Soil seed bank dynamics in post-fire heathland succession in south-eastern Australia

Soil seed banks can exert a strong influence on the path of vegetation succession following fire, with species varying in their capacity to persist in the seed bank over time, leading to changes in seed bank composition and propagules available for post-fire colonisation. This study examined the effect of time since fire on soil seed bank dynamics in a chronosequence of seven sites spanning 26 years in a south-eastern Australian sand heathland. No significant change was evident in the species richness and density of the germinable soil seed bank, but species composition differed significantly among young (0–6 years since fire), intermediate (10–17 years since fire) and old-aged (24–26 years since fire) sites (using presence/absence data). No significant trend was observed in the similarity between the extant vegetation and the soil seed bank with time since fire. A total of 32% of the species recorded in the soil seed bank were not present in the above-ground vegetation at the same site, which suggests that species requiring fire for germination may be present in the seed bank. Most species present in the extant vegetation were not recorded (63%) or were in very low abundances in the soil seed bank (29%). The mode of regeneration appears to be the major determinant of species absence in the soil seed bank, as 66% of species occurring in the extant vegetation but not in the seed bank have the capacity to regenerate by resprouting. This study shows that a major shift in the successional pathway after fire due to altered seed bank composition is unlikely in this vegetation; most species not recorded in the seed bank are either resprouters (obligate or facultative) or serotinous, suggesting that they will readily regenerate following fire. Unless fire frequencies are high and kill fire-sensitive obligate seeders before they reach maturity, the chance that the soil seed bank could substantially alter vegetation composition within the study area after fire is low. However, it is unclear how successional pathways may alter in response to severe fires with the potential to kill both seeders and resprouters.     

Markov models in the study of post-fire succession in heathland communities

Data on the presence of a number of vegetation states (defined in terms of species dominance in areas of 10×10 cm) and transition probabilities were derived from permanent quadrats in a number of recently burned heath stands. Data were taken from a species-rich community, a species-poor type and a high-level Calluna-Eriophorum bog. Simple Markovian models were constructed using these data, and the model predictions were compared with known or expected trends. Models for species-rich heath yielded poor simulations of expected trends since matrices derived from data for the first years after fire did not contain sufficient information on transitions to states important later in the developmental sequence. Model results for the simpler species-poor and bog communities were more satisfactory and simulated expected trends. In these types all species recovered quickly after fire and less rearrangement of species abundances took place. Maximum likelihood statistics carried out on the transition matrices produced inconclusive results for the species-rich and species-poor types, but indicated that the data from the Calluna-Eriophorum bog approximated a first-order time-homogeneous Markov chain. It was concluded that Markov models lack predictive ability except in relatively simple systems, but that they may be useful in illustrating variations in short-term community dynamics.Nomenclature follows Tutin et al. (1964–80) for vascular plants and Smith (1978) for mosses.I am grateful to Prof. C. H. Gimingham for help and advice during this study, which was carried out during tenure of a Natural Environment Research Council studentship. Thanks are also due to Dr M. B. Usher and an anonymous referee for their comments on the draft.     

Germination and establishment of seedlings in different phases of theCalluna life cycle in a Scottish heathland

Previous authors have suggested that cyclical succession takes place inCalluna-dominated heathland, and is determined by the different growth phases of the dominant. Characteristic species are said to invade at different stages of the life cycle ofCalluna, particularly in the initial and pioneer phases and in the degenerate phase. In the last phase, gaps are formed due to separation and dying of the oldest frame branches.This paper aims to analyze in which microhabitat and in which phase of theCalluna life cycle seedling emergence and establishment is possible.More seedlings appear in pioneerCalluna than in matureCalluna stands in experimentally manipulated micro-sites. In pioneerCalluna establishment is affected only by application of litter, in matureCalluna by increased illumination. The differences between treatments disappeared after three years, but not the difference in establishment between stands.Seedling emergence from species sown experimentally was different in the successive phases ofCalluna. Highest emergence was in the pioneer stand.In a seedling survey also most seedlings appeared and established in the pioneer phase. The number of recently germinated seeds was high in all phases. However, seedling survival was very low, except in the pioneer stands. This study does not of itself show evidence for cyclical succession inCalluna-dominated heath vegetation. Colonization of gaps by seedlings ofCalluna in mature or degenerate phases is possible, but must be an infrequent occurrence.Nomenclauture follows Clapham, Tutin & Warburg (1962).We wish to thank Dr G. R. Miller for stimulating discussions during the start of the experiments. E. de Hullu wishes to thank the Botany Department in Aberdeen for their hospitality during the research period.     

