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Invasive species in monocultures in introduced habitats experience different competitive conditions compared with their native habitats. Invasive monospecific stands can be composed of highly related individuals, creating high opportunity for kin selection in invaded habitats. We investigated the responses of North American and European ecotypes of the invasive species garlic mustard (Alliaria petiolata (M. Bieb.) Cavara & Grande) to aspects of the competitive environment including density, presence and identity of conspecific neighbors. Several aboveground morphological and performance traits responded to density independent of the origin of the plants. Belowground, however, North American ecotypes allocated more resources to roots and particularly to the taproot portion of their root system, while petiole elongation was density-dependent with North American ecotypes showing reduced elongation in high density compared to European ecotypes. These results were consistent with the evolution of reduced competitive ability in garlic mustard and indicated better resource storage in the introduced environment. Differential responses to neighbour identity indicated the ability for kin recognition in this species. Thus, a combination of natural and kin selection favouring better resource storage and less intraspecific competition, aided by the ability for kin recognition, may be responsible for the success of garlic mustard as an invasive species in North America. Key words: density, kin recognition, phenotypic plasticity, invasive, competition, allocation Les espèces envahissantes en monoculture dans les habitats introduits connaissent des conditions de compétition différentes de celles de leurs habitats d'origine. Les peuplements monospécifiques envahissants peuvent être composés d'individus hautement apparentés, ce qui crée une forte opportunité de sélection de parentèle dans les habitats envahis. Les auteurs ont étudié les réponses des écotypes nord-américains et européens de l'alliaire officinale (Alliaria petiolata (M. Bieb.) Cavara & Grande), une espèce envahissante, à des facteurs d'un environnement compétitif, y compris la densité, la présence et l'identité des voisins conspécifiques. Plusieurs caractéristiques morphologiques et de performance de surface répondaient à la densité indépendamment de l'origine des plantes. Sous terre, cependant, les écotypes nord-américains allouaient plus de ressources aux racines, et en particulier à la partie racine pivotante de leur système racinaire, tandis que l'élongation du pétiole dépendait de la densité, les écotypes nord-américains montrant une élongation réduite à haute densité par rapport aux écotypes européens. Ces résultats étaient cohérents avec l'évolution de la capacité compétitive réduite de l'alliaire officinale et indiquaient un meilleur stockage des ressources dans l'environnement introduit. Les réponses différentielles à l'identité du voisin indiquent la capacité de reconnaissance de la parenté chez cette espèce. Ainsi, la combinaison de la sélection naturelle et de la sélection de parentèle favorisant un meilleur stockage des ressources et une moindre compétition intraspécifique, aidée par la capacité de reconnaissance des parents pourrait être responsable du succès de l'alliaire officinale en tant qu'espèce envahissante en Amérique du Nord. [Traduit par la Rédaction] Mots-clés : densité, reconnaissance de parenté, plasticité phénotypique, invasif, compétition, allocation, Introduction Rapid evolution on introduced plant species is not uncommon (Buswell et al. 2011). When introduced into new habitats, plants can experience rapid evolution in dispersal ability (Cheptou et al. [...]

Coreopsis tinctoria (Asteraceae) is a widely-distributed North American prairie flowering [C.sub.3] annual. We hypothesized priority effects of planting C. tinctoria before a [C.sub.4] grass would decrease interspecific competition. Coreopsis tinctoria was grown in south-central Texas (29[degrees]35'N, 98[degrees]37'W) with Bouteloua curtipendula (Poaceae), a widespread C, prairie grass. The two species were planted in a de Wit replacement series with plant ratios of 12:0, 10:2, 8:4, 6:6, 4:8, 2:10, and 0:12 (C. tinctoria:B. curtipendula). There were five planting offsets with C. tinctoria planted 60 d before B. curtipendula (-60), 30 d before (-30), at the same time (0), 30 d after (30), and 60 d after (60). There were significant species by offset, species by frequency, and offset by frequency interactions. Coreopsis tinctoria produced the greatest total dry mass per plant in the 60 d before offset. Coreopsis tinctoria was competitively favored when planted 60 d before B. curtipendula and B. curtipendula was favored when planted 30 and 60 d before C. tinctoria. Results suggest a synergistic benefit from both spatial gaps and priority effects., INTRODUCTIONPlant competition is a long-studied ecological phenomenon, because of its role in controlling plant communities. Both intra- and interspecific competition are considered factors controlling plant community assembly, population density, distribution, [...]

In this study, we evaluated the performance of 18 competition indices composed of nine distance-dependent and nine distance-independent indices in explaining the variation in individual-tree diameter growth of Calabrian pine (Pinus brutia Ten.) in the central Mediterranean region of Turkey. The data were obtained from 432 sample plots with varying stand age, site index, and stand density. To evaluate the performance of each competition index, the mean square error reduction approach was used relative to the noncompetition. Also, this study compared fixed and mixed effects models to analyze diameter growth. Statistical analyses showed that the best distance-independent competition indices performed as well as the best distance-dependent competition indices. The distance-independent competition index of Schroder and Gadow(1999; Can. J. For. Res. 29(2): 280-283, doi:10.1139/x98-199) performed best and is recommended for use in future growth and yield models to be used in the central Mediterranean region of Turkey. Also, the best selection of competitive neighbors was achieved using the area of influence overlap method, whereas the fixed-radius and angle count sampling methods had no significant improvement in quantifying the competition effects. On the other hand, all mixed effects models provided much better fits than their fixed model counterparts. Key words: diameter growth, competition indices, selection of competitor, fixed and mixed models, Calabrian pine. Dans cette etude, nous evaluons la performance de 18 indices de competition, comprenant neuf indices dependants des distances et neuf indices independants des distances, pour expliquer la variation de la croissance en diametre d'individus de pin de Calabre (Pinus brutia Ten.) dans la region mediterraneenne centrale de la Turquie. Les donnees ont ete obtenues de 432 placettes echantillons couvrant une gamme d'age du peuplement, d'indice de qualite de station et de densite du peuplement. Pour evaluer la performance de chaque indice de competition, nous avons utilise l'approche de la reduction de l'erreur quadratique moyenne par rapport a l'absence de competition. De plus, cette etude compare des modeles a effets fixes et mixtes pour analyser la croissance en diametre. Les analyses statistiques montrent que les meilleurs indices de competition independants des distances sont aussi performants que les meilleurs indices dependants des distances. L'indice de competition independant des distances de Schroder et Gadow (1999; Can. J. For. Res. 29(2): 280-283, doi:10.1139/x98-199) est le plus performant et son utilisation est recommandee pour les futurs modeles de croissance et de production qui seront utilises dans la region mediterraneenne centrale de la Turquie. De plus, la meilleure selection des arbres voisins concurrents a ete realisee a l'aide la methode de chevauchement de l'aire d'influence alors que les methodes a rayon fixe et par balayage sous angle constant n'ont pas significativement ameliore la quantification des effets de competition. Par ailleurs, tous les modeles a effets mixtes ont produit de bien meilleurs ajustements que les modeles a effets fixes equivalents. [Traduit par la Redaction] Mots-cles: croissance en diametre, indices de competition, selection du competiteur, modeles a effets fixes et mixtes, pin de Calabre., Introduction Competition indices are the numerical expression of the degree of use of the growth potential limited by the genotype of a tree species (Pretzcsh 2009). The success of a [...]

