Shrub Population Research Articles

Old Juniper Troll stand - The oldest shrub population from Scandinavia

Old Juniper Troll stand - The oldest shrub population from Scandinavia

Responses of spatial distribution pattern of the dominant population Artemisia ordosica to enclosure restoration in desert steppe.

Shrubs play an important role in maintaining biodiversity, stability and ecological service in grassland. Exploring the effects of enclosure on dominant shrub population can provide scientific guidance for grassland restoration and tending management. In this study, we investigated main growth characteristics and spatial distribution pattern of Artemisia ordosica population in four enclosed grasslands with duration of 0, 5, 15, and 25 years. The results showed that population density increased first and then decreased with time extension, and peaked after enclosed for 15 years, which was 3.7 times that of unenclosed plot. The crown and projected area showed opposite responses trend to that of density, which decreased by 31.7% and 52.3% after enclosed 15 years, respectively. The height decreased by 25.3% after 5 years of enclosure, and then increased gradually. Semi-variance function analysis showed that population distribution in all grasslands conformed to Gaussian model. The spatial variation decreased gradually in the early stage of enclosure, and then increased after enclosed for 15 years. Structure ratio in each plot was higher than 0.75, but nugget was relatively small, indicating that spatial autocorrelation of population was mainly affected by structural factors rather than random factors. Spatial distribution of A. ordosica population was patchy and striped. Enclosure reduced spatial variation of population at small scale. However, spatial heterogeneity and scale dependence of population enhanced after enclosed 25 years as plaque dissociating. Our findings suggest that enclosure duration is the key factor affecting plant growth and spatial distribution of dominant population in desert steppe. Long-term fencing enhances the spatial heterogeneity of dominant population. Appropriate human intervention should be carried out after 15 years of enclosure.

Quaternary environmental changes in central Chukotka (NE Russia) inferred from the Lake El'gygytgyn pollen records

ABSTRACTThe 3.6‐Myr sedimentary record of Lake El'gygytgyn is crucial for understanding the response of the sensitive ecosystems in the Arctic to Quaternary climate variations at orbital timescales. In this study, we synthesize previously published pollen records and biome reconstructions and perform pollen diversity analysis of the deep‐drilling core ICDP 5011‐1 from Lake El'gygytgyn for periods during the Early Pleistocene (MIS 82 – MIS 79), Early–Middle Pleistocene (MIS 31 – MIS 18) and late Middle Pleistocene (MIS 7e – MIS 6f). The results indicate that the predominance of herb tundra in the regional vegetation was most characteristic during glacials/stadials. Interglacials, in contrast, can be distinguished by the expansion of shrub communities mainly composed of birch, alder and willow. The expansion of forest biomes in the region was influenced by peaks in obliquity values, which led to increases in daylight length, which was essential for plant growth in high latitudes. An apparent long‐term decreasing trend in the tree and shrub population, accompanied by a reduction in floristic richness, was induced by stepwise cooling and drying since the Mid‐Pleistocene Transition (MPT), which is linked to the modulation of extended global ice volume during the MPT via strong snow‐ and ice‐albedo feedback effects.

Stability and micro‐topographic effects of Sophora moorcroftiana population on a restored alluvial fan, Southern Tibetan Plateau

AbstractThe Tibetan Plateau, a vulnerable eco‐region undergoing global change, is hard to revegetate where land has become desertified due to the harsh habitat conditions. Sophora moorcroftiana shrubs, which are widely distributed in the middle reaches of the Yarlung Zangbo River basin of Tibet, China, have been naturally restored on desertified land during the last few decades. However, the influences of micro‐topography on the distribution and structure of the S. moorcroftiana population have been rarely reported. Here, we first describe the spatial variation of S. moorcroftiana population on a restored alluvial fan, and then explore the effects of micro‐topography on the restored S. moorcroftiana population, through a series of field surveys combined with the data collected by terrestrial laser scanning. The results indicate that the dominant age structure was adult shrubs (30–60 cm). Although the topographic effect had no significant impacts on soil organic matter and some other structural traits, which remained stable on the restored alluvial fan, it changed population density, plant growth, and distribution. The habitat conditions on the southwest aspect wapproximately 20°–25° slope with altitudes ranging between 3,593–3,643 m, conditions that mostly favouring the distribution of the shrub population. The micro‐topography played an important role in species distribution and growth of the S. moorcroftiana population. Our findings could help to understand the effect of micro‐topography on plant population structure and distribution for ecological restoration of desertified land in the Southern Tibetan Plateau.

