Demographic structure of California chaparral in the long-term absence of fire

Abstract. Demographic structure of 12 chaparral sites unburned for 56 to 120 years was investigated. All sites were dominated by vigorous shrub populations and, although there was colonization by seedlings of woodland tree species in several stands, successional replacement of chaparral was not imminent. Although successional changes in community composition were evident, there was no indication of a decline in species diversity. Non-sprouting species of Ceanothus suffered the greatest mortality at most, but not all, sites. Sprouting shrubs, such as Quercus and Heteromeles had very little mortality, even in stands more than a century old. All postfire resprouting species had multiple stems of different ages indicating these shrubs were capable of continuously regenerating their canopy from basal sprouts. Ceanothus populations were highly clumped and there was a significant correlation across all sites between variance/mean ratio and percentage mortality. As Ceanothus populations thinned, they became less clumped. In mixed chaparral stands, Quercus and Heteromeles were significantly taller than associated Ceanothus shrubs and overtopped the Ceanothus; at two sites, the density of live Quercus per plot was correlated with the density of dead Ceanothus. Thus, mortality of Ceanothus stems is likely related to both intra and interspecific interations. Seedling recruitment was observed for most shrub species that regenerate after fire by resprouting; seedling and sapling densities ranging from 1000–36 500 ha^-1 were recorded for Quercus dumosa, Rhamnus crocea, Prunus ilicifolia, Heteromeles arbutifolia and Cercocarpus betuloides. For all but the last species, seedlings and saplings were most abundant beneath the canopy cover and not in gaps. Across all sites, recruitment was significantly correlated with depth and bio-mass of the litter layer. Cercocarpus betuloides was present in several stands, but seedling establishment was found only in one very open, disturbed stand. Regardless of stand age, taxa such as Adenostoma, Arctostaphylos and Ceanothus, which recruit seedlings after fire, had no significant seedling production.     

Genotypic structure in clonal Rhododendron ferrugineum L. (Ericaceae) populations: origin and maintenance

Genotypic structure of a closed population of the clonal ericaceous shrub Rhododendron ferrugineum is examined in the light of two independent studies previously conducted on this species. In the first study, spatial distribution of genotypes in the closed population was inferred from the amplified fragment length polymorphism (AFLP) procedure. Age of clones was estimated using their spatial extent and the annual growth rate of shoots. In the second study, ramet demography was studied in the three most representative stages of shrub invasion on two different sites, including the site where the population investigated by AFLP grew. The demographic data recorded were the area occupied by ramets and ramet age, and from this information the developmental pattern of Rhododendron populations was determined. Additional data such as genet density and distance between genets were calculated.These two sources of information allow us to propose that all or most of the clones detected in the closed population established at the early successional stage, and that the present genotypic structure was established several hundred years ago, long before the population reached total closure. Hypotheses concerning the future development of this genotypic structure are discussed.     

Effects of disturbance caused by traditional Spanish rural land use on the regeneration of Cytisus multiflorus

Abstract. Cytisus multiflorus is a leguminous matorral shrub native to the NW Iberian Peninsula, where it is one of the most important species in the extension of matorral at the expense of set-aside agricultural land. Dehesas have traditionally been used for extensive livestock raising and matorral was periodically burnt, cut or pulled out. The two latter practices are now out of use. However, burning is more frequent than in the past. The effects of fire, cutting, and pulling out of C. multiflorus on its regeneration was studied in order to ascertain whether the presently increasing distribution of the species relates to fire-stimulated regeneration or to the reduction of other traditional practices. Three years after treatment two sets of parameters were determined: 1. Plant origin: seedlings and different ramet types (ramets=resprout clumps), density, weight, and biomass as well as the percentage of resprouting. 2. Seed persistence at various soil depths. The possible mechanisms of breaking dormancy and plant emergence in different years after fire were studied in other experiments. The results suggest that the regeneration mechanism in C. multiflorus is stimulated by fire, but it is not an exclusive relationship. Stimulation occurs also after other disturbances leading to the total elimination of aerial biomass. The present expansion of the species does not appear to result from the abandonment of some traditional practices, such as cutting or pulling out, but from frequent fires (resulting in aerial-biomass elimination).     

