Patches Of Bare Ground Research Articles

Runoff and Erosion After Cutting Western Juniper

Western juniper (Juniperus occidentalis spp. occidentalis Hook.) has encroached on and now dominates millions of acres of sagebrush/bunchgrass rangeland in the Great Basin and interior Pacific Northwest. On many sites western juniper has significantly increased exposure of the soil surface by reducing density of understory species and surface litter. We used rainfall and rill simulation techniques to evaluate infiltration, runoff, and erosion on cut and uncut field treatments 10 years after juniper removal. Juniper-dominated hillslopes had significantly lower surface soil cover of herbaceous plants and litter and produced rapid runoff from low-intensity rainfall events of the type that would be expected to occur every 2 years. Direct exposure of the soil to rainfall impacts resulted in high levels of sheet erosion (295 kg · ha−1) in juniper-dominated plots. Large interconnected patches of bare ground concentrated runoff into rills with much higher flow velocity and erosive force resulting in rill erosion rates that were over 15 times higher on juniper-dominated plots. Cutting juniper stimulated herbaceous plant recovery, improved infiltration capacity, and protected the soil surface from even large thunderstorms. Juniper-free plots could only be induced to produce runoff from high-intensity events that would be expected to occur once every 50 years. Runoff events from these higher-intensity simulations produced negligible levels of both sheet and rill erosion. While specific inferences drawn from the current study are limited to juniper-affected sites in the Intermountain sagebrush steppe, the scope of ecosystem impacts are consistent with woody-plant invasion in other ecosystems around the world.

Intersexual niche segregation in Cepero’s Ground-hopper, Tetrix ceperoi

Sexual differences in habitat preferences have been reported from a variety of animal taxa. However, the ultimate causes for this intersexual niche segregation remain poorly understood. It has been suggested that sexual dimorphism is a consequence of dimorphic niches based upon different reproductive costs and activities of the sexes. Here we provide evidence from field data to examine this hypothesis by studying the behavioral background of niche segregation in Tetrix ceperoi. Our data revealed distinct sexual differences in the substrates on which the insects perched and in the solar radiation of these locations. Males were found at brighter locations and more often on bare ground than females. Incorporation of behavioral data in our analysis showed that patches of bare ground were mainly utilized during mating behavior, in which males invested more time than females. In contrast, females spent more time resting and feeding in the vegetation. Intersexual differences in the proportion of autotomized individuals indicate that males might suffer higher predation risks. These patterns support the dimorphic niches hypothesis, which suggests that differential habitat utilization is caused by differences in the life history strategies of males and females, since males should accept a higher predation risk due to the benefits of multiple matings. Females should invest more time in gaining nutrients and energy for egg production and survival, whereas males should spend more time with searching for mates. We suggest that behavioral covariates should more often be implemented in ecological analyses, since these might have a strong explanatory power.

Disturbance persistence in managed grasslands: shifts in aboveground community structure and the weed seed bank

The length of time and form in which disturbances persist in systems depends on the intensity and frequency of disturbance and on the abilities of resident species to recover from such events. In grazed grasslands, trampling by large mammalian herbivores can periodically facilitate weed establishment by exposing patches of bare ground but whether an intense soil disturbance event results in a temporary increase in weed abundance or a persistent weed problem remains unclear. In May 2002, cattle trampling following heavy rain caused severe damage to nine-month old, rotationally grazed, cool-season pastures (Midwest USA). In September 2002, we compared the aboveground composition of paddocks (i.e., fenced pasture sections) that were heavily disturbed to those that received no damage. Relative to undisturbed paddocks, forage species relative cover was 17% lower in disturbed paddocks, and weed species and bare ground relative cover was 61% and 100% higher, respectively. By September 2004, paddock types did not differ in all aboveground community components. However, the abundance and species richness of weed seeds in the soil seed bank averaged respectively 82% and 30% higher in disturbed paddocks between 2003 and 2004. These findings indicate that a spatially extensive, intense soil disturbance event may soon become undetectable in components of aboveground pasture structure but can persist as an augmented weed seed bank. Because of high weed seed bank longevity, disturbances to formerly disturbed pastures would likely result in higher weed recruitment, with more species represented, than in those which lack previous disturbance. Disturbance history may thus be a useful predictor of weed community composition following subsequent disturbance. Based on empirical data supporting this proposition, we recommend that grassland managers explicitly incorporate disturbance history into dynamic management planning and do not rely exclusively on aboveground characters to evaluate the invasion status or colonization potential of an area by undesirable plants. We emphasize that the ecological legacies of past soil disturbance events cannot only influence the contemporary patterns and processes of grasslands, but importantly, affect their compositional trajectories following subsequent perturbation.