Persistent soil seed bank and standing vegetation at a high alpine site in the central Chilean Andes

The persistent soil seed bank (viable seeds >1 year) and standing vegetation were investigated in the upper alpine belt (3250 m) in the Andes of central Chile, 33° S. Nine species (eight in standing vegetation) were found in a total persistent seed bank of 899 seeds m⁻². Seven additional species were represented by physically intact, non-viable seeds. Over 90% of the persistent seed bank was concentrated in Montiopsis sericea (Portulacaceae), Pozoa coriacea (Umbelliferae), Phacelia secunda (Hydrophyllaceae) and Oxalis compacta (Oxalidaceae). Examination of the seed/cover ratio revealed different propensities for persistent seed bank formation among species, and annuals formed persistent seed banks more frequently than perennial species. Abundance in the standing vegetation had predictive value for abundance in the persistent seed bank only when non-persistent seed bank species in the standing vegetation were discarded from the analysis. At the local scale, species diversity in the persistent seed bank and standing vegetation were correlated, but compositional similarity was low. Secondary down-slope dispersal promoted by frost heaving in combination with runoff, and life-form correlates are discussed as possible factors accounting for poor correspondence between the persistent seed bank and the standing vegetation at a local scale. The high Andean seed bank is similar to or larger than that reported for two Arctic tundra sites, but smaller than for a northern hemisphere subalpine site. If seed bank size is considered in relation to plant cover, the Andean seed bank greatly exceeds that of one Arctic site. Our study constitutes the first demonstration of a sizable persistent seed bank at an alpine site in the South American Andes and in southern hemisphere temperate mountains in general. Received: 26 May 1998 / Accepted: 21 November 1998     

Role of soil seed bank along a disturbance gradient in an alpine meadow on the Tibet plateau

We examined the role of the soil seed bank along a grazing disturbance gradient and its relationship with the vegetation of alpine meadows on the Tibet plateau, and discussed the implications for restoration. The seed bank had a high potential for restoration of species-rich vegetation; 62 species were identified in the vegetation and 87 in the seed bank, 39 species being common to both. Mean seed density was 3069–6105 viable seeds m⁻². The density of buried seeds increased significantly with increasing disturbance, indicating that restoration of disturbed areas is not seed limited. Seed density and species richness decreased with depth. The proportion of perennial species decreased with decrease in disturbance both in seed bank and in vegetation. A large portion of species with persistent seeds in the disturbed areas indicate that this seed type can be regarded a strategy of adaptation to current disturbances. Detrended correspondence analysis (DCA) showed significant differences of species composition between seed bank and vegetation, except for the seriously disturbed site. Our results suggest that the establishment of new species in severely disturbed areas is more dependent on the seed bank. By contrast, the restoration in less-disturbed and mature meadows does not rely on seed banks, and the establishment of the vegetation in these communities is more likely to rely on seed dispersal from the standing vegetation and on species with vegetative reproduction.     

Seed bank versus seed rain in the regeneration of a tropical pioneer tree

Summary We used the tropical pioneer tree, Cecropia obtusifolia to evaluate the relative importance of different sources of seeds in the regeneration of species that depend on ephemeral sites. We studied seed production in a population established in a 5 ha plot, and dispersal, dormancy and seed predation in two recent treefall gaps (<1 year-old), two building or successional forest patches (10–15 since disturbed), and two mature forest patches (>35 years since disturbed) for a one year period at Los Tuxtlas (Mexico). Flowers and fruits were counted at monthly intervals. Annual fecundity per tree ranged from 1.4×10⁴ to 1.4×10⁷ seeds. Seeds were continuously available on the trees and on the ground. Average annual seed rain per m² (as measured by 0.5×0.5 m seed traps) varied from 184 to 1925 among the six sites. Distance to nearest seed source and patch type explained more than 60% of the seed rain variation among sites. Soil seed density, estimated by counting seeds from ten samples (78.5 cm²×10 cm deep) collected from each site in October and January, ranged among the six sites from 269 to 4485 seeds per m² in January and from 204 to 5073 in October. Soil seed viabilities were much lower (17.1% in October and 5.1% in January) than those of rain seeds (48.26%). Annual survivorships of 2.2% were estimated for seeds artificially sown on the soil surface of a gap and a mature patch, and 3.75% in a building patch. In two other experiments seed removal rates ranged from 27% to 98% in 4 days. Removal rates were significantly higher in gap and mature patches than in building patches. Ants (Paratrechina vividula) and grasshopper nymphs (Hygronemobius. sp.) were the main predators. We draw three main conclusions from our data: (1) Pathogens and predators determine low survivorship of C. obtusifolia's seeds in the soil and a rapid turnover rate (1.07 to 1.02 years) of its seed bank; (2) a continuous and copious seed production and an abundant and extensive seed rain replenish the soil seed pool in patches with different disturbance ages at least up to 86 m from nearest source; (3) more than 90% of the seeds contributing to C. obtusifolia seedling recruitment in gaps are less than one year-old. We discuss our results in the context of previous similar studies for tropical forests.     

