Facilitation and competition influence succession in a subtropical old field

The effects of neighboring vegetation and soil fertility on the establishment, survival and growth of tree species were studied in a subtropical old-field area in south Brazil. Seed damage, germination and seedling establishment of four tree species plus growth and survival of two transplanted tree species were monitored under factorial combinations of the following treatments: (1) pioneer vegetation (presence and absence); (2) soil fertility (addition of NPK and control). Facilitation was the main process affecting plant performance. The presence of pioneer vegetation significantly improved germination, establishment, growth and survival of most study species. Around 90% of sown seeds were damaged and the removal of pioneer vegetation significantly increased seed damage for all species studied, decreased germination in three out of four species, and decreased establishment in one species. Moreover a significantly higher seedling growth rate of Inga virescens was found after the first year of the experiment in plots where vegetation was present. The presence of vegetation significantly increased seedling survival of I. virescens by protecting seedlings from leaf loss due to winter frosts. Competition was detected by the second year when a higher growth of transplanted seedlings of the species Araucaria angustifolia occurred in plots where vegetation was present and fertilizer were applied. A lower growth rate was detected in plots where vegetation was present but fertilizer was not applied. These results indicate a balance between competition for soil nutrients and protection by neighbor vegetation. Damage of seedlings by leaf cutter ants was an important barrier for plant survival. Damage occurred in 80% of the A. angustifolia seedlings and 58% of these damaged seedlings died. The presence of neighboring vegetation tended to protect seedlings from ant damage. Although competition occurred, facilitation seems to be the main process driving early successional changes in this subtropical old field. This was mainly due to the improvement of local microclimatic conditions and protection against herbivores by neighboring vegetation. Facilitation occurred during establishment and growth phases in a subtropical area that is considered a productive, low stress environment. Our results indicate that facilitation may be more frequent in productive environments than previously thought.     

Mycorrhizal symbiosis increases growth,reproduction and recruitment of Abutilon theophrasti Medic. in the field

We examined in the field the effect of the vesicular-arbuscular (VA) mycorhizal symbiosis on the reproductive success of Abutilon theophrasti Medic., an early successional annual member of the Malvaceae. Mycorrhizal infection greatly enhanced vegetative growth, and flower, fruit and seed production, resulting in significantly greater recruitment the following year. In addition, the seeds produced by mycorrhizal plants were significantly larger and contained significantly more phosphorus than seeds from non-mycorrhizal plants, an effect which may improve offspring vigor. Infection by mycorrhizal fungi may thus contribute to the overall fitness of a host plant and strongly influence long-term plant population dynamics.     

Density-dependent response to mycorrhizal infection in Abutilon theophrasti Medic

Summary One purpose of this study was to determine whether an increase in plant density would result in a decrease in response to mycorrhizal infection (particularly as measured by phosphorus content). Increases in plant density generally result in increases in root density in the volume of soil occupied by the plants. Root density, in turn, largely determines phosphorus uptake. If mycorrhizal plants had significantly higher effective root densities than non-mycorrhizal plants due to the fungal hyphae and thus were more thorough in exploiting a given volume of soil for phosphorus, then a given increase in root density might result in a greater proportional increase in phosphorus uptake for non-mycorrhizal plants than for mycorrhizal plants. Two experiments were performed in which mycorrhizal infection and available soil volume per plant were manipulated; one in which the number of plants within a given pot size was varied (experiment 1), and another in which single plants were grown in pots of differing volume (experiment 2). The two experiments yielded similar results but for apparently different reasons. In the first experiment, for a given increase in root density, non-mycorrhizal plants had a greater proportional increase in phosphorus uptake than mycorrhizal plants. Thus, as predicted, response to mycorrhizal infection was greatest at the lowest planting density (highest available soil volume per plant, lowest root density). In experiment 2, response to infection was also greatest at the highest available soil volume per plant (largest pot), but pot size did not influence root density. These results show that the benefit from mycorrhizal infection may be partly determined by root density and they suggest that plants either occurring in patches of contrasting root density in a given community, or occurring in different communities with inherently different root densities may differ in their reliance upon mycorrhizal fungi for phosphorus uptake.     