Modelling the spatial–temporal variability of spring snowmelt in an arctic catchment

Arctic spring landscapes are usually characterized by a mosaic of coexisting snow-covered and bare ground patches. This phenomenon has major implications for hydrological processes, including meltwater production and runoff. Furthermore, as indicated by aircraft observations, it affects land-surface–atmosphere exchanges, leading to a high degree of variability in surface energy terms during melt. The heterogeneity and related differences when certain parts of the landscape become snow free also affects the length of the growing season and the carbon cycle. Small-scale variability in arctic snowmelt is addressed here by combining a spatially distributed end-of-winter snow cover with simulations of variable snowmelt energy balance factors for the small arctic catchment of Trail Valley Creek (63 km2). Throughout the winter, snow in arctic tundra basins is redistributed by frequent blowing snow events. Areas of above- or below-average end-of-winter snow water equivalents were determined from land-cover classifications, topography, land-cover-based snow surveys, and distributed surface wind-field simulations. Topographic influences on major snowmelt energy balance factors (solar radiation and turbulent fluxes of sensible and latent heat) were modelled on a small-scale (40 m) basis. A spatially variable complete snowmelt energy balance was subsequently computed and applied to the distributed snow cover, allowing the simulation of the progress of melt throughout the basin. The emerging patterns compared very well visually to snow cover observations from satellite images and aerial photographs. Results show the relative importance of variable end-of-winter snow cover, spatially distributed melt energy fluxes, and local advection processes for the development of a patchy snow cover. This illustrates that the consideration of these processes is crucial for an accurate determination of snow-covered areas, as well as the location, timing, and amount of meltwater release from arctic catchments, and should, therefore, be included in hydrological models. Furthermore, the study shows the need for a subgrid parameterization of these factors in the land surface schemes of larger scale climate models. Copyright © 2005 John Wiley & Sons, Ltd.

Radial Dispersion of Neighbors and the Small‐Scale Competitive Impact of Two Annual Grasses on a Native Perennial Grass

Abstract In California's Mediterranean type grasslands, native perennial grasses such as Nassella pulchra are surrounded by introduced annual species and these annuals are thought to have displaced natives through much of their range. Amongst other invaders, two grasses Lolium multiflorum and Bromus hordeaceus, commonly dominate portions of the grassland with potential for N. pulchra restoration. We hypothesized that competitor species differences and small‐scale gaps (150 cm2) could be important determinants of N. pulchra survival and performance on these sites. Lolium multiflorum and B. hordeaceus were planted in 20 cm diameter circular plots at a constant rate of 1 seed per cm2 surrounding newly transplanted N. pulchra plants. Nassella pulchra showed no significant effect of the species of competitor or from the distribution of the competitors. Both interspersion of patches of bare ground and separation of competitors into patches did not increase N. pulchra pre‐dawn water potential, basal area change, number of seeds produced, or average weight of seeds. The presence of L. multiflorum was associated with a decrease in N. pulchra survival compared with plots with only B. hordeaceus. Plants with increases in basal area of less than 0.75 cm2 during the growing season had 74% mortality compared with no mortality in plants with more growth. However, initial N. pulchra plant size was not a good predictor of mortality. Limiting competition from annuals may increase survival of N. pulchra plantings, but 60% of the plants survived for at least 1 year, despite being transplanted into soil containing substantial annual grass seed.

