Potential and constraints for grasses to cope with spatially heterogeneous radiation environments

Placing plant organs into upper canopy layers or gaps is considered advantageous for avoiding neighbours. Current research only covers the lack of selective branching in response to heterogeneous radiation in dicots. Due to some unique shade avoidance regulatory pathways and clonal structure, directional response may occur in grasses. I used Festuca rubra L., a grass with two types of branches: intravaginal tillers developed from young buds close to the primary shoot, and extravaginal tillers developed from older buds and placed on rhizomes in a certain distance from the mother tussocks. Tussocks grown from vegetativelly multiplied initial tillers were exposed to three radiation regimes in two green-house experiments: full sunlight, full shading simulating canopy and heterogeneous radiation. Interaction of the treatments with orientation of the initial tillers relative to their mother tillers, which may constrain morphology of the developing tussocks, were studied. Even though shading decreased the number of intravaginal tillers, heterogeneous radiation did not influence direction of their outgrowth. However, shading activated dormant buds at the tussock base and thus stimulated extravaginal tillering, with a tendency to develop more extravaginal tillers towards the shaded sites.     

Performance of two Picea abies (L.) Karst. stands at different stages of decline

Summary The annual replacement of tillers of Agropyron desertorum (Fisch. ex Link) Schult., a grazing-tolerant, Eurasian tussock grass, was examined in the field following cattle grazing. Heavy grazing before internode (culm) elongation seldom affected tiller replacement. Heavy grazing during or after internode elongation, which elevates apical meristems, increased overwinter mortality of fall-produced tillers and reduced the number and heights of these replacement tillers. Unexpectedly, tussocks grazed twice within the spring growing season tended to have lower overwinter tiller mortality, greater tiller replacement, and larger replacement tillers than tussocks grazed only once in late spring. These responses of twice-grazed tussocks, however, were still less than those of ungrazed tussocks or tussocks grazed moderately in early spring. The presence of ungrazed tillers on partially grazed tussoks did not increase the replacement of associated grazed tillers relative to tillers on uniformly grazed plants. This result indicates that resource sharing among tillers, if present, is short-lived or ecologically unimportant in this species. Although A. desertorum is considered grazing-tolerant, tiller replacement on heavily grazed tussocks, particularly those grazed during or after internode elongation when apical meristems were removed, was usually inadequate for tussock maintenance. These observations at the tiller (ramet) level of organization in individual tussocks (genet) may explain the often noted reduction in stand (population) longevity with consistent heavy grazing.     

Light field heterogeneity among tussock grasses: Theoretical considerations of light harvesting and seedling establishment in tussocks and uniform tiller distributions

Although the tussock growth form of caespitose graminoids is widespread, the effect of this growth form on light interception and carbon gain of tillers has received little attention. Daily incident photosynthetic photon flux density (PFD_inc) and carbon gain in monospecific stands of tussock grasses were compared with those of a hypothetical distribution with the equivalent tiller density per total ground area, but evenly distributed rather than clumped in tussocks. This was computed for two tussock grasses Pseudoroegneria spicata (Pursh) A. Löve (bluebunch wheatgrass) and Agropyron desertorum (Fisch, ex Link) Schult. (creasted wheatgrass) at different plant densities. Daily PFD_inc and net photosynthesis (A) were greater if tillers were distributed uniformly rather than clumped in tussocks, except when the density of tussocks was so great as to approach a uniform canopy. When tussock density per ground area was low, much of the difference between tussock and uniform tiller densities in PFD_inc and A was due to shading within the tussocks; up to 50–60% of the potential carbon gain was lost in A. desertorum due to shading within tussocks. In a matrix of tussocks, the light field for establishing seedlings was very heterogeneous; potential A ranged from 7 to 96% relative to an isolated seedling. The mean of daily PFD_inc and A for seedlings in a tussock stand were nearly identical to the values in corresponding stands of uniform tiller distributions. It is hypothesized that the loss of A resulting from clumping tillers into tussocks is offset by benefits of protecting sequestered belowground resources from invasion by seedlings of competitors.     