Asymmetric competition between plant species

Despite extensive interest in the role of plant size in competition, few formal attempts have been made to quantify the magnitude of asymmetric competition, particularly for interactions between members of different species. This paper introduces the concept of asymmetric interspecific competition at the population livel (i.e. mean plant performance) in mixtures of species. It proposes an index of interspecific competitive asymmetry which allows for a progressively greater asymmetric effect as the average size differences between competing species increase, and allows for such an effect whether individuals of focal species are larger or smaller, on average, than competitors. This index of competitive asymmetry is evaluated in the study of interactions between two widely coexisting annuals of disturbed habitats, Stellaria media and Poa annua. An experiment was conducted in which the density, relative frequency and relative seedling sizes (emergence times) of Poa and Stellaria individuals were varied. The relative growth rate (RGR) for both species was measured over a 22-day period. An inverse linear model was fitted for each species, relating the RGR of the focal species to the initial biomass of each species. Each response model included an asymmetry coefficient () to assess whether the impact of a unit of initial biomass of the associate species changed with the relative sizes of seedlings of the two species. A zero value of implies symmetric competition between the two populations; i.e. the competitive effect of a unit of associate species biomass does not change with its initial seedling size. If is positive the smaller the initial relative size of seedlings of the associate species, the smaller their per unit biomass effect on the response of the focal species. The model fitted our data for Stellaria and Poa well and was validated by an alternative modelling approach. Asymmetry coefficients were estimated as 0.508 (P<0.05) for the effect of Poa in the Stellaria model, and 0.0001 (NS) for the effect of Stellaria in the Poa model; i.e. the effect of Poa on Stellaria was asymmetric while the effect of Stellaria on Poa was symmetric. Differences in interspecific species asymmetric competitive effects are discussed within the context of shoot architecture, and the relative importance of competition for light versus soil resources. Finally, we discuss the relationship of this model to earlier models of competitive asymmetry, and consider the implications of interspecific competitive asymmetry for a number of current theories of plant competition and community organisation.     

Mycorrhizal fungi and the nutrient ecology of three oldfield annual plant species

Summary Three oldfield annual species (Abutilon theophrasti Medic., Ambrosia artemisiifolia L. and Setaria lutescens (Weigel) Hubb.) were investigated. All three developed substantial mycorrhizal infections when inoculated with Glomus etunicatum Becker & Gerd. Mycorrhizal infection dramatically increased phosphorus content and dry weight of both Abutilon and Ambrosia, but did not significantly affect dry weight and only modestly increased phosphorus content of Setaria. These results were consistent with a lower level of infection and much greater root density in Setaria than in the other species. When Abutilon was grown in the presence of Setaria, mycorrhizal infection had no effect on Abutilon phosphorus content or dry weight. The depressive effect of Setaria on the response to inoculation in Abutilon was probably not caused by water soluble allelopathic chemicals from Setaria roots, but soil leachate from Abutilon plants did inhibit infection in other Abutilon plants. The data were consistent with the hypothesis that the very high root density and effective soil exploitation of Setaria reduced the benefit from mycorrhizal infection in Abutilon via phosphorus depletion in a large proportion of the available soil volume. Furthermore, even if mycorrhizal infection were capable of increasing phosphorus content of Abutilon in the presence of Setaria, the very high competitive ability of Setaria for nitrogen in the soil could have reduced the benefit of an enhanced phosphorus content. Carbon isotope ratios were reduced in Abutilon by mycorrhizal infection, indicating a possible reduction in water use efficiency.     

Phenological and demographic behaviour of an exotic invasive weed in agroecosystems