Long‐term dynamics of shrub facilitation shape the mixing of evergreen and deciduous oaks in Mediterranean abandoned fields

Abstract Recovery of Mediterranean forests after field abandonment is a slow process, even without propagule limitations. This is mainly due to stressful conditions for seedling establishment. In this context, shrubs play a critical role in facilitating tree recruitment, but how this process unfolds after field abandonment is not entirely known. We evaluated the long‐term dynamics of facilitation by the nurse shrub Retama sphaerocarpa in the recruitment of two ecologically contrasting oaks, the evergreen Quercus ilex and the deciduous Quercus faginea. Thirty years after field abandonment, we dated shrubs and oaks established in an old field to estimate the annual recruitment rates and investigate temporal recruitment patterns. For oaks, we differentiated recruitment at two microsites: open areas or under shrub. To assess how nurse shrubs modulated environmental stressors, we modelled oak recruitment as a function of climatic variables. For the evergreen oak, we assessed these effects within each microsite. Finally, we estimated the annual interaction index between shrubs and evergreen oak juveniles as a function of climatic conditions. Each species showed different recruitment pulses during colonization. Recruitment rate was the highest for the shrub, followed by the evergreen oak. Oak colonization under shrubs was appreciable 20 years after field abandonment, when shrub cover reached 2.2%, and concentrated under medium and large shrubs older than 7 years. Shrubs not only accelerated evergreen oak colonization but also attenuated the fluctuations of recruitment pulses. For the evergreen oak, the interaction index indicated facilitation dominance in years with more arid summers (precipitation < 47 mm and heat waves longer than 8 days) and competition in wetter summers. Synthesis. Oak colonization in Mediterranean abandoned fields progressed slowly, and only two decades after abandonment pioneer shrub population grew to a level in size and number that could effectively trigger facilitation and accelerate tree recruitment. The shrub nurse effect was unbalanced between oak species, only benefiting the evergreen oak, and it was more prevalent in arid years. Our study illustrates the build‐up of shrub facilitation during forest recolonization and the varying nature of this process among climatically different years and ecologically distinct species. This information provides insights for assessing and managing Mediterranean forest recovery.

A probabilistic, biologically informed model of desert shrub population dynamics with the granularity appropriate for geomorphic simulations

AbstractAcross arid landscapes, desert shrubs affect where and how sediment is transported by various physical processes such as overland flow, wind, and rain splash. Simplistic biological models such as logistic growth curves offer important first steps towards representing and linking life to landscape dynamics. More sophisticated descriptions of desert shrub dynamics on scales commensurate with downslope sediment transport, however, are essential for more rigorously understanding how complex shrub‐sediment interactions may be affecting hillslope geomorphology. Here we present such a model that features a strong biophysical foundation by including, for example, basic aspects of desert soil‐water hydrology and population ecology such as recruitment, growth, and mortality. Model input parameters can also be modified to account for the influence of different environmental conditions and stressors (e.g. precipitation, soil types, droughts, grazing, fires, and climate change). Model behaviors mimic well documented aspects of how desert shrub populations respond to changes in precipitation, for example, productivity decreases with increasingly arid conditions and density declines during prolonged periods of drought. Model output (position and size of shrubs occupying a hillslope in a given year) represents the basic biological input variables necessary for calculating, for example, how rain‐splash induced mound building by individual shrubs may be affecting downslope sediment fluxes. Future research aimed at coupling this biological model with existing sediment transport models can therefore help advance our understanding for how desert shrub populations affect hillslope erosion across a broad range of scenarios. © 2018 John Wiley & Sons, Ltd.

A 150-year fire history of mulga (Acacia aneura F. Muell. ex Benth.) dominated vegetation in semiarid Queensland, Australia

Changes to fire regimes associated with European colonisation are implicated in declines in biodiversity and productivity in rangelands globally. However, for many areas there is incomplete knowledge of historical fire regimes and purported changes can become accepted wisdom with little empirical evidence. In the Mulga Lands of south-western Queensland, the dominant narrative implicates reduced fire frequency as a cause of woody vegetation thickening. We present a fire history of the Mulga Lands since pastoral exploration in the 1840s based on a review of explorer and early pastoralist journals, newspaper articles, interviews with long-term landholders and collation of satellite imagery. Fires in mulga communities are infrequent and only occur after at least two years of above-average summer rainfall. The assumption of regular pre-pastoral fires is not supported by available evidence. Since pastoral settlement in the 1860s, fire events affecting >1000 km2 have occurred seven times (1891–1892, 1904, 1918, 1950–1951, 1956–1957, 1976–1979 and 2011–2013), with only the 1950s fires affecting a >10% of the total area of mulga-dominated vegetation. We argue that fire is limited by fuel loads, which are in turn limited by rainfall events occurring only a few times a century. Even in the absence of grazing and active fire suppression fire intervals would be extremely long, perhaps 30–50 years in relatively fire-prone communities and much longer throughout most of the region. Combined with quantitative studies of fire and tree and shrub population dynamics, detailed fire histories will allow for more informed and nuanced debates about the role of fire in rangelands subject to abrupt management upheavals.

