Establishment, competition and the distribution of native grasses among Michigan old-fields

1 In this study the potential role of competition in influencing the distribution of three displaced native perennial grasses across complex gradients of plant productivity and species composition was investigated in Michigan old-fields. To do this plant removal and propagule addition experiments were conducted at nine old-field sites to examine the effects of living plant neighbours and litter on seedling establishment and growth of target species in relation to community biomass.
2 For two target species, Andropogon gerardi and Schizachyrium scoparium , living plant neighbours suppressed establishment from seed at most sites, and suppressed the growth of transplants at all sites.
3 Plant litter strongly inhibited the seedling establishment of both Andropogon and Schizachyrium at sites of high community biomass and litter accumulation, but had little impact on the growth rate of transplants at any of the sites.
4 The total suppressive effect of the plant community on seedling establishment and transplant growth of both Andropogon and Schizachyrium increased in magnitude in a non-linear fashion with community biomass. These effects increased in magnitude more rapidly across sites of low to medium biomass than sites of medium to high biomass.
5 The results suggest that these native grasses may be restricted to low productivity habitats within this landscape because of strong competitive interference with establishment by the existing vegetation in the most productive sites.     

Competition between tree seedlings and herbaceous vegetation: support for a theory of resource supply and demand

1 We measured competition intensity (CI) between herbaceous vegetation and tree seedlings ( Quercus macrocarpa and Q. ellipsoidalis ) along an experimental moisture–light gradient. Contrasting theories were tested by comparing variation in competition intensity to changes in neighbour biomass and resource supply and demand.
2 CI based on survival was inversely correlated with net soil water supply (gross supply minus demand by herbaceous vegetation). CI was not positively correlated with either gross resource supply or neighbour biomass, contrary to predictions of Grime's triangular model for plant strategies.
3 Many of the inconsistencies and conflicting results that have characterized the recent literature on plant competition could be eliminated if changes in competition intensity along a resource gradient are compared with changes in net resource supply rather than changes in productivity or neighbour biomass.
4 Tree seedling success in savannas and grasslands may be strongly influenced by the intensity of competition from herbaceous vegetation. Factors that reduce soil water content are likely to increase competition intensity (and reduce seedling success) in these environments, while factors that increase soil water content will favour seedling success through decreased competition for water with herbaceous vegetation.     

Are variations of direct and indirect plant interactions along a climatic gradient dependent on species’ strategies? An experiment on tree seedlings

Investigating how interactions among plants depend on environmental conditions is key to understand and predict plant communities’ response to climate change. However, while many studies have shown how direct interactions change along climatic gradients, indirect interactions have received far less attention. In this study, we aim at contributing to a more complete understanding of how biotic interactions are modulated by climatic conditions. We investigated both direct and indirect effects of adult tree canopy and ground vegetation on seedling growth and survival in five tree species in the French Alps. To explore the effect of environmental conditions, the experiment was carried out at 10 sites along a climatic gradient closely related to temperature. While seedling growth was little affected by direct and indirect interactions, seedling survival showed significant patterns across multiple species. Ground vegetation had a strong direct competitive effect on seedling survival under warmer conditions. This effect decreased or shifted to facilitation at lower temperatures. While the confidence intervals were wider for the effect of adult canopy, it displayed the same pattern. The monitoring of micro‐environmental conditions revealed that competition by ground vegetation in warmer sites could be related to reduced water availability; and weak facilitation by adult canopy in colder sites to protection against frost. For a cold‐intolerant and shade‐tolerant species (Fagus sylvatica), adult canopy indirectly facilitated seedling survival by suppressing ground vegetation at high temperature sites. The other more cold tolerant species did not show this indirect effect (Pinus uncinata, Larix decidua and Abies alba). Our results support the widely observed pattern of stronger direct competition in more productive climates. However, for shade tolerant species, the effect of direct competition may be buffered by tree canopies reducing the competition of ground vegetation, resulting in an opposite trend for indirect interactions across the climatic gradient.     