An experimental work was conducted in Lleida (Spain) aiming to characterise the phenology and to quantify the demographic processes regulating the populations of Abutilon theophrasti Medicus in maize fields. Seedling emergence started a few days after crop sowing in early May and continued during two more months. The vegetative phase was very long due to the late seeding emergence; these later emerged plants showed a slower development, and many of them did not reach the fertility stage. A flowering peak was observed 12 weeks after emergence in late July, and fruit dehiscence and seed setting started in mid August, several weeks before crop harvest. Four different cohorts were identified, and two main peaks of emergence were determined 21 and 49 days after crop sowing nearest related with field irrigation. A functional logarithmic relationship between cumulative growing degree-days (GDD) and cumulative emergence was also described. The resulting demographic diagram reflects greater values relating to seedling survival for May cohorts (90.2 vs 7.9%), to fertility (100 vs 75%) and to fecundity (3774 vs 92 seeds pl⁻¹) than those determined for the June cohorts. The late emerged plants are subjected to a high density and are strongly affected by light competition, and their reproductive phase initiation delay is of about 10–20 days. In an assay conducted in Petri dishes, the seeds provided from plants emerged earlier were found more vigorous and germinated more than those from late emerged plants, which seem to be affected by incomplete fruit and seed ripening. Following the crop cycle without any weed control, the population rate increase was about 21.2. These values explain the high invasion capacity of this weed in the local summer irrigated fields, which consists in assuring their presence through a persistent soil seed bank and increasing the probability to spread to other fields. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nitrogen addition does not reduce belowground competition in a salt marsh clonal plant community in Mississippi (USA)

Previous studies have suggested that belowground competition for nutrients influences plant zonation in salt marshes. In this study, I tested the hypothesis that competition for nitrogen structured a clonal plant community in a nitrogen-limited salt marsh in coastal Mississippi, USA. In contrast to most previous field studies that have investigated mechanisms of competition, I examined clonal growth responses of established genets of a nitrogen-demanding low-intertidal species (Spartina alterniflora) to nitrogen addition and the removal of a nitrogen-conserving high-intertidal species (Juncus roemerianus). Nitrogen addition stimulated clonal invasion of the Juncus zone by Spartina but did not reduce the significant competitive effects of Juncus on Spartina. Simulated Juncus shade did not reduce invasion of the Juncus zone by Spartina, indicating that belowground competition reduced clonal invasion. In the last year of the study, the border shifted unexpectedly towards the Spartina zone, resulting in competitive displacement of Spartina by Juncus. Nitrogen addition did not prevent or slow this displacement, further contradicting the nitrogen competition hypothesis. Although growth rates were much more strongly limited by nitrogen in Spartina than in Juncus, nitrogen addition did not cause the displacement of Juncus by Spartina after three growing seasons. I conclude that zonation of Spartina and Juncus is maintained by preemption of space and greater tolerance of low nitrogen supplies by Juncus in the high marsh. These results contrast sharply with findings of reduced belowground competition with nutrient addition in previous studies and highlight the important role of nutrient-mediated competition for space between clonal plants.     

Assessing determinants of community biomass composition in two-species plant competition studies

A method is proposed for assessing the relative importance of species identity, neighbour species influence and environment as determinants of change in community biomass composition in two-species short-term competition experiments. The method is based on modelling the differences in relative growth rates (RGR) of species (hence called the RGRD method). Using a multiple regression approach it quantifies the effects of initial species abundance, species identity and environment on RGRD and hence on change in community biomass composition. The RGRD approach is relatively simple to use and deals readily with statistical difficulties associated with correlated responses between species from the same stand. It can be easily adapted to analyse sequential harvest data. An example based on data from two-species mixtures of the annual species Stellaria media and Poa annua is used to illustrate the method. The main determinant of change in community biomass composition was species identity, reflected in the difference in growth rates between the species. Change in community composition was not, in general, significantly affected by the influence of neighbours or fertiliser level. The unimportance of the influence of neighbours in affecting the composition of these communities contrasts with the strong role of intra- and interspecific competition in determining the size of individuals of both species (Connolly et al. in Oecologia 82:513–526, 1990).     

Evidence for competition between mudsnails (Hydrodiidae): a field experiment

The paper describes an experimental investigation of competition between Hydrobia ulvae and H. ventrosa using enclosed populations at a site at which the species coexist naturally. lntraspecific competition is more intense than interspecific competition and may have a regulatory influence on snail densities. Other experimental studies of competition between mudsnails are reviewed and found to infer strongly the occurrence of competition in natural field populations. However, features of the species' field distributions previously attributed to interspecific competition can be more convincingly explained by other processes.     

