic Structure and Community Composition of Polychaetes Inhabiting Some Sabellaria alveolata (L.) Reefs Along the Valencia Gulf Coast, Western Mediterranean

Abstract. A total of 24 samples from Sabellaria alveolata reefs have been collected and subjected to cluster analysis using the trophic dominance of each feeding guild as grouping variables. The result is the arrangement of samples by colony physical structure and S. alveolata dominance.Relating cluster groups to reef development phases reveals an inverse relationship between reef growth and the organization of the associated polychaete taxocoenosis. During the growth phase of the reef, S. alveolata dominates the community, excluding other taxa from colonization. Growth is followed by the destruction phase. Negative physical and biological factors for reef enlargement may open patches on the reef surface, allowing recruitment of other species and colony erosion. The end of this phase is represented by eroded reefs with high structural complexity and high species richness.Summary Sabellaria alveolata reefs reported from 13 localities in the study area have been classified according to their structural features into two main types: a) developed reefs, with considerable volume and a shape independent of the underlying substrate and b) eroded reefs, which form tiny encrusting plates covering the substrate. Based on cluster analysis each one of these may be subdivided according to SAQD values.