Effects of resource availability on seedling recruitment in a fire-maintained savanna

The herbaceous ground cover of the longleaf pine ecosystem harbors the highest plant species richness in North America, with up to 50 species per square meter, but the mechanisms that regulate this diversity are not well understood. In this system, variability in seedling recruitment events may best explain the extremely high small-scale species richness and its relationship to soil moisture and system net primary productivity. To understand the potential mechanistic controls on species richness, we used a long-term resource manipulation study across a natural soil moisture gradient to assess environmental controls on seedling recruitment. We considered the availability of resources to be an indicator of seedling safe-site supply, and also manipulated seed availability to examine the relative importance of recruitment limitations on seedling diversity. We found that water availability regulated the number of species in the seedling community regardless of the underlying natural moisture gradient, and that this effect may result from differential responses of seedling guilds to resource availability. Water supply was more important than seed supply in determining seedling establishment, suggesting that appropriate sites for regeneration are a factor limiting seedling success. This is the first study that shows that the episodic supply of microsites for recruitment could influence species richness in the highly threatened and biodiverse longleaf pine savanna.     

Effect of the canopy of Retama sphaerocarpa on its understorey in a semiarid environment

1. A dense understorey of annual and perennial herbs grow under the canopy of Retama sphaerocarpa shrubs in semiarid environments of south-east Spain, influencing plant productivity and diversity at a regional scale. We investigated the facilitation by the shrub on its understorey in field and laboratory experiments with Barley designed to explore the mechanisms of interaction between both vegetation layers and their spatial variation.
2. There was a gradient of spatial heterogeneity in soil chemical fertility under the shrub canopy, with organic matter and soil nitrogen contents higher at the centre than at the edge of the canopy. Dry mass production of Barley was also higher in soils from intermediate positions, and lower in soils from both the centre and edge of the canopy.
3. In the field, pots sown with Barley placed near the centre, at an intermediate position and at the edge of the canopy of Retama shrubs showed significant differences in productivity, suggesting a mulching effect of the canopy that also affects seedling establishment.
4. Micro-climatic measurements showed significant differences in total radiation reaching the soil, mean air and soil temperatures and maximum temperature among different positions in the understorey, increasing radially from the centre to the edge of the canopy.
5. These results and field observations suggest that the optimal association of climatic factors under the canopy would combine with a high soil fertility mediated by litter decomposition to increase biomass production mid-way between the centre and the edge of the canopy. Overstorey and understorey thus interact to increase nutrient retention locally, which benefits both the shrub and the herb layer.     

Contrasting responses of web-building spiders to deer browsing among habitats and feeding guilds

We examined web-building spider species richness and abundance in forests across a deer density gradient to determine the effects of sika deer browsing on spiders among habitats and feeding guilds. Deer decreased the abundance of web-building spiders in understory vegetation but increased their abundance in the litter layer. Deer seemed to affect web-building spiders in the understory vegetation by reducing the number of sites for webs because vegetation complexity was positively correlated with spider density and negatively correlated with deer density. In contrast, the presence of vegetation just above the litter layer decreased the spider density, and deer exerted a negative effect on this vegetation, possibly resulting in an indirect positive effect on spider density. The vegetation just above the litter layer may be unsuitable as a scaffold for building webs if it is too flexible to serve as a reliable web support, and may even hinder spiders from building webs on litter. Alternatively, the negative effect of this vegetation on spiders in the litter may be as a result of reduced local prey availability under the leaves because of the reduced accessibility of aerial insects. The response to deer browsing on web-building spiders that inhabit the understory vegetation varied with feeding guild. Deer tended to affect web-invading spiders, which inhabit the webs of other spiders and steal prey, more heavily than other web-building spiders, probably because of the accumulated effects of habitat fragmentation through the trophic levels. Thus, the treatment of a particular higher-order taxon as a homogeneous group could result in misleading conclusions about the effects of mammalian herbivores.     