土壤铅胁迫对苘麻光合特性及生物量的影响

为明确不同浓度铅污染对苘麻(Abutilon theophrasti)光合特性及生长的影响程度,揭示苘麻对铅胁迫的光合适应能力,在铅污染土壤区域提供潜在物种。该研究设置五个铅浓度梯度(0、500、1 000、1 500、2 000 mg·kg~(-1)),分析了在不同浓度铅污染胁迫下,苘麻的铅积累、气体交换参数、叶绿素荧光特性及生物量的变化。结果表明:当铅浓度在1 000 mg·kg~(-1)以下时,净光合速率(P_n)和气孔导度(G_s)、光化学效率(F_v/F_m)、电子传递速率(ETR)的值分别与空白组的P_n、G_s、F_v/F_m、ETR值相比均不显著(P0.05),表明铅浓度小于1 000 mg·kg~(-1)时,苘麻的光合活性未受到损伤,有很强的光合适应能力。苘麻生物量在500、1 000 mg·kg~(-1)铅胁迫下为空白组的88.6%、106%,苘麻能够正常生长,说明苘麻能够适应铅污染的胁迫。当铅浓度低于1 000 mg·kg~(-1)时,苘麻地上部与地下部对铅富集与转移的效果较好,表明在铅污染区苘麻也具有较强的铅积累能力。根据铅污染土壤区域的实际情况,结合苘麻在低浓度铅污染修复效率高的特点,认为苘麻可以作为铅污染地区修复的潜在物种。     

The effects of chrysomelid beetle grazing and plant competition on the growth of Rumex obtusifolius

Summary The interaction between grazing by Gastrophysa viridula and interspecific plant competition was investigated for Rumex obtusifolius growing in the field. During an eight-month growing season non-competing R. obtusifolius grew larger than competing plants although herbivore loads (beetle productivity per unit leaf area) were similar for competing and non-competing plants. However, grazing significantly reduced the growth of R. obtusifolius only in competing plants thus demonstrating a synergism between plant competition and invertebrate herbivory. The relevance of these findings for the distribution of plant species is discussed.     

Patterns and mechanisms of plant succession after fire on Artemisia-grass sites in southeastern Idaho

Cover data for plant species on eight environmentally similar sites that were each burned in a different year (from 2 to 36 years ago) were used to construct a composite sequence of vegetational change after fire on Artemisia-grassland sites in southeastern Idaho. Some species were early successional such as Lithospermum ruderale, and some late successional: Artemisia tridentata, A. tripartita, and Gutierreza sarothrae. But many species: Purshia tridentata, Symphoricarpos oreophilus, Amelanchier alnifolia, Chrysothamnus viscidiflorus, Achillea millefolium, Agropyron dasystachyum, and A. spicatum were present in both early and later stages. Shannon and Simpson indices of diversity and species richness indicated little change in alpha diversity through time. This was attributed mainly to the limited change in species composition from early to later stages. The general pattern of succession is compatible with the tolerance model of Connell & Slatyer (1977) in most respects. Species traits relating to persistence through a disturbance or re-establishment on the site, and tolerance of competition shape the course of succession on a site. Perennial grasses and forbs which sprout from the base after fire are the first species to dominate the sites. Sprouting shrubs, which require some years to regrow to their pre-fire form, are prominent by the sixth year. Shrubs which rely on dispersal become co-dominants in later stages, at which time some herbaceous species are reduced oreliminated. The pattern of succession can differ due to presence or absence of species with particular traits.     

Cultivar and pH effects on competition for nodule sites between isolates of Rhizobium in beans

Two experiments were carried out to evaluate the effect of acidity on bean-Rhizobium competition for nodule sites. SevenPhaseolus vulgaris host cultivars differing in acid-pH tolerance were grown in sand culture, and irrigated using a sub-irrigation system and nutrient solutions of pH 4.5, 5.0, 5.5, and 6.0. A mixed inoculant of two antibiotically markedRhizobium leguminosarum bvphaseoli strains CIAT899 (acid-tolerant) and CIAT632 (acid-sensitive) was used. The acid-tolerant CIAT899 dominated CIAT632 in nodule occupancy across all cultivars and pH treatments. Although several of the varieties had previously been identified as PH-tolerant, and these cultivars performed better than those reported to be acid sensitive, all showed a marked increase in nodulation and plant development when the pH was raised from 4.5 to 6.0. The second experiment using a modified Leonard jar system varied the inoculation ratio between CIAT899 and UMR1116 (acid-sensitive, inefficient in N₂-fixation) and contrasted nodulation response for the bean varieties Preto 143 (pH-tolerant) and Negro Argel (pH-sensitive) at 3 pH treatments (4.5, 5.5, 6.5). There was a significant effect of host cultivar, ratio of inoculation, and pH on the percentage of nodule occupancy by each strain. At low pH CIAT899 had higher nodule occupancy than UM1116 in the variety Negro Argel but had the same percentage of nodulation when the variety was Preto 143. Increasing the cell concentration of UMR1116 produced more inefficient nodules at all treatment combinations and reduced plant growth for both cultivars used.     

