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Arbuscular mycorrhizae (AM) are widespread in plant communities and are important mediators of plant community structure. However, patterns of variation among species and linkages with above-ground traits have received much less attention. To address this research need, we surveyed plant species from a long-term study of old field succession for AM colonization. Root tissue was collected from five individuals for each of 46 species that represented a range of origins and life forms. Root samples were cleared, stained, and examined to determine the percentage of colonized hyphae area and the number of vesicles and arbuscules. Nonnative species had greater hyphal colonization than native taxa, and the number of vesicles produced in woody species was lower than in the herbaceous species of the system. AM were associated with above-ground traits in both woody and herbaceous taxa, suggesting mycorrhizae integrate with other plant traits to generate resource acquisition strategies. AM decreased as species peaked later in succession for herbaceous species but was not associated in woody species. These results suggest AM may play an important role in species invasion and overall plant functional strategies, but these relationships may vary based on plant life form., INTRODUCTION The vast majority of terrestrial plant species form symbiotic associations mycorrhizal fungi, with arbuscular mycorrhizae (AM) the most common type (Smith and Read, 1997; Quilambo, 2003). The increased surface [...]

Plant species distribution and plant community composition vary along environmental gradients. At the continental scale, climate plays a major role in determining plant distribution, while at the local and regional scales vegetation patterns are more strongly related to edaphic and topographic factors. The projected global warming and alteration of the precipitation regime will influence tree physiology and phenology, and is likely to promote northward migration of tree species. However the influence of soil characteristics on tree species migration is not as well understood. Considering the broad tolerance of most tree species to variations in soil factors, soils should not represent a major constraint for the northward shift of tree species. However, locally or regionally, soil properties may constrain species migration. Thus, while climate change has the potential to induce a northward migration of tree species, local or regional soil properties may hinder their migratory response. These antagonistic forces are likely to slow down potential tree migration in response to climate change. Because tree species respond individualistically to climate variables and soil properties, new tree communities are likely to emerge from climate change. Key words: climate change, tree migration, soil properties, temperate forest, boreal forest. La distribution des especes et la composition des communautesveegetales varient le long de gradients environnementaux. A l'eechelle continentale, le climat joue un riale primordial dans la distribution des especes vegetales, alors qu'aux echelles locale et regionale la distribution des especes est surtout influenceee par les variables edaphiques et la topographie. Le rechauffement climatique projete et l'alteration du regime des precipitations sont susceptibles d'influencer la physiologie et la pheenologie des arbres, et d'entrainer leur migration vers le nord. Toutefois, l'influence des caracteeristiques des sols sur le potentiel migratoire des arbres est peu connue. Puisque la plupart des especes d'arbres possedent une grande tolerance aux variations edaphiques, les sols ne devraient pas constituer un obstacle majeur a leur migration vers le nord. Toutefois, localement ou regionalement, les sols pourraient posseder des proprietes qui pourraient restreindre la migration des especes. Alors que les changements climatiques ont le potentiel de provoquer la migration des especes vers le nord, localement ou regionalement les proprietes des sols pourraient limiter leur capacite; migratoire. Ces forces en opposition pourraient restreindre le potentiel migratoire des arbres en reponse aux changements climatiques. Parce que les especes d'arbres repondent de maniere individuelle aux variables climatiques et aux proprietes des sols, de nouvelles communautes d'arbres pourraient resulter des changements climatiques. Mots-cles: changements climatiques, migration des arbres, proprietes des sols, foret temperee, foret boreale., Introduction Plant species distribution and plant community composition vary along environmental gradients (Allen and Hoekstra 1990; Levin 1992), and several community and landscape studies have linked species responses to environmental [...]

* Premise of the study: Around the world, tropical glaciers and ice caps are retreating at unprecedented rates because of climate change. In at least one location, along the margin of the Quelccaya Ice Cap in southeastern Peru, ancient plant remains have been continually uncovered since 2002. We used genetic analysis to identify plants that existed at these sites during the midHolocene. * Methods: We examined remains between 4576 and 5222 yr old, using PCR amplification, cloning, and sequencing of a fragment of the chloroplast trnL intron. We then matched these sequences to sequences in GenBank. * Key results: We found evidence of at least five taxa characteristic of wetlands, which occur primarily at lower elevations in the region today. * Conclusions: A diverse community most likely existed at these locations the last time they were ice-free and thus has the potential to reestablish with time. This is the first genetic analysis of vegetation uncovered by receding glacial ice, and it may become one of many as ancient plant materials are newly uncovered in a changing climate. Key words: ancient DNA; Andes; bofedal; Holocene; puna. doi: 10.3732/ajb.1000058