Forest decline is frequently associated with infection; however, infections habitually affect trees that have been previously debilitated by environmental stress. Nevertheless, the causes and physiology of noninfectious forest decline are not well known. Some Juniperus thurifera L. forests presented severe (noninfectious) declines, with defoliations over 50%. The goal of this study was to determine the causes and characterize the physiology of this noninfectious decline, and we hypothesized that it could be related to environmental stress from increasing interspecific competition resulting from land abandonment. We randomly assigned 60 trees to either a control group or one of two competition-release treatments, i.e., vegetation clearing or soil ploughing. We characterized the physiological state of the trees both before treatment application and 14 months after treatment application. The J. thurifera trees that were declining experienced significantly lower soil nutrient and water availability, which resulted in lower leaf nutrient concentrations, lower photosynthetic rates, higher water stress, and arrested growth and reproduction. We confirmed that competition release increased nutrient availability and acquisition, reduced water deficit, improved photosynthetic rates, and abruptly stopped defoliation. Competition plays an increasingly critical role in forest conservation, particularly with the reported increase in the number of species colonizing previously unfeasible habitats due to global change and the absence of traditional activities that used to buffer competition. Key words: forest management, forest decline, global change, Juniperus thurifera, land abandonment. Le deperissement des forets est souvent associe a des agents biotiques; cependant, les agents biotiques infectent habituellement les arbres deja affaiblis par un stress environnemental. Malgre tout, les causes et la physiologie des deperissements qui ne sont pas associes a des agents biotiques sont peu connues. Certaines forets de Juniperus thurifera L. ont subi des deperissements severes d'origine abiotique qui ont cause plus de 50 % de defoliation. Nous avons cherche a determiner les causes et caracteriser la physiologie de ce deperissement d'origine abiotique et nous avons emis l'hypothese qu'il pourrait etre relie a un stress environnemental provoque par l'augmentation de la competition interspecifique a la suite de l'abandon des terres. Nous avons aleatoirement assigne 60 arbres a un groupe temoin ou a l'un ou l'autre des deux traitements visant a eliminer la competition : le defrichage et le labourage. Nous avons caracterise l'etat physiologique des arbres avant et 14 mois apres l'application des traitements. Les arbres qui deperissaient ont souffert d'une disponibilite en eau et en nutriments du sol significativement reduite entrainant une plus faible concentration de nutriments dans les feuilles, un taux de photosynthese plus bas, un stress hydrique plus prononce ainsi que l'arret de la croissance et de la reproduction. Nous confirmons que l'elimination de la competition a accru la disponibilite et l'acquisition des nutriments, reduit le deficit hydrique, ameliore le taux de photosynthese et brusquement arrete la defoliation. La competition joue un role de plus en plus important dans la conservation de la foret, particulierement avec l'augmentation rapportee du nombre d'especes qui colonisent des habitats jusque-la impraticables a cause du changement global et en l'absence d'activites traditionnelles qui avaient l'habitude de reguler la competition. [Traduit par la Redaction] Mots-cles: amenagement forestier, deperissement de la foret, changement global, Juniperus thurifera, abandon des terres., Introduction Forest decline is frequently associated with infection (Franklin et al. 1987; McGraw et al. 1990); however, pathogens are normally the executioners, the newcomers to a list of pre-existing factors [...]

Encroachment of woody species is a significant threat in native grasslands. This study exams the interaction between shrub-sapling overstory and ground layer species and functional group (FG) cover, richness, and diversity along a woody encroachment gradient. The study area is a 65 ha mosaic of tallgrass prairie and shrubland in northeastern Illinois. The study questions were: (1) how do plant species and FG cover, richness, and diversity respond to increasing levels of woody encroachment, (2) are there levels of encroachment relevant to restoration opportunities, and (3) are there patterns of FGs associated with increasing levels of encroachment that can serve as ecological indicators? Data on ground layer and overstory structure including stem density, canopy cover, and leaf area index (LAI) were recorded; LAI explained the most variance in the ground layer data. There was a significant decline in several ground layer species and FG parameters with increasing woody encroachment (LAI) and percent bare ground increased. For most parameters, change followed a linear response; however, native species diversity and FG density declined only after LAI reached intermediate levels and the changes are best fit by a quadratic function. Cover and species richness of perennial dicot (PD) forbs, [C.sub.3] and [C.sub.4] grasses, legumes and hemi-parasites declined with increasing LAI. Native FGs most strongly associated with the lowest LAI levels were hemi-parasites, [C.sub.4] grasses, legumes, and PD forbs. Efficacious opportunities for restoration remain at this site following low-to-intermediate levels of woody encroachment and results highlight ordered patterns of decline in cover and richness of FGs that may guide evaluating restoration potential of sites undergoing woody encroachment., INTRODUCTION Increasing woody encroachment in savanna and grassland communities has been widely reported (Bragg and Hurlbert, 1976; Archer et al., 1995; Hoch and Briggs, 1999; Brown and Archer, 1999; Price [...]

Early establishment of competing vegetation often presents an obstacle to the success of planted white spruce (Picea glauca (Moench) Voss) seedlings. We followed growth and development of white spruce and associated vegetation for up to 17 years in Alaska's boreal forests to quantify roles of overtopping plant cover in suppressing conifers. The three study areas represented a range of site conditions of varying productivity and species of competing cover, different site preparation and release treatments, and different bare-root and container white spruce stock types. Herbaceous overtopping peaked early after planting and decreased as white spruce were able to outgrow competitors. Overtopping by shrubs and hardwoods, especially aspen (Populus tremuloides Michx.) and resin birch (Betula neoalaskana Sarg.) peaked somewhat later than herbaceous overtopping and decreased over time for most sites and treatments. In a model that combined all sites, vegetation management treatments, and years, overtopping and previous year's volume explained approximately 85% of the variation in volume growth. Increasing the size of planting stock helped reduce overtopping, hence suppression, even in treatments dominated by hardwood species. Results suggested that control of overtopping was essential for maximum growth and long-term or increasing levels of overtopping severely suppressed white spruce seedling growth. Resume: L'etablissement precoce de la vegetation concurrente compromet souvent le succes des semis plantes d'epicea blanc (Picea glauca (Moench) Voss). Nous avons suivi la croissance et le developpement de l'epicea blanc et de la vegetation associee pendant 17 ans dans les forets boreales de l'Alaska, dans le but de quantifier le role que joue le couvert vegetal domine par les especes concurrentes dans la suppression des coniferes. Les trois zones d'etudes representaient un eventail de conditions de station incluant differentes productivites et compositions en especes du couvert forme par la vegetation concurrente, differentes preparations de terrain et differents traitements de degagement, ainsi que differents types de plants d'epicea blanc, a racines nues ou en recipient. La domination des plantes herbacees a atteint un sommet tot apres la plantation et a diminue par la suite etant donne que l'epicea a reussi a depasser les competiteurs. La domination des arbustes et des feuillus, particulierement le peuplier faux-tremble (Populus tremuloides Michx.) et le bouleau d'Alaska (Betula neoalaskana Sarg.), a culmine un peu plus tard que celle des plantes herbacees et a diminue avec le temps dans la plupart des stations et des traitements. Dans un modele qui combinait toutes les stations, tous les traitements de gestion de la vegetation et toutes les annees, la domination des especes concurrentes et le volume de l'annee precedente expliquaient approximativement 85 % de la variation de l'accroissement en volume. L'augmentation de la taille des plants a contribue a reduire la domination des especes concurrentes, et par consequent la suppression, meme dans les traitements domines par des especes feuillues. Les resultats indiquent que la maitrise de la domination des especes concurrentes est essentielle pour une croissance maximale et que la domination de longue duree ou de forte intensite des especes concurrentes reduit serieusement la croissance des semis d'epicea blanc. [Traduit par la Redaction], Introduction Forests have been identified as critical components to a sustainable future (FAO 2012). Planting trees can be used to establish or restore biodiversity and ecosystem function (Ciccarese et al. [...]