Nitrogen redistribution to sorghum grains as affected by plant competition

An experiment was conducted to study nitrogen absorption and translocation in grain sorghum plants during their reproductive growth. Sorghum was grown in four row spacings: 50 and 70 cm in single rows, 80 and 120cm in double rows 20 cm apart. Plant populations were 71000, 142000 and 213000 plants/ha. After flowering, samples were taken at 12 day intervals, and the plants were divided into grains and stover, where N was analyzed. There was an increase in N concentration in lower plant populations and in wider row spacings. However, total nitrogen accumulation (in kg/ha) increased as the number of plants was increased. In the vegetative parts of the plants there were higher N concentrations in lower populations showing that there was a higher N absorption and a lower translocation to the grains. When grain sorghum was grown in 50 cm rows, there was a high N accumulation, a high N translocation to the grains and the highest yield. This row spacing led to the highest N use efficiency.     

Effects of CO2 elevation on canopy development in the stands of two co-occurring annuals

Elevated CO₂ may increase dry mass production of canopies directly through increasing net assimilation rate of leaves and also indirectly through increasing leaf area index (LAI). We studied the effects of CO₂ elevation on canopy productivity and development in monospecific and mixed (1:1) stands of two co-occurring C₃ annual species, Abutilon theophrasti, and Ambrosia artemisiifolia. The stands were established in the glasshouse with two CO₂ levels (360 and 700 l/l) under natural light conditions. The planting density was 100 per m² and LAI increased up to 2.6 in 53 days of growth. Root competition was excluded by growing each plant in an individual pot. However, interference was apparent in the amount of photons absorbed by the plants and in photon absorption per unit leaf area. Greater photon absorption by Abutilon in the mixed stand was due to different canopy structures: Abutilon distributed leaves in the upper layers in the canopy while Ambrosia distributed leaves more to the lower layers. CO₂ elevation did not affect the relative performance and light interception of the two species in mixed stands. Total aboveground dry mass was significantly increased with CO₂ elevation, while no significant effects on leaf area development were observed. CO₂ elevation increased dry mass production by 30–50%, which was mediated by 35–38% increase in the net assimilation rate (NAR) and 37–60% increase in the nitrogen use efficiency (NUE, net assimilation rate per unit leaf nitrogen). Since there was a strong overall correlation between LAI and aboveground nitrogen and no significant difference was found in the regression of LAI against aboveground nitrogen between the two CO₂ levels, we hypothesized that leaf area development was controlled by the amount of nitrogen taken up from the soil. This hypothesis suggests that the increased LAI with CO₂ elevation observed by several authors might be due to increased uptake of nitrogen with increased root growth.     

A field investigation into the effects of interspecific competition on the distribution and cover of Ulothrix flacca

Ulothrix flacca is a filamentous green alga found in its greatest abundance in the littoral fringe of the rocky intertidal during the winter months. It is much less abundant below this level. This experiment was undertaken tot determine if the lower abundance below the littoral fringe is due to the effects of interspecific competition or physiological limitations. Clearing experiments were performed on the Coast of Maine, USA to eliminate competition. Belts 10 cm wide that ran from the littoral fringe downward into the eulittoral zone were cleared of all attached organisms down to bare rock. A second condition was set up in which only the brown algal canopy was removed from a 60 cm wide belt. Unaltered belts were designated as controls. Squares 0.25 m² in area were cleared in the littoral fringe and eulittoral zone. U. flacca was able to colonize the cleared areas of the eulittoral zone and reach abundances equal to those found in the unaltered control areas of the littoral fringe. This indicates that physiological limitations do not restrict the abundance of U. flacca in the eulittoral zone. However, evidence of grazing in cleared areas adjacent to high concentrations of the barnacle Semibalanus balanoides suggest that competition alone is not responsible for the limited abundance of U. flacca in the eulittoral zone. Herbivores associated with the presence of barnacles may also have an impact on the abundance of U. flacca, even during the winter months.     

