Ramet Density Research Articles

Spatial variation in vegetation and abiotic factors related to the occurrence of a ring‐forming sedge

Abstract. The clonal sedge Carex humilis forms rings of densely aggregated ramets in a dry grassland community in Central Europe. We describe the small‐scale spatial variation, both in abiotic factors and vegetation, in relation to these rings. Compared to the surrounding vegetation the cover of plants, other than C. humilis, was significantly lower both in the central area of rings and within the rings themselves. The vegetation structure was also different. The soil was more fertile in the central area and within the ring than in the surroundings, measured both directly and by the abiotic response values of the vascular plants. We conclude that neither resource depletion nor competition from other plants were likely to be responsible for the low ramet density in the central area of C. humilis rings. Instead, we suggest that the ring form is caused either by the deposition of growth inhibiting substances or by intrinsic morphological rules.

Can nutrients alone shift a sedge meadow towards dominance by the invasive Typha × glauca

Abstract Where wetlands receive urban runoff, Typha spp. and other invasive plants often displace the native vegetation. We tested the ability of nutrients (N and P) to increase vegetative growth of T. × glauca (a hybrid of T. latifolia and T. angustifolia). In the greenhouse, 17 treatments revealed that T. × glauca required both N and P for growth, and total leaf length was most stimulated where a higher proportion of P was added (7N:1P vs. 14N:1P, with N constant and P changed), regardless of concentration (the High treatment was 4× the Low treatment). In Gardner Marsh (Madison, Wisconsin, USA), we set up 28 plots (1 × 6 m) that bisected the boundary between sedge meadow (graminoids) and T. × glauca, and we added a common lawn fertilizer (9N:1P:4K) at high (62.5 g/m2), medium (31.3 g/m2), low (15.6 g/m2), and control (0 g/m2) rates on five dates, with n=7 plots/treatment. After one growing season, fertilizer addition increased T. × glauca ramet density, height, and biomass, especially where the sedge me...

Spatial variation in vegetation and abiotic factors related to the occurrence of a ring-forming sedge

Abstract. The clonal sedge Carex humilis forms rings of densely aggregated ramets in a dry grassland community in Central Europe. We describe the small-scale spatial variation, both in abiotic factors and vegetation, in relation to these rings. Compared to the surrounding vegetation the cover of plants, other than C. humilis, was significantly lower both in the central area of rings and within the rings themselves. The vegetation structure was also different. The soil was more fertile in the central area and within the ring than in the surroundings, measured both directly and by the abiotic response values of the vascular plants. We conclude that neither resource depletion nor competition from other plants were likely to be responsible for the low ramet density in the central area of C. humilis rings. Instead, we suggest that the ring form is caused either by the deposition of growth inhibiting substances or by intrinsic morphological rules.

The contrasting clonal architecture of two bromeliads from sandy coastal plains in Brazil

The vegetation of the Brazilian sandy coastal plains (‘restingas’) consists of vegetation patches of different sizes, separated by bare sand. Clonal growth seems to play an important role in this system, since this patchy resource-poor environment imposes several constraints on the flora. Aechmea nudicaulis and Vriesea neoglutinosa are two bromeliad species that occur in the restinga of Maricá, southeastern Brazil. The clonal growth patterns of these species were studied in order to understand their clonal architecture and ramet growth dynamics. For A. nudicaulis, we mapped six clonal fragments and measured the number of ramets produced by each mother-plant, rhizome length and branching angle. Height and diameter were measured in live plants. For V. neoglutinosa, we measured the number of ramets produced by each mother-plant and the respective branching angles. Leaf number, height, and diameter were recorded over a period of five months. For both species, variation in these parameters was correlated with life stage of the ramets. Clonal fragments of A. nudicaulis concentrate live ramets at the edge of vegetation islands, increasing ramet density. This strategy allows a single clonal fragment to potentially occupy a large area, forming large ramet systems. In contrast, the clonal growth of V. neoglutinosa, with ramets that develop only after sexual reproduction and arise inside the mother-plants, practically without spacer structures, lead to the formation of compact small clonal fragments, retaining their original site of occupation, maintaining the patch size