Ecological correlations between the persistence of the soil seed bank and several plant traits,including seed dormancy

It was empirically showed that seed size and life history correlate with the formation of a soil seed bank. Although no empirical data are available that indicate a close relationship between seed dormancy and the soil seed bank, dormancy has been considered essential to the formation of a soil seed bank. I have considered the formation of the soil seed bank and survival of seeds for more than a year in the soil, and the persistence and survival of the seed bank for more than 5 years. These periods were derived from the definition of a persistent seed bank and the criterion for seed banks of long-term persistence. Plant traits that are closely related to the formation or persistence of a seed bank and their relationships to dormancy were analysed using two pre-existing databases of seed longevity in soil and comparative ecology. The integrated database comprised 18 plant traits and seed bank formation or persistence data. This approach was used to identify more reliable general empirical rules. The results of a regression tree analysis and common statistical tests of plant traits indicated that only life history and seed size were closely related to seed bank formation, and dormancy was not essential for the formation and persistence of a seed bank. However, the contribution of dormancy differed slightly between dormancy types. Scarification or dry storage requirements to break dormancy slightly enhanced the formation and persistence of a seed bank, whereas a chilling requirement decreased the formation and persistence of a seed bank. In contrast, fluctuating temperature requirements clearly contributed to the formation and persistence of a seed bank.     

Soil seed bank in different habitats of the Eastern Desert of Egypt

The floristic composition and species diversity of the germinable soil seed bank were studied in three different habitats (desert salinized land, desert wadi, and reclaimed land) in the Eastern Desert of Egypt. Moreover, the degree of similarity between the seed bank and the above-ground vegetation was determined. The seed bank was studied in 40 stands representing the three habitats. Ten soil samples (each 25 × 20 cm and 5 cm depth) were randomly taken per stand. The seed bank was investigated by the seedling emergence method. Some 61 species belonging to 21 families and 54 genera were identified in the germinable seed bank. The recorded species include 43 annuals and 18 perennials. Ordination of stands by Detrended Correspondence Analysis (DCA) indicates that the stands of the three habitats are markedly distinguishable and show a clear pattern of segregation on the ordination planes. This indicates variations in the species composition among habitats. The results also demonstrate significant associations between the floristic composition of the seed bank and edaphic factors such as CaCO₃, electrical conductivity, organic carbon and soil texture. The reclaimed land has the highest values of species richness, Shannon-index of diversity and the density of the germinable seed bank followed by the habitats of desert wadi and desert salinized land. Motyka’s similarity index between the seed bank and the above-ground vegetation is significantly higher in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%).     

The role of the soil seed bank in the unpredictable habitat of Anthemis chrysantha J. Gay,a critically endangered species

The ability of Anthemis chrysantha to form a soil seed bank (SB) was investigated in order to understand better the adaptation of this rare annual species to its arid and unpredictable natural habitat. The natural seed bank population was analyzed in five consecutive plant cohorts (2006–2010) by sampling at two different times: in May, after the germination period, and in October, after the first dispersal episodes due to the beginning winter rainfall. In addition, to determine the persistence in the soil of the two achene morphs of this species’ (white and dark achenes), an artificial SB was created where achenes were buried and exhumed successively after each season, during two years. In all the cohorts studied, seedling emergence from May samples indicated the existence of a persistent seed bank (PSB). Moreover, the dark achenes were largely responsible for the permanent fraction because, after two years of burial in the artificial SB, 85.0% of them remained apparently healthy without having germinated, versus 9.9% of white achenes. Both types of buried achenes exhibited an annual conditional dormancy/non-dormancy cycle, induced by low winter temperatures. The PSB dynamics appeared to oscillate between the minimum values at the end of the germination period in spring (up to 2000 achenes per square meter) and the maximum values of the dispersal episodes in early autumn (up to 6000 achenes per square meter), with fluctuations of achene density due to the variability in annual rainfall. Hence, the SB showed a decline due to the failure of fruiting in the 2008 cohort, caused by drought, although the low value of ca. 560 achenes per square meter was able to establish the following population. Our study highlights the importance of the PSB, which, in “bad” years, may be critical for the persistence of this species.     