Interactions between plants are a complex combination of positive and negative interactions, with the net outcome depending on environmental contexts. The more frequent association of Trifolium alpinum (legume) with Festuca eskia than with Nardus stricta (grasses) in many Pyrenean subalpine meadows suggests a differential ability to use nitrogen (N) derived from [N.sub.2] fixation. In the field, we investigated the interactions between the legume and grasses and, in the glasshouse, the transfer of [sup.15]N from the legume to the grasses. In one grass-Trifolium mixture, the legume had a strong positive effect on the biomass and N content of the grass as compared to pure grass stands. When both grasses grew together with the legume, only Festuca benefited from the presence of Trifolium but, surprisingly, the benefit decreased with increasing Trifolium abundance. Leaf labeling experiments with [sup.15]N-N[H.sub.4.sup.+] revealed a higher transfer of [sup.15]N from Trifolium to Festuca than to Nardus, suggesting a more direct N pathway between the two species. This more direct pathway could prevent Nardus from benefiting from the legume N in the three-species mixtures. Thus, the positivc interactions bctween N-fixers and nonlixers appear to be largely species-specific and to depend strongly on the species in the plant assemblage. Key words: atmospheric [N.sub.2] fixation; facilitative interaction; Festuca; grasses; legume; [sup.15]N transfer; Nardus; productivity; subalpine meadow; Trifolium. doi: 10.3732/ajb.0800405

ABSTRACT. An inventory of the vascular flora occurring in the Bennett wetland complex in Henry County, Indiana, revealed 264 species and varieties representing 187 genera in 61 families. Of the [...]

Natural hybridization between introduced species and their native congeners occurs frequently and can create serious conservation concerns. Ulmus pumila (Siberian elm) is an introduced Asian elm species that has naturalized in the United States and is now considered invasive in 41 states. Red elm (U. rubra), a native to the eastern United States, often occurs in sympatry with Siberian elm, and the two species are thought to hybridize. Here, we genetically characterized reference populations of the two elm species to identify species-specific microsatellite alleles. These markers were used to classify individuals in putative hybrid zones as parental species or hybrids, assess the extent of hybridization, and track patterns of introgression. We identified nine U. rubra, 32 U. pumila, and 51 hybrid individuals in our hybrid zones. Of the 51 hybrids. 35 were classified as first-generation hybrids and 16 as backcrosses. The majority of the backcrosses (88%) were introgressed toward U. pumila. Our classification of genotypes was consistent whether we used manual classification, principal coordinate analyses or Bayesian clustering. We observed greater genetic diversity and new combination of alleles in the hybrids. Our study indicates widespread hybridization between U. pumila and U. rubra and an asymmetric pattern of introgression toward U. pumila. Key words: Dutch elm disease; hybrid zones; invasive species; microsatellites: red elm; Siberian elm; species-specific markers. doi: 10.3732/ajb.0800334

Understory light is essential to the establishment and growth of understory plants and varies temporally and spatially within gaps. The previously defined gap light index (GLI) is a good model for assessing understory light levels, but it is time-consuming to determine gap coordinates, which are crucial to GLI, for numerous points within a gap. This paper introduces the geometric calculation (GeoCalc) of gap coordinates. GeoCalc quickly obtains gap coordinates for any specified point within a canopy gap and takes into account the tridimensional profile of the gap and the slope and aspect of the ground. The GeoCalc-based GLI was validated by the GLI derived from hemispherical photographs taken at 93 sampling points within seven natural gaps. Our results demonstrate that GeoCalc-based GLI was strongly positively correlated and not significantly differed from the GLI derived from hemispherical photographs. Next, to analyze gap light regimes and the effects of gap size, canopy height, and topography, three natural gaps of various size were selected and simulated as nine gaps with 1 and 1.5 times canopy height or on the opposite slope. Finally, we have summarized characteristics of GeoCalc-based GLI and its application. La lumiere sous le couvert est essentielle a l'etablissement et a la croissance des plantes du sous-bois et varie de facons temporelle et spatiale dans les trouees. Le coefficient de trouees lumineuses (CTL) est un bon modele pour estimer le niveau de lumiere sous le couvert, mais necessite beaucoup de temps pour determiner les coordonnees des trouees, ce qui est essentiel au calcul du CTL pour plusieurs points dans une trouee. Cette etude propose le calcul geometrique (GeoCalc) des coordonnees des trouees. Les coordonnees des trouees sont rapidement obtenues par GeoCalc pour tout point situe dans une trouee du couvert en tenant compte du profil tridimensionnel de la trouee de meme que de la pente et de l'exposition du terrain. Le CTL obtenu grace a GeoCalc a ete compare au CTL derive de photographies hemispheriques prises a 93 points d'echantillonnage a l'interieur de sept trouees naturelles. Nos resultats indiquent que le CTL obtenu grace a GeoCalc n'est pas significativement different et est fortement et positivement correle au CTL derive de photographies hemispheriques. Ensuite, pour analyser le regime des trouees lumineuses et les effets de la taille de la trouee, de la hauteur de la canopee et de la topographie, nous avons selectionne trois trouees naturelles de differente taille que nous avons simulees comme s'il s'agissait de neuf trouees situees a l'interieur d'une canopee avec une hauteur equivalente a 1,0 et 1,5 fois la hauteur originale ou situees sur une pente opposee a la pente originale. Finalement, nous avons resume les caracteristiques et les applications du CTL obtenu grace a GeoCalc. [Traduit par la Redaction], Introduction Understory light levels (ULL) are essential to photosynthesis and other plant processes, such as the emergence, establishment, survival, and growth of understory plants (Gray and Spies 1996; Page and [...]