Regeneration of a dominant Mediterranean dwarf-shrub after fire

Abstract. The regeneration after fire of the dominant east Mediterranean dwarf-shrub Sarcopoterium spinosum was studied under various habitat and geographic conditions in northern Israel, northern Crete and the Aegean coast of Turkey. Age structures of regenerating stands were determined 3–9 yr after fire on 13 1 mx 1 m sample stands by analysis of growth rings in the taproot crown of 874 plants. S. spinosum regenerated either by massive seedling recruitment in some habitats or by resprouting from surviving meristems in others. This versatility facilitates adaptation of the species to widely different environmental conditions and is a factor in the maintenance of its dominance after fire at occupied sites. In the years after fire, the growth of younger plants within the stand at all sites was severely suppressed by the older shrubs. Consequently, the age structure of the stands was dominated by one or two older cohorts that became established soon after fire. In addition, on most habitats, older shrubs (usually older than 6 yr) were often composed of clusters of plants that suggested clonal proliferation of ramets. The strongly peaked age structure, together with the appearance of clonal regeneration on the older plants, suggests that subsequent maintenance of the stand was not dependent on recurrent seedling recruitment. This can explain the persistence of S. spinosum in stands of herbaceous vegetation despite the extreme sensitivity of the seedlings to competition under such conditions.     

Defining grassland fire events and the response of perennial plants to annual fire in temperate grasslands of south-eastern Australia

Many species-rich Themeda triandra grasslands in south-eastern Australia have been burnt annually in summer as a land management practice for decades. The characteristics of annual fires (maximum surface temperatures, maximum soil temperature changes at 10 mm depth, Byram fire intensity and duration of surface heating) were compared to fires that occur less frequently (2, 4 or 7-year inter-fire interval). The impacts of annual summer fires on seed survival, perennial plant dynamics and flowering were also documented at two sites over two years.Annual grassland fires differ principally from other grassland fire types in their duration of surface temperatures above 100°C (i.e., <1 min versus >2–3 min in grasslands burnt at 4–7-year intervals). This was correlated to fuel load, but not rate of spread or Byram fire intensity. Maximum surface temperatures were variable within annual fire events (98–458°C ) and did not differ significantly from those observed in other grassland fires. All sites experienced temperatures above 350°C but no site had its soil temperature increase at 10 mm depth by >10°C during fire. Byram fire intensity was lower in annually burnt sites, but positively correlated with rate of spread moreso than fuel load.Many perennial species (34–55%) and most perennial individuals (70–83%) avoid the annual summer fire event by being dormant at the time of burning. All perennial species regenerated rapidly by vegetative means after fire and seedlings were absent or rare for most species. As a result, turnover of species at the small-scale was low.Annual burning permits high perennial species richness (mean 18 species/0.25 m²) and high numbers of individual perennial plants (range 189–1036 plants/0.25 m²) to co-exist at the small-scale. Seasonal plant density maxima peaked in spring, 6–8 months after fire, and the number of plants in a species' population each year following fire depended on their seasonal response more than their direct fire response. The direction and magnitude of population change from one year to the next was species and site specific and did not correlate with life form or a plant's ability for vegetative spread. All perennial species have a short secondary juvenile period (i.e., 6–11 months) but relatively few individual plants flower in any one year. Seed that has made its way into the soil is completely protected from the direct effects of fire and hence, occasional post-fire seedling recruitment may be possible for all species.Maintaining annual burning in grasslands as a management regime is unlikely to lead to a decline in richness and plant density in the short-term. Rather, delayed burning (i.e., >3 year intervals), and the impact that this may have on interval dependent processes such as above-ground competition, are predicted to have more substantial long-term effects on the small-scale dynamic of this community.     

