Ramet Density Research Articles

Comparative Analysis of Goldenseal (Hydrastis canadensis L.) Population Re-growth Following Human Harvest: Implications for Conservation

Abstract Goldenseal is an uncommon woodland herb whose rhizomes are widely harvested for their medicinal properties. Goldenseal populations regenerate from vegetative propagules that are broken-off from the primary rhizome during harvesting activities. While previous studies reported significant variability in re-growth among harvested populations, it is not entirely clear what drives differences in population re-growth. One hypothesis is that goldenseal populations re-grow at greater rates when harvested during the fall compared to mid-summer. Over a 4 y period, we biennially censused the re-growth of a goldenseal population that was wild-harvested during the fall 2001 from the Wayne National Forest in southern Ohio. Data were compared to previous studies that quantified the re-growth of a goldenseal population wild-harvested during the fall in West Virginia and to goldenseal populations (n = 3) experimentally harvested during mid-summer. Among the two fall-harvested populations, ramet densities increase...

Floral distribution, clonal structure, and their effects on pollination success in a self-incompatible Convallaria keiskei population in northern Japan

In plant species, when clonal growth produces a patchy structure and flowering ramets are clustered, the amount of pollen contributing to reproductive success is often regulated by pollinator efficiency and geitonogamy. The spatial population structure may influence reproductive success. We examined the clonal structure, the spatial ramet distribution, and their combined effects on fruit set in a natural population of the insect-pollinated, self-incompatible clonal herb, Convallaria keiskei, in northern Japan. The number of shoots, flowers, and fruits in 1-m2 quadrats were counted at every 5 m grid point in an established 100 × 90-m study plot. From all the quadrats where shoots existed, leaf samples were collected for allozyme analysis. Using the two spatial parameters of flowering ramet densities and genotypes, we then constructed individual-based fruit-set models. A total of 236 quadrats contained shoots, and 135 contained flowering ramets, which indicated expanded distribution of this plant throughout the study plot, while shoots, flowers and fruits all showed clustering distributions. Allozyme analysis of 282 samples revealed 94 multilocus genotypes. The largest clone extended to more than 40 m, whereas 56 genotypes were detected in only one sample. Several large clones and many small clones were distributed close to each other. Fine-scale spatial modelling revealed that the neighbouring flower numbers of different genotypes, compared with local genet or flower diversity, more influenced fruit set, in which the range of the neighbour was 14.5 m. These findings indicate that the compatible pollen dispersed by insect pollinators has a significant effect on sexual reproduction, in this C. keiskei population. Consequently, the spatial structure, which includes both genet distribution and clonal expansion by ramets, had a significant effect on pollination success.

Posible influencia de los rametes en la estructura poblacional y distribución del árbol gigante Warszewiczia uxpanapensis, Cordillera de Tilarán, Costa Rica

Se estudio los rametes, la distribucion y estructura de la poblacion de Warszewiczia uxpanapensis, en 2 ha de bosque pluvial premontano. Este arbol alcanza hasta 45 m de altura, DAPs hasta de 1.40 m y gambas de 8 m de altura. Su frecuencia fue de 49 % y su cobertura promedio de 6.5 %. La distribucion de los adultos fue aleatoria (distancia promedio al vecino mas cercano en la parcela 1 de 14.26 m, IC-E= 1.07, n= 15 en la parcela 2 de 15.33 m, n= 18, IC-E= 1.34). Sus plantulas representaron el 60.3 % de la poblacion estudiada (parcela 1, n= 94; parcela 2, n= 85). Se descubrio que esta especie tiene una alta capacidad de propagarse vegetativamente por rametes, los cuales influyen en la estructura de poblacion y distribucion de la especie (con una densidad de rametes parcela 1, 0.8 y parcela 2, 0.3 rametes/100 m2). Su propagacion vegetativa puede ser a partir de troncos caidos o ramas desprendidas (las cuales abren claros en el bosque), rebrotes de arboles, o ramas todavia adheridas a los arboles que son capaces de producir raices adventicias aereas.