Growth and tillering patterns within tussocks of Eriophorum vaginatum

Growth patterns were investigated for Eriophorum vaginatum tussocks from disturbed and undisturbed tussock tundra at two sites in Alaska. Total basal area of tussocks decreased with increased cryoturbation but mean basal area per tussock did not. Flowering was observed in tussocks of significantly smaller size on disturbed compared to undisturbed tundra. For tussocks with < 10% cover by shrubs and moss, number of tillers per tussock was linearly related to tussock diameter in most disturbed and undisturbed sites. Exceptions occurred in an area that had been bladed with a bulldozer 7 yr before our survey where tiller number increased as the square of diameter and in an area with much frost activity where tiller number was not emulated with diameter. The ratio of daughter tillers to adult tillers decreased with diameter in disturbed tundra, whereas the trend was less pronounced in undisturbed tundra.
Microsuccession in undisturbed tussock tundra was investigated by sampling tussocks with different amounts of shrub and moss cover for number of daughter tillers per adult tiller, weight per tiller, percent nitrogen, and percent phosphorus. Small tussocks without cover by other species and large, partially covered tussocks were not significantly different by any measures, but tussocks that were almost completely buried had significantly (p < 0.05) lower values of tillering index, weight per tiller, and percent phosphorus.     

Leaf demography of <Emphasis Type="Italic">Festuca pallens</Emphasis> in dry grassland communities

Leaf blade parameters and leaf demography of Festuca pallens Host were studied in two types of dry grasslands. The field work was carried out in the Považsky Inovec Mts (Western Carpathians) during 1993–1995. The permanent plot in the Poo badensis-Festucetum pallentis was located on a steep, strongly eroded S-facing slope covered with dolomite outcrops, scree and sparse vegetation (20%) dominated by Festuca pallens. The permanent plot in the Festuco pallentis-Caricetum humilis was located on the even ridge plateau with shallow stony soil and vegetation covering about 70% dominated by Carex humilis and Festuca pallens. In comparison to other grasses Festuca pallens had a very low rate of leaf turnover. The highest leaf birth rates and the lowest leaf death rates were observed in June. Leaf mortality was uniformly distributed in time without a distinct minimum or maximum. For the surviving tillers the leaf production exceeded the leaf mortality during the whole growing season. The steady net gain of leaves in tillers was not interrupted by the parallel process of tillering. Among the leaf cohorts the leaves produced in May had the longest leaf blades. Leaves grew during the whole year. The winter cold and summer drought might slow down the growth rate or interrupt the growth. The growth of a leaf blade took five to eight weeks. Leaf life span was estimated to 150–200 days (time from leaf appearance at the apex to the complete loss of its green assimilating parts). In comparison to other grasses Festuca pallens belongs to the species with the longest leaf life span. The effect of environmental factors on leaf demography was followed by the comparison of two populations belonging to two phytosociological associations differing mostly in habitat xericity. Differences were revealed in the following characteristics: length of leaf blade in cohorts born during May and June, maximum length of a leaf blade in a tiller and daily increments in May and June. The course of leaf natality and mortality was similar in the studied populations.     

Spatial arrangement of tiller replacement in Agropyron desertorum following grazing

Summary The spatial arrangement of tiller replacement was assessed on grazed and ungrazed tussocks of Agropyron desertorum (Fisch. ex Link) Schult. for three annual cycles. Frequency distributions of the number of replacement tillers per single progenitor were also determined. Tiller replacement was usually greater on the perimeter of tussocks than within the core, with or without grazing. Replacement was inversely related to grazing intensity, both on the perimeter and within the core of tussocks. Heights of replacement tillers on the perimeter or within the core seldom differed. Furthermore, grazing seldom affected the number of replacement tillers per progenitor. Greater tillering on the perimeter than within the core indicates that the tussocks were expanding. Apparently, grazing neither enhances tussock expansion and subsequent disintegration, nor does it necessarily lead to patches of tillers (multiple tillering per progenitor) within tussocks of A. desertorum.     

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.     