Size distribution and expansion of canopy gaps in a northern Appalachian spruce-fir forest

Canopy gap area/age distributions and growth mechanisms were examined in a virgin subalpine forest in the White Mountains, New Hampshire, USA. The gap area distribution was negative exponential in form. Whithin gap tree ages varied widely in response to stepwise gap expansion caused by windthrow of peripheral trees or death of standing mature Picea rubens at gap edges. As a consequence, the density of small gaps may have been underestimated and the density of large gaps overestimated. The estimates of canopy turnover time, 303 yr, and of patch birth rate on an area basis, 3.3×10^-3 ha new patches/ha land area/yr, were not affected by the gap expansion phenomenon. However, any estimate of patch birth rate as numbers of new patches formed per year would have been too low. Because of increasingly widespread Picea death, the patch area/age distribution of this forest may not currently be in steady-state.     

Responses of Microplitis croceipes to host and nonhost plants of Heliothis virescens in a wind tunnel

The relative attractiveness of velvet leaf (Abutilon theophrasti Medicus), cotton (Gossypium hirsutum L.) (host plants) and groundcherry (Physalis angulata L.) (nonhost plant), and cotton plants with or without nectaries and with or without glands to Microplitis croceipes Cresson (Hymenoptera: Braconidae) was determined in a wind tunnel. Female parasitoids flew significantly more to glandless than to glanded cotton; response to nectaried and nectariless cotton was similar. Velvet leaf and cotton were favored significantly over groundcherry; parasitoids being equally responsive to both host plant species. Addition of larval frass alone or in combination with host larvae significantly improved the attractivity of the nonhost plant (groundcherry) to the parasitoids. There was no difference in attractiveness of groundcherry terminals with or without host larvae. Parasitoid search time was significantly increased with addition of larval frass. In the presence of cotton, however, kairomone-treated groundcherry remained unattractive.

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Résumé L'influence des nectaires et des glandes du coton, et l'attractivité des feuilles veloutées du coton (plante hôte) et de Physalis angulata sur Microplitis croceipes, ont été déterminées dans des expériences avec tunnel à vent. Les résultats ont montré que les parasitoïdes femelles sont significativement plus attirés par les cotons sans glandes que par les cotons glanduleux, tandis que la présence ou l'absence de nectaires ne modifie pas l'attractivité du coton. Elles réagissent de la même façon aux feuilles veloutées. P. angulata traité avec des crottes de chenilles, présenté seul ou en combinaison avec du coton, attire plus de femelles que des pousses de P. angulata non traitées. L'addition de chenilles hôtes seules n'améliore pas significativement l'attractivité de la plante. P. angulata traité avec des kairomones est demeuré inattractif quand il était proposé en même temps que du coton.

     

Habitat-specific estimates of competition in stream salmonids: A field test of the isodar model of habitat selection

Summary The population densities of sympatric Atlantic salmon,Salmo salar and brook charr,Salvelinus fontinalis, were measured in riffle and pool stream habitats to test whether non-linear isodars, a multispecific model of habitat selection based on ideal distribution assumptions, could (1) predict the distribution of densities between habitats and (2) reproduce the processes postulated to underlie spatial segregation and species interactions in previous laboratory and field studies. The model provided a good fit to observed density patterns and indicated that habitat suitability declined non-linearly with increased heterospecific competitor densities. Competitive effects in riffles appeared to be due to exploitative resource use, with salmon always emerging as the superior competitor. No evidence was found for interference competition in riffles. In contrast, interspecific competition in pools seemed to occur through exploitation and interference. The specific identity of the superior competitor in pools depended on the density of both species; pools provided the charr with refuge from competition with the salmon, presumably through the adoption by the charr of density-dependent behaviours, such as schooling and group foraging, that mitigated the negative impact of the salmon. Charr were displaced from the riffles toward the pools as the total salmon density increased. The isodar analysis, based on limited density data, successfully reproduced the processes suggested to underlie spatial segregation in previous field and laboratory studies and provided new insights into how changes in competitor densities modify habitat suitability in this system.