Competition in forest stands has long been of interest to researchers. However, much of the knowledge originates from empirical studies that examined the effects of competition. For instance, many studies were focused on the effects of the presence of herbaceous species on the development of tree seedlings or the decrease in individual tree growth with increases in stand density. Several models that incorporate competitive effects have been developed to predict tree and stand growth, but with simplified representations of competitive interactions. While these studies provided guidance useful for forest management, they contributed only partially to furthering our understanding of competitive mechanisms. Also, most competition studies were conducted in single-species stands. As competitive interactions occurring in mixed stands are characterized by a higher degree of complexity than those in single-species stands, a better understanding of these mechanisms can contribute to developing optimal management scenarios. The dynamics of forest stands with at least two species may be affected not only by competition, but also by facilitation or complementarity mechanisms. Thus, knowledge of the mechanisms may provide insight into the relative importance of intra- versus inter-specific competition and whether competition is symmetric or asymmetric. Special attention to the implementation of field experimental designs is warranted for mixed stands. While traditional spacing trials are appropriate for single-species stands, the examination of competitive interactions in mixed stands requires more complex experimental designs to examine the relative importance of species combinations. Forest productivity models allow resource managers to test different management scenarios, but again most of these models were developed for single- species stands. As competitive interactions are more complex in mixed stands, models developed to predict their dynamics will need to include more mechanistic representations of competition. Key words: competition, facilitation, complementarity, experimental designs, site resources, modelling. Resume : Les chercheurs s'interessent depuis longtemps a la competition dans les peuplements forestiers. Cependant, la majeure partie des connaissances provient d'etudes empiriques n'examinant que les effets de la competition. Par exemple, plusieurs etudes se sont concentrees sur les effets de la presence d'especes herbacees sur le developpement des plantules d'arbres ou sur la diminution de la croissance des arbres individuels avec l'augmentation de la densite. Plusieurs modeles incorporant les effets de la competition ont ete developpes pour predire la croissance des arbres et des peuplements, avec cependant des representations simplifiees des interactions competitives. Bien que ces etudes fournissent des pistes utiles pour l'amenagement forestier, elles contribuent seulement partiellement a ameliorer notre comprehension des mecanismes competitifs. De plus, la plupart des etudes sur la competition ont ete conduites dans des peuplements monospecifiques. Puisque les interactions competitives survenant dans les peuplements mixtes se caracterisent par un degre de complexite plus eleve que celles des peuplements monospecifiques, une meilleure comprehension de ces mecanismes pourra contribuer au developpement de scenarios d'amenagement optimises. La dynamique des peuplements comportant au moins deux essences peut se voir affectee non seulement par la competition, mais aussi par des mecanismes de facilitation ou de complementarite. Ainsi, la connaissance des mecanismes peut jeter un nouvel eclairage sur l'importance relative de la competition intra- vs inter-specifique et determiner s'il s'agit de competition symetrique ou asymetrique. Une attention speciale lors de la conception de plans experimentaux est essentielle pour les peuplements melanges. Alors que les essais d'espacement traditionnels conviennent pour les peuplements monospecifiques, l'examen des competitions interactives necessite des plans experimentaux plus complexes pour evaluer l'importance relative des combinaisons d'especes. Les modeles de productivite forestiere permettent a l'amenagiste des ressources de verifier divers scenarios d'amenagement, mais encore une fois la plupart de ces modeles ont ete developpes pour des peuplements monospecifiques. Les interactions competitives etant plus complexes dans les peuplements mixtes, les modeles developpes pour predire leur dynamique devront desormais inclure de plus fortes representations des mecanismes de competition. [Traduit par la Redaction] Mots-cles: competition, facilitation, complementarite, plans experimentaux, ressources des sites, modelisation., Introduction Although competition is a widespread phenomena [sic] exhibited by organisms from microbes to mammals, empirical evidence is elusive. Controlled laboratory experiments usually involve simplified competitive systems whose results cannot [...]

: In a forest stand, competition plays a central role, affecting individual growth. The size-growth relationship (SGR) indicates whether large trees grow proportionally more than (asymmetric SGR), equal to (symmetric), or less than (inversely asymmetric) smaller trees. SGR is thus an indicator of the growth partitioning and competition intensity within a stand. Using tree-ring analysis, we investigated long-term trends and interannual variability of SGR in several Norway spruce (Picea abies (L.) Karst.) stands in the Paneveggio Forest (eastern Italian Alps) over a 100-year period. The study plots were characterized by different stand structures (one multilayered and two monolayered) and disturbance histories (different dates of logging). Logging conducted until the 1940s induced an inversely asymmetric SGR in all the plots. During the successive five decades, in the monolayered plots, it shifted to direct asymmetric (plot 1) and to symmetric (plot 2). In the multilayered plot (plot 3), SGR remained inversely asymmetric. A direct effect of climate on SGR interannual variability was not found. However, fast-growing trees had a stronger climatic signal than slow-growing trees, indicating that growth rate affects tree response to climate. Moreover, we observed that sensitivity to climate was reduced in the monolayered plots over the study period, possibly as a consequence of increased competition. Resume: La competition joue un role central dans les peuplements forestiers etant donne l'influence qu'elle exerce sur la croissance des individus. La relation taille-croissance (RTC) indique si les gros arbres ont une croissance proportionnelle-ment plus grande (RTC asymetrique), egale (RTC symetrique) ou plus faible (RTC inversement asymetrique) que les arbres plus petits. La RTC est par consequent un indicateur de la repartition de la croissance et de l'intensite de la competition a l'interieur d'un peuplement. Al'aide de la dendrochronologie, nous avons etudie les tendances a long terme et la variabilite interannuelle de la RTC dans plusieurs peuplements d'epicea commun (Picea abies (L.) Karst.) de la foret de Paneveggio (Alpes italiennes orientales) sur une periode de 100 ans. Les parcelles echantillons etaient caracterisees par differentes structures de peuplement (une parcelle multietagee et deux parcelles monoetagees) et des historiques de perturbation distincts (differentes dates de recolte). Jusque dans les annees 1940, la recolte a engendre une RTC inversement asymetrique dans toutes les parcelles. Au cours des cinq decennies qui ont suivi, la RTC est devenue asymetrique (parcelle 1) et symetrique (parcelle 2) dans les parcelles monoetagees. La RTC est demeuree inversement asymetrique dans la parcelle multietagee (parcelle 3). Aucun effet direct du climat n'a ete observe. Cependant, les arbres a croissance rapide avaient un plus fort signal climatique que les arbres qui croissaient lentement, ce qui indique que le taux de croissance influence la reaction des arbres au climat. De plus, nous avons observe que la sensibilite au climat etait reduite dans les parcelles monoetagees pendant la duree de l'etude, possiblement a cause d'une plus forte competition. [Traduit par la Redaction], Introduction Competition between plants affects the ability of individuals to acquire resources, influencing growth and mortality patterns. In plant populations, partitioning of resources is defined as size-asymmetric when larger plants, [...]

An efficient and accurate detection of the contribution of legume dry matter (DM) in legume--grass mixtures is of great importance for a targeted management of legume-based swards. Legume coverage is a frequently used parameter to describe the dominance structure in vegetation because it is easily accessible. The present study investigates the relationships between legume coverage and different legume species and growth stages. A dataset was generated from a pot experiment that was conducted under controlled conditions across a wide range of legume species (Trifolium repens L., T. pratense L., Medicago sativa L.), proportions of legumes (0-800 g [kg.sup.-1]), and growth stages (tillering to heading). Multiple regression analysis revealed a positive relationship between legume contribution and coverage for the separate legume species ([R.sup.2] = 0.98-0.99), as well as for the combined dataset including all legumes ([R.sup.2] = 0.98). Total biomass was related to clover coverage in a complex manner, reducing legume contribution by up to 180 g [kg.sup.-1] with an increase of total biomass from 300 to 1500 kg DM [ha.sup.-1]. The validation of the models with mixed swards of grass and either white or red clover that were grown in the field showed good results ([r.sup.2] = 0.98). doi: 10.2135/cropsci2009.10.0592

Management practices following forest harvesting that modify' organic matter (OM) inputs and influence changes in the soil environment have the potential to alter soil C pools, but there is still much uncertainty regarding how these practices influence soil C flux. We examined the influence of varying amounts of logging-debris retention (0, 40, and 80% coverage) and vegetation control (initial or annual applications) on in situ bulk soil respiration, microbial respiration, and total soil C at two Doughs-fir [Pseudotsuga menziesii (Mirb.) Franco] sites. Annual vegetation control decreased bulk respiration, which was attributed to reduced root respiration and OM inputs when competing vegetation was absent. There was no difference in potential microbial respiration or total soil C pools between vegetation-control treatments, indicating that OM inputs from competing vegetation were rapidly consumed in situ. Logging-debris retention reduced bulk respiration, microbial respiration, and soil temperature, but the significance and magnitude of the difference were variable. A soil temperature function explained between 44 and 76% of the variation in microbial and bulk respiration, but there was no effect of reduced temperature on microbial respiration with 40% coverage. Total soil C at the end of the experiment was higher with 80% coverage at the site with relatively low initial total soil C, but there was also a significant increase in total soil C at both sites when the logging debris was removed. The results suggest that root decomposition following harvesting causes an increase in total soil C, which is dependent on the magnitude of logging-debris retention, its influence on the soil environment, and ultimately the microbial response. Abbreviations: DOC, dissolved organic carbon; OM, organic matter; SOM, soil organic matter; SWC, soil water content. doi: 10.2136/sssaj2009.0234