Aerial seed bank and dispersal traits in Anthemis chrysantha (Asteraceae), a critically endangered species

Mature plant density and fruit production were monitored in the main population of four successive cohorts of the endangered winter annual Anthemis chrysantha (Asteraceae) in southeastern Spain. Experiments were conducted with artificial rainfall and a wind tunnel to determine the temporal and spatial dispersal pattern of the species and the relationship with rain and wind.     

持久土壤种子库研究综述

土壤种子库是指存在于土壤上层凋落物和土壤中全部存活种子,简单地可分为短暂土壤种子库和持久土壤种子库。即使给予理想的萌发条件如季节、温度、湿度等,土壤种子库中也仍有部分种子保持休眠状态,休眠的种子组成了持久土壤种子库。持久土壤种子库具有在承受了空间或时间上不可预测的干扰的植被中发挥繁殖能力的潜势,因此,其在植被承受干扰后的恢复、管理和重建中常常起关键作用。研究持久土壤种子库能丰富生物多样性的内容,同时,可以为深入了解植被更新提供更多的信息。本文从持久土壤种子库的研究方法、分类方法、指示因子、生态意义,以及持久土壤种子库研究中存在的问题和今后的工作进行综述,试图为将来的工作奠定基础。     

森林土壤种子库与天然更新

论述了森林土壤种子库的特点：（1）森林土壤种子库密度小于耕作土壤和草地,并且受森林类型和年龄的影响,变化幅度很大;（2）森林土壤种库的种类组成在森林的不同演替阶段,受其地上植被影响的程度不同;（3）森林土壤种子库的动态变化是周期化因数和随机性因数的结合,并从土壤种子库的时间动态和空间分布角度,分析了土壤种子库对天然更新的影响,森林土壤种子库是天然更新的物质基础,种子库的结构和功能影响天然更新能力的方向,因此能影响土壤种子库格局变化的各种因子或措施,均能显著地促进或妨碍森林的天然更新,根据森林土训种子库的时空格局特点及影响更新的障碍因子,可以采用人工的辅助措施促进更新,同时,对于森林土壤种子库的研究应着眼于应用干扰生态学的基本理论,以退化生态系统的重建和群落交错区的演替为主要内容,研究方法应以长期定位观测为宜。     

Effects of altitude and seed size on germination and seedling survival of heathland plants in north Spain

The effects of altitude and seed size on germination and seedling survival were studied in Calluna vulgaris, Erica cinerea and Erica vagans. Experiments were carried out in the laboratory over a one year period. Seeds collected from heathlands of different altitudes were divided in two size classes. They were sowed on moist filter paper inside Petri dishes which were placed in chambers at 20 °C and a photoperiod of 12 h light/12 h darkeness. The seeds of Calluna vulgaris were the first to begin germination and had the highest rate and percentage of germination. The germination of Erica vagans was moderate, while seeds of Erica cinerea germinated later and the germination was very low. Seeds of Calluna vulgaris and E. cinerea collected at the highest altitudes had the highest germination percentages. Seed size in Calluna vulgaris and E. cinerea did not affect germination. However, large seeds of Erica vagans had higher germination rates and percentages than small seeds. The large seed size of Calluna vulgaris contributes to a better survival and growth of its seedlings.     