Annual plant–shrub interactions along an aridity gradient

Annual plants in semi-arid and arid areas are often closely associated with shrubs. The degree of association largely depends on the balance of negative and positive effects between these contrasting plant life-forms, ranging from interference to facilitation. Since positive interactions are predicted to become less important with increasing rainfall, the interaction balance is expected to shift along aridity gradients. However, this prediction has not been tested on a community level and for different life-history stages across large geographical gradients. Here, we show such changes for annual plant populations and communities in four contrasting sites along a steep climatic gradient, ranging from the arid desert to mesic Mediterranean regions in Israel. Above-ground productivity, richness, seedling density, and seed bank density of the annual plant community, as well as fecundity of annual plant populations, were generally higher under shrubs than in areas between shrubs at the arid end of the gradient, but significantly lower at the humid end. Net effects of shrubs on annuals expressed as relative interaction intensity indicated a steady and consistent shift from net positive or neutral effects in the desert to net negative effects in the mesic part of the gradient. These findings emphasize the usefulness of studies along large-scale gradients encompassing a wide range of environmental conditions for understanding community level interactions among coexisting species.     

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

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

Strong microsite control of seedling recruitment in tundra

The inclusion of environmental variation in studies of recruitment is a prerequisite for realistic predictions of the responses of vegetation to a changing environment. We investigated how seedling recruitment is affected by seed availability and microsite quality along a steep environmental gradient in dry tundra. A survey of natural seed rain and seedling density in vegetation was combined with observations of the establishment of 14 species after sowing into intact or disturbed vegetation. Although seed rain density was closely correlated with natural seedling establishment, the experimental seed addition showed that the microsite environment was even more important. For all species, seedling emergence peaked at the productive end of the gradient, irrespective of the adult niches realized. Disturbance promoted recruitment at all positions along the environmental gradient, not just at high productivity. Early seedling emergence constituted the main temporal bottleneck in recruitment for all species. Surprisingly, winter mortality was highest at what appeared to be the most benign end of the gradient. The results highlight that seedling recruitment patterns are largely determined by the earliest stages in seedling emergence, which again are closely linked to microsite quality. A fuller understanding of microsite effects on recruitment with implications for plant community assembly and vegetation change is provided.     

Litter legacy after spruce plantation removal hampers initial vegetation establishment

Restoration of areas used for intensive even-aged Norway spruce (Picea abies Karst.) plantations often involves felling and subsequent spontaneous vegetation succession. However, the accumulated litter layer may hamper vegetation development, and thereby postpone recovery or even change the outcome.We studied effects of the litter layer on vegetation establishment during two seasons following a clear-cut of Norway spruce in Denmark. We experimentally assessed the response of multiple vegetation properties to litter removal, with and without wildlife exclusion by fencing, and in combination with sowing of trees, while fencing. Burning was tested as an alternative way to remove the litter layer.Vegetation establishment was poor, when the litter layer was intact, and cover developed slowly remaining below 10% after two years, irrespective of fencing. In contrast, litter removal and fencing together gave significantly faster recovery and reached nearly 60% mean cover. Vegetation cover was driven by few dominant species, especially the sedge Carex pilulifera. Species richness was similar in all treatments, but increased with sowing of trees. Fencing resulted in taller birch seedlings independently of litter removal, but enhanced by seedling density. Litter removal seemed to favor species with lighter seeds, lower specific leaf area (SLA) and lower Ellenberg N value, i.e. associated with relative infertile conditions. Disturbing the litter by burning seemed to have an effect comparable to mechanical removal, and could be a management alternative.Our results showed that a persistent litter layer after spruce plantation removal may hamper the initial vegetation establishment. Actively removing litter may serve as an additional restoration intervention to overcome this legacy. However, as grazing can keep this potential in check, wildlife exclusion may be necessary as well. To speed up recovery and diversify vegetation structure after spruce plantation removal, we suggest patchy disturbance of the litter, essentially combined with wildlife exclusion.     