Effects of neighbourhood structure and tussock dynamics on genet demography of Festuca rubra in a mountain meadow

Summary We examined whether genet demography in the clonal grass Festuca rubra in a mountain grassland in the Krkonoše Mountains (Czech Republic) changes in response to local environment. The data were also used to compare genet recruitment with estimates of genet mortality. We sampled F. rubra from four permanent plots in which ramet densities of all species have been recorded at a fine scale for a decade. Identities of 224 ramets were assessed by means of RAPD, yielding 145 different genets, of which most (68%) were found only once. The genet with the highest number of sampled ramets was recorded nine times. By assuming that the probability that two ramets are genetically identical is a function of distance between ramets, we estimated that successful seedling recruitment rates were between 0.15 and 16 genets m−2 yr−1 depending on the plot. Genet mortality was estimated from ramet mortality using the assumption that ramet mortality was uncorrelated over space. In three of the four plots, genet mortalities fell into the same range as the genet recruitment rates. The total number of genets per unit area must be known to enable determination of per capita recruitment and mortality rates. We developed an estimation technique involving simulation of the sampling process, which yielded values of 231–968 genets m−2. Genet turnover was therefore low (0.1–1% annually) and the high genet diversity is maintained by a very low recruitment. Spatial analysis showed that two ramets were more likely to be genetically identical when they came from a microhabitat where Festuca ramet density had recently increased; patches in which Festuca ramet density had decreased were more likely to contain ramets from several genets. Expanding tussocks are thus more likely to be composed of ramets of one genet only. Density and biomass of other species in the neighbourhood showed little correlation with the genet structure of Festuca rubra. Although ramets of Festuca rubra intermingle with other species at a fine scale, the population biology of its genets is driven mainly by the dynamics of its own ramets. Interactions with other species and response to local environments have little effect on genet structure.

Growth strategy of the stoloniferous rattan Calamus javensis in Mt. Halimun, Java

Calamus javensis is a widespread understorey rattan in Southeast Asia and is one of the dominant rattan species in Mt. Halimun National Park, West Java. This species can establish clumps either sexually with seeds or clonality with stolons. To analyze the role of sexual reproduction and clonality for clump establishment in C. javensis, we studied the vegetative and genetic structures of the population. Totals of 2,777 ramets comprising 1,321 clumps were found in three 0.16 ha plots. The number of ramets per clump showed reverse J-shaped distribution with a maximum of 15. Clustering and stoloniferous ramets accounted for 62 and 20% of all ramets, respectively. This indicates that C. javensis is actively engaged in clustering clonal growth. The longest stem was 14 m although 75% of all stems were shorter than 1 m long. The stolon ranged from 6 to 538 cm in length, which decreased the local density of ramets. Using randomly amplified polymorphic DNA (RAPD) analysis, we obtained 56 polymorphic bands for 118 clumps sampled from one of the plots. All samples showed different RAPD banding patterns, except one pair at a distance of 225 cm from each other. The genetic similarity among samples closer than 10 m was significantly higher than the expected value from a purely random distribution. These results suggest that most separate clumps are established by recruitment from seeds, and most seeds dispersed near mother plants. Hence, in C. javensis, clonality is more a growth strategy to increase the size of the genet than a dispersion-propagation strategy to expand the habitat.

Patterns in the Distribution of American Beachgrass (Ammophila breviligulata) and the Density and Reproduction of Annual Plants on a Coastal Beach

Beach communities of eastern North America are commonly dominated by the native perennial grass Ammophila breviligulata. It typically co-occurs with a variety of annual grasses and herbs in the pioneer zone. To determine the potential significance of this vigorous clonal species to the annual community, density, spatial distribution, seed production and potential seed rain were quantified for three focus species (Triplasis purpurea, Cenchrus tribuloides, and Heterotheca subaxillaris) in a 45×40 m section of coastal beach on Staten Island, New York, USA. During autumn 2000, five 40 m transects (10 m apart) were established perpendicular to shore; ramet density for Ammophila, and densities and per capita seed production for the three species, were estimated in 150 contiguous quadrats (35 cm × 30 cm) per transect. Both Triplasis and Cenchrus were more likely to occur in quadrats without Ammophila. Per capita seed production of Cenchrus and Heterotheca showed a curvilinear decrease with increasing numbers of Ammophila ramets. Mean seed rain per m2 was significantly reduced in the presence of Ammophila for all three focus annuals. In this beach community, Ammophila acts as an ecosystem engineer, but has a mostly negative influence on the distribution, density, and reproduction of the co-occurring annuals. Results suggest that continual, deliberate planting of A. breviligulata could be detrimental to the abundance and diversity of native annual herbs on the eastern coasts of North America.