Influence of sward height on bite dimensions of horses

AbstractThe management of pastures used by horses poses various problems: swards are often grazed down rapidly, patches of bare ground and refused areas develop. In order to gain a better understanding of the plant-animal interaction behind this problem the influence of sward height on bite dimensions (bite depth, weight, volume, area) of horses was investigated. Perennial ryegrass (Lolium perenne) microswards were grown on artificial substrate to four different heights (3, 8, 15, 19 cm) but the same plant density. Biomass and bulk density for each sward height was determined. Eight horses (predominantly Thoroughbred ×) were allowed to take six bites of each grass height. Bite depth, weight, volume and bite area were calculated.Bite depth, weight, volume and area were not fixed, but increased with sward height (P< 005). For all sward heights, the horses removed between 51% and 68% of the initial grass length. The horses adjusted their bite dimensions to sward height but did not maximize food intake per bite. The response to sward height might be explained by a preference for easily digested food or the trade-off between energy intake and the cost for food processing.This experiment suggests that grass height has a major influence on bite dimensions of horses. However there may be more complex factors influencing ingestive behaviour of horses (e.g. age, dietary experience and social behaviour) that have to be considered when horses are grazing pasture.

Prairie dog engineering indirectly affects beetle movement behavior

Previous studies have shown that landscape structure influences animal movement and population structure. In this study, we show an indirect interaction between beetles and prairie dogs due to prairie dog ecosystem engineering. Gunnison's prairie dog (Cynomys gunnisoni Hollister) towns have more bare ground and are structurally less complex than adjacent unmodified grasslands. This results in bare ground facilitating beetle movement. Differences in landscape structure between prairie dog towns and unmodified grasslands had a significant effect on the movement of the darkling beetle, Eleodes hispilabris Say. Beetle movement tended to be more linear (pathway fractal dimension approached 1) on prairie dog habitats and more sinuous on adjacent grasslands. Beetle velocity was 44% greater and net displacement 63% farther on the prairie dog habitat. These differences were also evident at fine scales when beetles moved between grass and bare ground patches regardless of habitat. Beetles moved 2.3 times faster and displaced 3.0 times farther after making this microhabitat transition. Beetles avoided grass and selected bare ground for movement 12–22% more than the percent of bare ground on the landscape. Since beetles exhibited directed movements and grasslands inhibit movement, it might be expected that beetles would accumulate in this habitat upon encounter. However, beetles were approximately twice as abundant on prairie dog towns as on adjacent grasslands. A difference in beetle movement behavior, between prairie dog towns and grasslands, suggests that prairie dog towns are an important component of this desert grassland landscape for these beetles.

Boundary‐layer integration approach to advection of sensible heat to a patchy snow cover

AbstractIn the calculation of the melting of a patchy snow cover, the energy advected from the adjacent bare soil to the snow surface is an important consideration. The quantity or rate of energy advected depends on the fetches and sizes of snow and bare ground patches. Any successful method to estimate advection will necessarily require the incorporation of relationships describing the same. Complex boundary‐layer methods require detailed spatial knowledge of the patch sizes, wind direction and fetch distances, and are computationally intensive. A physically based approach that can be spatially applied using distributions of snow patch geometry is required.This paper presents a new approach, in which boundary‐layer integration is used to provide a means of calculating the amount of energy removed by the snow patch surface as warmer air moves over it. The method is reduced to a simple parametric form, and the relationships describing the coefficients required for its application are developed.The applicability of this new approach is discussed in light of the fractal nature of snow patches, the relationship between the individual patch length and area, and the distribution of patch sizes as they develop and disappear on the landscape. Copyright © 2002 Crown in the right of Canada. Published by John Wiley & Sons Ltd.

The influence of grass height on bite dimensions of horses

The management and maintenance of swards grazed by horses is known to be a difficult task. In a short period of time (2 to 3 weeks) pastures used for horse grazing develop patches of bare ground, short grass and areas which horses refuse to graze because of faecal contamination. Bite depth and bite area are important in determining the effects of grazing on the vegetation. The dimensions of the bite have been investigated in cattle (Ungar et al., 1999), sheep and goats (Concha, 2000), but not horses. The interaction of the horse with the pasture surface is poorly understood but important to improve management and maintenance of areas grazed by horses. The study reported examines the influence of grass height on bite dimensions, i.e. bite depth, bite volume and bite area of the horse at grazing.

木曽駒ケ岳高山帯での風食ノッチの後退とパッチ状裸地の拡大

Mt. Kisokoma is located in the northern part of the central Japanese Alps. On the southwest-facing slope of Mt. Kisokoma, patches of bare ground are well developed. This paper presents the extending direction of the patches of bare ground in relation to wind direction, and seasonal changes in the amount of retreat of turf scarps measured with erosion bars. Extending direction of patches of bare ground correspond to the direction of the prevailing westerly wind, which is dominant throughout the year. The results show that the prevailing wind controls the extension of patches of bare ground. The turf scarps retreated most in the Baiu-season. Strong wind accompanied by heavy rain is observed several times during the season, and rain splash wash seems to be the dominant cause of retreat. Negligible retreat of the turf scarps in winter suggests that the turf scarps are protected by snow beds in winter. In spring and autumn, the combination of needle ice and wind erosion might have caused some retreat of the turf scarps.