Seasonal herbivory and mortality compensation in a swan–pondweed system

Many birds feed on submerged macrophytes during a temporally discrete period every year, for instance during migratory stopover or at the wintering grounds. Hence, seasonal herbivory is a common feature of the life cycle in many aquatic macrophytes. We are interested in the effect of Bewick's swans ( Cygnus columbianus bewickii) feeding on the tubers of fennel pondweed ( Potamogeton pectinatus) in the Netherlands every autumn. For that purpose, we developed a sequential macrophyte population model, including seasons of tuber production, herbivory and winter mortality as distinct and unambiguously defined events. The model is characterised and parameterised with both field and laboratory data. Tuber consumption inevitably decreases the density of ramets sprouting next spring, but it may actually increase the density of tubers produced in the following autumn. Hence, we can only understand the effect of grazing on the fennel pondweed population by recognising the seasonal structure of density-dependence. The mean density of fennel pondweed and the yield taken by swans are dependent on the foraging threshold below which no grazing takes place. Furthermore, the consumption has a stabilising effect for a wide range of parameter values.

The role of species richness for recruitment in a seminatural grassland

I examined two aspects of how recruitment is influenced by species richness in a seminatural grassland: effects of species richness among colonisers in a seed mixture, and effects of species richness, ramet density, grass and moss cover in the colonised vegetation. The results suggest that recruitment is higher in more species‐rich seed mixtures (for three of the four target species: Anthyllis vulneraria, Centaurea jacea, Filipendula vulgaris and Primula veris). Average recruitment of the target species was negatively related to ramet density in the colonised vegetation, but not affected by grass or moss cover. The results imply that mechanisms other than interspecific competition among species are important in structuring the community during the recruitment phase. The negative effects of ramet density and the positive effects of species richness may be a result of strong intra‐specific competition, or species‐independent competition in combination with diversity effects enhancing microsite variability.

Stand-Level Effects on the Role of Decaying Logs as Vascular Plant Habitat in Adirondack Northern Hardwood Forests

GREGORY G. MCGEE (State University of New York-College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210). Stand-level effects on the role of decaying logs as vascular plant habitat in Adirondack northern hardwood forests. J. Torrey Bot. Soc. 128:370-380. 2001.-The objectives of this study were to determine (1) if decaying logs provide critical habitat for any vascular plants in Adirondack northern hardwood forests; and (2) whether environmental factors that vary at the stand-scale influence the role of decaying logs as vascular plant habitat. Herbaceous ramet densities and woody seedling/sapling basal area were characterized on well-decayed logs and adjacent forest floor microsites in three northern hardwood stand types: old-growth, partially-cut, and 15to 29-year-old shelterwoods. No common herbaceous species were restricted to the log microsites, although Dryopteris intermedia had greater densities on logs. Betula alleghaniensis, Tsuga canadensis and Abies balsamea all attained greater basal areas (stems --15.0 cm diameter at the root collar) on decaying logs, while Acer pensylvanicum, Acer saccharum, Fagus grandifolia and Viburnum alnifolium had greater basal areas on forest floor microsites. Average (+ 1 SD) total basal area of woody stems was greatest in the shelterwoods (1775 + 615 mm2 m-2), compared to the partially-cut (714 + 266 mm2 m-2) and old-growth (503 + 657 mm2 m-2) stands. Betual alleghaniensis accounted for a substantial proportion of the basal area in the shelterwoods. In these shelterwood stands B. alleghaniensis basal areas were not statistically discernible between the forest floor and log microsites (P = 0.07). However, in partially-cut stands, B. alleghaniensis basal area was greater on the decaying log microsites (P = 0.05). In the old-growth stands, no B. alleghaniensis stems > 2 cm diameter were sampled. This study indicates that decaying logs provide early establishment sites for some tree species and the fern, D. intermedia, in northern hardwood forests. While B. alleghaniensis favored logs as establishment sites in the partially-cut stands, this species was capable of widespread establishment throughout the shelterwood stands. Even though B. alleghaniensis readily germinated on decaying logs, its recruitment into larger sapling size classes appeared to be consistently limited by factors varying at the stand level (e.g., light levels).

Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced.The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.

A simulation study of the effects of architectural constraints and resource translocation on population structure and competition in clonal plants

(1) Spatially explicit simulation of clonal plant growth is used to determine how rametlevel traits affect ramet density, spatial pattern of ramets and competitive ability of a clonal plant. The simulation model used combines elements of (i) an individual-based model of plant interactions, (ii) an architectural model of clonal plant growth, and (iii) a model of resource translocation within a set of physiologically integrated plant individuals. (2) The effects of two groups of parameters were studied: growth and resource acquisition parameters (resource accumulation, density-dependence of resource accumulation, resource translocation between ramets) and architectural rules (branching angle and probability of branching, internode length). The model was parameterised by values approximating those of clonally growing grasses as closely as possible. The basic parameter values were chosen from a short-turf grassland. Sensitivity analysis was carried out on relevant parameters around three basic points in the parameter space. Both single-species and two-species systems were studied. (3) It is shown that increasing resource acquisition and growth parameters increase ramet density, genet number and competitive ability. Translocation parameters and architectural parameters modify the effects of resource acquisition and growth, but their effect in single-species stands was smaller. (4) The simulations of species with fixed ramet sizes showed that ramet density in single-species stands cannot be used for predicting competitive ability. Increase in resource acquisition and growth parameters was correlated with an increase in equilibrium ramet density and competitive ability. Increasing branching angle, branching probability or internode length lead to an increased competitive ability, but did not affect equilibrium ramet density. Change of architectural parameters could therefore affect competitive ability independently of their effect on the final ramet density. (5) Spatial pattern both in single-species and two-species stands was also highly parameter-dependent. Changes in architectural parameters and in translocation usually lead to pronounced change in the spatial pattern; change in growth and resource acquisition parameters generally had little effect on spatial pattern.

Regeneration strategies of dominant boreal forest dwarf shrubs in response to selective removal of understorey layers

Abstract.The deciduousVaccinium myrtillusand the evergreenVaccinium vitis‐idaeawere subjected to five removal treatments of understorey layers: control, removal of the moss layer, removal of the field layer, removal of both moss and field layers and removal of moss, field and humus layers. A second factor, sowing, was included to investigate sexual reproduction after disturbance. Density of new ramets and seedlings and growth of annual shoots were studied for the first two growing seasons, whereas cover was measured for five growing seasons after disturbance treatment.Initially, vegetative production of new ramets and species cover increased rapidly in all disturbed plots, except for the most severe treatment, in which production of new ramets was virtually absent throughout the study. Full recovery following removal of the field layer only or both field and moss layers was reached after four years forV. myrtillus. V. vitis‐idaearecovered more quickly, after one year (removal of field layer only) and four years (removal of field and moss layers). The relative growth ofV. myrtillusandV. vitis‐idaeaincreased in the latter treatment in terms of production of annual shoots and length of annual shoots, respectively. Seedling density increased after sowing in the most severe treatment. The results underscore the importance of vegetative growth for recovery of these species at moderate‐level disturbances. The high rate of sexual reproduction in the most severe treatment implies that strong mechanical disturbance is needed to enhance the establishment of new genotypes in these species.

The effect of nitrogen on stolon and ramet growth in four genotypes of Fragaria chiloensis L.

Plant foraging response is a process in which clonal plants proliferate in nutrient-rich sites by shortening stolon length and increasing ramet density. Conversely, stolon length increases and ramet density decreases in nutrient-poor sites. Four genotypes of strawberry ( Fragaria chiloensis (L.) Duch.) were grown in a greenhouse for 10 weeks and treated with different concentrations of nitrogen. Genotypes differed in plant size, stolon and ramet production, and nitrogen distribution between parent and ramets. Genotype Q18 were the smallest plants with the greatest number of stolons and ramets, typical of the phalanx morphology. The other genotypes had fewer but longer stolons, typical of the guerrilla morphology. Number of stolons and ramet density increased with increased N more in Genotype Q18 than the other genotypes. Results indicate that vegetative growth changed in response to increasing N treatment of the parent plant by shortening the average stolon length, increasing the number of stolons, and increasing the number of ramets while maintaining total stolon length. Foraging response characteristics were observed in strawberry but varied among genotypes.