Ramet dynamics in a centrifugally expanding clonal sedge: a matrix analysis

Carex humilis is a clonal sedge that can form distinct rings of densely aggregated ramets. We hypothesize that rings form because both production of new ramets and ramet dispersal are positively correlated to ramet size. This would lead to an overrepresentation of fast-moving and large ramets with high ramet production at the periphery, whereas slow-moving and small ramets with low ramet production would mainly be found in the interior of rings. We use matrix models to analyse how ramet populations both at the periphery and in the interior develop in the absence of ramet dispersal. We found that the stable size class distributions of ramets predicted by the models were not different from the distributions found in the field. Also, the asymptotic ramet population growth rates (λ1) were the same. Hence, we conclude that rings would form even in the absence of a link between ramet dispersal and ramet production. Further analysis of the matrix models showed that the ramet population increases at the periphery but decreases in the interior of rings because medium and large ramets produce fewer large ramets in the interior than at the periphery. We also found that the temporal variance in λ1 and transitions rates during the four study years was much higher at the periphery than in the interior. Our results suggest that rings may form because C. humilis ramets use below-ground resources from a much larger area than the one covered by the shoots. As the clone grows larger, the soil volume available to the ramets in the interior decreases because their access to soil outside the ring is cut-off by the ramets at the periphery. Ramet density in the interior is therefore decreasing.

High shoot plasticity favours plant coexistence in herbaceous vegetation

Several theoretical considerations imply that high shoot morphological plasticity could increase competition symmetry and favour plant coexistence. We tested whether mean plasticity across co-occurring species is a key trait for explaining ramet density and species richness in herbaceous vegetation. We used three data sets to test the hypotheses: (a) experimentally achieved estimates of plasticity to light availability for 35 herbaceous species; (b) richness, ramet density and canopy architecture data from 17 herbaceous communities; (c) species richness data from a 5-year permanent-plot study in a calcareous grassland. In herbaceous communities containing species with relatively higher shoot plasticity, ramet density was significantly higher. Consequently, relatively more species were growing per unit area-a greater proportion of the community species pool was represented on 1 m(2). In the permanent plot study species-richness was higher in those 40 x 40 cm quadrats where species with high shoot plasticity prevailed-there was a positive regression of richness on the mean plasticity of species. This relationship was highly significant in five consecutive years. Our results suggest that shoot plasticity to light availability is evidently one of the key traits in processes that alter the density of co-existing plants and, therefore, species diversity in herbaceous communities.

Ramet performance in two tussock plants — Do the tussock-level parameters matter?

Studies in plant demography are primarily done at the level of ramets and typically collect ramet-related parameters such as ramet size, type and history. This approach ignores possible effects of factors associated with higher levels, such as genet or tussock. This is particularly important in perennial resprouting herbs with persistent root that consists of many ramets as interaction between ramets, both by competition and by resource sharing, are likely to be intense in these plants. This study investigates effects of tussock-level parameters (age, size and ramet position within tussock and ramet density) on performance of individual ramets in two tussock-forming resprouting herbs ( Tanacetum vulgare and Centaurea jacea). The results show that position of a ramet within tussock did not affect ramet growth, but had significant effects on flowering and survival in both species. The direction of the effect differed between the two species; marginal ramets were more successful in T. vulgare, while central ramets were more successful in C. jacea. In addition, tussock age had a significant effect on ramet flowering and survival in T. vulgare. Both these effects are likely to be due to the more competitive life form of T. vulgare, which is restricted to temporary habitats with intense competition. C. jacea is a species of mown or grazed grasslands with lower productivity, which are more stable and where competition is weaker. The effects of the number of neighboring ramets and of the tussock size were significant, but often locality-specific. The results indicate that although many important effects of tussock-related parameters exist, their direction and magnitude differ between species or even populations and are thus not easily predictable. Neglecting these effects, however, is likely to make the demographic models weaker.

Is two company or a crowd: How does conspecific density affect the small-scale dispersal of a weed biocontrol agent?

To predict the growth and spread of an insect population introduced for the biological control of weeds, one must first understand the factors affecting the movement of individuals in the population. The purpose of this study was to determine how the dispersal rate of Aphthona lacertosa (Rosenhauer) (Chrysomelidae) was affected by conspecific density and by the characteristics of leafy spurge (Euphorbia esula L.: Euphorbiaceae) in patches where these beetles feed. In 2002 in Manitoba and in 2003 in Alberta, Canada, between 200 and 2500 insects were released in small patches (<10 m2) of spurge. The number and location of beetles within patches was monitored over subsequent days. In 1 m2 plots within patches, spurge ramet density, the proportions of vegetative and reproductive ramets, and ramet height were measured. In both years, beetle movement within patches and emigration from patches, was not affected by conspecific density. In Manitoba in 2002, beetles aggregated non-randomly on either vegetative or reproductive ramets within plots, but plot characteristics were not related to the formation of aggregations. In Alberta in 2003, plots in which beetles aggregated had significantly higher spurge density but did not differ in either the proportion of vegetative ramets or in the amount of non-spurge vegetation. These results suggest that density-dependent dispersal does not limit the population's ability to reach densities up to 2500 beetles/m2.