Effects of Sand Dune Stabilization on the Spatial Pattern of Artemisia ordosica Population in Mu Us Desert, Northwest China.

Vegetation patterns are strongly influenced by sand mobility in desert ecosystems. However, little is known about the spatial patterns of Artemisia ordosica, a dominant shrub in the Mu Us desert of Northwest China, in relation to sand fixation. The aim of this study was to investigate and contrast the effects of sand dune stabilization on the population and spatial distribution of this desert shrub. Spatial autocorrelation, semi-variance analysis, and point-pattern analysis were used jointly in this study to investigate the spatial patterns of A. ordosica populations on dunes in Yanchi County of Ningxia, China. The results showed that the spatial autocorrelation and spatial heterogeneity declined gradually, and the distance between the clustered individuals shortened following sand dune fixation. Seedlings were more aggregated than adults in all stage of dune stabilization, and both were more aggregated on shifting sand dunes separately. Spatial associations of the seedlings with the adults were mostly positive at distances of 0–5 m in shifting sand dunes, and the spatial association changed from positive to neutral in semi-fixed sand dunes. The seedlings were spaced in an almost random pattern around the adults, and their distances from the adults did not seem to affect their locations in semi-fixed sand dunes. Furthermore, spatial associations of the seedlings with the adults were negative in the fixed sand dune. These findings demonstrate that sand stabilization is an important factor affecting the spatial patterns of A. ordosica populations in the Mu Us desert. These findings suggest that, strong association between individuals may be the mechanism to explain the spatial pattern formation at preliminary stage of dune fixation. Sand dune stabilization can change the spatial pattern of shrub population by weakening the spatial association between native shrub individuals, which may affect the development direction of desert shrubs.

Population Density and Diversity of Plants Species in Ikot Efre Itak Community Forest, Akwa Ibom State, Nigeria

This study was aimed at assessing the diversity and population of plants species in Ikot Efre Itak Community forest in Nigeria. A total of 59 plant species were identified in the study area comprising 33 tree species, 12 shrub, 7 herbs, 34 climbers and 3 palms species. Mimusops heckelii and Canarium schweinfurthii, had the highest tree population density of 16 and 12 individual/0.6 ha respectively, all other tree species has a density of less than 10 individual/ha. In the shrub category, Bambusia vulgaris had the highest shrub population density with 31 individual/0.6 ha. Also, Costus afar and Hippocratea africana had the highest density of 53/0.6 ha and 11/0.6 ha respectively in the herb category. The result further shows that Ancistrophyllum secondiflorum had the highest population density of 23 in the palm category. Given the high rate of forest destruction in the country, there is need for to ensure sustainable conservation of the forest area to avoid further destruction by provision of alternative means of livelihood for the local population to reduce their dependence on these forest.

Livestock Grazing Impacts on Herbage and Shrub Dynamics in a Mediterranean Natural Park

Shrub encroachment can be explained by the abandonment of extensive livestock farming and changes to land use, and it is a common problem in the Mediterranean mountain pastures of Europe, with direct effects on biodiversity and landscape quality. In this paper, the effects of livestock exclusion vs. grazing on the dynamics of shrub and herbaceous vegetation were analyzed in a Spanish natural park located in a dry Mediterranean mountain area over a 5-yr period. Twelve 10 × 10m exclosures were set up in six representative pasture areas of the park (with two replicates per location). Each year, the shrub number, volume, and biomass were measured in April, and the herbage height, biomass, and quality were measured in April and December (which represent the start and end of the vegetative growth season). A sustained increase of the shrub population and individual biomass was observed throughout the study, which was reflected in total shrub biomass per ha. Growth was greater in nongrazed exclosures (2 563kg dry matter [DM] · ha−1 · yr−1), but it also happened in the grazed control areas (1 173kg DM · ha−1 · yr−1), with different patterns depending on the location and shrub species. Herbage biomass did not change when grazing was maintained, but it did increase in places where grazing was excluded (291kg DM · ha−1 · yr−1), mostly as a consequence of the accumulation of dead material, with a concomitant reduction in herbage quality. It was concluded that at the current stocking rates and management regimes, grazing alone is not enough to prevent the intense dynamics of shrub encroachment, and further reductions in grazing pressure should be avoided.