Facilitation and interference of seedling establishment by a native legume before and after wildfire

In semi-arid ecosystems, heterogeneous resources can lead to variable seedling recruitment. Existing vegetation can influence seedling establishment by modifying the resource and physical environment. We asked how a native legume, Lupinus argenteus, modifies microenvironments in unburned and burned sagebrush steppe, and if L. argenteus presence facilitates seedling establishment of native species and the non-native annual grass, Bromus tectorum. Field treatments examined mechanisms by which L. argenteus likely influences establishment: (1) live L. argenteus; (2) dead L. argenteus; (3) no L. argenteus; (4) no L. argenteus with L. argenteus litter; (5) no L. argenteus with inert litter; and (6) mock L. argenteus. Response variables included soil nitrogen, moisture, temperature, solar radiation, and seedling establishment of the natives Elymus multisetus and Eriogonum umbellatum, and non-native B. tectorum. In both unburned and burned communities, there was higher spring soil moisture, increased shade and reduced maximum temperatures under L. argenteus canopies. Adult L. argenteus resulted in greater amounts of soil nitrogen (N) only in burned sagebrush steppe, but L. argenteus litter increased soil N under both unburned and burned conditions. Although L. argenteus negatively affected emergence and survival of B. tectorum overall, its presence increased B. tectorum biomass and reproduction in unburned plots. However, L. argenteus had positive facilitative effects on size and survival of E. multisetus in both unburned and burned plots. Our study indicates that L. argenteus can facilitate seedling establishment in semi-arid systems, but net effects depend on the species examined, traits measured, and level of abiotic stress.     

Productivity and species richness across an environmental gradient in a fire-dependent ecosystem

The fire-dependent longleaf pine-wiregrass (Pinus palustris Mill.-Aristida beyrichiana Trin. & Rupr.) savannas of the southeastern United States provide a unique opportunity to examine the relationship between productivity and species richness in a natural ecosystem because of the extremely high number of species and their range across a wide ecological amplitude (sandhills to edges of wetlands). We used a natural gradient to examine how plant species richness and plant community structure vary with standing crop biomass (which in this system is proportional to annual net productivity) as a function of soil moisture and nitrogen mineralization rates in a frequently burned longleaf pine-wiregrass savanna. Highest ground cover biomass and highest species richness were found at the same position along the gradient, the wet-mesic sites. Relative differences in species richness among site types were independent of scale, ranging from 0.01 m(2) to 100 m(2). Nitrogen availability was negatively correlated with species richness. Dominance of wiregrass (in terms of biomass) was consistent across the gradient and not correlated with species richness. Regardless of site type, the community structure of the savannas was characterized by many perennial species with infrequent occurrences, a factor in the low temporal heterogeneity (percent similarity between seasons and years) and high within-site spatial heterogeneity (percent dissimilarity of vegetation composition). The coexistence of numerous species is likely due to the high frequency of fire that removes competing hardwood vegetation and litter and to the suite of fire-adapted perennial species that, once established, are able to persist. Our results suggest that soil moisture is an important factor regulating both the number of species present and community production within the defined gradient of this study.     

浇水频率和凋落物覆盖量对黧蒴锥种子萌发及幼苗存活的影响

黧蒴锥是华南用于人工造林和生态恢复的优良乡土树种.选取浇水频率和凋落物覆盖量两个控制因子,研究在不同的土壤湿度和凋落物覆盖量条件下,黧蒴锥种子萌发及幼苗存活和生长特征.结果表明: 凋落物对黧蒴锥种子萌发和幼苗生长的影响与环境的潮湿程度相关.当浇水频率为每天1次时,凋落物覆盖会抑制黧蒴锥种子萌发和幼苗早期生长,导致种子萌发率降低、幼苗死亡率提高; 在浇水频率为2 d 1次和3 d 1次时,凋落物覆盖可以改善土壤的水分状况,有助于种子萌发和幼苗存活.凋落物覆盖还可以显著地提高幼苗的生物量.在通过播种来实现亚热带灌木林地更新时,可利用凋落物覆盖促进黧蒴锥的种子萌发和幼苗存活.     