DEVELOPMENTAL PLASTICITY ALLOWSBETULA NANATO DOMINATE TUNDRA SUBJECTED TO AN ALTERED ENVIRONMENT

We investigated how three co-dominant arctic shrubs (Betula nana, Salix pulchra, and Ledum palustre ssp. decumbens) responded to long-term treatment with N+P fertilizers and greenhouses in a factorial field experiment at Toolik Lake, Alaska. Our goal was to understand the relationship between growth of individuals and species abundance in the community, and the mechanism by which one species achieves dominance under changed environmental conditions. We compared aboveground growth and allocation patterns in individual ramets 15 yr of age with community abundance measured by quadrat harvests. Ramets of all three species substantially increased their stem biomass with fertilization, but the increase was much larger for Betula than for the other two species. In quadrat sampling, only Betula appreciably increased its biomass per unit area with fertilization or greenhouse treatment. For Salix in all treatments, and Ledum in the two fertilizer treatments, ramet density per unit area decreased more than growth of surviving 15-yr-old ramets was promoted, so community biomass of these species declined. In contrast, Betula increased its ramet density in all treatments by producing new shoots from stems older than 15 yr, even though stem mortality was also increased in the two fertilizer treatments. Betula increased its growth in part by a major change in allocation, from producing mostly short shoots to producing many more long shoots. As a result, the number of branches and the rate of production of new meristems greatly increased. This developmental plasticity allowed extensive growth that led to development of a dense canopy and imposed light limitation on the other species. The flexible growth strategy of Betula points to the importance of meristem availability and developmental constraints in determining plant response to environmental change. Developmental controls over meristem availability are not usually considered in ecological paradigms for allocation, but they may be useful for predicting plant response to changes in nutrient availability in other ecosystems.

Factors affecting browsing by moose (Alces alces L. ) on European aspen (Populus tremula L.) in a managed boreal landscape

There is considerable circumpolar concern regarding the regeneration of several tree species in the temperate and boreal landscape due to heavy browsing. We analyzed the risk of browsing on aspen (Populus tremula L.) at two different scales in a managed boreal forest in northern Sweden with one dominating browser in the system, the moose (Alces alces L.). At the stand level, we found that a high density of aspen ramets in connection to or surrounded by young forest (predominantly Scots pine Pinus sylvestris L.) attracted moose relatively more than aspen stands in mature forest and interior forest, respectively. If a stand was being used, a single aspen ramet faced the best chance of escaping browsing in a stand with a high density of aspen ramets, located far from arable land. This utilization pattern by the herbivores suggests that older forest may function as a temporal refuge for aspen regeneration in the managed boreal landscape, but this situation may change as remaining old forest stands eventually turn into young forest. Although cutting will favour aspen regeneration, our study highlights an apparent paradox, as the emerging aspen ramets will face a high browsing risk from attracted herbivores.

RAMET DYNAMICS FOR THE CLONAL SEAWEED PTEROCLADIELLA CAPILLACEA (RHODOPHYTA): A COMPARISON WITH CHONDRUS CRISPUS AND WITH MAZZAELLA CORNUCOPIAE (GIGARTINALES)

Little is known about the dynamics and the ecological interactions among ramets (fronds) from populations of clonal red seaweeds. Small ramets are very difficult to tag, so their growth cannot be monitored directly. The temporal variation of the relationship between stand biomass and ramet density offers information on ramet performance. We calculated this relationship for an intertidal population of Pterocladiella capillacea (Gmelin) Santelices et Hommersand (Gelidiales) from Baja California, Mexico. Biomass and density were positively correlated on an annual basis, indicating that biomass accumulated without involving self‐thinning among ramets. This contrasts with nonclonal seaweeds, for which self‐thinning among individuals occurs during growth, but agrees with other clonal red seaweeds, such as Chondrus crispus Stackhouse and Mazzaella cornucopiae (Postels et Ruprecht) Hommersand (both Gigartinales). The growth pattern for these members of the Gelidiales and of the Gigartinales holds despite differences in holdfast morphology and ramet branching degree and despite differences in the capacity of coalescence during early stages, known only for the Gigartinales. The positive slope for the dynamic biomass–density relationship, on a bilogarithmic scale, was statistically steeper for M. cornucopiae than for P. capillacea and for C. crispus. This suggests that the addition of new ramets during the growth season may be relatively more beneficial for biomass accumulation rates for M. cornucopiae. This would be expected for high‐intertidal species subjected to strong abiotic stress, for which ramet crowding constitutes a key protection. Pterocladiella capillacea occurs at the mid‐intertidal zone and C. crispus at the subtidal zone, so ramets would be relatively less important in that respect.

