Biomass Allocation and Light Partitioning Among Dominant and Subordinate Individuals inXanthium canadenseStands

Patterns of above-ground biomass allocation and light captureby plants growing in dense stands or in isolation were studiedin relation to their height. A canopy model was developed tocalculate light absorption by individual plants. This modelwas combined with data on canopy structure and patterns of biomassallocation for solitary plants and for plants of different heightsin dense mono-specific stands of the dicotyledonous annualXanthiumcanadenseMill. There were four stands, and stand height increasedwith age and nutrient availability. The allometric relationshipbetween height and mass differed considerably between plantsin stands and those growing in isolation and also between plantsof different heights within stands. The proportion of shootmass in leaf laminae (LMR) decreased with increasing plant height,but solitary plants had a higher LMR than competing plants ofthe same height. Thus, in contrast to previous assumptions,LMR of competing plants is not strictly determined by biomechanicalconstraints but results from a plastic shift in biomass allocationin response to competition. Average leaf area per unit leafmass (SLA) decreased with increasing photosynthetic photon fluxdensity (PPFD) independent of nutrient availability. Consequently,taller, more dominant plants in stands had a lower leaf arearatio (LAR: LAR=LMRxSLA) than shorter, more subordinate plants.Dominant plants absorbed more light both per unit leaf area(     

Demography of the invasive shrub Scotch broom (Cytisus scoparius) at Barrington Tops,New South Wales: insights for management.

The exotic shrub Scotch Broom (Cytisus scoparius) has invaded large areas of eucalypt woodland at Barrington Tops, New South Wales, where it forms dense stands that have significant impacts on vegetation structure, flora and fauna. Data are presented from four 25 m² plots, which have been studied since 1985. Two plots were located in uniform broom thickets of different ages, and two were located across the margins of broom stands, which have since expanded to cover the entire plots. All broom plants in the plots (other than young seedlings, which were counted) were mapped, tagged and monitored annually. New seedlings appeared annually, but there was no relationship between their numbers (varying between years) and subsequent recruitment of older plants. The probability of seedlings reaching first flowering was less than 2%, and of surviving to mature size (>10 cm² basal area) was negligible. Seedlings mainly died through suppression (shade). Individuals less than 50 cm high were also browsed. Recruitment occurred only where light levels were high, either before closure of the broom canopy or after senescence had led to canopy opening. From approximately 12–30 years after initial invasion, broom stands underwent self‐thinning of mature plants, accelerated by collapse of plants on to each other. Recruitment of new, maturing plants, after this period produced a stand that was less dense than that found after initial invasion. Broom is creating more disturbance‐prone environments due to its impacts on other biota, likely alterations to the fire regime, and by harbouring feral pigs. Further disturbance favours broom, and elsewhere it has resulted in massive seedling regeneration. While fire or other disturbance can be used to stimulate germination, and thereby reduce a large part of the soil seed bank, denser broom infestations are likely to result unless follow‐up treatments can be applied over long time periods. A wiser management option, at least in the short term, may be avoidance of all disturbance, especially for stands of mature broom.     

Effects of an artificial protection structure on the sandy shore macrofaunal community: the special case of Lido di Dante (Northern Adriatic Sea)

Biomanipulation of eutropicated peaty lakes has rarely been successful; clear water with dense macrophyte stands fails to develop in most cases. It was unclear whether (1) high turbidity due to resuspension by benthivorous fish or wind is the major cause of low macrophyte density or whether (2) the establishment of submerged macrophyte stands is prevented by a lack of propagules, low cohesive strength of the lake sediment, high concentrations of phytotoxics, grazing by waterfowl and/or shading by periphyton growth. These hypotheses were tested in an experiment in a shallow peat lake in the Netherlands (Terra Nova). Removal of fish from a 0.5 ha experimental site resulted in clear water and the development of a dense (90% coverage) and species-rich (10 species) submerged vegetation. At a fish-stocked site and a control site the water remained turbid and dense macrophyte stands did not develop. The establishment of submerged macrophytes appeared not to be limited by a lack of propagules. Introduced plants grew poorly in turbid water, but very well in clear water. Exclosures showed that bird grazing reduced the plant biomass. In clear water grazing seemed to enhance the vegetation diversity. Periphyton development did not prevent plant growth in clear water. After the experiment, the fish stock was greatly reduced in the whole lake (85 ha), to test if (3) in a large lake, submerged macrophyte stands will not develop after biomanipulation. In the first season after fish reduction, transparency increased and species-rich submerged macrophyte stands developed, covering 60% of the shallow parts of the lake. Most of the species known to have occurred in the past re-established. The results indicate that high turbidity caused by benthivorous fish in combination with bird grazing were the major causes of the absence of submerged macrophyte stands in this lake. Abiotic conditions after the clearing of the lake were suitable for the growth of macrophytes. We infer that the restoration potential of submerged macrophyte stands in eutrophicated peaty lakes can be high, and results can be obtained quickly.     