Using plant traits to compare sward structure and composition of grass species across environmental gradients

Abstract. Plant traits which may give an indication of a plant's strategy for nutrient acquisition and regeneration are known for numerous grassland species. This study aimed to establish whether there is any relationship between two plant traits: specific leaf area (SLA) and number of reproductive tillers, and sward structural characteristics which influence herbage intake by grazers (bulk density and digestibility, leaf:stem ratio). Comparison is made for nutrient‐rich (Dactylis glomerata) and nutrient‐poor (Festuca rubra) grass species. We hypothesized that these traits are responsive to environmental gradients and also act on the processes of the ecosystem. Both grasses were compared with two P‐fertilizer rates in two localities (200 and 1300 m a.s.l.) which differed in their temperature:radiation ratios. For the vegetative phase SLA was well correlated with sward characteristics: D. glomerata, which has the higher SLA, has the lower bulk density and higher digestibility. The values of SLA and vegetation bulk density varied according to growing conditions (P‐rate and temperature:radiation ratio), but the ranking of the species remained the same because the phenotypic plasticity that exists for plant traits was also observed for sward structure and composition. That suggested the possibility of grouping natural grassland species for their relevant characteristics for grazers according to SLA values. Over the reproductive phase, the proportion of stems was well correlated to the percentage of reproductive tillers. However, the percentage of reproductive tillers was a very plastic trait for both species, depending on the growing conditions, and resulting in a density‐dependent effect, particularly for F. rubra. The species studied were too plastic and too similar in their regenerative strategy so that there is no unique relationship between percentage of reproductive tillers and stem proportion, regardless of the species and the growing conditions. The number of reproductive tillers is not a suitable plant trait which could be used to rank species for leaf and stem proportions in the sward.     

Clone-specific response of Festuca rubra to natural variation in biomass and species composition of neighbours

A long-term implant experiment with four clones of Festuca rubra was performed to identify fine-scale spatial variation in a competitive environment within a grassland sward of natural composition and density variation. Total above-ground biomass and shoot counts of all species were recorded in 10×10 cm neighbourhoods of each implant, and their effect on the growth response of the implant was tested. Two types of response were recorded: (1) shoot sizes and vertical shoot growth dynamics, and (2) horizontal space encroachment by means of new shoot natality, mortality and their mode of formation (intravaginal or extravaginal). The vertical growth of individual shoots showed the strongest response to neighbourhood composition; it responded to the overall aboveground biomass of the neighbours, but not to their species composition. The responses in parameters of horizontal growth of individuals (natality, mortality, proportion of extravaginal shoots) were much weaker and not consistent over the observation period; however, both total biomass of the neighbours and species composition affected the response of the target plants.
The overall response was rather weak in spite of a tenfold variation in neighbouring density and a thirtyfold variation in neighbouring biomass. This indicates that the response to this variation is rather flat under field conditions, either due to high overall values of density or due to interactions with below-ground processes. Consequently, though the plant is capable of remarkable plastic responses in both vertical growth and morphogenetic change, under field conditions this capacity for plastic response is expressed only to a limited degree.     

Tiller dynamics of Spartina maritima in successional and non-successional mediterranean salt marsh

Tiller demography was compared in two populations of Spartina maritima present at similar elevations in the coastal saltmarshes of Odiel (Huelva, S.W. Spain). The successional population consisted of colonizing tussocks in a littoral lagoon, and the non-successional population comprised a stable sward that had fringed a major channel for 40 years. At both sites S. maritima was replaced by Arthrocnemum perenne at higher elevation, where sediments were less reducing. Rapid, consistent sediment accretion confirmed the successional nature of the lagoon site but there was little net accretion in the stable sward.Census of permanent quadrats at the successional site chronicled moving concentric waves of high tiller density as tussocks expanded. Initially high densities declined after one year to low values at the end of the second year but they had almost recovered after 3 years. The decline represented a combination of reduced numbers of births and increased numbers of deaths. Tiller densities were substantially higher in the stable sward and showed relatively small fluctuations with time. The underlying risk of tiller mortality was similar in the two populations for much of the time but after two years there was increased mortality, mainly associated with flowering, at the successional site; very few tillers flowered in the sward. This mortality contributed to a shift to a younger age structure in the successional population.Data aggregated over consecutive 3-monthly periods were examined for density dependence. None was found in the successional population. In the sward population there was evidence of density-dependent adult and juvenile mortality of tillers, particularly over the first 18 months of the study, when there were compensatory responses to subtle variations in density. The lack of density dependence and relatively low peak density of about 2000 m^-2 near to the leading edges of the expanding tussocks at the successional site suggest that tiller placement there was regulated mainly by physiological mechanisms affecting rhizome growth and bud development in well integrated clones.     

