Interactions between root and shoot competition vary among species

Understanding how the competition varies with productivity is essential for differentiating among alternative models of plant community organization. Prior attempts to explain shifts in root and shoot competition along gradients have generally assumed an additive interaction between the two competitive forms, using an experimental design which does not fully separate both above‐ and belowground processes. At the most basic level, few field studies have separated root and shoot competition, and we have limited knowledge about both the relative importance of these processes, and how they interact to affect plant growth in the field. Presented here are findings from a field study in which root and shoot competition were experimentally separated by using root exclusion tubes and neighbor tiebacks in an early successional community. Individuals of four species (Abutilon theophrasti, Amaranthus retroflexus, Rumex crispus, and Plantago lanceolata) were grown at two levels of fertilization with full competition, aboveground competition only, belowground competition only, or neither above‐ nor belowground competition. Competition was measured as competitive response, which is the natural log of the relative biomass of a target plant grown with competition compared to growth without competition. In contrast to predictions from current models of productivity‐competition relationships, but in agreement with other experimental studies, there was no change in the strengths or root, shoot, or total competition with a modest increase in productivity. Despite no effect of fertilization on the strength of competition, the form of interaction between root and shoot competition varied both as a function of species identity and fertilization. For both of the rosette forming species, the combined effects of root and shoot competition were less than predicted assuming no interaction (a “negative interaction”), with one species switching from a negative to an additive interaction with fertilization. The fact that fertilization caused a shift in the root‐shoot interaction, but not in the total strength of root and shoot competition, suggests that the root‐shoot interaction is itself a highly labile variable. If root‐shoot interactions are common in natural systems, then simply measuring the strength of one form of competition in no way provides any information about the overall importance of that competitive form to plant growth.     

植物竞争对3种移植树苗生长的影响

在植物幼苗生长过程中,总是受到包括地下根竞争在内的各种竞争影响。植物间的竞争主要同有效光辐射、水分和各种营养相关。当外来植物侵入森林群落时,可能受到群落中其它植物竞争的影响。该文通过移植尾叶桉(Eucalyptus urophylla)、大叶相思(Acacia auriculaeformis)两种外来种和本地种荷木(Schima superba)幼苗,挖沟排除根竞争和砍树创造林窗来排除地上竞争的野外实验,研究植物竞争对幼苗生长的影响。有根竞争时,荷木、尾叶桉和大叶相思幼苗的生物量和净初级生产力均小于没有根竞争时,可见地下根竞争对3种幼苗生长有抑制性影响。尾叶桉、大叶相思和荷木3种幼苗受到的总竞争强度分别是0.357 9、0.338 3和0.198 9,受到的地下根竞争强度则分别是0.104 3、0.053 04和0.118 8,受到的地上竞争强度则分别是0.285 1、0.277 0和0.090 85。尾叶桉和大叶相思两种幼苗受到的总竞争强度间的差异不显著,但都显著大于荷木;3种幼苗受到的地上竞争强度间的差异同总竞争强度情况相似;尾叶桉和荷木两种幼苗受到的地下根竞争强度间的差异不显著,但都显著大于大叶相思。地上竞争对阳性树种尾叶桉和大叶相思两种幼苗的生长影响大,而地下根竞争则对耐阴性强的荷木幼苗影响大。尾叶桉和大叶相思两种外来种幼苗受到的总竞争强度均大于本地种荷木幼苗,这反映了这两种外来树种侵入次生林这样的群落受到竞争影响大。     

Water and nitrogen addition differentially impact plant competition in a native rough fescue grassland

We examined how water and nitrogen addition and water–nitrogen interactions affect root and shoot competition intensity and competition–productivity relationships in a native rough fescue grassland in central Alberta, Canada. Water and nitrogen were added in a factorial design to plots and root exclusion tubes and netting were used to isolate root and shoot competition on two focal species (Artemisia frigida and Chenopodium leptophyllum). Both water and nitrogen were limiting to plant growth, and focal plant survival rates increased with nitrogen but not water addition. Relative allocation to root biomass increased with water addition. Competition was almost entirely belowground, with focal plants larger when released from root but not shoot competition. There were no significant relationships between productivity and root, shoot, or total competition intensity, likely because in this system shoot biomass was too low to cause strong shoot competition and root biomass was above the levels at which root competition saturates. Water addition had few effects on the intensity of root competition suggesting that root competition intensity is invariant along soil moisture gradients. Contrary to general expectation, the strength of root competition increased with nitrogen addition demonstrating that the relationship between root competition intensity and nitrogen is more complex than a simple monotonic decline as nitrogen increases. Finally, there were few interactions between nitrogen and water affecting competition. Together these results indicate that the mechanisms of competition for water and nitrogen likely differ.     