Shrub encroachment control by browsing: Targeting the right demographic process

Exploring key demographic parameters for establishment and growth of shrub populations enables us to identify the life stages or plant organs that browsers need to consume to effectively influence shrub population dynamics. However, there is a lack of concrete knowledge on how browsing can efficiently meet this goal. We therefore tested the impact of different levels of browsing intensity on a key demographic parameter (survival of juveniles) of a major colonizing shrub species (broom, Cytisus scoparius) in order to control population growth, and designed a browsing management strategy focused on this target stage. Three browsing intensities, representing pertinent management practices, were simulated on juveniles (2 years old) in a broom population. Juvenile broom plants were either left untouched (control treatment; n = 126) or had 50% or 90% of their total edible stem biomass removed for “light-intensity browsing” (n = 127) and “high-intensity browsing” (n = 77) treatments, respectively. Survival and fecundity data were collected over 6 years. Standard matrix modeling was used to analyze the impact of browsing on changes in population growth rate (λ), and the results were 6.34 (control), 2.26 (light browsing) and 0.85 (heavy browsing). Therefore, the natural expansion of broom populations may be slowed by light browsing or even reversed by heavy browsing (λ < 1). This multi-year survey confirms that focusing browsing on juveniles is an efficient strategy for controlling broom dominance. Shrub control strategies should therefore target early-growth-stage populations and repeat the browsing strategy at the same intensity over several years to achieve cumulative effects.

Temporary cropping in semi-arid shrublands increases native perennial grasses

Thickening of native shrubs is a major problem in many ‘semi-arid woodlands’ as significant increase in shrub density is often negatively correlated with herbaceous vegetation and leads to reduced pasture production and soil erosion. This project aimed to test the hypothesis that temporary cropping (up to three crops in 15 years) consistently increases the density of native perennial grasses following the removal of shrubs. A total of 30 paddocks that had been cropped during the last 20 years were randomly selected using a satellite-based database that documented annual clearing and cropping history from 1987 to 2003. Paddocks were classified into four types based on clearing and cropping history and grazing management – not cleared (shrubs), regrowth (re-invaded by shrubs), set stocked (cropped and grazed), light/rotationally grazed (cropped and grazed). The responses of vegetation and soil (chemical and physical) properties to clearing and cropping were evaluated. Results indicated that ground cover, native perennial grass cover and standing dry matter were highest under light/rotationally grazed conditions. The shrub state represents a stable state within the Cobar pediplain brought about due to land-use change in the form of overgrazing and/or the removal of fire from the system. An alternative stable state was achieved as a result of disturbance in the form of clearing, cropping and grazing management thereby directly altering the shrub population. The resilience of this state is largely dependent on the grazing management system used and on the prevention of shrub from re-establishing while failure to control shrubs could lead to the re-emergence of the Shrub State. We conclude that native grasslands do regenerate following cropping after removal of shrubs. The importance of grazing management for restoring perennial ground cover following removal of shrubs and temporary cropping has been clearly demonstrated by the study.

Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia

*Shrub expansion in alpine and arctic areas is a process with possibly profound implications for ecosystem functioning. The recent shrub expansion has been mainly documented by remote sensing techniques, but the drivers for this process largely remain hypotheses. *Here, we outline a dendrochronological method, adapted to shrubs, to address these hypotheses and then present a mechanism for the current shrub expansion by linking recent climate change to shrub growth performance in northern Sweden. *A pronounced increase in radial and vertical growth during recent decades along an elevational gradient from treeline to shrubline indicates an ongoing shrub expansion. Age distribution of the shrub population indicates the new colonization of shrubs at high elevations. *Shrub growth is correlated with warm summers and winter snow cover and suggests the potential for large-scale ecosystem changes if climate change continues as projected.