Seed size commonly varies by five to six orders of magnitude among coexisting plant species, a pattern ecologists have long sought to explain. Because seed size trades off with seed number, small-seeded species clearly have the advantage in fecundity, but what is the countervailing advantage of large seeds? Higher competitive ability combined with strong competitive asymmetry can in theory allow coexistence through a competition--colonization trade-off, but empirical evidence is inconsistent with this mechanism. Instead, the key advantage of large seeds appears to be their tolerance of stresses such as shade or drought that are present in some but not all regeneration sites. Here I present a simple, analytically tractable model of species coexistence in heterogeneous habitats through a tolerance--fecundity trade-off. Under this mechanism, the more tolerant species win all of the more stressful regeneration sites and some of those that are less stressful, whereas the more fecund species win most but not all of the less stressful sites. The tolerance--fecundity trade-off enables stable coexistence of large numbers of species in models with and without seed limitation. The tolerance--fecundity mechanism provides an excellent explanation for the maintenance of diversity of seed size within plant communities and also suggests new hypotheses for coexistence in animal and microbial communities. species coexistence | habitat partitioning | competition--colonization tradeoff | stress tolerance | seed mass doi/10.1073/pnas.0911637107

Transgenic herbicide-resistant cultivars of creeping bentgrass (Agrostis stolonifera L.) have been developed and are being considered for commercial release. Concerns regarding transgenic species include potential for outcrossing of the crop, which may produce novel genotypes with increased weediness. Persistence and multiyear growth of interspecific hybrids between creeping bentgrass and velvet bentgrass (A. canina L.), colonial bentgrass (A. capillaris L.), dryland bentgrass (A. castellana Boiss. and Reut.), and redtop bentgrass (A. gigantea Roth) and the backcross progeny of the hybrids were compared with the parental species in field experiments. The plants were evaluated over 2 yr in residential Kentucky bluegrass (Poa pratensis L.) and a low maintenance roadside turf of mixed species. With the exception of redtop bentgrass by creeping bentgrass interspecific hybrids, all interspecific hybrids between creeping bentgrass and colonial, dryland, and velvet bentgrasses were equally or less competitive relative to their parental species. Backcross progeny of interspecific hybrids, including those of redtop bentgrass by creeping bentgrass hybrids, were equally or less competitive than parental species. Bentgrasses that were highly competitive in residential turf were also competitive under roadside maintenance, suggesting bentgrasses have weediness potential regardless of management intensity. doi: 10.2135/cropsci2008.07.0431

In high-diversity communities, rare species encounter one another infrequently and therefore may compete more intensely with common species or guilds for limiting space and resources. In addition, rare species may be strongly recruitment limited because of their low abundances. Under these conditions, stochastic dispersal and immigration history can have an important influence on community structure. We tested the hypothesis that local immigration and competition from common, large-stature guilds interact to structure local biodiversity in high-diversity longleaf pine savanna groundcover assemblages (>30 species/[m.sup.2]). In two factorial field experiments, we increased local immigration by adding seeds of 38 mostly rare, small-stature forbs and sedges to plots physically dominated by either a common, large-stature bunchgrass or shrub species and to plots in which competition from these dominant guilds was reduced. We measured species richness and abundance at two spatial scales (0.01 and 0.25 [m.sup.2]) over two years. Immigration increased total species richness and richness of focal seed addition species regardless of levels of competition with bunchgrasses and shrubs, indicating that many rare, small-stature species can recruit in the face of potential competition from dominant guilds. Removal of dominant guilds increased total and focal species richness in shrub-dominated but not bunchgrass-dominated plots. In addition, competition from both dominant guilds had no clear effect on rank--abundance distributions of focal species. Our results suggest a key role for dispersal assembly in structuring local biodiversity in this high-diversity plant community, but the importance of this mechanism depends on the strength of local niche assembly involving competition from some, but not all, dominant guilds. Key words: biodiversity; bunchgrass; community assembly; dispersal assembly; dominant guild,' local immigration; longleaf pine savanna; niche assembly; recruitment limitation; shrub; species coexistence; species-rich community.

Biological invasions can change ecosystem function, have tremendous economic costs, and impact human health: understanding the forces that cause and maintain biological invasions is thus of immediate importance. A mechanism by which exotic plants might displace native plants is by increasing the pressure of native consumers on native plants, a form of indirect interaction termed 'apparent competition.' Using experimental exclosures, seed addition, and monitoring of small mammals in a California grassland, we examined whether exotic Brassica nigra increases the pressure of native consumers on a native bunchgrass, Nassella pulchra. Experimental plots were weeded to focus entirely on indirect effects via consumers. We demonstrate that B. nigra alters the activity of native small-mammal consumers, creating a gradient of consumption that dramatically reduces N. pulchra establishment. Previous work has shown that N. pulchra is a strong competitor, but that it is heavily seed limited. By demonstrating that consumer pressure is sufficient to curtail establishment, our work provides a mechanism for this seed limitation and suggests that, despite being a good competitor, N. pulchra cannot reestablish close to B. nigra within its old habitats because exotic-mediated consumption preempts direct competitive exclusion. Moreover, we find that apparent competition has a spatial extent, suggesting that consumers may dictate the rate of invasion and the area available for restoration, and that nonspatial studies of apparent competition may miss important dynamics. Key words: apparent competition; biological invasion; Brassica nigra; California (USA) grassland; consumers; granivory; herbivory; Nassella pulchra.

Interactions among plants have been hypothesized to be context dependent, shifting between facilitative and competitive in response to variation in physical and biological stresses. This hypothesis has been supported by studies of the importance of positive and negative interactions along abiotic stress gradients (e.g., salinity, desiccation), but few studies have tested how variation in biotic stresses can mediate the nature and strength of plant interactions. We examined the hypothesis that herbivory regulates the strength of competitive and facilitative interactions during succession in Argentinean marshes dominated by Spartina densiflora and Sarcocornia perennis. Spartina densiflora is preferred by the dominant herbivore in the system, the crab Chasmagnathus granulatus. We experimentally manipulated crab herbivory, plant structure, and shade, and we found that, when herbivory was low in the spring and summer, competitive interactions between plants were dominant, but in the fall, when herbivory was highest, facilitative interactions dominated, and Spartina densiflora survival was completely dependent upon association with Sarcocornia perennis. Moreover, experimental removal of Sarcocornia perennis across recently disturbed tidal fiats revealed that, while Sarcocornia perennis positively affected small Spartina densiflora patches by decreasing herbivory, as patch size increases and they can withstand the impact of herbivory, competitive interactions predominated and Spartina densiflora ultimately outcompeted Sarcocornia perennis. These results show that herbivory can mediate the balance between facilitative and competitive processes in vascular plant communities and that the strength of consumer regulation of interactions can vary seasonally and with patch size. Key words: Chasmagnathus granulatus; competition; facilitation; herbivory threshold; Sarcocornia perennis; southwest Atlantic; Spartina densiflora.

How resource availabilities affect the competitiveness of Canada blue joint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] is poorly understood. Bluejoint is a widespread grass species in boreal forests and competes with tree species such as white spruce [Picea glauca (Moench) Voss] for belowground resources (e.g., soil N and water) when their supply is limited. In this greenhouse-based study, we tested the following hypotheses: (i) bluejoint competition reduces white spruce growth when belowground resource availabilities are limited; (ii) greater N and water availabilities may increase bluejoint competition and its adverse effects on white spruce growth; and (iii) white spruce foliar [delta][sup.13]C and [delta][sup.15]N are affected by soil N and water availabilities and bluejoint competition. A 2 x 2 x 2 (competition x N availability x water availability) factorial experiment was conducted using pots of planted white spruce seedlings with or without bluejoint. Bluejoint competition reduced the volume index ([diameter.sup.2] x height) of white spruce by 50%. The competitiveness of bluejoint appeared to be independent of resource availabilities, but bluejoint had greater growth response to increased N availability than white spruce. Bluejoint competition depleted white spruce foliar [delta][sup.13]C and [delta][sup.15]N by 1.2 and 1.2 [per thousand], respectively, even under adequate water supply, indicating that N deficiency caused by bluejoint competition had a dominant effect (increasing [sup.13]C discrimination during photosynthesis) compared with the potential effect of drought stress on foliar [delta][sup.13]C, and that strong N[H.sub.4] uptake by bluejoint may have prevented significant soil N losses and [sup.15]N enrichment through nitrification and subsequent denitrification.