A comparison of plant communities on the basis of their clonal growth patterns

This study was performed to analyse how a vegetative propagation pattern of plants affects the coexistence of species and subsequent species richness of the community. We compared community average clonal growth in the herbal communities of forests, wooded meadows, and open meadows in Laelatu, Estonia. The parameters used for the calculation of the community averages and measured for each species were ramet life span, rhizome branching, and clonal mobility. We also examined the intrinsic (i.e. independent of the environment) relationship between community clonal growth and plant species density. We found strong correlations between the environmental factors (productivity, light availability, and mowing regime) and community averages of clonal growth parameters, while species density was (negatively) correlated only with community average of rhizome increment. The community average of ramet life span decreased with the increasing biomass of the herb layer. No evidence was found to support the hypothesis that species-rich communities may consist of species with more contrasting mobility compared with species-poor communities. Independent of the effect of the environmental factors, species density was positively correlated with ramet density. There was intrinsic positive relationship between species density and community average of ramet life span at open meadow sites and intrinsic negative relationship between species density and community average of rhizome increment at wooded meadow sites. We conclude that in forest communities the capability of clonal plants to forage for light is favoured, while in unmown meadows a competitively strong phalanx growth form is advantageous. We established that ramet turnover increases and vegetative mobility decreases with increasing species diversity, although these two relationships depend strongly on the type of the studied community.

Dominance not richness determines invasibility of tallgrass prairie

Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume,Melilotus officinalis, in intact, native Kansas grassland. We manipulated dominance of C4grasses by reducing their abundance (i.e. ramet densities) by ∼25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001–2002), invasibility, measured as peak establishment ofMelilotus, was not affected by a 3‐fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space‐filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.

Demographic Effects of Collecting Rattan Cane and Their Implications for Sustainable Harvesting

Abstract: Rattan, Old World climbing palm, is an extremely valuable nontimber forest product whose canes are gathered for both market and nonmarket uses. I evaluated the effects of harvesting commercial rattan, Calamus zollingeri Becc., on genet survival and ramet demography in two primary forest sites near Lore Lindu National Park in Sulawesi, Indonesia. I monitored 168 permanently marked C. zollingeri genets for 4 years and surveyed random transects for C. zollingeri genet and ramet populations and evidence of cane harvesting in 1996 and 2000. Cane harvesting had no significant effect on genet survival or mean ramet densities. However, current cane extraction rates significantly reduced mean cane lengths and total available cane throughout the area during the study period. Based on observed genet and ramet populations and average cane growth rates of 1.4 m/year, the sustained‐yield harvesting potential of C. zollingeri is approximately 101 m and 56 m/ha/year in the two study sites. Although C. zollingeri exhibits life‐history traits well suited to sustained‐yield harvesting, including production of multiple canes, cane resprouting following harvest, rapid cane growth, and widespread abundance below 1100 m, current harvest rates exceed growth and yield, and supplies of cane are being depleted. Rattan harvesting is widespread in Lore Lindu National Park. Approximately 18% of the park (42,000 ha) is likely subject to intensive and unsustainable extraction of C. zollingeri.

The Life of a Fire-adapted Florida Goldenrod, Solidago odora var. chapmanii

Abstract We studied the ramet dynamics of Chapman's goldenrod [Solidago odora var. chapmanii (A. Gray) Cronquist] in burned Florida scrub and sandhill by monitoring permanent quadrats for 10 y, excavating to reveal clonal structure and postfire responses and taking extensive notes on the plant's life history. Chapman's goldenrod was attacked by a wide variety of herbivores and grazing mammals. However, leaf damage from herbivores and pathogens was relatively minor. This rhizomatous perennial forb produced conspicuous flowering displays after fires. Censuses in 23 permanent quadrats set out after 12 fires revealed that the density and size of goldenrod ramets declined over the years following fire. Densities dropped most rapidly in the first few years following fire and declined more slowly in formerly grazed areas than in ungrazed Florida scrub. Season of burn did not influence subsequent ramet dynamics. Flowering was concentrated during the first 2 y after fires. Repeated burns produced a rebound in rame...