Herbivory and competition slow down invasion of a tall grass along a productivity gradient

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced aboveground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.Plant nomenclature follows Van der Meijden et al. (1990)     

Effects of microclimate,litter type,and mesh size on leaf litter decomposition along an elevation gradient in the Wuyi Mountains,China

Litter decomposition is an important ecosystem process regulated by both biotic factors (e.g., decomposers and litter types) and abiotic factors (e.g., temperature and moisture). This study examined the regulatory effects of soil fauna and microclimate on decomposition of two substrates (Castanopsis carlesii and Pinus taiwanensis) along an elevation gradient in four ecosystems of zonal vegetation types in southeastern China: evergreen broadleaf forest (EVB), coniferous forest (COF), dwarf forest (DWF), and alpine meadow (ALM). Our objective was to identify the mechanisms by which microclimate, substrate, and fauna control litter decomposition, especially where variations in ecosystem structure and environment are markedly shown across an elevation gradient. The hypotheses were as follows: (1) litter decomposition within the same litter type would decrease across the elevation gradient, (2) litter decomposition would be lower in poorer nutrient quality substrate across the four sites, and (3) litter dynamics, influenced by strong interactions among ecosystem type, litter type, and decomposers, would vary by elevation gradient due to microclimate effects (i.e., temperature and moisture). The decomposition rates of C. carlesii were significantly higher than those of P. taiwanensis at EVB, COF, and DWF sites; however, they were not significantly different at the ALM site. Low elevation forests possessed a microclimate (warm and humid) that favors decomposer activities and also appeared to possess a decomposer community adapted to consuming large amounts of leaf litter, as indicated by the rapid leaf litter loss. Litter decomposition in micro-mesh bags proceeded more slowly compared to litter in meso-mesh and macro-mesh litterbags across the elevation gradient, indicating that restricting some detritivore access to litter reduced litter mass loss. We suggest that microclimate and faunal contributions to plant litter decomposition differ markedly across the ecosystems in the Wuyi Mountains.     

Conspecific versus heterospecific litter effects on seedling establishment

Plant litter is an important determinant of seed germination and seedling establishment. Positive effects of litter have received considerable attention, but few studies have explicitly tested whether seedlings are more facilitated by conspecific litter compared to heterospecific litter. In order to contrast conspecific and heterospecific facilitative effects on seedling establishment, we used Anthriscus sylvestris, Angelica sylvestris, Pimpinella saxifraga and different combinations of their seeds and litter seedbeds as a model system. Although litter had a significant species-specific effect on seedling emergence, we found no evidence of strictly conspecific facilitation. Anthriscus sylvestris displayed a positive response to all types of litter. In contrast, there was a clear negative effect of conspecific litter in Pimpinella saxifraga. Activated carbon did not modify the negative effect, indicating that chemical compounds were not the cause. Our study suggests a high level of idiosyncrasy in response to litter at the species level.     

A fungal endosymbiont affects host plant recruitment through seed- and litter-mediated mechanisms

1.  Many grass species are associated with maternally transmitted fungal endophytes. Increasing evidence shows that endophytes enhance host plant success under varied conditions, yet studies have rarely considered alternative mechanisms whereby these mutualistic symbionts may affect regeneration from seed.
2.  We performed a microcosm experiment to evaluate whether infection with Neotyphodium occultans affects recruitment in the annual grass Lolium multiflorum either directly, by infecting the seeds, or indirectly, by altering the suitability of recruitment microsites through the litter shed by host plants. Endophyte effects on establishment were tested for different litter depths and watering regimes under natural herbivory by leaf-cutting ants.
3.  Seed infection increased seedling emergence through the litter as well as final recruitment, irrespective of microsite conditions. However, litter produced by infected plants delayed emergence and decreased density of both infected and non-infected grass populations.
4.  Individual plant biomass did not change with seed infection but was increased under deep litter from endophyte-infected plants. Although seed infection did not protect establishing plants from leaf-cutting ants, herbivory was reduced in the presence of deep litter shed by infected plants.
5.  We conclude that fungal endophytes may affect host plant recruitment across subsequent generations not only by infecting the seeds but also through the host's dead remains. While the former effect entailed an advantage to infected plants, litter-mediated effects did not discriminate by infection status, and generally promoted the establishment of fewer and larger plants. Thus hidden foliar symbionts may play an underappreciated role in maintaining host species dominance through the litter produced by prior patch occupants.     