Effects of fire severity in a north Patagonian subalpine forest

Abstract. Question: What is the relative importance of fire‐induced canopy mortality, soil burning and post‐fire herbivory on tree seedling performance? Location: Subalpine Nothofagus pumilio forests at Challhuaco valley (41°13'S, 71°19'W), Nahuel Huapi National Park, Argentina. Methods: We fenced and transplanted soils of three burning severities along a fire severity gradient produced by a fire in 1996. Over two growing seasons we monitored soil water, direct incoming solar radiation, seedling survival, final seedling total biomass and root/shoot ratio. Additionally, we assessed severity‐related changes in soil properties. Results: Incoming radiation (an indicator of the amount of canopy cover left by the fire) was the primary factor influencing spring and summer top soil water availability, first and second‐year seedling survival and seedling growth. While seedling survival and soil water content were negatively affected by increased radiation, seedling final biomass was highest in very open microsites. Burned soils showed lower water holding capacity and soil carbon; however these changes did not affect topsoil water, and, contrary to expectation, there was a slight tendency toward higher seedling survival on more heavily burned soils. Herbivory significantly reduced seedling survival, but only under high‐radiation conditions. While the effect of radiation on final seedling biomass was not affected by herbivory, R/S ratios were significantly reduced by herbivory in high radiation micro sites. Conclusions: Despite inducing faster aerial growth, increased radiation and herbivory in severely burned sites may effectively prevent post‐fire regeneration in north Patagonian subalpine forest where seed sources are not limiting.     

Mortality of pregnant females in Arikara villages: osteological evidence

High infant mortality and high mortality for late adolescent and young adult females suggest that obstetrical hazards may be one explanation for differences in male and female mortality curves. This possibility is investigated in Arikara skeletal series by determining the frequency of females who died with fetal remains in utero. Two females (0.9%) were so identified. Examination of the females and the fetal remains do not provide evidence that stress of childbearing was the cause of death in these cases.     

Clonal growth in Ailanthus altissima on a natural site in West Virginia

Abstract. : The exotic tree Ailanthus altissima is usually confined to open sites. As an exception, Ailanthus established in a densely closed forest in West Virginia, which was analyzed with the aim to elucidate possible pathways of persistence in forest succession of this light-demanding pioneer species. Demographic analysis revealed a seedling mortality of 100 %. Instead, the understory is populated by clonal ramets, ranging from one to more than 19 yr of age, with a mean of 5 yr. Growth averages only 0.11 m/yr, and height is correlated with age. This clonal growth contrasts with the performance of Ailanthus on open sites. The possible ecological benefits of establishing a ramet bank in a resource-poor habitat are considered in terms of space occupation of a pioneer species.     

The role of water level and salinity in the regulation of Juncus gerardi populations in former ricefields in southern France

Abstract. In the Rhône delta (southern France) Juncus gerardi is a dominant, strongly aggregating species in artificially flooded former rice fields. In order to explain this pattern, the effects of water depth, salinity and their interaction were measured on (1) seed germination and seedling development and (2) vegetative growth of J. gerardi in a controlled-environment experiment. The germination pattern of J. gerardi was affected by salinity. Low salinity (2 g/l NaCl) delayed germination while moderate salinity (12 g/l NaCl) reduced germination rate. In contrast, the germination of J. gerardi was not affected in the range of water depths tested (i.e. 0–10cm). Salinity negatively affected the development of below-ground parts, shoots and inflorescences. This negative effect of salinity on the vegetative growth of J. gerardi was amplified when combined with flooding. Flooding with fresh water (0–20 cm depth) did not limit biomass production during the experiment. However, a decrease in the ratio of below-ground/above-ground dry weight at deeper water depths suggests a limitation of the vegetative propagation of J. gerardi under prolonged flooding conditions. This hypothesis is supported by the negative correlation between the cover of J. gerardi and water depth found in an abandoned rice field. The limitation on seedling recruitment imposed by salinity and the depression of vegetative growth of J. gerardi due to a combination of salinity and water depth could explain the aggregate distribution of J. gerardi in former rice fields.     

Afforestation causes changes in post‐fire regeneration in native shrubland communities of northwestern Patagonia,Argentina

Question: What are the effects of fire in native shrubland communities and in pine plantations established in these shrublands? Location: Northern Patagonia, Argentina. Methods: We surveyed four sites in Chall‐Huaco valley, located in northwest Patagonia. Each site was a vegetation mosaic composed of an unburned Pinus ponderosa plantation, a plantation burned in 1996, and an unburned matorral and a matorral burned by the same fire. We recorded the cover of all vascular plant species. We also analysed species richness, total cover, proportion of exotic species, abundance of woody species and herb species, cover of exotic species, abundance of woody and herb species and differences in composition of species. For both shrubs and tree species we recorded the main strategy of regeneration (by resprouting or by seed). Results: We found that fire had different effects on native matorral and pine plantations. Five years after fire, plantations came to be dominated by herbs and exotic species, showing differences in floristic composition. In contrast, matorral communities remained very similar to unburned matorral in terms of species richness, proportion of woody species, and herb species and proportion of exotics. Also, pine plantations were primarily colonized by seedlings, while matorrals were primarily colonized by resprouting. Conclusions: Matorrals are highly fire resilient communities, and the practice of establishing plantations on matorrals produces a strong reduction in the capacity of matorral to return to its original state. The elimination of shrubs owing to the effect of plantations can hinder regeneration of native ecosystems. Burned plantations may slowly develop into ecosystems similar to the native ones, or they may produce a new ecosystem dominated by exotic herbs. This study shows that plantations of exotic conifers affect native vegetation even after they have been removed, as in this case by fire.     