Forest structure and regeneration along the altitudinal gradient in the Binsar Wildlife Sanctuary, Uttarakhand Himalaya, India

Forest structure and regeneration were studied along the altitudinal gradient in the Binsar Wildlife Sanctuary of Uttarakhand Himalaya in India. Stratified random sampling of tree species was done by placing minimum 15 quadrats of 10 × 10 m at each aspect and altitude. The results reveal that along the altitudinal gradient there were three types of forest communities in the Sanctuary viz., chir pine (Pinus roxburghii), oakchir pine (Quercus leucotrichophora and Pinus roxburghii) and oak (Quercus floribunda and Quercus leucotrichophora). Rhododendron arboreum, Cornus macrophylla and Lyonia ovalifolia were main associates of these forest communities. The oak forests had larger shrub population as compared to chir pine. Myrsine africana was the most dominant shrub across the altitudinal gradient, whereas few shrub species were restricted to a certain altitudinal range. The regeneration of chir pine was best at lower altitude on south and east aspects, which indicates that it mostly regenerates on warm and dry slopes. In general, the regeneration potential in most of the tree species declines with the altitude. The density of saplings and seedlings also represented the dominant species at each altitudinal range, which indicates the cyclic regeneration of forests in the Sanctuary area.

Demographic analysis of a dominant shrub (Cytisus scoparius): Prospects for encroachment control

Shrub encroachment in pastoral areas subsequent to land-use changes has become a major problem for biodiversity and pastoral resource conservation. Controlling encroachment by grazing is the most promising prospect at this time, but the impact of small domestic ruminants on the population dynamics of dominant shrubby species is still poorly understood. To control the rapid densification of dominant shrub populations means that we have to focus on the early stages of shrub colonisation. In this paper, we present a modelling approach that includes deterministic matrix modelling to understand demography strategy at the very beginning of shrub population growth and to identify potential targeted life stages for control. We collected demographic data over a period of 5 years on the European dominant shrub, Cytisus scoparius (Scotch broom; Fabaceae), which is often part of the heathlands that establish themselves on former permanent grasslands in Europe. Our results reveal a very high variation in flowering frequency between years and a large contribution of the seed bank to the total number of seedlings that emerge each year. Modelling shows that population growth is very vigorous and is not stopped by repeated “bad years” for seed production. Sensitivity analyses indicate that early life stage survival (seedling, juvenile) has the greatest impact on population growth. These results make it possible to identify juveniles as one of the best targets for control since they combine a high degree of sensitivity in terms of population demography, good palatability for domestic herbivores and easy accessibility within a management schedule.

Why do some species in arid lands increase under grazing? Mechanisms that favour increased abundance of Maireana pyramidata in overgrazed chenopod shrublands of South Australia

AbstractWhile the abundance of some plant species decreases under high grazing intensity, others become more abundant. Release from competition by decreaser species contributes to this pattern in mesic systems, but this may not be the case in xeric systems where competition may be less intense. Here we examine three mechanisms that may be involved: (i) increased recruitment and growth because of soil changes produced by grazing, for example, increased soil nutrient availability through dung accumulation; (ii) increased recruitment favoured by the breaking up of the lichen crust; and (iii) reduced competition because of the decline of decreaser species. We used field and glasshouse experiments to determine the possible contribution of these mechanisms to the increase of the chenopod Maireana pyramidata around a watering point in a chenopod shrubland of South Australia. There was no evidence of nutrient accumulation close to the watering point, and while seedlings of M. pyramidata responded to nutrient addition, their growth was the same in soil collected from areas with different grazing intensity. While a broken lichen crust increased the emergence of both M. pyramidata and the decreaser Atriplex vesicaria, the effect was larger for the former. We found no competition between seedlings of the two species or between juveniles of A. vesicaria and seedlings of M. pyramidata in glasshouse experiments. Adult plants of both A. vesicaria and M. pyramidata produced similar growth reduction in seedlings of M. pyramidata. Furthermore, a field removal experiment failed to detect any competitive effect of A. vesicaria on M. pyramidata. Our data indicate that the disintegration of the soil crust by grazer activities can be a major factor controlling floristic changes in overgrazed rangelands. These results imply that factors that control establishment may be more important than competition in shaping shrub population dynamics in these systems. Ground surface itself can affect establishment opportunities, and this should be taken into account in management and restoration efforts in arid lands.

Deciduous shrubs for ozone bioindication: Hibiscus syriacus as an example

Ozone-like visible injury was detected on Hibiscus syriacus plants used as ornamental hedges. Weekly spray of the antiozonant ethylenediurea (EDU, 300 ppm) confirmed that the injury was induced by ambient ozone. EDU induced a 75% reduction in visible injury. Injury was more severe on the western than on the eastern exposure of the hedge. This factor of variability should be considered in ozone biomonitoring programmes. Seeds were collected and seedlings were artificially exposed to ozone in filtered vs. not-filtered (+30 ppb) Open-Top Chambers. The level of exposure inducing visible injury in the OTC seedlings was lower than that in the ambient-grown hedge. The occurrence of visible injury in the OTC confirmed that the ozone sensitivity was heritable and suggested that symptomatic plants of this deciduous shrub population can be successfully used as ozone bioindicators. EDU is recommended as a simple tool for diagnosing ambient ozone visible injury on field vegetation.