Growth and carbon allocation of Agropyron desertorum following autumn defoliation

Summary Growth and carbon allocation of a cool season tussock grass, Agropyron desertorum, following defoliation of newly initiated tillers in the autumn of 1988 and 1989 were investigated. Tiller density and mortality, reproductive shoot density, root density, biomass, individual tiller weight, carbon allocation, and soil water depletion were used to evaluate the response of A. desertorum to autumn grazing. Tiller recruitment was lower in the autumn-defoliated treatment in both years compared with the control because of the cessation of tiller development following autumn defoliation. Autumn defoliation also significantly reduced the movement of ¹³C to the roots in 1988 but not in 1989. Soils were cooler and drier in 1989. Other plant growth measurements and soil water depletion rates were not different between treatments. Autumn defoliation in 1988 did not influence tiller recruitment in the following autumn. Two consecutive years of autumn defoliation did not affect tiller overwinter mortality or peak standing crop in 1990.     

Growth response of threeFestuca rubra clones to light quality and arbuscular mycorrhiza

The influence of the arbuscular mycorrhizal fungus—Glomus etunicatum and changes in light quality (decrease of red/far-red ratio) on the growth of threeFestuca rubra clones, ecotypes originating from a mountain grassland region, was studied in a growth chamber experiment. Inoculation with the arbuscular mycorrhizal fungus and low red/far-red ratio decreased both the number of tillers and the biomass of treated plants. Significant interactions between the treatments were found and most of the growth characteristics were reduced further when both treatments were applied simultaneously. Inoculation with the arbuscular mycorrhizal fungus also resulted in reduced maximum height of tillers, whereas low red/far-red ratio caused the maximum height of tillers to increase. Differences in plasticity were found for the threeFestuca rubra clones. Response to one treatment was strongly modified by the other treatment. This indicates that the arbuscular mycorrhizal fungus and red/far-red ratio can differentially influence the growth ofFestuca rubra clones and thus modify their relative competitive abilities which can consequently have implications for the coexistence mechanisms within plant population, thereby potentially influencing plant canopy and community structure.     

Ecotypic differentiation within Festuca rubra L. occurring in a heterogeneous coastal environment

In order to study the genetic differentiation between Festuca rubra L. individuals growing in a heterogeneous environment, indices of salt tolerance, mean relative growth rates and the numbers of tillers formed by plants grown in a Hoagland solution, were determined. It was found that plants from salt marsh sites have a high index of salt tolerance, a high mean relative growth rate and numerous tillers; plants from coastal sand dunes are less tolerant, grow slowly and form few tillers; plants from the inland polder sites are rather salt sensitive, fast growing and form a high number of tillers. The heritability of the mean relative growth rate and the tiller number appeared to differ from zero. Apparently, these characters have been under recent selection and thus give a picture of the adaptations of individual plants to the different environments encountered. An indication of gene flow has been found, although the effect of gene flow seems to be small in the face of the force of selection. It was concluded that the distinction of three ecotypes within the species F. rubra is insufficient to describe the differentiation found. Considering the differences observed, it seems more reasonable to speak of ecotypic variation.     