Investigating the relationship between neighbor root biomass and belowground competition: field evidence for symmetric competition belowground

Little is known about how small-scale variation in neighbor biomass can influence the strength of root competition experienced by an individual plant. In this study, modified root exclusion tubes were used to vary the accessibility of the soil space surrounding Amaranthus retroflexus target plants to the neighboring plants. A gradient of root accessibility was created by drilling varying numbers of holes into standard root exclusion tubes, made of 15 cm diameter PVC pipe. Belowground competitive intensity, defined as biomass reduction due to root interactions alone, relative to growth in the absence of neighbors, was then measured along the resulting gradient of neighbor root densities. At low neighbor root abundances the strength of belowground competition was proportional to neighbor root biomass, consistent with prior evidence that belowground competition is symmetric. If belowground competition were asymmetric, neighbor roots should have had little effect on target plants when they are rare relative to those of the target plant. At higher neighbor root abundances, belowground competitive intensity should increase rapidly. The strong relationship found between neighbor root biomass and belowground competitive intensity suggests relatively small variations in root biomass could lead to large variations in belowground competition. Reduced belowground competition in areas with low root biomass could have important implications for the establishment and growth of poor belowground competitors, suggesting a mechanism by which species coexistence may occur despite extremely intense root competition.     

Root allocation and foraging precision in heterogeneous soils

Root growth patterns respond to small-scale resource heterogeneity and the presence of roots of neighboring plants, but how a plant integrates its responses to these cues is not well understood. In the presence of neighbors, plants may shift allocation to roots as a consequence of plant size and root:shoot allometry, as a response to resource depletion by neighbors, or through a direct response to neighbor presence. The same response pathways also have the potential to alter proliferation in resource-rich patches in soil.Four species of grassland plants were grown in the greenhouse as single plants, monocultures, and mixtures. Root length allocation as a function of shoot mass was examined for background soil and fertilized patches. Plants grown with same-species neighbors followed the same allometric trajectory as single plants for root length in background soil, so any change in root allocation was due only to reduced plant size. Root proliferation in patches declined with neighbors, consistent with a response to resource depletion. Mixtures overproduced roots in both background soil and in patches, relative to plants of the same size in monocultures.     

Competition and facilitation structure plant communities under nurse tree canopies in extremely stressful environments

Nurse plant facilitation in stressful environments can produce an environment with relatively low stress under its canopy. These nurse plants may produce the conditions promoting intense competition between coexisting species under the canopy, and canopies may establish stress gradients, where stress increases toward the edge of the canopy. Competition and facilitation on these stress gradients may control species distributions in the communities under canopies. We tested the following predictions: (1) interactions between understory species shift from competition to facilitation in habitats experiencing increasing stress from the center to the edge of canopy of a nurse plant, and (2) species distributions in understory communities are controlled by competitive interactions at the center of canopy, and facilitation at the edge of the canopy. We tested these predictions using a neighbor removal experiment under nurse trees growing in arid environments. Established individuals of each of four of the most common herbaceous species in the understory were used in the experiment. Two species were more frequent in the center of the canopy, and two species were more frequent at the edge of the canopy. Established individuals of each species were subjected to neighbor removal or control treatments in both canopy center and edge habitats. We found a shift from competitive to facilitative interactions from the center to the edge of the canopy. The shift in the effect of neighbors on the target species can help to explain species distributions in these canopies. Canopy‐dominant species only perform well in the presence of neighbors in the edge microhabitat. Competition from canopy‐dominant species can also limit the performance of edge‐dominant species in the canopy microhabitat. The shift from competition to facilitation under nurse plant canopies can structure the understory communities in extremely stressful environments.     