Interspecific competition between plants and herbivory by specialized insects can have synergistic effects on the growth and performance of the attacked host plant. We tested the hypothesis that competition between plants may also negatively affect the performance of herbivores as well as their top-down effect on the host plant. In such a case, the combined effects of competition and herbivory may be less than expected from a simple multiplicative response. In other words, competition and herbivory may interact antagonistically. In a greenhouse experiment, Poa annua was grown in the presence or absence of a competitor (either Plantago lanceolata or Trifolium repens), as well as with or without a Poa-specialist aphid herbivore. Both competition and herbivory negatively affected Poa growth. Competition also reduced aphid density on Poa. This effect could in part be explained by changes in the biomass and the nitrogen content of Poa shoots. In treatments with competitors, reduced aphid densities alleviated the negative effect of herbivory on above- and belowground Poa biomass. Hence, we were able to demonstrate an antagonistic interaction between plant-plant interspecific competition and herbivory. However, response indices suggested that antagonistic interactions between competition and herbivory were contingent on the identity of the competitor. We found the antagonistic effect only in treatments with T. repens as the competitor. We conclude that both competitor identity and the herbivore's ability to respond with changes in its density or activity to plant competition affect the magnitude and direction (synergistic vs. antagonistic) of the interaction between competition and herbivory on plant growth. Key words: antagonistic interactions; aphids; insect herbivory; Plantago lanceolata; plant competition; Poa annua; Rhopalosiphum padi; Trifolium repens.

Abstract.--Interactive effects of red brome grass (Bromus rubens) density and time of establishment on the early survival and growth of blackbrush (Coleogyne ramosissima) seedlings were quantitatively investigated. Seeds of Coleogyne [...]

Crop seed size affects the competitive relationship between spring wheat (Triticum aestivum L.) and wild oat (Avenafatua L.). However, the mechanisms associated with the process are not known. The effect of wheat seed size on wild oat competition was assessed by a mechanistic approach involving yield and its determinants in these species. Wheat plants established from large and small seed were evaluated under different seeding rates and wild oat densities during 1999-2001 near Kalispell, MT. Linear structural model systems based on ontogenic diagrams were constructed for each seed size class. Spikes [m.sup.-2] and panicles [m.sup.-2] had the greatest positive effect on yield within each species. For wheat, the impact of the two later-formed yield components on yield decreased in an ontogenic manner, whereas for wild oat, their relative contributions were similar in magnitude. Wheat plants derived from large seed had a noticeable negative effect on wild oat via a reduction in panicles [m.sup.-2] and seed weight, whereas wheat established from small seed mainly affected wild oat panicles [m.sup.-2]. Wild oat competition reduced wheat spikes [m.sup.-2] and kernels [spike.sup.-1] in both seed size classes. However, these reductions were less for plants derived from large seed, which demonstrated enhanced compensatory ability. In summary, nongenetic variations in crop seed size affected the competitive dynamics between these species, where the major crop--weed interference mechanism involved wild oat seed weight., WILD OAT is a weed of economic importance in the Pacific Northwest and northern Great Plains cereal production regions of the USA and Canada. Wild oat competition can reduce wheat [...]

Over the past 30 years, numerous attempts to understand the relationship between offspring size and fitness have been made, and it has become clear that this critical relationship is strongly affected by environmental heterogeneity. For marine invertebrates, there has been a long-standing interest in the evolution of offspring size, but there have been very few empirical and theoretical examinations of post-metamorphic offspring size effects, and almost none have considered the effect of environmental heterogeneity on the offspring size/fitness relationship. We investigated the post-metamorphic effects of offspring size in the field for the colonial marine invertebrate Botrylloides violaceus. We also examined how the relationship between offspring size and performance was affected by three different types of intraspecific competition. We found strong and persistent effects of offspring size on survival and growth, but these effects depended on the level and type of intraspecific competition. Generally, competition strengthened the advantages of increasing maternal investment. Interestingly, we found that offspring size determined the outcome of competitive interaction: juveniles that had more maternal investment were more likely to encroach on another juvenile's territory. This suggests that mothers have the previously unrecognized potential to influence the outcome of competitive interactions in benthic marine invertebrates. We created a simple optimality model, which utilized the data generated from our field experiments, and found that increasing intraspecific competition resulted in an increase in predicted optimal size. Our results suggest that the relationship between offspring size and fitness is highly variable in the marine environment and strongly dependent on the density of conspecifics. Key words: Botrylloides violaceus; competition; egg size; maternal effects; progeny size.

Dogma asserts that desert shrubs should become uniformly dispersed through time through a process of self-thinning caused by competition for water. We present long-term data (1985-2001) on two nearby populations of the perennial shrub, Chrysothamnus nauseosus ssp. consimilis, in sand-dune ecosystems near Mono Lake, California. Because previous studies established that water limited the growth, survival, and reproduction of this shrub, we searched for population-level patterns of competition. A seven-year history of survival, growth, and flowering of marked individuals delineated five demographic stage classes corresponding roughly to age and plant size: seedlings, nonflowering juveniles, juveniles that later flowered, adults that flowered intermittently, and 'immortal' adults that flowered annually. For all stages except nonflowering juveniles, initial size was a strong predictor of life expectancy, and for the reproductive classes, flowering was associated with larger size and higher survival. Survival of nonflowering juveniles was strongly related to depth to groundwater (consistent with the biology of phreatophytes, defined as plants that use deep soil moisture) corresponding to a critical demographic shift at this phreatophytic threshold necessary for long-term survival and reproduction. We therefore predicted that only the pre-phreatophytic stage classes dependent on depletable upper soil moisture should compete with close neighbors. Neither mixed-age population of C. nauseosus nor any single stage class within them was uniformly dispersed. Younger classes (seedling and juveniles) were aggregated, and older classes (from newly reproductive through oldest adults) were randomly dispersed with respect to their nearest neighbor. The younger the stage, the more aggregated was the dispersion. All five seedling cohorts became significantly less aggregated with time during the study, consistent with a hypothesis of competitive thinning. However, mortality was statistically density independent at a landscape scale in all classes and all years and resulted in thinning toward reduced aggregation, just as expected under competition. Two distinct stage-specific patterns occurred. Whereas the phreatophytic stage classes experienced only density-independent mortality, the pre-phreatophytic stage classes experienced opposing density dependence at different spatial scales. Seedling cohorts in particular exhibited strong direct density dependence within the first 1-3 years in a local neighborhood of 20 cm or less, in both survival (higher mortality in dense aggregations) and growth (seedlings in groups were smaller and grew more slowly than those not in groups). In contrast, these classes exhibited inversely density-dependent survival, and often growth, on a scale of 1-4 m caused by spatial heterogeneity and not facilitation such as by nurse plants. Although competition occurred for only the first 1-3 years of the life history, it caused size differences among individuals that increased through time and that significantly predicted long-term survival and eventual recruitment to the reproductive class. Despite this potential for long-lasting effects on population dynamics, competitive thinning was too brief and stage-structured spatial processes were too complex to result in a uniform dispersion of individuals in this mixed-age population. Key words: Chrysothamnus nauseosus; competitive thinning; density dependence; desert shrub demography; life history; phreatophyte; spatial dispersion.

1 In an asexually reproducing plant (Potamogeton pectinatus L.) the competitive advantage of a larger propagule over a smaller one was determined by a single parameter ([alpha] the competitive asymmetry coefficient), independent of sowing density or average propagule size. Competitive advantage was determined by the relative size difference between propagules instead of their absolute difference. 2 Productivity per germination site increased with the number and size of propagules present, even at densities high enough to result in decreased per capita yield due to plant competition. 3 Both the number of propagules produced by a plant and their size increased with increasing per capita resource capture. Therefore, propagule size was neither equal to the size of the propagule from which the plant germinated nor independent of resource status. 4 Our results clarify what assumptions should be made in ecological and evolutionary models dealing with competition for resources between seedlings. In particular, the use of relative size differences instead of absolute ones should lead to the evolution of smaller propagules than those expected if competitive advantage grew with absolute propagule size. Key-words: Competitive advantage, germination sites, Potamogeton pectinatus L., seed size.

