Overstorey Cover Research Articles

The influence of buffel grass (Cenchrus ciliaris) on biodiversity in an arid Australian landscape

Buffel grass [Cenchrus ciliaris L. syn. Pennisetum ciliare (L.) Link] is an exotic species that has been widely planted in Australian arid and semi-arid grazing lands, and has become an important resource for livestock. It establishes readily and has expanded into such a diversity of land types beyond grazing lands that it is also regarded as a serious environmental weed. Although there is an abundance of literature on the production benefits of buffel grass, there is relatively little about its influence on native flora and fauna in arid Australia, particularly when its cover levels are low. This study attempted to clarify the influence of buffel grass and environmental patterns on the occurrence of ground vegetation, birds, reptiles and ants in a gneissic hill habitat in central Australia where buffel grass has encroached. Despite poor conditions for growth, we were able to distinguish the influence of buffel grass from that of other variables like overstorey cover, soil pH, fire and transect orientation. Cover of buffel grass did not exceed ~20% but it still accounted for a small amount of the variation in the composition of ground vegetation and birds, and of the ‘ground-dwelling’ bird guild and the ‘hot climate specialist’ functional group of ants. There were insufficient reptiles for analysis. We conclude that, even when cover is low, buffel grass can have a detectable influence on some aspects of community dynamics. Given the evidence from published literature and from this study, we expect the influence of buffel grass on the diversity of native flora and fauna to increase, particularly if buffel grass expands into land types previously thought unsuited to its environmental needs.

Biodiversity benefits of alley farming with old man saltbush in central western New South Wales

Agricultural production systems that also provide opportunities to conserve biodiversity will be a crucial component of integrated and sustainable land use in mixed farming landscapes and should be considered and evaluated. Alley farming is an innovative farming system that aims to increase farm profitability while also enhancing environmental outcomes. Alley farming incorporates belts of woody perennial plants such as trees or shrubs, interspersed with alleys of conventionally rotated cropping and livestock grazing land. In the present study, we assessed the impacts on terrestrial biodiversity of alley farming with the native perennial chenopod shrub old man saltbush (Atriplex nummularia Lindl.) in central western New South Wales. Terrestrial biodiversity conservation status was assessed by site surveys conducted in spring 2005, 2006 and 2007 at 15 old man salt bush alley farming sites (OMSB), 15 conventionally managed sites and three native woodland remnants in and around the Condobolin Agricultural Research and Advisory Station in the central western plains of New South Wales. Biodiversity surveys included an assessment of ‘site condition’ – a metric of biodiversity conservation status at the site scale based on measurement of 10 habitat and vegetation condition attributes, compared against benchmark values for the appropriate native ecosystems with relatively little recent anthropogenic modification. Bird surveys were also conducted to assess the diversity and abundance of birds in OMSB, conventional and remnant woodland sites in four functional response groups. Site condition was significantly higher at remnant woodland sites than at conventional farming and OMSB alley farming sites. Remnant woodland sites had greater native overstorey cover and native ground cover of forbs, more trees with hollows, presence of at least some overstorey regeneration and the presence of fallen logs. Site condition was also significantly higher at OMSB sites than at conventional sites and increased significantly across 3 years. By the third year after establishment, OMSB sites had higher native plant species richness and native mid-storey cover than did conventionally farmed sites. These attributes increased markedly over time at the OMSB sites whereas they did not increase at conventional or remnant woodland sites. Native grasses and forbs established under and around the saltbush plants, indicating that OMSB alley plantings can provide habitat for a wide range of native plant species, enhancing biodiversity values of these areas through improved structure and composition. Improved habitat condition at the OMSB sites after 3 years did not lead to a significantly higher diversity or to a higher overall abundance of birds at the OMSB than at conventional sites. Furthermore, diversity and abundance of birds at both OMSB and conventional sites remained significantly below those of remnant woodland sites. Some decliner bird species were observed using OMSB sites, but not conventional sites. Old man saltbush alley farming can provide direct on-site benefits for native biodiversity by improving the structure, function and composition of vegetation at the site or paddock scale. If proposed as a replacement to conventional crop–pasture rotation, OMSB alley farming can enhance biodiversity conservation values, and where production benefits are likely, could play an important role in the integration of production and conservation as a synergistic ‘win–win’ system in mixed farming enterprises.