Trompe L'Oeil

Trompe L'Oeil

Bryophytes of chalk grassland in the Chiltern Hills, England

A survey of the bryophytes of 13 chalk grassland sites in the Chilterns was undertaken from 1992 to 1996. Despite changes in land-use and grazing regimes, loss of habitat and the possible effects of atmospheric pollution, the bryophyte interest of the grasslands remains high.Amongst the nationally scarce plants found were Barbula acuta, Ephemerum recurvifolium, Pollia caespitosa, Pterygoneurum ovatum, Seligeria calcarea, Thuidium abietinum ssp. hystricosum and Tortella inflexa. Other notable species include Entodon concinnus, Dicranum bonjeanii, Ditrichum crispatissimum, Phascum curvicolle, Neckera crispa, Rhodobryum roseum, Seligeria paucifolia, Thuidium philibertii and Tortella tortuosa.Although some species have shown a decline, a review of old floras, past site accounts and records shows that species losses in the chalk grassland of the Chilterns have been remarkably few. Only two mosses, Pleurochaete squarrosa and Thuidium abietinum ssp. abietinum, and one liverwort, Porella arboris-vitae, were not re-found during the present survey. The hepatic mat association of north-facing slopes, characterized by Frullania tamarisci and Scapania aspera, is also now very rare in the Chilterns. A reduction in grazing and neglect of chalk grassland has brought about an increase in coarse grasses and scrub encroachment reducing the area of habitat available to bryophytes. Rabbits have probably played a vital role in maintaining a mosaic of short species-rich turf with patches of bare ground. Ancient earthworks, trackways and chalk pits with thin, free-draining soils may also act as refugia for many chalk grassland bryophytes.

Contingent effects of plant species on soils along a regional moisture gradient in the Great Plains.

The central grassland region of the United States encompasses major gradients in temperature and precipitation that determine the distribution of plant life forms, which in turn may influence key ecosystem processes such as nutrient cycling and soil organic matter dynamics. One such gradient is the threefold increase in precipitation from the eastern Colorado shortgrass-steppe, in the rain shadow of the Rocky Mountains, to the tallgrass prairie in eastern Kansas. We investigated the relative roles of plant species and plant cover in influencing soil C and N cycling in three sites along this gradient. Plant cover (i.e., the presence or absence of an individual plant) was relatively more important than plant species in explaining variability in soil properties at the dry site, the Central Plains Experimental Range in␣northeastern Colorado. However, plant species explained relatively more of the variability in soil properties than did plant cover at the two wetter sites, Hays and Konza, in central and eastern Kansas. The wetter sites had more continuous plant cover, resulting in less plant-cover-induced variation in soil C and N, than did the dry site, which had distinct patches of bare ground. Plant species at the wetter sites had higher and more variable levels of tissue C:N than plant species at the dry site, due to both within species changes and changes in species composition. Aboveground tissue C:N was better correlated with net nitrogen mineralization rates at the wet sites than the dry site. Thus, tissue chemistry appears to exert more control on nitrogen dynamics at the wet than the dry sites. The results suggest that plant species traits that are relevant to nutrient cycling (e.g., tissue C:N ratios, spatial patterns, productivity) reflect environmental limitations as well as species' physiological potentials. Furthermore, a dominant environmental driver such as precipitation may ameliorate or exaggerate the importance of individual species traits for nutrient cycling.