Population dynamics of the tropical annual grass Sorghum intrans in relation to local patchiness in its abundance

Communities of Sorghum intrans, a tropical annual grass of north-west Australia, contain sparse patches several metres in diameter within otherwise dense swards. These patches are persistent for at least ca. 4 years, despite the fact that these species are annuals which lack a persistent seed-bank. Three possible causes of the sparse patches were investigated at a site near Darwin: loss of viable seeds through granivory or death; high plant mortality; or poor fecundity. This was done by augmenting the seed-banks of S. intrans in the sparse patches, and thinning stands of seedlings in the surrounding, naturally dense areas, and comparing the results of these perturbations with controls. Granivory did not appear to be a cause of the sparse patches. Furthermore, the seeds in the sparse patches did not become inviable over the dry season. Compared with the plants of S. intrans in the naturally high-density areas, survival of the plants in the augmented patches was not reduced, but vigour and seed production were. Conversely, plants in the thinned stands had a higher survival rate and very much greater vigour and seed production than plants in either the undisturbed patches or the undisturbed surrounding areas. Biotic constraints to plant growth such as grazing and disease appeared to be unimportant. These sparse patches result from poor site potential for S. intrans which limits its growth and reproductive output.     

Intraspecific competition and subterranean herbivory: individual and interactive effects on bush lupine

High mortality of plants growing in dense monospecific stands (i.e. self-thinning) usually results from intense intraspecific competition. However, inconspicuous below-ground insect herbivory might be a potent but overlooked source of mortality within dense stands of plants, particularly if crowding limits a plant's ability to compensate for herbivore damage. Here I ask how high conspecific density influences a plant's ability to cope with heavy below-ground insect herbivory.
I manipulated conspecific density and exposure to an abundant root-borer, the ghost moth ( Hepialus californicus ), and examined the impacts on the fecundity, growth, and survival of bush lupine ( Lupinus arboreus ), a fast-growing shrub that grows in dense monospecific stands in coastal grasslands. Both herbivory and intraspecific competition affected seed production, size, and mortality of bush lupine over the two years of the experiment. Plants consistently produced fewer seeds when growing at high versus low density and ghost moth herbivory also significantly reduced seed production. The negative effects of herbivory on plant fecundity were similar, regardless of plant density. In contrast, plant survival was affected by both competition, herbivory, and the interaction of these factors. In high density plots, plant survival was uniformly low (averaging 0.45–0.50); plants exposed to herbivores died from heavy herbivory, and plants protected from herbivores died due to intense intraspecific competition that compensated for losses due to herbivory. In low density plots, ghost moth herbivory similarly reduced lupine survival, from an average survival probability of 0.94 in plots protected from these herbivores to 0.55 in plots exposed to herbivory. Thus, results show that regardless of plant density, below-ground herbivory can be a potent source of mortality.     

Current and future threats to plant biodiversity on the Cape Peninsula, South Africa