Resprouting of Erica multiflora after experimental fire treatments

Abstract. The effect of fire intensity - both temperature and duration - on the resprouting pattern of the evergreen Mediterranean shrub Erica multiflora in relation to plant size, was experimentally investigated by subjecting plants to the flame of a propane torch, and observing mortality and resprouting after 5 and 20 months. Pre-treatment plant size was not important in determining post-fire plant survival, but it did influence the resprouting vigour, increasingly so with time. High temperatures induced higher mortality rates within populations, but duration of fire did not effect mortality. Biomass of resprouts was lower following more intense fire treatments, but this effect progressively disappeared over time, except in plants subjected to the most intense fire treatment. This is probably because of the increasing importance of the below-ground organs for the regeneration of the above-ground biomass. Some of the plants which were clipped but not exposed to fire also died 20 months after treatment. The effect of clipping onmortality andresprout-ing response, estimated as biomass of resprouts, was not significantly different from the effect induced by either low or medium temperature treatments, but was significantly different when compared with the effect of high-temperature treatments. Field observations show that the establishment of seedlings of E. multiflora is rare both after fire and between fires. Thus, our data support the idea that both a single fire or clipping can diminish the population size of a resprouting species. We conclude that fire may have a stochastic effect on E. multiflora populations, due to the variation in fire intensity existing within a single burning stand.     

Growth patterns of Juncus gerardi clonal populations in a coastal habitat

Juncus gerardi populations demonstrated a logistic growth curve during the colonization stage. Shoot production by vegetative multiplication was virtually continuous from December to June. Experiments suggested that the stabilisation stage of the demographic curve reflected water deficit. Taller, fertile, winter and early spring cohorts could be distinguished from shorter, infertile end of spring and beginning of summer cohorts. Shoot emergence began in March and terminated at the end of June, when water becomes a limiting factor due to a period of water shortage, typical of the thermo-atlantic climate. Spatial extension of populations was due to rhizome growth, which ceased during flowering.Flowering in May temporarily checked growth in shoot height of all emerged cohorts. No cost of reproduction was demonstrated concerning the rate of appearance of new shoots.Although fertile shoots were taller than vegetative shoots, their growth rates were significantly lower from April onwards. The tallest fertile shoots produced the most capsules.Energy allocation to seed production is the only possible means for long-term establishment of new genotypes, and vegetative multiplication appears as the principal source of recruitment of new modules in Juncus gerardi.Resource allocation patterns in this clonal species are discussed in relation to the ecological background in the concerned marshlands and with theoretical proposals derived from models of spatial colonization strategies in clonal plants.Nomenclature: follows Flora Europaea (Tutin et al., 1964ndash;1980).     

Demographic analyses of the dusky shark, <Emphasis Type="Italic">Carcharhinus obscurus,</Emphasis> in the Northwest Atlantic incorporating hooking mortality estimates and revised reproductive parameters

Demographic analyses were conducted for the dusky shark, Carcharhinus obscurus, in the Northwest Atlantic. Fishery-independent, fishery-dependent, and previously published data were used to estimate age specific by-catch mortality and reproductive cycle for incorporation into the analyses. Correspondence analyses were conducted for viability (dead or alive), shark length, month, soaktime, and region. Dead sharks were associated with small size, long soak times, and the spring season, while live sharks were associated with large size and shorter soak times. Data on reproduction suggested a 3-year reproductive cycle consisting of a 2-year gestation period and a 1-year resting period. Litter sizes ranged from 3–12 embryos. Decreasing age-specific fishing mortality and a 3-year reproductive cycle were used in age structured life tables which incorporated stochasticity in life history parameters through Monte Carlo simulation to estimate annual population growth. Demographic analyses showed that the dusky shark population would decline even at low levels of fishing mortality despite low natural mortality for neonates. The demographics of this species make it among the most vulnerable to excessive fishing mortality, and suggest stringent regulatory measures may be required to recover collapsed populations.     