Biomass of Grazed, Burned, and Undisturbed Paramo Grasslands, Colombia. I. Aboveground Vegetation

Estimations of the amount of aboveground vegetation mass were made for four piramo grassland sites with different grazing management and burning histories in the Central Cordillera of Colombia. The total mass of live plus dead grassland vegetation showed a decrease from 2820 ? 190 g m-2 at the undisturbed to 868 + 73 g m-2 at the intensively grazed and burned sites. Stem rosette mass was highest at both the undisturbed and the heavily grazed sites (666 ? 168 and 1029 + 245 g m-2, respectively), but considerably lower at the burned sites (397 ? 94 and 285 ? 78 g m-2). Eighty percent of the total undisturbed vegetation mass consisted of standing dead material and litter. The decrease of dead material mass along the disturbance gradient may have large implications for ecosystem stability. The proportion of live material increased along the grazing and burning gradient, resulting in a similar live material mass at all sites. Under high grazing intensities and in the absence of burning, the vegetation can transform into ground-covering mats, attaining a moderately high biomass. Where burning took place, this transformation does not occur, and both grassland and stem rosette biomass were reduced, leaving many patches of bare ground.

Temporal Variation in Snowcover Area During Melt in Prairie and Alpine Environments

Information on the temporal variation in snow-covered area of watershed during melt is requisite for accurate predictions of runoff. The amount of the gross watershed area that is snow-covered affects runoff primarily in two ways: a) it influences the melt rate, because patches of bare ground affect the energy balance of the snow field, and b) it governs the contributing area of runoff. This paper examines the area-frequency and perimeter-area characteristics of soil and snow patches that form during ablation of seasonal snowcovers in Prairie and Alpine environments. It uses fractal geometry as a basis for quantifying these properties. Image analyses are applied to aerial photographs taken during snowmelt on two small watersheds: one in the West-central part of the Province of Saskatchewan in the Canadian Prairies, the other in the alpine region of the Austrian Alps. The results of the study suggest that the soil and melting snow patches behave as fractals, that is their perimeter-area and area-frequency characteristics can be described by simple power equations with patch area. The perimeter-area ratio of the soil and snow patches decreases with increasing size of patch, but at a smaller rate than for Euclidean objects. The area-frequency characteristics of snow patches follow a hyperbolic distribution with relatively few large patches and numerous small patches. It is suggested that the soil and snow patches have the same fractal dimension. It is concluded that snow patches are not random and their size distribution is predictable. The variation in the edge length of a snow field per unit basin area during ablation is demonstrated. A maximum value of the ratio is reached when a basin has 45-65 % snowcover. With snow coverage in this range the potential for local advection increasing melt under a specific set of climatic conditions is greatest.

Grasshopper (Orthoptera: Acrididae) and Plant Relationships Under Different Grazing Intensities

Grasshopper (Orthoptera: Acrididae) density and species composition were monitored in three southwestern Montana grazing areas. Periodic comparisons were made of percent cover, percent crude protein, percent water content, and total phenolics for grasses grazed by cattle versus grasses protected from grazing. Grasshopper densities and composition differed in three areas. Grazing reduced vegetative cover, accompanied by increases in ground litter and patches of bare ground. Plant nitrogen and water content decreased during the spring and then increased in late summer following rains. Phenolics levels were independent of grazing and weather fluctuations. Changes in grasshopper nymph densities roughly corresponded to springtime decreases of crude protein and water content in grasses. Nymphal mortality was inversely related to density. Grazing, weather, or changes in plant chemistry had little effect on grasshopper parameters of this study.

On the impact of snow cover on daytime pollution dispersion

A preliminary evaluation of the impact of snow cover on daytime pollutant dispersion conditions is made by using conceptual, scaling, and observational analyses. For uniform snow cover and synoptically unperturbed sunny conditions, observations indicate a considerate suppression of the surface sensible heat flux, the turbulence, and the development of the daytime atmospheric boundary layer (ABL) when compared to snow-free conditions. However, under conditions of non-uniform snow cover, as in urban areas, or associated with vegetated areas or bare ground patches, a milder effect on pollutant dispersion conditions would be expected. Observed concentrations of atmospheric particles within the ABL, and surface pollutant concentrations in urban areas, reflect the impact of snow cover on the modification of ABL characteristics.