Competition components along productivity gradients – revisiting a classic dispute in ecology

Competition is ubiquitous in plant communities with various effects on plant fitness and community structure. A long-standing debate about different approaches to explain competition is the controversy between David Tilman and Philip Grime. Grime stated that the importance of competition relative to the impact of the environment increases along a productivity gradient, while Tilman argued that the intensity of competition is independent of productivity. To revisit this controversy, we assumed that the effects of plant–plant interactions are additive and applied the new competition indices by Díaz-Sierra et al. (2017) in a field experiment along a productivity gradient in S-Germany, using the rare arable plant Arnoseris minima as a study species. The ‘target technique' was applied, to separate the effects of root and shoot competition. The study plants were exposed to five competition treatments with three replicates in 18 sites, respectively. We investigated the expectation that root competition is more intense in unproductive sites than shoot competition. Additionally, we predicted survival to be less affected by competition than growth-related plant parameters. Using the biomass of individuals without competition as a proxy for site productivity there was a positive relationship with competition importance but no relationship with competition intensity when plants experienced full competition. Survival of the target plants was unaffected by competition. Root competition was the main mechanism determining the performance of the target plants, whereas the effect of shoot competition was relatively low albeit increasing with productivity. We conclude that when considering plant–plant interactions additive both Grime's and Tilman's theories can be supported.     

When competition does not matter: grassland diversity and community composition

We examined whether the intense root competition in a rough fescue grassland plant community in central Alberta, Canada, was important in structuring plant species diversity or community composition. We measured competition intensity across gradients of species richness, evenness, and community composition, using pairs of naturally occurring plants of 12 species. One plant in each pair was isolated from neighbors to measure competition; community structure and environmental conditions were also measured at each pair. We used structural equation modeling to examine how competition influenced community structure. Competition intensity was unrelated to species richness and community composition, but increased competition intensity was associated with a slight decline in evenness. Size-symmetric root competition was probably unimportant in structuring this plant community because there are no feedback mechanisms through which size-symmetric competition can magnify small initial differences and eventually lead to competitive exclusion. In plant communities with little shoot competition, competition and community structure should be unlinked regardless of competition intensity. In more productive systems, we propose that interactions between root and shoot competition may indirectly structure communities by altering the overall asymmetry of competition.     

Root architecture of Arabidopsis is affected by competition with neighbouring plants

How roots detect and respond to the presence of neighbors is relevant to understand plant belowground interactions. The aim of the present work was to evaluate the effect of the presence of neighboring plants and the limited availability of phosphorus on root architecture. A target plant of Arabidopsis thaliana (Ler or Col) was surrounded by combinations of two individuals (Ler and Col), and subjected to different growth conditions (levels of activated charcoal (AC) and phosphorus). Both accessions consistently concentrated their roots towards the competition zone shared with a neighbor of the same accession, avoiding the side shared with the other accession. All these competition strategies disappeared when plants were limited by phosphorus or when activated charcoal was added to the growth media. Plants produced consistently fewer but longer lateral roots when activated charcoal was added to the growth media irrespective of the neighbors. Our results indicate a direct role of secondary metabolites present in the root exudates and phosphorus availability in the response of presence and identity of neighboring roots.     

Tight Coupling Between Shoot Level Foliar N and P,Leaf Area,and Shoot Growth in Arctic Dwarf Shrubs Under Simulated Climate Change

Nutrient availability limits productivity of arctic ecosystems, and this constraint means that the amount of nitrogen (N) in plant canopies is an exceptionally strong predictor of vegetation productivity. However, climate change is predicted to increase nutrient availability leading to increases in carbon sequestration and shifts in community structure to more productive species. Despite tight coupling of productivity with canopy nutrients at the vegetation scale, it remains unknown how species/shoot level foliar nutrients couple to growth, or how climate change may influence foliar nutrients–productivity relationships to drive changes in ecosystem carbon gain and community structure. We investigated the influence of climate change on arctic plant growth relationships to shoot level foliar N and phosphorus (P) in three dominant subarctic dwarf shrubs using an 18-year warming and nutrient addition experiment. We found a tight coupling between total leaf N and P per shoot, leaf area and shoot extension. Furthermore, a steeper shoot length-leaf N relationship in deciduous species (Vaccinium myrtillus and Vaccinium uliginosum) under warming manipulations suggests a greater capacity for nitrogen to stimulate growth under warmer conditions in these species. This mechanism may help drive the considerable increases in deciduous shrub cover observed already in some arctic regions. Overall, our work provides the first evidence at the shoot level of tight coupling between foliar N and P, leaf area and growth i.e. consistent across species, and provides mechanistic insight into how interspecific differences in alleviation of nutrient limitation will alter community structure and primary productivity in a warmer Arctic.     