Changes in microorganisms populations in the soil after fumigation

Soil fumigation with dazomet, metam sodium, chloropicrin and chloropicrin + 1.3 D resulted in significant decrease of fungi and increase of bacteria populations in trials carried out in four farms located in different areas. Depending on the farm and the active substance applied, the fungi population was decreased by 1.4- to 3500-fold in comparison to control. Metam sodium and chloropicrin showed the best efficacy, both of them almost totally eliminated the fungi from the soil environment.The total number of bacteria was increased by the chemical fumigation with all tested products. While the population of fluorescent Pseudomonads in all treated plots increased from 2- to 100-fold, depending on the farm, the number of Bacillus spp. was not changed or decreased compared with non fumigated soil.The nematology analysis of the soil indicated that any chemical fumigant significantly limited the population of plant parasitic nematodes, which number was, anyway, below the damage threshold. However, in most cases dazomet and metam sodium reduced the total number of all nematodes present in the soil.The fumigation with chloropicrin and 1,3 D at dose of 30 g/m2 resulted in an increase of the total number of all nematodes in soil.     

Changes in mediterranean shrub communities with Cytisus purgans and Genista scorpius

Summary Shrub communities are important in the mediterranean landscape and linked very much with human activities. The permaneney and the spreading of these communities are related to biological characteristics of the dominant species which control the patterns of changes in structure, richness, types of life forms, and ability of colonization by trees. In this paper, changes in communities are analysed and compared for Cytisus purgans and Genista scorpius (both Papilionaceae). For both communities the schemes of origin and spreading are rather the same: overgrazing and fire play a prominent role. Changes in structure show different stages emphasized by the types of distribution of the chlorophyll containing stratum. There is a direct relation between changes in structure and changes in species richness and life form spectrum: richness decreases generally from the second or third year after fire. According to the dryness of the stand therophytes may or may not be important in the youngest stands. The colonization of the shrub communities by trees is dependent on several conditions, either on the shrub community itself, or on the plant-matrix surrounding the shrub community patch and also on animals; man, too, plays an important role. In conclusion we think that the above-mentioned changes in shrub communities fit the inhibition model proposed by Connell & Slatyer (1977); but, more generally it seems that the plant successions observed in our regions follow both, this model and the facilitation and tolerance models of the same authors. Linked with this fact, it seems that the successions show generally fluctuating values of richness as is suggested by Whittaker (1975). We think that plant succession must be well analyzed at different levels of organization and include the study of the influences man and animals may have.Plant nomenclature follows Fournier (1961); for birds see Peterson et al. (1967).We are very much indebted to Ch. Rimbault for her contribution to the translation of this paper.     

Effects of resource availability on plant recruitment at the community level in a Mediterranean mountain ecosystem

Coexisting plant species usually differ in resource requirements, which may also vary within species at successive demographic stages. Such differences become extremely important during the early life stages, since these are the most critical phases in woody-species recruitment, they depend heavily on resources, and they may determine future community composition. Under a global-change scenario, where climatic conditions, nutrient availability, and habitat characteristics are expected to be altered, it is difficult to predict the way in which plant recruitment will be affected. To understand the impact of different global-change drivers on community recruitment, we sowed a set of species representative of the different successional groups of a complete Mediterranean woody community under field conditions, and studied their emergence, growth, and survival along the main resource gradients of light, water, and nutrients. The light and nutrient gradients followed the natural range of conditions in the study area, but water availability was manipulated to simulate three contrasting climatic scenarios: wetter, drier, and current conditions. Structural equation modelling was used to provide a comprehensive analysis of the factors and relations governing plant recruitment. Overall, seedling emergence was determined directly by light; growth was determined by light and summer soil moisture; and survival was determined by summer soil moisture. Light was the main factor indirectly affecting the demographic stages of all species. However, the magnitude of the direct and indirect relationships varied among species. Particularly, species differed in their response to the expected drier climatic conditions, some (e.g. Pinus sylvestris, Acer opalus) being much more vulnerable than others (e.g. Cytisus scoparius, Salvia lavandulifolia). These differential responses could translate as major shifts in the structure of the overall plant community. Our results support the idea that the analysis of complex relations among essential resources is critical for accurate forecasts of the impact of climate change on community dynamics.     