The interspecific biomass-density relationship for terrestrial plants: where do clonal red seaweeds stand and why?

For crowded stands of terrestrial plants, ranging from mosses to trees, plant (or ramet, for clonal plants) density is negatively related to stand biomass. Stand biomass and ramet density were determined for Mazzaella cornucopiae and for Pterocladiella capillacea, two morphologically distinct intertidal clonal red seaweeds, to compare them with terrestrial plants. For these seaweeds, ramet densities were similar to the highest values reported for terrestrial plants (mosses, specifically). Stand biomass was higher than average values expected from the terrestrial interspecific biomass–density relationship, but lower than the limits expected from the terrestrial ultimate biomass–density line. These seaweeds show unexpectedly low ramet slenderness and high biomass packing per unit of volume, compared with the trend observed for terrestrial plants. Possible explanations for these differences are related to the particular physiology and habitat of intertidal clonal seaweeds.

Competition and coexistence of rhizomatous perennial plants along a nutrient gradient

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

Persistence of colonizing plant species along an inferred successional sequence on a subarctic coastal dune (Québec, Canada)

Coastal dune systems have been extensively studied in various parts of the world, but few investigations have been made in the Subarctic where temperatures remain relatively low during the short growing season. Here, we report the results of a study performed on a subarctic coastal dune system on the east coast of Hudson Bay, Canada. We determined the spatiotemporal dynamics of some important substrate characteristics, described the spatial dynamics of the vascular plant community, and followed temporal changes in the populations of the two major plant species, Leymus mollis (Poaceae) and Lathyrus japonicus (Fabaceae), along a toposequence, from the embryo dunes to the stabilized dune. The observed spatial changes in substrate characteristics were consistent with those reported in studies of lower-latitude dune systems, but they were generally of lesser magnitude. The subarctic climate appears to have unusual effects on the spatial gradient of substrate salinity and water content. For instance, salinity did not vary along the toposequence early in the season while the bay was still partly frozen, but it showed a significant gradient during the summer, with lower salinity values on the stabilized dune. Life-form turnover, from the upper beach to the stabilized dune, was lower than what has been reported for lower-latitude dune systems because of the rarity of annual plant species. Although the density and biomass of Leymus and Lathyrus ramets decreased along the toposequence, both species persisted onto the stabilized dune where they were the most frequent vascular plants. Such persistence appears to be related to the ability of both species to physiologically adjust to local conditions. Because of the climatic context, subarctic coastal dunes constitute unique systems, with characteristics quite different from those of lower-latitude coastal dune systems.

Ramet distribution, leaf morphometry and elemental composition of Caltha palustris L. along a water depth gradient

Summary Ramet distribution, leaf morphometry and elemental composition of Caltha palustris L. were studied along a hydrologic gradient in Nyarias marsh, N. E. Hungary, to identify patterns associated with water depth. Water-table height was linearly correlated with distance over most of the plant's distribution range. Caltha ramets were not dispersed uniformly but appeared in an aggregated pattern of a 12m scale. However, no clear relationship was found between water depth and the density of ramets, leaf number per ramet, aspects of plant performance (leaf blade and petiole dry weight), water status (water content of leaf parts), or leaf morphometry (lamina length and breadth, petiole length). Leaf size decreased towards both ends of the transect, and was highest at intermediate position. Contrastingly, foliar concentrations of several elements (e.g. Na, Mn, Cu) and especially the Fe: Mn ratio were strongly correlated with water-table height. Principal components analysis revealed two groups of elements: one discriminating between the lamina and the petiole but uncorrelated with the gradient (Ca, S, Fe, P, AI, K), and another not discriminating but positively correlated with the gradient (Na, Mn, Mg). The complex patterns of Caltha population traits in Nyarias demonstrate the ecologic importance of small-scale spatial (i.e. microtopographic) differences in wetlands. Besides water depth and duration per se, biotic factors such as gradients of standing crop, species richness or competition intensity may have intricate effects on single populations. Foliar Fe: Mn ratios, however, seem to be directly determined by water-table height and associated soil chemistry, thus can be a subtile indicator of wetland water conditions.