Habitats vary considerably in the level of invasion (number or proportion of alien plant species they contain), which depends on local habitat properties, propagule pressure, and climate. To determine the invasibility (susceptibility to invasions) of different habitats, it is necessary to factor out the effects of any confounding variables such as propagule pressure and climate on the level of invasion. We used 20 468 vegetation plots from 32 habitats in the Czech Republic to compare the invasibility of different habitats. Using regression trees, the proportion of alien plants, including archaeophytes (prehistoric to medieval invaders) and neophytes (recent invaders), was related to variables representing habitat properties, propagule pressure, and climate. The propagule pressure was expressed as the proportion of surrounding urban and industrial or agricultural land, human population density, distance from a river, and history of human colonization in the region. Urban and industrial land use had a positive effect on the proportion of both archaeophytes and neophytes. Agricultural land use, higher population density, and longer history of human impact positively affected the proportion of archaeophytes. Disturbed human-made habitats with herbaceous vegetation were most invaded by both groups of aliens. Neophytes were also relatively common in disturbed woody vegetation, such as broad-leaved plantations, forest clearings, and riverine scrub. These habitats also had the highest proportion of aliens after removing the effect of propagule pressure and climate, indicating that they are not only the most invaded, but also most invasible. These habitats experience recurrent disturbances and are rich, at least temporarily, in available nutrients, which supports the hypothesis that fluctuating resources are the major cause of habitat invasibility. The least invaded habitats were mires and alpine-subalpine grasslands and scrub. After removing the effect of propagule pressure and climate, some habitats actually invaded at an intermediate level had very low proportions of aliens. This indicates that these habitats (e.g., dry, wet, and saline grasslands, base-rich fens, and broad-leaved deciduous woodlands) are resistant to invasion. Key words: archaeophyte; biological invasions; Central Europe; Czech Republic; disturbance; exotic species; invasion resistance; neophyte; plant community; propagule pressure.

Outbreaks of infectious agents in natural ecosystems are on the rise. Understanding host-pathogen interactions and their impact on community composition may be central to the conservation of biological diversity. Infectious agents can convey both exploitive and facilitative effects that regulate host populations and community structure. Parasitic angiosperms are highly conspicuous in many plant communities, and they provide a tractable model for understanding parasite effects in multispecies communities. I examined host identity and variation in host infectivity of a holoparasitic vine (Cuscuta salina) within a California salt marsh. In a two-year parasite removal experiment, I measured the effect of C. salina on its most frequent host, a rare hemiparasite, and the plant community. C. salina clearly suppressed the dominant host, but rare plant fitness and plant species diversity were enhanced through indirect effects. Priority effects played a role in the strength of the outcome due to the timing of life history characteristics. The differential influence of parasites on the fecundity of multiple hosts can change population dynamics, benefit rare species, and alter community structure. The continuum of negative to positive consequences of parasitic interactions deserves more attention if we are to understand community dynamics and successfully restore tidal wetlands. Key words: Bodega Bay, California, USA; community structure; Cordylanthus maritimus ssp. palustris; Cuscuta salina; facilitation; indirect effects; parasite-host interactions; parasitic plant; Plantago maritima; removal experiment; salt marsh; salt marsh dodder.