Age structure and dynamics of Rhododendron ferrugineum L. populations in the northwestern French Alps

Abstract. The ericaceous shrub Rhododendron ferrugineum occupies large areas at the subalpine level (1600–2200 m) of the northwestern French Alps. It frequently reaches 90–100 % cover, accumulates 60–70t/ha (dry weight) above-ground biomass and is able to dominate many subalpine landscapes by outcompeting other species. We formulated the hypothesis that the species shows a rapid and invasive seedling establishment and a fast population development. We investigated the age structure and development of two populations. Although classic colonization stages can be identified, and variation did occur depending on the site, R. ferrugineum populations showed a surprisingly slow development compared with other ericaceous species. Curves obtained from polynomial regression equations of cover on age allowed us to predict the future development of populations. Thus, depending on the site, it would take 150 to 250 yr to achieve total cover. We discuss the factors involved that could explain this slow development and discuss which biotic and abiotic factors may be responsible for the large extension of this species in the subalpine belt.     

Host-pathogen interactions in natural populations of Linum marginale and Melampsora lini

Summary The epidemiology of rust caused by the fungus Melampsora lini and the effects of infection by this pathogen on its host, the herbaceous perennial Linum marginale, were determined in the field and in garden experiments. There was considerable natural variability in disease levels over the four years (1986–1989) of the study. In two years (1986, 1989) major rust epidemics occurred. In the field, the main effect of disease was to reduce survivorship during the winter following infection. Plants which were heavily infected during the 1986 or 1989 growing seasons had reduced survivorship relative to more lightly infected plants. Melampsora lini infections did not appear to affect survivorship in either 1987 or 1988. Flowering was correlated with environmental factors and the number of stems a plant possessed. A severe drought in 1987 completely inhibited flowering. In the other three years the number of flowers produced by a plant was strongly positively correlated with the number of stems it possessed. Disease levels had no consistent effect on flowering. Controlled garden experiments were also used to examine the response of seedlings and adult plants to infection. These showed that both the timing and severity of disease appears to determine the effect of M. lini infections on L. marginale. Early, severe infection reduced growing season and overwintering survivorship as well as capsule production. However, plants in the field were most often infected only after flowering had begun, and the predominant effect of infection was a reduction in overwintering survivorship. The high variability in disease levels from year to year and the deferred nature of the effect of the rust on its host have significant implications for the design of experiments aimed at assessing the role of diseases in plant communities.     

Seed and microsite limitation of recruitment in plant populations

Summary Availability of seed and microsites, respectively, are two factors that potentially may limit recruitment in plant populations. Microsites are small-scale sites suitable for germination and survival of seedlings. We discuss this dichotomy of recruitment limitation both from a theoretical and empirical point of view. Investigations of recruitment in 14 woodland species showed that 3 species were seed limited, 6 species were limited by a combination of seed and microsite availability, and 5 species were found not to be seed limited, but the limiting factor was not identified. A combination of seed and microsite limitation implies that recruitment is promoted by increasing both seed and microsite availability. We suggest that the importance of seed limitation in plant populations has been underestimated, and that the operating limiting factors may be dependent on spatial and temporal scale. We expect that many species, if adequately studied, will turn out to be both seed and microsite limited. Experimental field studies that incorporate a range of seed and microsite densities in various spatial and temporal scales are needed to examine the extent to which plant populations are seed and microsite limited.     

Tiller demography in seashore populations of Agrostis stolonifera,Festuca rubra and Poa irrigata

Abstract. The dynamics of tillers in natural populations of three cohabiting perennial grass species, Agrostis stolonifera, Festuca rubra and Poa irrigata (= Poa pratensis ssp. irrigata) were studied for five years in a Baltic seashore meadow. The process of tiller population maintenance was very dynamic. Both birth and death rates of tillers were high, particularly in A stolonifera, and the turnover rate of the populations was high. Recruitment was mainly by vegetative tillers, produced continuously throughout the growing season. The proportion of flowering tillers was low, but varied between years. Considerable year-to-year variation was also found in birth and death rates. Despite this between-year variation and the differences found between species in flowering frequency, pattern of survivorship and tiller longevity, population sizes of the species remained relatively constant.