The biodiversity of the Cape Peninsula (49127 ha in extent) has been considerably affected by various factors since European settlement in 1652. Urbanization and agriculture have transformed 37% of the original area of natural vegetation. Lowland vegetation types have been worst affected, with almost half of the transformation occurring in one of the 15 recognized vegetation types. Vegetation at high altitudes has been little affected by urbanization and agriculture, but alien trees and shrubs are now threatening biodiversity in these areas. Of the area not affected by urbanization and agriculture 10.7% is currently under dense stands (>25% canopy cover) of alien plants and another 32.9% is lightly invaded. Dense stands of Acacia cyclops, the most widespread invader, cover 2510 ha, 76% of the total area under dense alien stands. This paper assesses the impacts of these factors on aspects of the plant biodiversity of the area, namely, the distribution of major vegetation types and of endemic, rare and threatened plant taxa and of taxa in the Proteaceae (a prominent element in almost all communities, taken as an indicator of overall plant biodiversity).Possible future impacts on biodiversity are assessed by considering the effects of several scenarios involving increased urbanization and changes to alien plant control strategies and further spread over the next 50–100 years. The worst-case scenario for urbanization sees the area under natural vegelation reduced to 12163 ha (39.3% of its extent in 1994, or 24.8% of its original extent). Under this scenario almost a quater of the 161 endemic, rare and threatened (special) taxa are confined totally to urban areas; 57.4% of the known localities of these taxa, and 40.1% of the remaining localities of Proteaceae taxa are transformed. Dense alien stands currently affect 29.8% of the localities of special taxa known from herbarium records and 8.4% of these taxa currently occur only in areas at present affected by aliens. Clearing all dense stands would result in 62.9% of special taxa having less than half of their known localities affected (49.1% at present). Under this scenario, 92% of Proteaceae taxa have more than 75% of their localities unaffected by aliens. If clearing is confined to specific areas (the Cape Peninsula Protected Natural Environment or all publicly-owned land) and the aliens spread further outside these areas, the area of natural vegetation remaining shrinks (to 82.4% of the current extent if control is confined to public land). The further losses in biodiversity associated with these scenarios are described. If control programmes collapse and all potentially invadable land is occupied by dense alien stands, only 407 ha of natural vegetation would remain (1.5% of the current extent).The probability of the various scenarios materializing is discussed. To minimize further losses in biodiversity it is essential that: (1) a major initiative is launched immediately to clear (firstly) the 10184 ha of lightly invaded vegetation and then the 3313 ha of densely invaded vegetation; (2) no urban development be permitted within the boundaries of the Cape Peninsula Protected Natural Environment; (3) a systematic programme of prescribed burning (linked to the alien control programme) is initiated; and (4) contingency measures are implemented to improve the status of seriously threatened taxa, habitats and vegetation types.     

SPATIAL DYNAMICS OF A HIMANTHALIA ELONGATA (FUCALES, PHAEOPHYTA) POPULATION

In dense monospecific stands of plants intraspecific competition usually results in self-thinning, the concurrent increase in biomass and decrease in density over time. Self-thinning may also result in a change in the spatial pattern of individuals, but so far the spatial dynamics of marine plants has not been investigated. The brown alga Himanthalia elongata ( L.) S. F. Gray forms dense monospecific stands on many northern temperate rocky shores, and various attributes (including its simple form) facilitated the study of the spatial dynamics of this species .
The spatial pattern of settling zygotes was examined in the laboratory. In the absence of water movement, substratum heterogeneity, and a point source, zygotes usually settled in clumps rather than randomly. Within the clumps zygotes appeared to be regularly distributed at a scale similar to the size of the zygotes themselves. Furthermore, the clumps themselves seemed to be regularly distributed. On the shore, well-established stands of "button-stage" Himanthalia populations were examined during a period of extensive growth and self-thinning. Individual plants were initially highly regular in spatial pattern but became less so over time. The pattern of plants dying during self-thinning was also highly regular and probably reflected existing spatial regularity. However, using a hypothesis of mortality as a random event, I found that smaller plants had a less than average survival potential, while larger plants had a greater than average chance. A consideration of the spatial pattern of plants alive at the end of the study revealed regularity at a scale of 2–7 mm but a random spatial pattern at larger scales, which might indicate a small sphere of influence of competing individuals. The best predictor of mean nearest neighbor distances in the populations was mean plant diameter .     