Inundation,Vegetation, and Sediment Effects on Litter Decomposition in Pacific Coast Tidal Marshes

The cycling and sequestration of carbon are important ecosystem functions of estuarine wetlands that may be affected by climate change. We conducted experiments across a latitudinal and climate gradient of tidal marshes in the northeast Pacific to evaluate the effects of climate- and vegetation-related factors on litter decomposition. We manipulated tidal exposure and litter type in experimental mesocosms at two sites and used variation across marsh landscapes at seven sites to test for relationships between decomposition and marsh elevation, soil temperature, vegetation composition, litter quality, and sediment organic content. A greater than tenfold increase in manipulated tidal inundation resulted in small increases in decomposition of roots and rhizomes of two species, but no significant change in decay rates of shoots of three other species. In contrast, across the latitudinal gradient, decomposition rates of Salicornia pacifica litter were greater in high marsh than in low marsh. Rates were not correlated with sediment temperature or organic content, but were associated with plant assemblage structure including above-ground cover, species composition, and species richness. Decomposition rates also varied by litter type; at two sites in the Pacific Northwest, the grasses Deschampsia cespitosa and Distichlis spicata decomposed more slowly than the forb S. pacifica. Our data suggest that elevation gradients and vegetation structure in tidal marshes both affect rates of litter decay, potentially leading to complex spatial patterns in sediment carbon dynamics. Climate change may thus have direct effects on rates of decomposition through increased inundation from sea-level rise and indirect effects through changing plant community composition.     

Effects of pH and resources on a processing chain interaction in simulated treeholes

1.  Both resources and abiotic factors may affect biotic interactions. One interaction that occurs in treehole habitats involves leaf shredders that facilitate growth of detritivores, and it may be affected by both leaf litter quantity and changes in water quality.
2.  Water chemistry in central Pennsylvania treeholes has been impacted by acid deposition, and the most common insects therein have differential survival under low pH conditions. Experimental microcosms that mimic treehole habitats were used to test the hypothesis that this abiotic factor, pH, also affects facilitative interactions. Leaf litter resources and pH were varied independently of presence of leaf-shredding scirtid beetles ( Helodes pulchella and Prionocyphon discoideus ), and the mosquito Aedes triseriatus , to examine interactions among pH, resources and insects.
3.  pH affected the interaction between the insects, such that effects of scirtids were more evident at pH 4·5 than at 6·5. Female mosquitoes were larger in the presence of scirtids, low resource and low pH conditions than in absence of scirtids, low resource and low pH conditions.
4.  There were also effects of A. triseriatus on scirtids. The size of individual scirtids was smaller in the presence of A. triseriatus , but total scirtid biomass was unaffected as survival was also higher in the presence of A. triseriatus .
5.  The effects observed on a resource-mediated biotic interaction led to the conclusion that this interaction is pH dependent, and gives support to the concept that abiotic factors play a role in determining the outcome of biotic interactions, and that acidification can have complex effects on communities.     

Negative and neutral marsh plant interactions dominate in early life stages and across physical gradients in an Oregon estuary

Conceptual models and some empirical studies in plant systems show that species interactions can shift from competitive under low stress and high productivity conditions to positive under high stress and low productivity conditions. In this study, we explore the relative strength and direction of marsh plant interactions at early life stages across a stress and productivity gradient in South Slough, Coos Bay, Oregon, USA. Germination and survival of five plant species (Atriplex patula L., Distichlis spicata (L.), Plantago maritima (Lam.) A. Gray, Sarcocornia pacifica (Standl.) A. J. Scott, Triglochin maritimum L.) were examined in the presence and absence of neighboring vegetation at three intertidal levels in each of three marsh sites along an estuarine gradient. We found that many of the interactions measured across these gradients were negative or neutral. The direction of the interactions depended on life stage, with neighboring plants having mostly negative and neutral effects on seed germination, and mostly neutral and rarely positive effects on seedling survival especially at lower intertidal locations. The exception was in the high intertidal at all sites, where competition was common. We found that the intensity of the interactions varied depending on marsh site, intertidal elevation, and plant species identity and that life stage and regional climate may be responsible for the general lack of positive interactions. We suggest that measuring species interactions across complex gradients of physical stress at different life stages can help refine our conceptual models and lead to better predictions of the factors controlling community structure.