Relationships between spatial environmental heterogeneity and plant species diversity on a limestone pavement

No empirical studies have examined the relationship between diversity and spatial heterogeneity across unimodal species richness gradients. We determined the relationships between diversity and environmental factors for 144 0.18 m2 plots in a limestone pavement alvar in southern Ontario, Canada, including within‐plot spatial heterogeneity in soil depth, microtopography and microsite composition. Species richness was unimodally related to mean soil depth and relative elevation. Microsite heterogeneity and soil depth heterogeneity were positively correlated with species richness, and the richness peaks of the unimodal gradients correspond to the maximally spatially heterogeneous plots. The best predictive models of species richness and evenness, however, showed that other factors, such as ramet density and flooding, are the major determinants of diversity in this system. The findings that soil depth heterogeneity had effects on diversity when the effects of mean soil depth were factored out, and that unimodal richness peaks were associated with high spatial heterogeneity in environmental factors represent significant contributions to our understanding of how spatial heterogeneity might contribute to diversity maintenance in plant communities.

Optimal allocation to vegetative and sexual reproduction in plants: the effect of ramet density

Vegetative reproduction is a very common alternative by which plants can contribute to the next generations. There are many considerations predicting which mode of reproduction, vegetative or sexual, should be favored and numerous experimental studies to verify them. However, the results are inconsistent especially when the effect of plant density is considered. I apply here a dynamic optimization model to predict the rate of vegetative and sexual reproduction in plants as a response to changes in the local plant density. The population is assumed to occupy a heterogeneous environment consisting of patches in which growth and reproduction of plants are possible and unfavorable space between them. As the environment is globally stable, the seeds, which can disperse without restriction, exhibit a constant recruitment rate. The ramets are assumed to settle only within the patch of the mother plant. The rate of ramet production effects local density, which in turn determines ramet recruitment. The optimal strategy maximizes the expected lifetime genetic contribution, realized via both vegetative and sexual reproduction. The solutions obtained under these assumptions are dualistic. The model predicts that different approaches applied in studying the effect of ramet density should give opposite outcomes. When the comparison is between patches in natural populations, a positive relationship between relative ramet allocation and density is expected. When the density is experimentally manipulated or its effect is analyzed across different successional stages, a negative relationship should be found. The results seem to be confirmed by empirical studies.

Clonal expansion in the deciduous understory shrub, devil's club (Oplopanax horridus; Araliaceae)

To examine clonal growth and the ability of devil's club (Oplopanax horridus (Sm.) Torr. &amp; A. Gray ex Miq.) to persist throughout stages of forest succession, we sampled populations in three classes of stand development: clearcuts (1–10 years old), young stands (11–50 years old), and maturing stands (51–200+ years old). We completely excavated and mapped all clonal fragments (systems of ramets connected by decumbent stems) in a plot at each site and determined ramet and clonal fragment age using annual growth rings on stem cross sections. Clonal fragment density and size, ramet density and age, decumbent stem length, annual stem elongation, annual ramet recruitment, lateral meristem formation, and the number of persistent dead ramets were significantly (P &lt; 0.05) related to stand development class. Clonal fragments in clearcut stands were large, predating the year of stand establishment, with many dead, old ramets, but many young stems. Ramet recruitment and lateral meristem formation were highest in clearcut stands, which contributed to replacement of older ramets lost to the disturbance. Clonal fragments in young stands were few and small, consisting of a few ramets and short decumbent stems. In maturing stands, clonal fragments were numerous but consisted of few ramets with extensive decumbent stem connections. No devil's club seedlings were observed in any of the stands sampled. Devil's club populations are maintained by prolific basal stem sprouting following disturbance and continual layering and clonal fragmentation throughout stand development.Key words: age structure, clonal architecture, clonal shrubs, demography, layering, population dynamics.

Effect of different manuring and defoliation patterns on broad‐leaved dock (Rumex obtusifolius) in grassland