1 A field experiment in a native grassland in Central Alberta, Canada, tested whether plants alter relative allocation to roots vs. shoots in response to below-ground competition, and whether the mass of a species' root system accounts for interspecific differences in below-ground competitive response. 2 Seedlings of each of 10 native species were transplanted into the naturally occurring vegetation in the field at the start of the growing season. Root interactions between the target plants and their neighbours were manipulated through the use of PVC root exclusion tubes, with target plants grown with or without potential root interactions with their neighbours. Neighbour shoots were also tied back, forcing any target-neighbour interactions to be below ground. 3 Below-ground competition generally reduced plant growth, with its relative magnitude varying among species. 4 An allometric analysis indicated that competition below ground did not result in an increase in the relative biomass allocated to roots for any of the 10 target species. This is counter to the growth-balance hypothesis (and optimal foraging theory). Belowground competition did increase root: shoot ratios, but this was due to reduced plant size (small plants have larger root: shoot ratios), rather than adaptive plasticity. 5 A species' below-ground competitive ability was not related to its root system size. Although this finding is counter to commonly made assumptions, it is supported by other work demonstrating below-ground competition to be generally size-symmetric. 6 Despite the majority of plant-plant interactions in grasslands being below ground, our understanding of plant competition above ground is significantly more robust. Several wide-spread assumptions regarding below-ground competition are suspect, and more multispecies studies such as this are required to provide a fuller picture of how plants respond to, and compete for, soil resources. Key-words: balanced-growth hypothesis, below-ground competition, community ecology, growth allometry, size-symmetric competition

1 Competition usually involves the allocation of limiting resources to non-reproductive functions. Natural selection is expected to favour mechanisms that increase competition with non-self neighbours and limit wasteful competition with self. 2 We used Pisum sativum plants that had two roots and 'double plants' with two shoots and two roots that could be either longitudinally separated into two genetically identical but physiologically distinct individuals or left intact. 3 Root development was significantly greater in split-root plants whose neighbours belonged to different plants. Furthermore, root development was relatively greater in the presence of roots of a different plant, regardless of its identity. This discrimination had a vectorial component whereby plants developed more and longer lateral roots towards neighbouring roots of different plants than towards other roots of the same plant. 4 The results thus demonstrate a mechanism of avoiding self-competition that is based on physiological co-ordination among different organs of the same plant rather than on allochemical recognition that depends on genetic differences. 5 The ability to discriminate between self and non-self could be expected to increase resource use efficiency and ecological performance in plants. It could also be expected that tight physiological co-ordination will be selected for between organs of the same plant that have greater probability of being engaged in direct competitive interactions with each other. Key-words: allorecognition, clonality, competition, double plants, physiological coordination, physiological integration, phenotypic plasticity, roots, split roots, two-shoot plants

A species-addition experiment showed that prairie grasslands have a structured, nonneutral assembly process in which resident species inhibit, via resource consumption, the establishment and growth of species with similar resource use patterns and in which the success of invaders decreases as diversity increases. In our experiment, species in each of four functional guilds were introduced, as seed, into 147 prairie-grassland plots that previously had been established and maintained to have different compositions and diversities. Established species most strongly inhibited introduced species from their own functional guild. Introduced species attained lower abundances when functionally similar species were abundant and when established species left lower levels of resources unconsumed, which occurred at lower species richness. Residents of the C4 grass functional guild, the dominant guild in nearby native grasslands, reduced the major limiting resource, soil nitrate, to the lowest levels in midsummer and exhibited the greatest inhibitory effect on introduced species. This simple mechanism of greater competitive inhibition of invaders that are similar to established abundant species could, in theory, explain many of the patterns observed in plant communities. biodiversity | invasibility | resource competition | functional guilds | ecological niche

1 Plant community theory often invokes competition to explain why species diversity declines as productivity increases. Competition for all resources might become more intense and lead to greater competitive exclusion or, alternatively, competition for light only could become more intense and exclude poor light competitors. 2 To test these hypotheses, we constructed communities of seven old-field species using combined monocultures. Constructs experienced no interspecific competition, only shoot competition or only root competition, with and without fertilizer. Diversity in these limited interaction communities was compared to diversity in unfertilized and fertilized mixtures of the seven species. 3 Diversity decreased with fertilization in mixtures and in communities with only root competition. Shoot competition had small effects on the community and did not contribute to changes in diversity with fertilization. 4 Root competition may strongly impact plant community structure in unproductive communities where light never becomes limiting, or under non-equilibrium conditions following human disturbances. Key-words: community structure, fertilization, old field, null community, combined monocultures design

Populus deltoides subsp, wislizinii (Salicaceae), a cottonwood native to the Middle Rio Grande of New Mexico, must potentially compete against exotic Tamarix ramosissima (Tamaricaceae) during establishment after flooding. We investigated competitive interactions between seedlings of Tamarix and Populus in two substrates representing field textures and declining (i.e., draw-down) or stagnant water tables. The experiment was performed using a full-additive series design and interpreted with response surface models for each species. As reflected in both aboveground mass and height, Populus suppressed aboveground growth of Tamarix across all treatments, whereas competitive effects of Tamarix against Populus could only be seen at low Populus densities. Clay substrates with draw-down stimulated the greatest growth and created the most intense competitive environment for both species. Tamarix was competitively suppressed in every substrate tested, with the weakest response in sand with no draw-down, where growth of Populus was poorest. These results suggest that stream flow management that promotes Populus establishment could also aid in controlling Tamarix invasion across a range of substrates. Key words: competition; flooding disturbance; invasion; Populus deltoides subsp, wislizinii; riparian management; Salicaceae; Tamaricaceae; Tamarix ramosissima.

1 We studied mortality, growth and spatial distribution of individuals of Lysimachia rubida to elucidate causes and consequences of temporal and spatial variations in mortality. This short-lived monocarpic biennial grows in open dry habitats on rocky coastal cliffs of the subtropical Bonin (Ogasawara) Islands, Japan. 2 All individuals of two successive cohorts that appeared in a 8 x 8-m quadrat were mapped, and their fate and size were recorded at 2-3 month intervals from March 1998 to July 2000. We analysed spatial and temporal changes in the relative importance of plant size, local densities of neighbouring plants and ground surface conditions to determine plant mortality. We also examined spatially non-random mortality and its spatial scales. 3 Patchy seed dispersal from scattered mother plants resulted in a clumped distribution of seedlings. During the growth cycle of L. rubida, we recognized two growth stages that differed in the relative importance of the factors affecting plant mortality. 4 At the earlier growth stage, after germination in March until July, mortality was caused mainly by local density. Smaller plants within areas of high local densities were more likely to die. The spatial patterns of surviving and dead plants tended to be regular and aggregated, respectively, in comparison with those expected from random mortality. The spatial scale of aggregation of the dead plants was 5-25 cm, corresponding to the distance of direct interactions between neighbours. 5 At the later growth stages, in October until May of the following year, mortality was affected mainly by ground surface conditions. Both surviving and dead plants were spatially aggregated at spatial scales of 20-100 cm. Plants tended to survive in microhabitats of smaller gravel, which had a greater water-holding capacity. 6 This study also showed the consequence of spatial variations in mortality on population structures of L. rubida. Patchy seed dispersal and subsequent temporal and spatial variations in mortality are likely to contribute to the scattered distribution of L. rubida individuals observed at the study site. Key-words: density-dependent mortality, density-independent mortality, ground surface conditions, growth stage, spatial scale of mortality

1 We used a series of removal experiments to examine how species response to competition and climatic differences varied in three different years. We tested the interaction between removal of the dominant grass species, Festuca rubra, and year-to-year environmental variation in a mown mountain grassland. 2 In each year, we quantified shoot frequency and above-ground biomass of all remaining plant species. Above-ground responses were tested both by analysis of covariance and by redundancy analysis with randomization tests of changes in total species composition. 3 Analysis of above-ground biomass data showed that other species compensated for the removal of F. rubra biomass within 2 years and that the response in total biomass of the community did not differ among years in which the experiment was started. 4 Multivariate tests showed that species composition changed as a result of the removal; grass biomass and frequency increased more than that of dicotyledons. However, response of species composition to removal of F. rubra was significantly different between onset years. Specific conditions in individual years thus affect the competitive ability of individual species in a non-additive way. 5 Our results indicate that the year-to-year variation at the site has the potential to affect species coexistence and richness. As a consequence, year-to-year variation of climatic parameters may be an important driving factor in community dynamics and should be taken into account in studies of ecosystem response to climate. Key-words: competition, environment-competition interaction, Festuca rubra, growth forms, randomization tests, redundancy analysis, year-to-year variation, removal experiment, temporal heterogeneity