Restoration of alvar vegetation on Öland,Sweden

Abstract. Alvars in the Baltic region, particularly on the Swedish islands of Öland and Gotland and in western Estonia, are well‐known for their plant species richness and extensive populations of rare species. Grasslands make up most of alvar vegetation. The extent of these grasslands decreases because of bush enchroachment which takes place in most alvars when traditional land use practice changes, notably when grazing is ceased. The main threat for the alvar grassland is formed by the rapid expansion of Juniperus communis and Potentilla fruticosa in dry and wet sites, respectively. Applied research has been carried out during several years in order to develop plans for the restoration of alvar grassland. Thus it was shown that species richness in areas with a mixture of grassland and shrubland decreases in relation to the increased cover by shrubs. In the case of Juniperus there is a distinct drop in species number as soon as the cover exceeds 75‐80%, while the decrease is more gradual with increased cover of Potentilla. The seed banks under dense stands of these shrubs only contain ca. 20% of the species found in dry and wet alvar meadows. This was confirmed by clearing experiments. Long‐term recordings have shown that establishment of juniper seedlings takes place mainly in half‐open areas between already existing junipers. Intermediate‐sized junipers expanded faster than old and big shrubs. Potentilla shrubs recover fast from cutting or burning. After 2‐4 yr they have almost attained their former size. This recovery can be prevented when cattle grazing is introduced. In the framework of a local alvar restoration project on Öland, covering 7000 ha, grazing regimes are re‐established, fences erected and large‐scale clearings carried out. Recently established juniper shrubs are being cleared, intermediate sized junipers (30‐50 yr old) are selectively removed while creating a mosaic landscape with high biological diversity. Older dense juniper stands are left alone or are only partly cleared. Potentilla stands in moist areas are cut to create moist meadows which are breeding grounds for waders, and to establish corridors between remaining open areas. Three items are discussed (1) the importance of the Stora Alvar area; (2) re‐introduction of grazing and (3) re‐introduction of species. The outstanding importance of the area regarding species richness and endemism should be recognized through a ‘golden list’ to be used along with red and blue lists.     

Environmentally induced clonal reproduction and life history traits of the liana Machaerium cuspidatum in an Amazonian rain forest,Ecuador

The population structure of the canopy liana Machaerium cuspidatum Kuhlm. & Hoehne (Fabaceae) was studied in ten transects covering a total of 2.5 ha of tropical rain forest in Yasuní National Park, eastern Ecuador. The aim was to investigate how trellis availability, topographic position and light availability affected the population density and structure of the most abundant liana species in the area. The environmental variation affected plants in different size classes differently. Large plants (diameter 1 cm) were almost exclusively found in areas with low canopy and dense undergrowth. These areas had the highest density of suitable host plants. Seedling-sized plants (height < 30 cm) and saplings (height 30 cm but diameter < 1 cm) occurred more frequently and withhigher density in steep upland areas than in the floodplains, presumably due to elevated seedling mortality in the periodically flooded areas. Seedling-sized plants and saplings did not grow in areas with elevated light levels, and although plants of all sizes were highly clumped, this clumping rarely coincided with patches with abundant canopy gaps. Machaerium cuspidatum reproduces clonally when branches re-root and break off from the parentplant. The density of clonally produced plants was high in areas where the density of sexually produced plants was low, whereas the total density varied little among habitat types. A higher proportion of the seedling-sized plants were of sexual origin in the upland areas. An environmentally induced increase in the production of independent ramets enables M. cuspidatum to persist in the floodplains. This shows that plants may expand theirrealised niches to habitats with a low potential for seedling survival if high seedling mortality can be compensated for by increased clonal reproduction.     

Developmental changes in shoot N dynamics of lucerne (Medicago sativa L.) in relation to leaf growth dynamics as a function of plant density and hierarchical position within the canopy

Shoot N concentration in plants decreases as they get bigger, due to the fact that N accumulates less rapidly than dry matter in plants during the plant growth process, leading to an allometric relationship between shoot N content (N(sh)) and shoot mass (W(sh)): N(sh)=a(W(sh))b. The results obtained on lucerne plants growing either under controlled low density conditions or in dense stands under field conditions show that the value of the allometric coefficient b that represents the ratio between the relative N accumulation rate in shoots [dN(sh)/(N(sh)dt)] and the relative growth rate [dW(sh)/(W(sh)dt)], decreases from 0.88 for a low plant density to 0.72 for a dense stand. Therefore, the fractional increase of shoot N per unit of shoot dry matter is lower when plants are in competition for light in dense canopies. This decrease can be entirely explained by the parallel decline in the leaf area per unit of shoot mass. Thus, a remarkably constant linear relationship can be established between N(sh) and leaf area (LA): N(sh)=1.7 g m(-2) LA, regardless of the conditions (low versus high density, controlled versus field conditions). Moreover, in a field dense stand, the comparison of plants with contrasting positions between the top and the bottom of the canopy (dominant, intermediate or suppressed plants), also shows that the difference in N(sh) at similar shoot mass is explained by the proportion of leaf mass to shoot mass. These data support the idea that leaf growth drives the dynamics of shoot N accumulation. These results also indicate that competition for light among individual plants within a dense canopy induces developmental changes in plant morphology (leaf:stem ratio) that explain the differences observed in shoot N concentration. This last observation could be extrapolated to multi-specific plant stands. Therefore, the sharing of N resources among plant species could partially be the result of the sharing of light within the canopy.     