This paper addresses the capability of synthetic aperture radar and optical images in combination with theoretical models to detect the vegetation water content (VWC) at field level. In this paper, a retrieval algorithm for the estimation of VWC from AirSAR acquired on vegetated fields during the SMEX'02 experiment is addressed. The aforementioned campaign has been chosen because, along with sensor observations, extensive ground truth measurements were acquired. The retrieval procedure, which is based on a Bayesian approach, has been initially developed for soil moisture extraction. It consists of two modules: one is pertinent to bare soils and the other one has been modified for vegetated fields. The last one uses the synergy with optical images to correct for the contribution of VWC. The VWC, a variable in the inversion procedure, as well as soil moisture can be estimated. The results indicate a good correlation with both ground measurements and VWC calculated from Landsat images through the use of normalized difference water index (NDWI). Furthermore, in the inversion procedure, the introduction of the dependence on roughness improves the estimates. This indicates that, even for dense vegetation, the contribution from bare soil greatly influences the radar signal. Three main levels of VWC are discriminated in the inversion procedure: values below 1 kg/[m.sup.2], values between 1 and 3 kg/[m.sup.2], and values greater than 3 kg/[m.sup.2]. Index Terms--Inversion algorithms, optical images, radar images, roughness, vegetation.

Vegetation is traditionally regarded to reduce the erosion of channels in both fluvial and tidal landscapes. We present a coupled hydrodynamic, morphodynamic, and plant growth model that simulates plant colonization and channel formation on an initially bare, flat substrate, and apply this model to a tidal landscape. The simulated landscape evolution is compared with aerial photos. Our results show that reduction of erosion by vegetation is only the local, on-site effect operating within static vegetation. Dynamic vegetation patches, which can expand or shrink, have a contrasting larger scale, off-site effect: they obstruct the flow, leading to flow concentration and channel erosion between laterally expanding vegetation patches. In contrast with traditional insights, our findings imply that in tidal landscapes, which are colonized by denser vegetation, channels are formed with a higher channel drainage density. Hence this study demonstrates that feedbacks between vegetation, flow, and landform have an important control on landscape evolution. Keywords: salt marsh, tidal creek, gully erosion, biogeomorphology.

Consumer and resource control of diversity in plant communities have long been treated as alternative hypotheses. However, experimental and theoretical evidence suggests that herbivores and nutrient resources interactively regulate the number and relative abundance of coexisting plant species. Experiments have yielded divergent and often contradictory responses within and among ecosystems, and no effort has to date reconciled this empirical variation within a general framework. Using data from 274 experiments from marine, freshwater, and terrestrial ecosystems, we present a cross-system analysis of producer diversity responses to local manipulations of resource supply and/or herbivory. Effects of herbivory and fertilization on producer richness differed substantially between systems: (i) herbivores reduced species richness in freshwater but tended to increase richness in terrestrial systems; (ii) fertilization increased richness in freshwater systems but reduced richness on land. Fertilization consistently reduced evenness, whereas herbivores increased evenness only in marine and terrestrial ecosystems. Producer community evenness and ecosystem productivity mediated fertilization and herbivore effects on diversity across ecosystems. Herbivores increased producer richness in more productive habitats and in producer assemblages with low evenness. These same assemblages also showed the strongest reduction in richness with fertilization, whereas fertilization increased (and herbivory decreased) richness in producer assemblages with high evenness. Our study indicates that system productivity and producer evenness determine the direction and magnitude of top-down and bottom-up control of diversity and may reconcile divergent empirical results within and among ecosystems. fertilization | herbivory | evenness | species richness | metaanalysis

ABSTRACT. We have described the vegetation structure with respect to various community types of Highland Heights Community Park and adjoining territory. High values of Shannon's Diversity Indexes and Floristic Quality [...]

1 Small-scale plant species richness may enhance local production if the local environment promotes resource partitioning (a form of complementarity) or causes some species to perform better next to interspecific neighbours (facilitation) than when grown in monoculture. Little is known, however, about how species richness and environmental factors such as soil fertility and light levels interact to affect ecosystem production. 2 To determine whether some environments promote a positive richness-production relationship, I performed an experiment using artificial plant communities, selected from a pool of six annuals and one perennial species, with different levels of species richness and composition. Each community was grown in either high or low soil fertility and either full sun or 50% shade. 3 Higher species richness significantly enhanced above-ground production. This effect did not depend on environmental treatment due to consistent sampling effects for a particularly productive and competitive species. However, overyielding, which measures the combined effect of complementarity and facilitation, did depend on environmental conditions and was enhanced by increased soil fertility in plots in full sunlight. 4 Relative yield analyses of species of different height and growth form, coupled with data on the performance of each species in monoculture, support the hypothesis that mixtures overyielded by means of light partitioning (rather than facilitation), an effect that was precluded under shaded conditions. 5 The results therefore suggest that higher annual plant species richness may enhance production when environmental conditions allow for resource partitioning. Key-words: complementarity, facilitation, light partitioning, overyielding, sampling effect

Abstract Robinson Crusoe Island of the Juan Fernandez Archipelago, as is the case with many oceanic islands, has experienced strong human disturbances through exploitation of resources and introduction of alien [...]