Competition and coexistence of rhizomatous perennial plants along a nutrient gradient

I studied competition and coexistence of three tall clonal perennial plant species, Calamagrostis epigejos (L.) Roth, Solidago canadensis L., and Tanacetum vulgare L. along a gradient of soil productivity over five years. A replacement series field experiment was conducted with high, moderate and low fertility levels in 1m×1m plots. There were significant effects of soil type on ramet density (P<0.001), mean height (P<0.01), and total biomass (P<0.01). Ramet density, mean height, and total biomass increased with increasing soil fertility. There were also significant effects of mixture on ramet density (P<0.01), but not on mean height and total biomass for all species. Significant neighbor effects on ramet density and total biomass (P<0.01) were found for Solidago, showing that it is important whether Tanacetum or Calamagrostis is its neighbor within mixtures. During the five years there was only one case of competitive exclusion: Calamagrostis excluded Solidago on the most fertile substrate in the fifth growing season. In most cases species coexisted over the five years. Each of the three species was able to dominate in at least one combination of substrate type and mixture. The experiment showed that asymmetric competition for light on substrates of high fertility, symmetric competition for nutrients on nutrient-poor soil and positive interactions especially on substrates of intermediate fertility played a role. A founder effect was evident in aggregated mixtures of Calamagrostis and Solidago on the nutrient-rich substrate. A conceptual model of the relative importance of root competition for soil nutrients, shoot competition for light, and positive interactions along the fertility gradient is presented. The model emphasizes that positive interactions play an important role over a broad range of the productivity scale with a peak at intermediate levels of fertility. On the substrate of high productivity shoot competition for light is more important than positive interactions and root competition for soil nutrients as well. The competitive superiority of Calamagrostis on the most productive substrate was evident only in the long run. Rare events like extreme summer drought or selective herbivore pressure caused a switch in dominance in mixtures with Solidago, respectively Tanacetum. The guerrilla growth strategy of Calamagrostis and interference competition through a dense cover of aboveground biomass and litter could further cause competitive exclusion.     

The effects of gap disturbance on the seedling emergence,survival and growth of two different native species in Inner Mongolia

A field study was conducted to investigate the effects of gap disturbance on the seedling establishment process of two native species. Seeds of Agropyron cristatum and Stipa krylovii were reseeded to artificially created gaps in a degraded steppe in North China. There were seven treatments: shoot gaps and root gaps (10 cm, 20 cm and 40 cm in diameters), no gaps (control). Shoot gaps were formed by removing above ground vegetation and below ground biomass without restricting the re-growth of neighbor roots back into the gap. The root gaps were accomplished by using polyvinyl chloride pipes sunk in the soil of shoot gaps to exclude neighboring roots. Seedling emergence, survival and growth performance after 90 days of growing were recorded for both species. Gap significantly increased soil moisture, especially for root gaps. Emergence increased significantly for both species as gap size increased. Seedling emergence and survivorship of both species were greater in gaps than in controls. However, the gap size showed a significantly negative effect on Agropyron cristatum's survivorship. Growth performance of Agropyron cristatum and Stipa krylovii differ in their response to gap disturbance. Gap had positive effects on seedling growth (including seedling height, dry weight, and numbers of tillers and leaves) of Stipa krylovii, but had negative effects on seedling growth of Agropyron cristatum. The two species have significantly different responses to gap disturbance. All results suggest that Stipa krylovii is a gap-enhanced species, and Agropyron cristatum is not. Predation by insects may be one of the key reasons to explain the stand dominance in this grassland.     

Defoliation and competition effects in a productivity gradient for a semiarid Mediterranean annual grassland community

Grazing and competition are two main factors shaping range plant communities; however, few studies have investigated their interaction. The current study aimed to investigate the effects of defoliation, competition and their interaction on production of annual grasses in semiarid Mediterranean areas. Competition treatments (absence/presence of neighbors) were combined with three defoliation intensities (0%, 30% and 60%) in a complete factorial design. Competition significantly reduced grass biomass. However, the role of competition was eliminated under heavy defoliation or under dry growth conditions. Defoliation showed variable results on final biomass (FB) and cumulative biomass (CB). While heavy defoliation (60% clipping intensity) reduced grass FB down to 80% during the two growing seasons, light defoliation (30%) significantly increased CB. Results showed that competition may limit the direct effect of defoliation on dominant grass species. Further, the relationship between site productivity and competition effect was best explained by a negative linear model. This hypothesized model may suggest that facilitation and competition alternatively affect grassland communities along a productivity gradient. The results suggest that light grazing may sustain or even enhance grassland productivity. The results also indicated the suitability of annual grass species to re-vegetate degraded rangeland in semi-arid climate. Further, optimum grazing practices to conserve biodiversity of Avena grassland may involve moderate stocking rate.     