Spatial pattern and process at the plant neighbourhood scale: insights from communities dominated by the clonal grass Elymus repens (L.) Gould

Question: How do clonal traits of a locally dominant grass (Elymus repens (L.) Gould.) respond to soil heterogeneity and shape spatial patterns of its tillers? How do tiller spatial patterns constrain seedling recruitment within the community? Locations: Artificial banks of the River Rhône, France. Material and Methods: We examined 45 vegetation patches dominated by Elymus repens. During a first phase we tested relationships between soil variables and three clonal traits (spacer length, number of clumping tillers and branching rate), and between the same clonal traits and spatial patterns (i.e. density and degree of spatial aggregation) of tillers at a very fine scale. During a second phase, we performed a sowing experiment to investigate effects of density and spatial patterns of E. repens on recruitment of eight species selected from the regional species pool. Results: Clonal traits had clear effects – especially spacer length – on densification and aggregation of E. repens tillers and, at the same time, a clear response of these same clonal traits as soil granulometry changed. The density and degree of aggregation of E. repens tillers was positively correlated to total seedling cover and diversity at the finest spatial scales. Conclusions: Spatial patterning of a dominant perennial grass responds to soil heterogeneity through modifications of its clonal morphology as a trade‐off between phalanx and guerrilla forms. In turn, spatial patterns have strong effects on abundance and diversity of seedlings. Spatial patterns of tillers most probably led to formation of endogenous gaps in which the recruitment of new plant individuals was enhanced. Interestingly, we also observed more idiosyncratic effects of tiller spatial patterns on seedling cover and diversity when focusing on different growth forms of the sown species.     

Demographic variability in tiller populations of two perennial pampa grasses

Abstract. The general objectives of this study were: (1) to investigate the importance of internal influences in regulating the tiller dynamics in natural populations of the warm-season perennial grasses Paspalum dilatatum and Sporobolus indicus, coexisting in Argentine flooded pampa, in as much as they act independently of the underlying external environment, and (2) to evaluate the extent to which interactions between internal and external factors affect the variation in tiller dynamics within such populations. Within-population variation in seasonal development of plants and tillers with different neighbour composition was studied for an annual growth cycle. Tiller survival and tillering were significantly influenced by tiller size. Tiller age influenced tiller fate, as suggested by the additive effects of age and size of tillers. These relationships varied with season and with species. Size and age of tillers showed additive effects with their neighbouring species on the tiller fate of P. dilatatum, but the effects of age and size of S. indicus changed according their neighbourhood. Tiller survival of S. indicus during the early growth season was more size-dependent when the cold-season species Poa lanigera, was the principal neighbour. Flowering and tillering probabilities were positively related through their common positive responses to tiller size. Tiller survival and recruitment between different seasons were strongly related. Independently of neighbour composition, tiller survival was generally inversely related to tiller recruitment in previous seasons. Therefore, significant density-dependent mortality of tillers was found for both species during the early summer when tiller density was expressed by basal area units.     

Festuca campestris density and defoliation regulate abundance of the rhizomatous grass Poa pratensis in a fallow field

Invasion by the rhizomatous grass Kentucky bluegrass (Poa pratensis) is a global phenomenon, including into foothills rough fescue (Festuca campestris) grasslands of southwestern Alberta, Canada. In order to better understand the competitive relationships between these species, we conducted a fallow field study where rough fescue bunchgrass tussocks were transplanted at one of three planting densities (15, 30, or 45 cm spacing), and then subject to various treatments in a factorial design, including one‐time intensive summer defoliation and seeding of bluegrass into adjacent bare soil. Rough fescue plants exhibited marked intraspecific competition, as high planting densities increased tussock mortality, while decreasing plant tiller counts and relative inflorescence production, together with plant and tiller‐specific mass. However, high densities of the bunchgrass also reduced the cover and biomass of encroaching bluegrass, coincidental with reduced resource (soil moisture and light) availability in mid‐summer. Although summer defoliation increased rough fescue tiller counts, this disturbance reduced plant and tiller mass, and also increased Kentucky bluegrass. We conclude that while high densities of nondefoliated stands of rough fescue may increase resistance to bluegrass encroachment, a reduction in either fescue plant density or vigor via defoliation can increase the risk of bluegrass invasion within northern temperate grassland.     