Survivorship,growth and self-thinning in Banksia ericifolia

High-density (dense) and low-density (sparse) plots were set up in naturally sown monospecific stands of Banksia ericifolia in coastal heath, 3 years after fire. This was done both in high-growth and low-growth areas. Plant mortality was recorded quarterly, and two harvests were made at 6 and 9 years to sample growth. Density-independent mortality at an exponential rate was observed in the low-growth treatments at both densities, and in the high-growth sparse treatment. Growth level affected mortality, with the half-life of populations in the high-growth sparse plots being double that of populations in the low-growth plots. Density-dependent mortality (self-thinning) was seen only in the high-growth dense plots. Seasonal effects on mortality were slight; maximum mortality was observed in the spring-summer period in plots subject to density-independent mortality, and in the winter-spring quarter in plots that had self-thinned. Yields in the high-growth plots and the low-growth dense plots were high for heath vegetation. The self-thinning populations did not exceed White's (1985) upper boundary for thinning lines of log intercept (K) = 5 on standardized axes. The data suggested a log intercept value in the range 4.8–4.9 in the high-growth stands assuming a thinning-line slope of – 1.5. Banksia ericifolia (a large shrub/small tree) has a high mean plant weight per given thinning density compared with trees, where an upper limit of log K= .4 has been suggested by White (1985). The volume of canopy space per plant in B. ericifolia is not unusual compared with other species. The amount of biomass packed into a given volume of canopy space was high in this Banksia, achieved by having leaves with a low ratio of area to weight (specific leaf area, SLA). For given values of density, leaf area index and proportion of shoot as leaf, plants with a low SLA will be several times heavier than plants with a high SLA. This achieves a high biomass to volume ratio without an erectophile canopy and may explain the high intercept seen for thinning lines of conifers.     

Light spectral quality, phytochrome and plant competition

The light environment experienced by plants in natural vegetation is strongly dependent upon interactions with neighbors. For plants in dense stands, reduced irradiance can lead to reductions in growth and fitness. Spectral light quality is also altered beneath a leaf canopy, and can serve as an important signal of competition for light. Recent physiological studies indicate that plants can perceive the quality of light reflected from neighbors as an accurate predictor of future competition, and respond morphologically even before they are directly shaded. These findings have important implications for plant population biology, and provide a valuable opportunity for the study of adaptive plasticity.     

Architectural and physiological mechanisms of reduced size inequality in CO2-enriched stands of common ragweed (Ambrosia artemisiifolia)

Testing whether and how subordinate individuals differ from dominants in the utilization of enriched CO₂ atmospheres is important for understanding future stand and community structure. We hypothesized that subordinate and dominant Ambrosia artemisiifolia L. (Asteraceae) (common ragweed) plants growing in dense stands would not equally acquire or utilize carbon gains from CO₂-enrichment, and that the resulting disproportionate growth gains to subordinates would reduce size inequalities in competing stands. We grew experimental stands of A. artemisiifolia in either ambient (360 μL L⁻¹) or twice ambient (720 μL L⁻¹) levels of atmospheric CO₂. We compared the relative growth, photosynthetic capacity, and architecture of subordinate and dominant plants in each treatment, and assessed size inequalities using the stand-level coefficient of variation (CV). In elevated CO₂, plants grew larger, but subordinate plants shifted more mass to upper stem allocation than dominants. Dominant plants demonstrated reduced leaf-level photosynthetic gains in elevated CO₂ compared with subordinate plants. Reduced CVs in plant size reflected smaller proportional growth gains by dominants over subordinates in elevated vs. ambient stands. We conclude that differences in the architectural and physiological responses of subordinate and dominant ragweed plants reduce competition and allow subordinate plants to catch up to dominants in elevated CO₂ conditions.     