Cues Cattle Use to Avoid Stepping on Crested Wheatgrass Tussocks

This paper tests 2 hypotheses regarding the cues cattle use to avoid stepping on crested wheatgrass (Agropyron cristatum (L.) Gaertner) tussocks. The first hypothesis is that cattle are attentive to shade and avoid tussocks by stepping on light areas (soil interstices) and avoiding dark areas (tussocks). In an experiment with 90 Angus heifers placed in a short-duration grazing paddock of 8.5 ha, the animals stepped with equal relative frequency on 28 patches of bare ground, 37 disks painted the shade and color of bare ground, and 37 disks painted to match vegetation over a 24-h period. We therefore reject the shade-cue hypothesis. The second hypothesis is that cattle are attentive to the vegetation itself in their avoidance behavior, and that as they crop the vegetation the frequency of trampling increases. In experiments similar to the first, cattle stepped on 85 intact tussocks 9 times, on 85 clipped (3 to 4 cm above litter) tussocks 28 times, on 85 vegetation-free tussock mounds 107 times and on 35 patches of bare ground 130 times. These differences are statistically significant. The data are consistent with the vegetation-cue hypothesis, except that the cattle also were attentive to the elevated substrate upon which the tussock grew. We conclude that, under the test conditions, hoof action does not have an important impact on crested wheatgrass pastures used for short-duration grazing. The impact could approach importance, however, if the pasture was grazed more heavily and if the vegetation was dry and dusty.

Woody plant seed dispersal and gap formation in a North American subtropical savanna woodland: the role of domestic herbivores

The relationship between domestic cattle and vegetation change in a savanna woodland was evaluated with respect to dung deposition and the dispersal and establishment of mesquite (Prosopis glandulosa var. glandulosa, Mimosaceae), a cosmopolitan woody invader of grasslands in the southwestern USA. Dung deposited in autumn disintegrated rapidly, leaving patches of bare ground ranging from 50 to 900 cm2. Herbaceous cover on gaps created by dung deposition recovered to levels comparable to neighboring vegetation by the end of the following growing season. Vegetation colonizing gaps consisted primarily of grasses not found in the surrounding vegetation. Dung deposition increased species diversity and spatial heterogeneity of the herbaceous vegetation and contributed to the development of a fine-grain mosaic of small patches of varying successional age-states. The role of cattle in facilitating the ingress and establishment of mesquite has broader implications with regard to the conversion of grasslands to woodlands. On the site with cattle, mesquite seedlings were found in 75% of dung pats surveyed in September (mean =4.2 seedlings per pat; maximum =50). Although seedling survival in dung (79%) was only 16% greater than that of mesquite emerging from seeds experimentally sown away from dung, no seedlings were found on areas without cattle. Mean (± SE) density of mesquite seedlings ranged from 12±2 to 15±2 m-2 on the site with cattle. Seed densities away from parent plants averaged 10.7 m-2 and 0.0 m-2 on areas with and without cattle, respectively. Seed densities beneath adult plants were comparable between sites. The high density of seedlings on areas with cattle, in contrast to absence of seedlings on the area without cattle, suggests rates of invasion of grasslands by mesquite would have increased substantially in North America following the settlement and introduction of domestic ungulates. Prior to the introduction of livestock, poor seed dissemination and germination may have limited its Holocene spread.

Colonizing Abilities of 'Biennial' Plant Species in Relation to Ground Cover: Implications for their Distributions in a Successional Sere

The "biennial" plant series Verbascum thapsus, Oenothera biennis, Daucas carota, and Tragopogon dubius are all found in abandoned agricultural fields, but they differ in their time of first appearance, peak abundance, and length of time populations persist. In experimental field studies these species showed differences in their abilities to colonize various types of ground cover. Seedling emergence, survival, and growth were highest for all four species in patches of bare ground. The small—seeded species, Verbascum and Oenothera, survived only in bare ground. In contrast, the large—seeded species, Daucus and Tragopogon, survived in several ground cover types. Relative coverage of three ground cover types and differences in seedling performance in these cover types were compared in a 1—yr and 15—yr old field. Verbascum and Oenothera became established only in the 1—yr old field where bare ground was abundant and not in the 15—yr old field where these patches were rare (°1% of area). When bare—ground patches were created experimentally in the 15—yr old field, both species were able to establish, supporting the conclusion that it is availability of bare ground rather than age of field per se that confines these two species to recently disturbed areas. In contrast, Daucus and Tragopogon became established in both fields, although seedling emergence, survival, growth, and reproductive output were higher in the 1—yr old field. Under nonexperimental conditions, however, these two species are not generally found in newly abandoned fields. Their absence is due to the fact that they produce seeds that, although broadly dispersed, do not survive in the soil for >1—2 yr and are not often present in a newly distributed area. Once dispersed to and established in an area, they can persist for several generations because their seedlings can survive in several types of ground cover, including those with thick vegetation.