1 In the Great Basin of the western United States of America, the invasive annual grass Bromus tectorum has extensively replaced native shrub and bunchgrass communities, but the native bunchgrass Elymus elymoides has been reported to suppress Bromus. Curlew Valley, a site in Northern Utah, provides a model community to test the effects of particular species on invasion by examining competitive relationships among Elymus, Bromus and the native shrub Artemisia tridentata. 2 The site contains Bromus/Elymus, Elymus/Artemisia and monodominant Elymus stands. Transect data indicate that Elymus suppresses Bromus disproportionately relative to its above-ground cover. Artemisia seedlings recruit in Elymus stands but rarely in the presence of Bromus. This relationship might be explained by competition between the two grasses involving a different resource or occurring in a different season to that between each grass and Artemisia. 3 Time reflectometry data collected in monodominant patches indicated that in spring, soil moisture use by Bromus is rapid, whereas depletion under Elymus and Artemisia is more moderate. Artemisia seedlings may therefore encounter a similar moisture environment in monodominant or mixed perennial stands. However, efficient autumn soil moisture use by Elymus may help suppress Bromus. 4 In competition plots, target Artemisia grown with Bromus were stunted relative to those grown with Elymus, despite equivalent above-ground biomass of the two grasses. Competition for nitrogen in spring and autumn, assessed with [sup.15]N tracer, appears to be secondary to moisture availability in determining competitive outcomes. 5 Elymus physiology and function appear to play an important role in determining the composition of communities in Curlew Valley, by maintaining zones free of Bromus where Artemisia can recruit. Key-words: [sup.15]N tracer, Artemisia tridentata, Bromus tectorum, cheatgrass, Elymus elymoides, Great Basin, N uptake, resource competition, semiarid, Sitanion hystrix, squirreltail, time domain reflectometry

Invasions of nonindigenous species have caused ecological devastation to natural communities worldwide, yet the biological bases for invasiveness remain poorly understood. Our studies of invasive watermilfoil (Myriophyllum) populations revealed widespread polymorphisms in biparentally inherited nuclear ribosomal DNA sequences, which were not detected in populations of native North American species. Subclones of the polymorphic regions revealed the occurrence of distinct sequences matching those acquired from both nonindigenous and native North American species. Molecular data demonstrate clearly that invasive watermilfoil populations in North America have resulted from hybridization between nonindigenous and native species. These observations suggest that invasiveness in these aggressive aquatic weeds may be linked to heterosis maintained by vegetative propagation.

Invasive species are frequently regarded as superlative competitors that can vegetatively crowd out natives, but little is known about whether invasives can compete for pollination services with native plants. We hypothesized that, when the showy invasive species Lythrum salicaria (purple loosestrife) was present, pollinator visitation and seed set would be reduced in a native congener, L. alatum (winged loosestrife). To test this hypothesis, we constructed mixed and monospecific plots of the two species. Over two years of study, we found that L. salicaria significantly reduced both pollinator visitation and seed set in L. alatum. Furthermore, pollinators moved frequently between the two plant species, which may cause heterospecific pollen transfer. Thus, reductions in both pollen quantity and pollen quality may reduce L. alatum seed set. If similar patterns occur in the field, invasive plants may be an even greater threat to natives than previously thought. Key words: competition, invasive species; Lythrum alatum; Lythrum salicaria; pollination; pollinator visitation; purple loosestrife; seed set; winged loosestrife.

A simple, empirical method of detecting evidence of publication bias in meta-analyses is proposed, based on the relationship between the strength of the results in published studies and the quality of the journals in which they appear. In an illustration of its use, the method is applied to published meta-analyses of terrestrial plant competition, predation in streams, woody plant growth under elevated C[O.sub.2], and marine nutrient enrichment experiments. Statistically significant associations of effect strength and journal quality were found in two of the four meta-analyses. Key words: journal impact factor; log response ratio; meta-analysis; publication bias; research synthesis.

Connectivity is a fundamental concept that is widely utilized in spatial ecology. The majority of connectivity measures used in the recent ecological literature only consider the nearest neighbor patch/population, or patches within a limited neighborhood of the focal patch (a buffer). Meta-analysis suggests that studies using nearest neighbor connectivity measures are much less likely to find statistically significant effects of connectivity than studies that use more complex measures. Here we compare simple connectivity measures in their ability to predict colonization events in two large and good-quality empirical data sets. The nearest neighbor distance to an occupied patch is found to be an inferior measure. Buffer measures do much better, but their performance is found to be sensitive to the estimate of the buffer radius. For highly fragmented habitats, the best and most consistent performance is found for a measure that takes into account the size of the focal patch and the sizes of and distances to all potential source populations. When experimenting with reduced data sets, it was discovered that nearest neighbor measures fail to find a statistically significant effect of connectivity for a large range of data set sizes for which the more complex measures still detect a highly significant effect. We conclude that the simplicity of a nearest neighbor measure is not an adequate compensation for poor performance. Key words: buffer measure; colonization; connectivity; incidence function model; isolation; Melitaea cinxia; metapopulation model; nearest neighbor; Scolitantides orion.

Many clonal plants consist of many connected individual ramets, allowing them to share water and nutrients via physiological integration. Integration among ramets may also improve the ability of clonal plants to tolerate abiotic stress-or improve the competitive ability of individual ramets. Here I use a field experiment to determine whether clonal integration improves ramet performance for a widespread clonal tree species invading into native prairie. Aspen (Populus tremuloides) dominates the southern treeline in western Canada, has long-lived belowground connections between mother and daughter ramets, and reproduces vegetatively via resprouting rhizomes after disturbance. I applied two competition treatments (neighbors present or absent) and two clonal integration treatments (belowground rhizomes between mother and daughter ramets either severed or left intact) to 12 replicate Populus daughter ramets at each of three sites. Neighbors improved the survivorship of Populus ramets by 25-35% after 2 yr, but decreased growth by ~20%. Clonal integration tended to improve ramet survival and growth, but these trends were often not significant. Clonal integration did not alter the effects of competition from neighboring vegetation, suggesting that connections between ramets do not necessarily improve the competitive ability of Populus invading into native prairie. Key words: clonal plant; competition; grassland; physiological integration; Populus tremuloides; prairie; tree-grass competition; tree invasion.

Theory on costs of plant defense against herbivory in stressful environments predicts that costs should increase when competition is intense. This amplifies a fundamental dilemma that plants are thought to face: allocate limited resources to grow fast enough to compete, or invest these resources in secondary metabolites to maintain defense. We studied costs associated with genetic and environmental variation in secondary metabolite production of Brassica rapa in the presence and absence of the generalist competitor Lolium perenne. We used experimental quantitative genetics (artificial selection) to manipulate genetic variation, and herbivore-induction treatments to produce environmental variation in myrosinase and glucosinolate concentrations and resistance. Glucosinolates, and their byproducts after breakdown by myrosinase, are known to affect herbivory on plants in the Brassicaceae family. Defense costs were significant in the absence of competitors, but in contrast to theoretical predictions, costs of constitutive defense (measured as growth rates) were not detectable and the cost of induced defense remained the same in the competitive environment. To understand what factors made constitutive defense costs not detectable under competition we conducted several experiments to assess the effects of limited resources and allelopathy on costs and benefits of the defense chemicals. None of the experiments involving nutrient supply and weak competition supported the hypothesis that the lack of defense costs in competitive environments was due to limited resources. Instead, the breakdown products of the glucosinolate--myrosinase reaction appeared to function as allelopathic agents, which may benefit B. rapa plants in competition, thereby reducing net costs of chemical defense. We found that: (1) the effects of exogenous glucosinolates on Lolium root length depended on the presence of myrosinase. (2) In the absence of nutrients, Lolium root lengths were shorter when seeds germinated with B. rapa. (3) Genetic increases in glucosinolate concentration negatively affected Lolium seedling growth only when there were simultaneous genetic increases in myrosinase concentration. Activated carbon treatments designed to neutralize allelopathic effects and restore costs in the competitive environments were, however, not statistically significant. When plant defenses also function to benefit plants in competitive interactions, plants may evolve to compete and defend. Key words: allelopathy; artificial selection; Brassica rapa; constitutive defenses; costs of defense; herbivory; inducible defenses; interspecific competition; Lolium perenne; nutrient availability; plant stress; Plutella xylostella.