Field measurements of wind speed and reconfiguration in Arundo donax (Poaceae) with estimates of drag forces

The giant reed (Arundo donax) is well known as a species that can withstand high wind loads without mechanical damage. To examine wind impact, profiles of vertical wind speeds in the plant's natural habitat (southern France) were measured at the edge and within a stand in the main wind direction. Wind speed was recorded simultaneously at five heights. For 75 measurements of within-canopy wind speed profiles, the attenuation coefficient was 4.4 ± 0.5, a value typical for plant stands with very dense canopies. Video recordings proved that A. donax becomes streamlined with increasing wind speed, reducing the projected surface area of leaves and stem. The total projected surface area is a function of wind speed and can be characterized by a second-order polynomial regression curve. For small wind velocities up to 1 m/s, the calculated drag force is proportional to the square of the wind speed. However, when A. donax plants are subjected to higher wind speeds (1.5-10 m/s), the drag force becomes directly proportional to the wind speed. Streamlining is a potentially important adaptation for withstanding high wind loads, especially for individual plants and plants at the edge of stands, whereas in dense stands streamlining probably plays a minor role.     

Host plant dispersion, leaf hopper movement and disease transmission

Abstract.

• 1 The plant-to-plant movement of the corn leafhopper, Dalbulus maidis Delong & Wolcott, and the spread of the leafhopper-borne maize rayado fino virus were investigated in four patterns of maize (Zea mays) dispersion.
• 2 D. maidis was less abundant and the spread of the virus was slower in dense stands of maize than in sparse stands.
• 3 When plant density was held constant, leafhoppers were more abundant in maize stands with relatively equidistant plant spacing (uniform dispersion) than in stands with densely-sown rows (linear dispersion) or double-sown hills (clumped dispersion), but there was no difference in virus incidence among these plant dispersion patterns.
• 4 Leafhoppers were less likely to move to adjacent plants in uniform plant dispersion patterns than in either linear or clumped dispersion patterns. This result may explain the lack of higher virus incidence in uniform stands, despite higher leafhopper abundance.
• 5 Leafhopper movement was consistent with a simple rule: the shorter the distance to the next adjacent plant, the more likely a leafhopper is to move between plants.
• 6 These results demonstrate that host plant dispersion can affect the abundance and behaviour of highly mobile herbivorous insects even when plant density is constant.

     

Soil Characteristics of Rehabilitating and Unmined Coastal Dunes at Richards Bay, KwaZulu-Natal, South Africa

The postmining rehabilitation of coastal sand dunes north of Richards Bay (28°43′S, 32°12′E), South Africa, is resulting in the development of a series of known-aged stands of vegetation dominated by Acacia karroo (sweet thorn). Other broad-leaved species are establishing themselves in rehabilitating areas more than 12 years of age. Soils from rehabilitating stands 3–5, 9–12, 13–15, and 16–18 years of age, from two disturbed, unmined stands 30 and 58 years of age, and from a mature unmined stand were examined to assess age-related trends in selected soil properties. Individually, these stands represent a series of different developmental stages of a coastal dune successional sere. Soil organic material, percentage organic carbon and concentrations of sodium, potassium, magnesium, calcium, and nitrogen increased with an increase in habitat regeneration age. Concentrations of most of these elements were lower than those recorded on the 58-year-old unmined and mature unmined stands. Multivariate analyses suggest, however, that the similarity of these values for rehabilitating stands to those for the unmined stands increased with an increase in regeneration age. The growth response of Raphanus sativus (radish) plants, based on mass attained under experimental growing conditions in soil collected from these stands, suggests an increase in soil fertility with an increase in regeneration age.     

Spatial patterns and dynamics of woody vegetation in an arid savanna

The spatial distribution of woody plants was studied in an arid savanna in Botswana. The study included stands of mixed species and sizes as well as monospecific even-sized stands of different size classes of the tree Acacia erioloba and the shrub Acacia mellifera. In the case of A. mellifera both dense stands on overgrazed land and more open stands were included. The analysis used all plant-to-plant distances, and individuals were represented with a realistic canopy extension. The mixed stands showed aggregated distribution of individuals, mainly caused by strong clumping of small shrubs. In A. erioloba saplings were aggregated, small trees were randomly or regularly distributed and large trees were randomly spaced. In open stands of A. mellifera aggregation increased with size of the shrubs, while in dense stands with overgrazing aggregation decreased with increasing size. The different patterns are discussed in relation to the relative importance of inter- and intraspecific competition for water and of disturbance by fire as regulatory mechanisms for total amount and spatial distribution of woody plants in this savanna.     