SATCHMO: A spatial simulation model of growth, competition, and mortality in cycling savanna patches

Many mechanisms have been suggested to explain the coexistence of woody species and grasses in savannas, yet, evidence from field studies and simulation models has been mixed. Shrub encroachment is an ecological and economic problem in savannas worldwide which generally is attributed to overgrazing. Patch-dynamics is a new mechanism explaining tree-grass coexistence and the natural occurrence of shrub encroachment in savannas. A patch-dynamic savanna consists of patches in which cyclical succession between grassy and woody dominance proceeds spatially asynchronously. The spatially explicit, individual-based patch-scale simulation model SATCHMO was developed to investigate cyclical succession in the paradigm of patch-dynamics for arid and semi-arid savannas. SATCHMO is designed to capture within-patch shrub population dynamics based on a grid of 51 m side length and a resolution of 10 cm. The model shrub characteristics were derived from Acacia mellifera, the main encroaching species in African savannas. The aim of SATCHMO is to give a detailed small-scale understanding of above- and belowground growth, competition, and mortality of savanna woody plants and the influence of precipitation and fire on patch transition frequencies, shrub growth rates, and shrub size frequencies. With SATCHMO, we want to identify the conditions leading to cyclical successions in general and shrub encroachment in particular. Soil moisture is the most important parameter in SATCHMO influencing growth, reproduction, and mortality of shrubs and grass tufts, and that mediates competition. To acknowledge the importance of belowground interactions in savannas, shrub root growth and competition are modelled spatially explicitly. The model output was successfully validated with morphometrical and spatial data from the field site in the South African Kalahari thornveld and with recent literature data on savanna woody species cover. Global sensitivity analysis with Latin hypercube sampling shows that soil moisture is the most important driver of shrub cover dynamics in semi-arid savannas.

Landscape ecology of the burrowing bettong: fire and marsupial biocontrol of shrubs in semi-arid Australia

Prior to European settlement, medium-sized marsupials, especially bettongs (Bettongia spp.), were widely distributed across arid and semi-arid Australia. Most disappeared rapidly in the late 1800s in the earliest settled rangelands such as the West Darling region of western New South Wales following the spread of domestic herbivores, rabbit invasion, exotic predators and loss of habitat. Because the burrowing bettong (Bettongia lesueur) is the only fossorial macropod species, it left a clearly visible record of its past presence, distribution and habitat preferences in the form of substantial relict warrens, particularly in stony, ‘hard-red’ habitats. With the reduction in fire frequency because of excessive grazing pressures following European settlement in the 19th century, there was a rapid increase in the density of unpalatable native shrubs. We examine the hypothesis that periodic wildfires and browsing by bettongs were together able to regulate shrub densities in semi-arid rangelands in Australia. Information from various sources concerning the effects of fire, rainfall and browsing on the demography of shrubs was used to construct a model of shrub population dynamics. The model indicates the potential for two states for a given bettong density: first, a low shrub density maintained by a combination of periodic fire and bettong browsing; and second, a high shrub density in the absence of fire. These results have broad implications for pastoral and conservation management in Australian semi-arid rangelands.

Shrub age structure in northern Minnesota aspen stands

Age data from hazel (Corylus spp.) and total shrub populations of seven aspen stands in northern Minnesota were used to investigate two age-density distribution models: a negative exponential model and a power function model. The negative exponential model, implying a constant mortality rate for all age shrubs, is the better model for describing population dynamics of both hazel and total shrub populations in the study area. The mortality rate within a stand was independent of stem density and age, decreased when overstory basal area increased, and increased as site quality increased. The mortality rate of hazel increased with light availability, but the total shrub population mortality rate decreased with increasing light availability. The regeneration rate of new aerial stems differed among the sampled stands, indicating its importance in regulating shrub dynamics. The regeneration rate was negatively related to overstory basal area, and positively related to light availability and site index. Hazel regeneration was also positively related to overstory age, implying more regeneration of aerial stems in older stands. Coefficients of the negative exponential shrub age structure relationships are estimated from overstory characteristics and site conditions.