Competition for pollination has been hypothesized to select for the divergence of floral traits between species and populations. A primary prediction of this hypothesis is that the strength of competition for pollination, mediated by variation in plant abundance, should directly influence the strength of selection on floral traits. To test this prediction, I examined the relationships between multiple components of plant abundance and pollination, reproductive success, and phenotypic selection via female fitness on four floral traits in artificial and natural populations of the hummingbird-pollinated Ipomopsis aggregata. In the artificial arrays, I manipulated the absolute density and interspersion of neighboring I. aggregata and a competitor for pollination (Castilleja linariaefolia). I also measured natural variation in the absolute and relative density of these two species within a 2.5-m radius of focal I. aggregata plants in three natural populations. The strength of competition for pollination in the I. aggregata--C. linariaefolia system was only weakly influenced by local plant abundance. Both the absolute density and interspersion of plants in the arrays significantly influenced at least one component of I. aggregata's pollination and reproductive success, but the effects were not consistent across these components. For example, the treatment groups that received more conspecific pollen were not the same ones that set more seeds/fruit. Within the natural populations, variation in relative and absolute plant density influenced two components of I. aggregata's pollination and reproductive success, but only in one to two populations per component. As would be expected from these inconsistent effects of plant abundance on pollination and reproduction, the strength of selection on floral traits of I. aggregata in both the arrays and natural populations was also only weakly dependent on abundance. Previous studies have indicated (1) that self-incompatible species such as I. aggregata should experience strong, abundance-dependent effects on pollination, and (2) that variation in plant abundance at a local scale should have a stronger effect on pollination than variation at a larger spatial scale. In contrast, my results suggest that the effect of plant abundance on pollination cannot be easily predicted from simple diagnostic traits such as breeding system or spatial scale. Key words: Castilleja linariaefolia; competition; density; floral traits; Ipomopsis aggregata; natural selection; phenotypic selection; pollination.

We conducted a neighbor removal experiment in natural alpine plant communities of the southwestern Alps to test for the relative importance of competitive and facilitative interactions along elevational and topographical gradients. The experimental sites were chosen to encompass most of the floristic diversity observed along gradients of elevation and topography, which are the two main ecological gradients associated with alpine plant communities in the western Alps. The effects of neighbor removal on the survival, aboveground biomass, and reproduction of five target species were tested at each of six experimental sites. Using biomass data, we calculated relative competitive index (RCI) and log response ratio (LRR) as measures of interaction strength and direction. We found highly significant shifts from strong competitive effects in low and sheltered sites to strong facilitative responses in high and exposed sites. When experimental results were integrated with gradient analyses, we found that the responses of particular alpine plant species to neighbor removal generally depended on the species' position on elevational and topographical gradients. When neighbors were removed from around target species at experimental sites that were lower in elevation than the distributional mean of the target species, biomass generally increased. When neighbors were removed from around target species at experimental sites that were higher in elevation than the distributional mean of the target species, biomass decreased. In other words, facilitation appeared to allow species from lower elevations to move up the gradient, but competition at low elevations appeared to restrict species from higher elevations from moving down the gradient. In high and exposed sites, experimental evidence for facilitation was coupled to small-scale spatial associations among species, but spatial disassociation was not coupled to experimental evidence for competition at any sites. We conclude that the distribution and abundance of many species in high-elevation communities of the western Alps appears to be enhanced by neighbors, and that species continua commonly observed along environmental gradients are the result of both negative and positive plant interactions. Key words: alpine tundra; competition; elevation gradient; facilitation; log response ratio (LRR); niche theory; ordination; plant communities; plant interactions; relative competitive index (RCI); removal experiment; topography gradient.

Research on floodplain ecology in the southwestern US indicates that the native cottonwood species Populus deltoides is sufficiently dominant to withstand the potential introduction of the invasive species saltcedar, or Tamarix ramosissima. Although both are responsive to natural flooding cycles, Populus was found to have superior competitive ability.

Because reducing aboveground competition for photosynthetically active radiation (PAR) increases growth and survival rates of tropical rain forest seedlings, below-ground competition for nutrients is often assumed to be of little or no importance. We tested this assumption. We measured the growth, over one year, of seedlings of Aspidosperma carapanauba (shade tolerator) and Dinizia excelsa (light demander) transplanted into understory (high aboveground competition; 1% incoming photosynthetic photon flux density [PPFD]) and single treefall gap areas (lower aboveground competition; 6% PPFD in centers of gaps) without and with trenches cut around plots (high and low belowground competition). Over the wetter six months, location in gaps significantly increased relative height growth rates by 320% and 570%, and relative leaf production rates by 190% and 280%, while trenching significantly increased height by 330% and 520%, and leaf production by 170% and 260%, for Aspidosperma and Dinizia, respectively, with significant interactions between treatments. Trenching approximately doubled height growth in the understory. Leaf loss rates were unaffected by either treatment but were 3.7 times higher in Dinizia compared with Aspidosperma. Aspidosperma could persist in the understory, as leaf production and loss were balanced at 0.60% PPFD, whereas Dinizia could not, as it required 2.4% PPFD. Reducing either aboveground competition or belowground competition allowed Dinizia to persist as leaf production was increased above unchanging leaf loss rates. Gap and trenching treatments both significantly reduced mortality rates in Dinizia. Location in gaps significantly reduced mortality rates in Aspidosperma. Both species significantly increased allocation to leaves and significantly decreased allocation to roots in response to trenching. The increased growth in the wet season caused by trenching, and increased foliar nitrogen and phosphorus amount, showed that both species were probably competing with adult trees for nutrients as well as PAR. We deduced that phosphorus did not limit growth; however, we could not deduce which nutrient did limit growth. Given that 60-70% of the world's tropical rain forests occur on low-fertility oxisols, intense adult-juvenile competition both aboveground and belowground may be the norm, not the exception. Key words: Amazonia; competition experiments; lowland tropical rain forest; nitrogen; nutrient availability; phosphorus; photosynthetically active radiation (PAR); root competition; seedling growth; tree fall gaps; trenching.

1 Experiments on competition between plant species are frequently designed without considering the analysis stage of the study. We argue that this omission may lead to over-complication of the issue of designing experiments. 2 An overwhelming number of studies have shown that the effects on performance of competition in plant mixtures may be described by simple (hyperbolic) regression models. The most natural view of the problem of measuring plant competition is therefore as a problem in regression. 3 Only with experiments designed explicitly to apply regression analyses can phenomena such as frequency dependence and the size dependence of measures of competition be identified. In contrast to previous assertions, this means that designs that are based around, or allow, regression analysis are the most robust as such effects may be tested for using appropriate statistics. 4 Experiments are probably most easily designed to measure competition as a function of the density of interacting species, rather than biomass. This is because the per unit biomass effect of competition on performance is a function of density. Competition measures based on biomass will hence be dependent on the density at which the experiment is performed. Furthermore, the most effective way to manipulate biomass is through changing species' densities. 5 In terms of economy of design, we would recommend simple additive series. Whilst this does not allow the role of frequency dependence to be analysed, this phenomenon appears to be rare in any case. Keywords: experimental design, community structure, competitive hierarchy, nonlinear model

We conducted field experiments manipulating lichens, shrubs, and herbs along a time-since-fire gradient and assessing effects on three endemic herbaceous species of Florida scrub: Eryngium cuneifolium, Hypericum cumulicola, and Polygonella basiramia. Responses included seed germination, survival, biomass, and fecundity. Transplants into recently burned patches generally had higher survival, larger biomass, and greater reproductive output than transplants into long-unburned patches. Open areas and sites near oaks frequently were more favorable than sites near Florida rosemary. Ground lichens did not affect germination but increased mortality rate of seedlings. Neighboring small shrubby and herbaceous species did not affect the performance of these species. Of the three species, naturally occurring E. cuneifolium were farthest from large shrubs, and their microhabitats had the least ground lichens and shrubs. Eryngium cuneifolium and H. cumulicola are capable of forming persistent seed banks and their recruitment after fire depends mostly on these dormant seeds. Polygonella basiramia relies on seed dispersal and immediate seed germination to colonize recently burned patches. Management for these species should involve variable fire regimes to allow all three species to persist along with many other scrub endemics. Key words: competition; disturbance; Eryngium cuneifolium; Florida scrub; Hypericum cumulicola; Lichens; Polygonella basiramia; seed banks.