Methane efflux in relation to plant biomass and sediment characteristics in stands of three common emergent macrophytes in boreal mesoeutrophic lakes

Methane efflux was studied in stands of three emergent macrophyte species (Equisetum fluviatile, Schoenoplectus lacustris and Phragmites australis) commonly found in the littoral zone of boreal lakes. In vegetation stands with relatively low methane (CH₄) emissions (<0.3 mol m^?2 (ice‐free period)^?1), the seasonal variation of CH₄ efflux was better correlated with the dynamics of plant growth than variation in sediment temperature. In dense and productive vegetation stands that released high amounts of CH₄ (2.3–7.7 mol m^?2 (ice‐free period)^?1), the seasonal variation in CH₄ efflux was correlated with sediment temperature, indicating that methanogens were more limited by temperature than substrate supply. The bottom type at the growth site of the emergent plants significantly influenced the ratio of CH₄ efflux to aboveground biomass of plants (Eff : B). The lowest Eff : B ratio was found in E. fluviatile stands growing on sand bottom under experimental conditions and the highest in P. australis‐dominated littoral areas accumulating detritus from external sources. The future changes expected in the hydrology of boreal lakes and rivers because of climatic warming may impact the growth conditions of aquatic macrophytes as well as decomposition and accumulation of detritus and, thus, CH₄ effluxes from boreal lakes.     

The effect of an invasive alien vine, <Emphasis Type="Italic">Vincetoxicum rossicum</Emphasis> (Asclepiadaceae), on arthropod populations in Ontario old fields

Vincetoxicum rossicum is an invasive alien vine introduced from Europe in the late 1800s that is now an emerging pest in upstate New York and eastern Ontario. The plant can form dense, monotypic stands in woodlots and old fields, and may be displacing native vegetation. As a consequence, V. rossicum may be displacing arthropod fauna associated with native vegetation. In June and August 2002, we sampled V. rossicum and three other old field plants (Asclepias syriaca, Solidago altissima, and mixed graminoids) for arthropods using pitfall traps and by sampling individual plants. A total of 7868 arthropods were counted on plants and 18,195 individuals were trapped; these were sorted by feeding guild. Overall, stands of V. rossicum supported the lowest abundance of both stem- and ground-dwelling individuals, as well as the lowest number of arthropods in most phytophagous guilds. Some feeding guilds are entirely absent: V. rossicum stands are completely devoid of gall-makers and miners, and support few pollinators. This study suggests that arthropod diversity will decline if V. rossicum displaces native old-field plants.     

Fight or flight: plastic behavior under self-generated heterogeneity

Plants are able to plastically respond to their ubiquitously heterogeneous environments; however, little is known about the conditions under which plants are expected to avoid or confront their neighbors in dense stands, where heterogeneity is self-generated by non-uniform growth and feedback between plant interactions and stand heterogeneity. We studied the role of plasticity for spatial pattern-formation and the resulting stand-level fitness of clonal plants, assuming variable types of plastic behavior. Specifically, the adaptive values of behavior ranging from pure avoidance, to neutral and pure confrontation were assessed using a simulation model of stands of clonally growing plants with varying capacity of plastic behavior. The results demonstrated significant effects of the type of competitive behavior on mean final densities of single-species stands at equilibrium. Density was the lowest and aggregation was the highest in stands of purely confrontational plants, and density was highest in stands of neutral and purely avoiding plants. When competing against a neutral photometer (i.e. non-plastic but otherwise identical plant), the best competitors were plants that avoided their neighbors in 0.33–0.50 of the cases and were neutral otherwise. Differences in adaptive values of individual behaviors depended both on the distance over which the environmental structure (i.e. local density) was perceived, and on overall density. Density-independent ramet mortality profoundly changed the effectiveness of competitive behaviors. Under high levels of mortality, avoidance was the most effective and confrontation the least effective behavior. The results indicate that individual-based behaviors might affect higher organizational levels, and that their reciprocal interactions with resource levels and patchiness, and responsiveness to density-independent mortality might generate higher-order feedbacks that intricately affect the fate of individual ramets and the patterning of whole stands and communities.