Distribution and abundance of small insects and arachnids in relation to structural heterogeneity of grazed, indigenous grasslands

1. The species composition and spatial distribution of small insects (Hemiptera, Coleoptera, Lepidoptera) and arachnids (Araneae, Opiliones, and Pseudoscorpiones) were investigated in three indigenous, upland grasslands identified as the National Vegetation Classification Festuca–Agrostis–Galium typical subcommunity (code U4a), Festuca–Agrostis–Galium, Vaccinium–Deschampsia subcommunity (code U4e), and Nardus stricta species-poor sub-community (code U5a), on which grazing management was manipulated experimentally. 2. Two hypotheses were tested that predicted arthropod diversity in upland grasslands. The habitat heterogeneity hypothesis predicts that the species number and abundance of arthropods will have an asymptotic relationship with increasing numbers of plant species and greater structural heterogeneity in the vegetation. The symbiosis between patches hypothesis states that the species number and abundance of arthropods will express a unimodal relationship with the grain size of sward patches created by grazing. The sward patches must be large enough to be apparent to, and support populations of, arthropods, but small enough that interspersed tussocks provide shelter from weather and a deterrent to disturbance by grazers. 3. The hypotheses were tested by sampling arthropods from the geometrical patterns represented by the individual tussocks and intermediate sward components of three indigenous grasslands produced by different grazing treatments. Paired samples of arthropods were taken by motorized suction sampler, the first of the pair from the grazed sward and the second, the accumulated samples from the surrounding triad of tussocks (U4a and U5a grasslands) or hummocks (U4e grassland). The paired samples were taken from six randomly-selected locations across both replicates of each of the grazing treatments. 4. Arthropod species composition and abundance were compared between the paired sward and tussock samples and in turn with measures of the vertical and horizontal components of vegetation structure, i.e. the variance in vegetation height per unit area and the area covered by tussock compared with sward. 5. There were consistently more species and a greater abundance of arthropods associated with tussocks than with swards and the average species number and abundance for the combined pair of samples declined with increased grazing pressure. The relationship between vertical and horizontal components of vegetation structure and the species number and abundance of selected arthropods was asymptotic as opposed to unimodal, supporting the habitat heterogeneity hypothesis, rather than the symbiosis between patches hypothesis. 6. Small and relatively sedentary insects and arachnids are more sensitive to grazing intensity and species of grazer in these upland, indigenous grasslands than are larger Coleoptera and Araneae, which respond less directly to varied grazing management. The overall linear reduction of small herbivorous and predatory arthropods with increased grazing intensity was buffered in grasslands with substantial tussock patches.     

Is vegetation inside Carex sempervirens tussocks highly specific or an image of the surrounding vegetation?

Questions : How do species diversity, frequency and composition in tussocks differ from those in similar sized plots outside tussocks? Does the extent of the differences depend on community types or environmental conditions? Location : A sub‐alpine grassland in the Swiss National Park. Methods : In each of the two communities (short grass and tall graminoid) differing in species composition, grazing intensity and soil nutrient availability, relevés were made in 40 pairs of small circular plots, with one plot located inside a randomly selected Carex sempervirens tussock and the other outside. Results : We found 92 vascular species, of which 46 had a frequency higher than 5%. Species richness (S), pooled cover, Shannon's diversity (H) and cumulative species number (CS) were higher outside than inside the C. sempervirens tussocks, but evenness (J) was lower. S, H and CS differed more in the tall graminoid community than in the short grass community. However, dissimilarity between the paired relevés inside and outside tussocks did not differ between the two communities. Of the 46 most frequent species, 12 were statistically more and only one less frequent outside than inside the tussocks. Vegetation inside and outside tussocks could be clearly distinguished in the ordination space. Conclusion : Vegetation inside C. sempervirens tussocks is different from that in the surrounding area and represents an impoverished but homogenized version of the surrounding vegetation. Although tussocks of C. sempervirens were systematically avoided by grazers, there is little evidence that tussocks facilitate the species growing inside them.     