水曲柳苗木地下竞争与地上竞争的定量研究

对3种密度进行栽培试验,利用通径分析的方法,研究了水曲柳地下竞争和地上竞争的关系及对总竞争的影响。结果表明,水曲柳苗木的地下部分生物量、地上部分生物量和总生物量与营养空间有密切关系。随着苗木空间距离增加,由生物量计算的竞争指数下降。在同一密度条件下,地下竞争指数明显大于地上竞争指数。由于地下生长与地上生长的相互作用,各竞争指数之间具有明显的相关性。但是地下竞争和地上竞争对总竞争的影响是不同的,通径分析可以定量的区分地下竞争和地上竞争的相对大小。地下竞争对总竞争的直接作用范围在0．5543～0．7426之间,明显大于地上竞争对总竞争的直接作用(0．2851～0．5282)。随着距离的增加,单株苗木的生长空间加大,地上部分的竞争作用增加,地下部分的竞争程度减弱。但是,地下根系的竞争在水曲柳苗木总的竞争中占有重要地位。     

Influence of microbial activity on plant–microbial competition for organic and inorganic nitrogen

To investigate how the level of microbial activity in grassland soils affects plant–microbial competition for different nitrogen (N) forms, we established microcosms consisting of a natural soil community and a seedling of one of two co-existing grass species, Anthoxanthum odoratum or Festuca rubra. We then stimulated the soil microbial community with glucose in half of the microcosms and followed the transfer of added inorganic (¹⁵NH₄¹⁵NO₃) and organic (glycine-2-¹³C-¹⁵N) N into microbial and plant biomass. We found that microbes captured significantly more ¹⁵N in organic than in inorganic form and that glucose addition increased microbial ¹⁵N capture from the inorganic source. Shoot and root biomass, total shoot N content and shoot and root ¹⁵N contents were significantly greater for A. odoratum than F. rubra, whereas F. rubra had higher shoot and root N concentrations. Where glucose was not added, A. odoratum had higher shoot ¹⁵N content with organic than with inorganic ¹⁵N addition, whereas where glucose was added, both species had higher shoot ¹⁵N content with inorganic than with organic ¹⁵N. Glucose addition had equally negative effects on shoot growth, total shoot N content, shoot and root N concentrations and shoot and root ¹⁵N content for both species. Both N forms produced significantly more shoot biomass and higher shoot N content than the water control, but the chemical form of N had no significant effect. Our findings suggest that plant species that are better in capturing nutrients from soil are not necessarily better in tolerating increasing microbial competition for nutrients. It also appears that intense microbial competition has more adverse effects on the uptake of organic than inorganic N by plants, which may potentially have significant implications for interspecific plant–plant competition for N in ecosystems where the importance of organic N is high and some of the plant species specialize in use of organic N.     

Shading and the capture of localized soil nutrients: nutrient contents,carbohydrates, and root uptake kinetics of a perennial tussock grass

Summary The ability to exploit spatial and temporal heterogeneity in soil resources can be one factor important to the competitive balance of plants. Competition above-ground may limit selective plant responses to below-ground heterogeneity, since mechanisms such as root proliferation and alterations in uptake kinetics are energy-dependent processes. We studied the effect of shading on the ability of the perennial tussock grassAgropyron desertorum to take up nutrients from enriched soil microsites in two consecutive growing seasons. Roots of unshaded plants selectively increased phosphate uptake capacity in enriched soil microsites (mean increases of up to 73%), but shading eliminated this response. There were no changes in ammonium uptake capacity for roots in control and enriched patches for either shaded or unshaded plants. The 9-day shade treatments significantly reduced total nonstructural carbohydrate (TNC) concentrations for roots in 1990, but had no apparent effect on root carbohydrates in 1991 despite dramatic reductions in shoot TNC and fructan concentrations. Enrichment of the soil patches resulted in significantly greater phosphate concentrations in roots of both shaded and unshaded plants, with less dramatic differences for nitrogen and no changes in potassium concentrations. In many respects the shaded plants did surprisingly well, at least in terms of apparent nutrient acquisition. The effects of aboveground competition on nutrient demand, energy requirements, and belowground processes are discussed for plants exploiting soil resource heterogeneity.     