Organisation of the soil seed bank in mixed chaparral

Seed densities, estimated from germinations under glasshouse conditions, were determined for fire-treated and control soils sampled from north and south facing stands of mixedAdenostoma fasciculatum-Ceanothus greggii chaparral that last burned 10, 17, 36, 62 and 86 years before 1987. A total of 53 species was recorded. Total germinable seed density was not influenced by aspect nor by an experimental fire treatment, however, densities varied significantly, but not predictably, over the time since fire sequence. Species diversity was significantly lower in the 86 year old stand, but neither diversity or richness was influenced by the fire treatment. Soils from north facing slopes supported a higher species diversity overall, and seed densities of six species were significantly higher on north than on south slopes.We identify two functional groups in the seed bank on the basis of the direct effects of fire on seed densities: a fire-dependent group, comprising 6 species and about 21% of the total seed bank, in which germinable densities increased after the fire treatment, and a fire-independent group, made up of 11 species and almost two-thirds of the seed bank. The fire-independent group is further separated into two: a shrub-centred sub-group (Crassula erecta, Filago californica, Pterostegia drymarioides, Streptanthus heterophyllus andCryptantha intermedia) maintained larger seed banks in older stands and was associated with shrub cover; and an opportunistic sub-group (Camissonia hirtella, Gnaphalium chilense, Mimulus pilosus andSenecio vulgaris) tended to have larger seed banks in younger stands and was associated with canopy gaps.Overall, the soil seed assemblage from this mixed chaparral does not appear to be tightly coupled to fire-generated opportunities for recruitment. The reason for this is that the fire-dependent group represents only 6 species. In addition, among the fire-independent species, the shrub-centred group of 5 annuals made up almost 40% of total soil seed density. This group shows ecological and taxonomic affinities with desert annuals.     

Submerged macrophyte seed bank in a Mediterranean temporary marsh: abundance and relationship with established vegetation

The abundance and composition of the submerged macrophyte seed bank in the Doñana marsh (southwestern Spain) was evaluated to assess its relationship with the overlying vegetation. The results obtained demonstrate the existence of a dense seed-bank, both in terms of the number of seeds and their biomass, which represented about 10% (5% for angiosperms and >20% for Charophyta) of the total plant biomass, which ensures the maintenance of the annual submerged macrophyte populations of the seasonally inundated Donñana marsh. Seed bank and established vegetation were coupled, as reflected in the existence of significant correlations between their structure and abundance. This coupling was lacking for Charophyta, whose oospores are widespread and abundant across the marsh, even at locations where they are absent, or rare, in the established vegetation. These differences between the seed bank of annual angiosperms and Charophyta appear to reflect, in part, structural differences between angiosperm seeds and Charophyta oospores, with important ecological consequences. An important aspect of these differences is the allocation of Charophyta reproductive effort to many small propagules lacking embryo storage, compared to those of angiosperms, which ensures their efficient dispersal and numerical abundance in the seed bank.     

The persistence of calcareous grassland species in the soil seed bank under developing and established scrub

The relationships between the composition of the soil seed bank, the field layer vegetation, and the scrub canopy were investigated along a 69 m transect, grading from incipient woodland, through scrub, into intensively rabbit-grazed calcareous grassland. The results are used to assess the persistence of species associated with open calcareous grassland in the seed bank under developing scrub. Scrub age, composition and density, changed along the transect from the woodland to open grassland. A total of 35 forb and grass species were found in the field layer. The pattern evident in the scrub layer was also reflected in the herbaceous vegetation. The field layer in the most closed portion of the transect, where the scrub was oldest, was dominated by shade-tolerant species normally associated with woodland habitats. The abundance of these species decreased along the transect as the scrub age declined, and the field layer became increasingly dominated by species typical of open grassland. A total of 47 species germinated from the seed bank. Few species were recorded in the seed bank along the entire length of the transect. Overall, the seed bank was dominated by Hypericum perforatum and Centaurium erythraea, which accounted for 38.2% and 28.6% of emerging seedlings respectively. As with a number of similar studies, the composition of the seed bank had a low correspondence with the composition of the field layer vegetation. The results also emphasise that the composition of the seed bank can be viewed as an ecological palimpsest, with germinable seed of species from each stage of the old-field succession occurring in the soil. The seed bank is an important component in the re-vegetation of an area after disturbance such as scrub removal. This study supports the findings of previous research in showing that relatively few characteristic calcareous grassland species form persistent seed banks. The soil seed bank would therefore appear to be of limited value in the restoration of such grassland following scrub removal.     