Axillary bud banks of two semiarid perennial grasses: occurrence, longevity, and contribution to population persistence

The occurrence, longevity, and contribution of axillary bud banks to population maintenance were investigated in a late-seral perennial grass, Bouteloua curtipendula, and a mid-seral perennial grass, Hilaria belangeri, in a semiarid oak-juniper savanna. Axillary buds of both species were evaluated over a 2-year period in communities with contrasting histories of grazing by domestic herbivores. A double staining procedure utilizing triphenyl tetrazolium chloride and Evan's blue indicated that both viable and dormant axillary buds remained attached to the base of reproductive parental tillers for 18–24 months which exceeded parental tiller longevity by approximately 12 months. Bud longevity of the late-seral species, B. curtipendula, exceeded bud longevity of the mid-seral species, H. belangeri, by approximately 6 months. Younger buds located on the distal portion of the tiller base were 3.2 and 1.4 times more likely to grow out than older proximal buds of B. curtipendula and H. belangeri, respectively. The percentage of older proximal buds, which included comparable portions of viable and dormant buds, that grew out to produce tillers following mortality of parental tillers was 6.0% for B. curtipendula and 8.4% for H. belangeri. In spite of the occurrence of relative large axillary bud banks for both species, the magnitude of proximal bud growth did not appear sufficient to maintain viable tiller populations. We found no evidence to support the hypothesis of compensatory bud growth on an individual tiller basis for either species. Grazing history of the communities from which the buds were collected did not substantially affect the number, status, longevity, or outgrowth of axillary buds on an individual tiller basis for either species. However, long-term grazing by domestic herbivores influenced axillary bud availability by modifying population structure of these two species. Bud number per square meter for B. curtipendula was 25% lower in the long-term grazed compared to the long-term ungrazed community based on a reduction in both tiller number per plant and plant number per square meter. In contrast, bud number per square meter for H. belangeri was 190% greater in the long-term grazed than in the long-term ungrazed community based on a large increase in plant density per square meter. Minimal contributions of axillary bud banks to annual maintenance of tiller populations in this mid- and late-seral species underscores the ecological importance of consistent tiller recruitment from recently developed axillary buds. Consistent tiller recruitment in grasslands and savannas characterized by intensive grazing and periodic drought implies that (1) bud differentiation and maturation must be remarkably tolerant of adverse environmental conditions and/or (2) tiller recruitment may resume from buds that mature following the cessation of severe drought and/or grazing, rather than from mature buds that survive these disturbances. These scenarios warrant additional research emphasis given the critical importance of this demographic process to tiller replacement in species populations and the maintenance of relative species abundance in grasslands and savannas. Received: 12 August 1996 / Accepted: 30 December 1996     

Growth responses of arctic graminoids following grazing by captive lesser snow geese

The effects of grazing by captive goslings of the Lesser Snow Goose on coastal vegetation at La Pérouse By. Manitoba were investigated. Swards of Carex subspathacea, Festuca rubra and Calamagrostis deschampsioides were grazed once for different periods (0–180 min) and regrowth of vegetation determined, based on measurements of standing crop, net above-ground primary production (NAPP) and forage quality (leaf nitrogen content). The amounts of foliage removed from swards of Carex subspathacea increased with the length of the grazing period, but after 44 days of regrowth there were no significant differences in above-ground biomass between control and grazed plots. While the amount of foliage removed by goslings from swards of Festuca rubra increased with the length of the grazing period (except after 150 min of grazing), the increase in biomass following defoliation was similar among treatments. Goslings removed little biomass from swards of Calamagrostis deschampsioides, even when the opportunity for grazing was 180 min. No significant differences in standing-crop or NAPP between grazed and ungrazed plots were detected by the end of summer. Grazing had no significant effect on amounts of nitrogen in leaf tissue of all species, suggesting that faecal nitrogen was not rapidly incorporated into plant biomass within the growing season. Patterns of regrowth of these species are compared to that of Puccinellia phryganodes. An increase in goose numbers in recent years has led to birds foraging on less preferred species, such as Calamagrostis deschampsiodes and Festuca rubra. Their poor nutritional quality and a lack of a rapid growth response following defoliation may explain, in part, the decline in the weight of wild goslings recorded over the last decade.