SummaryThe effects of cutting frequency and cutting height on broad‐leaved dock (Rumex obtusifolius) in Lolium perenne‐based agricultural grassland at two levels of fertiliser input were investigated at North Wyke, Devon, UK. Two micro‐plot field experiments, containing immature dock plants at uniform densities, with a factorial design, were used to compare: (a) ‘organic’ and ‘low‐input’ fertilisation, i.e. cattle slurry only vs slurry plus mineral fertiliser (NPK at 100‐0‐64 kg ha−1 yr−1), (b) cutting heights of 5–6 cm vs 10–12 cm, and (c) four harvesting frequencies representative of different grassland management practices (regular 4‐weekly cutting, a ‘hay‐stage’ cutting, and two treatments with ‘silage‐stage’ cutting). Expt 1 was established in 1995 with 13 dock plants m−2 (from excised dock‐root sections) and Expt 2 established in 1996 with 25 plants m−2 (as seed‐grown plug plants). Treatments were assessed over the 2 subsequent years to determine treatment effects on total herbage dry matter (DM) yield and dock DM yield, and on in‐situ measures of dock ramets. In both experiments, total DM yield was increased by 1.0–2.01 ha−1 yr−1 for treatments receiving NK fertiliser; the proportion of dock was also higher than from slurry‐only treatments. In Expt 1, the dock ramet density, mean dock ramet height, mean leaf length and numbers of dock leaves per m2 were also greater on NK fertilised treatments in autumn of yr 1. Height of cut had no consistent effect on dock yield, but dock ramet density and leaf density in autumn were greater on the 5–6 cm than the 10–12 cm cutting treatment, Expt 2 only. In yr 2 of both experiments cutting at 4‐weekly intervals resulted in less dock in the herbage than hay‐stage cutting and, particularly in Expt 1, there were associated differences in leaf density and ramet height in autumn; silage‐stage treatments were intermediate. Results are discussed in relation to requirements for management options where there is a need to avoid or reduce herbicides.

Why does fertilization reduce plant species diversity? Testing three competition‐based hypotheses

Summary1 Plant species diversity drops when fertilizer is added or productivity increases. To explain this, the total competition hypothesis predicts that competition above ground and below ground both become more important, leading to more competitive exclusion, whereas the light competition hypothesis predicts that a shift from below‐ground to above‐ground competition has a similar effect. The density hypothesis predicts that more above‐ground competition leads to mortality of small individuals of all species, and thus a random loss of species from plots.2 Fertilizer was added to old field plots to manipulate both below‐ground and above‐ground resources, while shadecloth was used to manipulate above‐ground resources alone in tests of these hypotheses.3 Fertilizer decreased both ramet density and species diversity, and the effect remained significant when density was added as a covariate. Density effects explained only a small part of the drop in diversity with fertilizer.4 Shadecloth and fertilizer reduced light by the same amount, but only fertilizer reduced diversity. Light alone did not control diversity, as the light competition hypothesis would have predicted, but the combination of above‐ground and below‐ground competition caused competitive exclusion, consistent with the total competition hypothesis.

Population structure and establishment of the threatened long‐lived perennial Scorzonera humilis in relation to environment

Summary The intensification of agriculture has resulted in the decline of many plant species of nutrient‐poor wet grasslands. At some sites local populations of long‐lived characteristic species have persisted and might benefit from recent extensification schemes. However, little is known about the population biology of these plants, and the prospects for the populations are uncertain. We studied the population structure and establishment of the long‐lived Scorzonera humilis in 23 populations in Luxembourg and neighbouring Belgium. Two types of populations could be distinguished according to their population structure: regenerating populations, with a high proportion of plants with only one or a few rosettes, and aged populations, with a low proportion of small, young plants but a high proportion of individuals with many rosettes. The total density of Scorzonera individuals was higher in regenerating than in aged populations. Within sites, S. humilis was restricted to more open and nutrient‐poor patches. The composition of the vegetation in plots where S. humilis was present was significantly different from that of plots without the species, indicating that S. humilis is restricted to particular microhabitats. In multiple regression analyses, environmental variables explained a large part of the variation in the total density of genets, the density of genets of different size and the density of rosettes. The main variables of influence were site productivity and soil moisture. With increasing productivity and decreasing soil moisture the proportion of small genets decreased and that of large genets increased. Increased productivity had contrasting effects at the genet and ramet (rosette) levels. While genet density decreased, ramet density increased, indicating that if a site is fertilized, recruitment of new genets and survival of genets is reduced, but growth of surviving genets is increased. The results of a sowing experiment indicated that an aged population structure was due to a lack of recruitment. The number of seeds that germinated and the proportion of seedlings that survived until the next summer were positively correlated with soil moisture and negatively with productivity. Germination rate and establishment success were significantly higher in Molinion grassland than in the Calthion grasslands. The results suggest that for long‐lived species the size and number of populations may not be good indicators of the status of a species. In S. humilis large populations (&gt; 1000 genets) still exist, but all are of the aged type. In order to preserve existing populations of S. humilis, management should aim to reduce productivity and increase soil moisture.