The Effects of Crude Oil and Remediation Burning on Three Clones of Smooth Cordgrass (Spartina alterniflora Loisel)

Burning has been employed as an oil spill remediation technique in coastal marshes, even though the combined and interactive effects of oil and burning on vegetation are poorly understood. Variation among clones of perennial marsh grasses in response to these perturbations is not known. We performed a greenhouse experiment designed to assess the effects of Venezuelan crude oil alone and of oil followed by burning on three clonal genets ofSpartina alterniflora. The fully-crossed 6-mo experiment involved five dosages of oil (0 l m−2, 4 l m−2, 8 l m−2, 16 l m−2, and 24 l m−2) and two burn treatments (burned or unburned) applied to ramets from three clones. All oil-only dosages reduced survival, but burning after oiling (oil + burn treatments) increased survival relative to oil-only groups in all except the highest two oil dosages. Higher oil-only treatments also reduced ramet densities and inhibited density increases over 6 mo. Burning after treatment with the 16 l m−2 oil concentration allowed increased production of new ramets, but burning exacerbated the negative impacts on ramet density at the oil concentration of 24 l m−2. At some intermediate oil dosages, burning remediated the negative effects of oil on aboveground biomass production and growth in height. There was a significant effect of oil-only treatments on numbers of flowering ramets produced, in which two clones responded with decreased flower production and one exhibited increased flowering. There was no main effect of oil + burn on flowering. There were significant among-clones differences in all response variables to one or both treatments. Our experiment demonstrates that burning of oiledS. alterniflora marshes may have little measurable effect at low levels of Venezuelan crude oil, can remediate the effects of oil at intermediate oil concentrations, but can increase the negative impacts at high concentrations of oil. These results indicate that oil spills have the potential to adversely affect genetic diversity inS. alterniflora populations by eliminating some sensitive clonal variants or changing the relative dominance of genets. These results suggest certain clones may be better suited for phytoremediation or restoration planting following oil spills.

An isozyme study of clone diversity and relative importance of sexual and vegetative recruitment in the grass Brachypodium pinnatum

We asked whether differences in abundance and seed prodtiction of Brachypodium pinnatum after 16 yr of four different experimental land use regimes were reflected in differences in vegetative dispersal distance, clone diversity, clone area, and the proportions of sexual and vegetative recruitment. Mean vegetative dispersal distance was 5.5 mm yr‘. Electrophoresis of 5 polymorphic isozyme loci of 20 tillers sampled at defined positions in each of twelve 1 × 6 m sampling areas (3 per treatment) revealed considerable clonal diversity. Per sampling area we found on average 9.98 enzyme phenotypes (clones), mean Simpson index was 0.825. and mean Shannon index 0.801. The mean ratio of sexual vs vegetative recruitment was about 1:32000. Despite this low ratio, clonal diversity within the population of” B. pinnatum was higher than reported for other clonal plant populations, possibly because of its high ramet densities. Moan clone area was 5.73 m2‐. i.e. mean clone radius was 1.35 m. None of the 10 pairwise correlations between abundance and seed production on the one hand, and number of clones per plot sample, plot Simpson index, plot Shannon index, ratio of vegetative vs sexual recruitment, and clone area on the other, was significant. Mean clone radius was 245 times larger than the mean distance of yearly vegetative dispersal which suggests old ages and low turnover rates of clones. The time scale of the inert response of clonal diversity of B. pinnatum to changes in land use appears to largely exceed the experimental period of 16 yr.