The effects of shading and N status on root proliferation in nutrient patches by the perennial grass Agropyron desertorum in the field

Competition for light can affect exploitation of spatially heterogeneous soil resources. To evaluate the influence of shoot status on root growth responses in nutrient-rich soil patches, we studied the effects of shading and whole-plant nitrogen status on root growth in N-enriched and nonenriched patches by mature Agropyron desertorum plants growing in the field with below-ground competition. Roots in enriched patches had greater length to weight ratios (specific root length, SRL), indicating increased absorptive surface areas, compared with roots in control patches. Increased SRL was due to increased production and length of higher order laterals rather than morphological changes in roots of the same branching order. Although the pattern of root growth rates in patches was the same for shaded and unshaded plants, the magnitude of this response to enriched patches was damped by shading. Root relative growth rates (RGR) in N-enriched patches were reduced by more than 50% by short-term shading treatments (60% reduction in photosynthetic flux density), while root RGR in unenriched patches was unaffected by shading. Unexpectedly, plants with higher nitrogen status had greater root RGR in enriched patches than plants that had not received nitrogen supplement, again with no detectable effect on root RGR in the unenriched patches. Therefore, while both shading and plant N status affected the ability of roots to exploit enriched patches by proliferation, there was no stimulation or suppression of root growth in the unenriched, control patches. Thus, plants already under competitive pressure above ground for light and below ground for nutrients should be less able to rapidly respond to opportunities presented in nutrient patches and pulses.     

Effects of collembola (arthropoda) and relative germination date on competition between mycorrhizalPanicum virgatum (Poaceae) and non-mycorrhizalBrassica nigra (Brassicaceae)

In this experiment the separate and interactive effects of grazing of vesicular-arbuscular mycorrhizal hyphae by collembola and of relative germination date on competition between the mycorrhizal perennial grassPanicum virgatum and the non-mycorrhizal annual cruciferBrassica nigra were investigated. In the absence of competition,P. virgatum mass and P uptake were not affected by collembola grazing; grazing did reduce tissue N concentration and root: shoot ratio. Competition fromB. nigra plants of the same age/size (“simultaneous competition”) significantly reducedP. virgatum total, root, and shoot mass relative to control plants not subject to competition. In contrast, when in competition,B. nigra plants did not differ in biomass fromB. nigra controls grown without competition. Simultaneous competition also reduced N and P uptake byP. virgatum, but not byB. nigra. Grazing by collembola during simultaneous competition increased the differences in nutrient uptake and tended to shift the competitive balance further toward the non-mycorrhizalB. nigra. WhenP. virgatum plants were subjected to competition fromB. nigra plants which germinated three weeks later (“offset competition”) the situation was reversed: offsetB. nigra plants were negatively affected by competition while the larger, olderP. virgatum plants were not. Thus, relative germination date is important in determining the relative competitive ability of these two species. Grazing by collembola did not affect offset competition. The grazing of VAM hyphae by collembola appears to increase N availability in this experimental system. Under simultaneous competition, this N is taken up by the more extensive root system ofB. nigra; under offset conditions, the root system of the smallerB. nigra plants is insufficient to take advantage of the added resources. Thus, we suggest that grazing-induced transient changes in nutrient availability and the differential abilities of the two species to make use of these added resources constitute the mechanisms by which relative germination date and collembola grazing influence competition.     

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)     

Root foraging traits and competitive ability in heterogeneous soils

The responses of plant roots to nutrient patches in soil may be an important component of competitive ability. In particular, the scale, precision, and rate of foraging for patchy soil resources may influence competitive ability in heterogeneous soils. In a target–neighbor experiment in the field, per-individual and per-gram competitive effects were measured for six old-field species with known root foraging scale, precision, and rate. The presence and number of nutrient patches were also manipulated in a full factorial design. Number and presence of patches did not influence the outcome of competition. Competitive ability was not related to total plant size, growth rate, or root:shoot allocation, or to root foraging precision. Per-individual competitive effects were marginally correlated with root foraging scale (biomass of roots) and root foraging rate (time required to reach a patch). Therefore, competitive ability was more closely related to ability to quickly fill a soil volume with roots than to ability to preempt resource-rich patches.