Soil seed bank composition and diversity in a managed temperate deciduous forest

Little is known about the influence of forest management on the interaction between seed bank and aboveground vegetation. We surveyed seed banks and vegetation in 10 forest stands under similar abiotic conditions but submitted either to a coppice-with-standards treatment (n=5) or to a selective-cutting system (n=5). We analyzed species composition and diversity, community ecological profile, and distribution of taxa among different life forms, strategy, morphology and functional type categories. A total of 2085 seedlings (8296 seedsm^–2) germinated-corresponding to 28 species, among which Juncus effusus was the most abundant. Fifty-seven percent of the species were also recorded in the aboveground vegetation, the dominant species being Rubus fruticosus agg., but only 28% of the aboveground species were present in the seed bank. Our results suggest that (1) vernal geophytes and shade-tolerant perennials, which group most true forest species, are not incorporated in the seed bank, (2) parent plants of most seeds were present either in the stand in an earlier dynamic stage or apart from the stand and long-distance dispersed, (3) as expected, early-successional species are well represented in the seed bank, (4) forestry vehicles seem to be a major means of dispersion for stress-tolerant species normally found in forest lanes and wheel tracks. We conclude that seed banks contain species that have a potentially negative impact on the true forest flora and, thus, forest management should minimize soil disturbance and retain remnants of old-coppice woods to conserve disturbance-sensitive true forest species.     

Postfire seed bank dynamics in semiarid grasslands

We evaluated the combined effects of fire after drought on the seed bank composition and its role in the postfire recovery of NW Patagonia grasslands. During three years, we monitored the seed bank and the aboveground vegetation. Species were arranged in functional groups and Detrended Correspondence Analysis was used to separate sites according to species and functional groups. Similarity between aboveground vegetation and seed bank was calculated with SØrensen Index. In the first year, the seed density was similar in the control and burned sites and was lower than following years in all the sites. The species that survived the high temperatures were all annuals with the exception of the perennial species Fabiana imbricata and Rumex acetosella. In the second postfire year, the diversity and seed density increased due to the contribution of fugitive species (rare in the community) and exotic annual species. Seed bank of perennial species was the most affected by fire and just recovered in the third year. Drought did not affect the similarity between the seed bank and vegetation. Fire had low impact on the total seed bank, probably due to the heat buffering nature of the soil, whereas drought reduced significantly seed bank size and richness. Seed bank contributes to grassland richness maintenance.     

Effects of seed storage on germination of two succulent desert halophytes with little dormancy and transient seed bank

Seeds of both Salsola imbricata and Haloxylon salicornicum have high germination level and germination speed, and form a transient seed bank in nature. The impacts of storage period and condition on germination level and speed were assessed in the two species. Storage for three months significantly increased both germination level and speed of seeds stored under the different conditions, compared to that of fresh seeds. In both species, nine months storage did not affect germination percentage in cold storage seeds, but completely inhibited it in field seeds. Storage for longer time in room and warm temperatures resulted in significant reduction or complete inhibition in the germination of the two species, so this was more pronounced in H. salicornicum. Storage significantly increased germinate rate index of seeds stored in all conditions till 17 months in S. imbricata and till 12 months in H. salicornicum. In both species, fridge storage had little effects on final germination and germination speed of seeds incubated at the different temperatures, compared to fresh seeds. However, room temperature and warm storages significantly reduced final germination and germination speed at the different temperatures, so the reduction was more pronounced at 35 °C, especially in H. salicornicum.     

Spatial patterns of the germinable soil seed bank of coexisting perennial-grass species in grazed shrublands of the Patagonian Monte

We compared the size and spatial pattern of the germinable soil seed bank (GSB) of the three dominant perennial-grass species in the arid Patagonian Monte of Argentina. These species differ in plant functional traits, seed morphology, and in their preference by grazers. During three consecutive years, we sampled the soil seed bank at 1 m intervals and the intercepted diameter of perennial-grass species and shrub patches along 50-m linear transects at three sites with different grazing pressure. Soil samples were incubated at field capacity during 12 weeks and emerged seedlings counted. The largest-seeded and most xerophytic Stipa speciosa did not form a GSB. The other two grass species with more mesophytic traits and smaller seeds than S. speciosa formed different GSB. Poa ligularis formed a smaller and more heterogeneous GSB than Stipa tenuis. Grazing had not effect on GSB size of P. ligularis but increased the proportion of seeds under dense shrub canopies. In contrast, grazing reduced the size and also increased the proportion of the GSB of S. tenuis under shrub canopies. We conclude that the size and the spatial pattern of the GSB of the dominant perennial-grass species in the arid Patagonian Monte were related not only to seed and plant traits but also to the spatial distribution of grass plants and their exposure to grazers.