Forest canopy health and stand structure associated with bell miners (Manorina melanophrys) on the central coast of New South Wales

Summary Forest attributes including canopy health and stand structure were visually assessed to identify features commonly associated with the bell miner (Manorina melanophrys, Meliphagidae), a cooperatively breeding, insectivorous honeyeater endemic to south-eastern Australia. A stratified random sampling methodology, using a combination of SPOT5 satellite imagery and a 25-m pixel digital altitude model, was used to establish 130 circular plots 40 m in diameter within a 12800 ha study area in the Watagan mountain range, central coast of New South Wales (NSW). In this study site, the presence of bell miners was significantly associated with unhealthy eucalypt crowns. This supports the proposition that a dieback syndrome known as bell-miner-associated dieback (BMAD) exists in central coastal forests of NSW. Our results also demonstrate a strong association of a sparse eucalypt canopy over a dense lower midstorey with the presence of bell miners. Altitude and fire frequency were negatively correlated with bell miners while the topographical wetness index was positively correlated with bell miners. Logistic regression was used to quantify the effects of a series of explanatory variables including tree species, forest structure variables and selected topographic variables. Although there were significant differences in the ranking of crown condition between eucalypt species, tree species was not selected in the final model. The final variables identified by the logistic regression for the presence of bell miners included sparse eucalypt overstorey cover, dense lower midstorey cover, the absence of young vigorous regrowth trees and higher values of the topographic wetness index. Based on the results of our survey we discuss practical options for managing the forest dieback syndrome associated with bell miners in the central coast region of NSW and an approach to mapping forests at risk from BMAD.

The roles of habitat features, disturbance, and distance from putative source populations in structuring alien plant invasions at the urban/wildland interface on the Cape Peninsula, South Africa

Natural areas are becoming increasingly fragmented and embedded in an urban matrix. Natural and semi-natural areas at the urban/wildland interface are threatened by a variety of ‘edge effects’, and are especially vulnerable to invasion by introduced plants, with suburban gardens acting as significant sources of alien propagules. Urban/wildland interfaces also provide access for humans, leading to various types of disturbance. Alien plant invasions are one of the biggest threats facing remaining natural areas on the Cape Peninsula, South Africa. The area provides an ideal opportunity to study the dynamics of invasions at the urban/wildland interface, since the largest natural area, the Table Mountain National Park (TMNP), is surrounded by the city of Cape Town. We explored invasion patterns in Newlands Forest (a small section of the TMNP) and detailed the roles of habitat features and distance from putative source populations in three main habitat types: natural Afromontane forest, riverine woodland habitats, and plantations of exotic pines ( Pinus radiata and P. pinaster). We also examined the role of disturbance in driving invasions in two of these habitat types (Afromontane forest and pine plantations). We hypothesized that alien richness and alien stem density would decrease with distance from the urban/wildland interface, and that alien richness and alien stem density would increase with increasing levels of human disturbance. Distance from putative source populations and levels of anthropogenic disturbance influenced alien richness in Newlands Forest but not alien stem density. Alien richness decreased significantly with distance from presumed sources in the pine habitat, and increased significantly with disturbance in the forest habitat. Percentage overstorey cover and soil pH were important environmental variables associated with alien plant species. A socio-economic approach is discussed as being the most effective approach to the management and prevention of alien plant species in Newlands Forest.

Applicability of landscape and island biogeography theory to restoration of riparian understorey plants

Summary Ecosystem recovery is influenced by processes at different spatial scales, yet land managers lack specific predictions on the relative importance of such processes that might guide management decisions. We tested whether ideas from landscape ecology (local vs. landscape scales) and island biogeography theory (patch size and isolation) predict restoration success for understorey plant communities in a highly fragmented riparian landscape, in an effort to provide guidance on how to allocate scarce restoration resources. We sampled naturally colonizing riparian forest understorey plant communities in 15 riparian forests restored by planting native woody species along a 150‐km stretch of the Sacramento River in central California. We analysed native and exotic understorey species richness and cover as a function of biotic and abiotic local and landscape variables. Cover and species richness of exotic understorey plants decreased strongly with increasing overstorey cover, and were lower in quadrats closer to river base flow. Native understorey species richness and cover were negatively related to exotic cover and positively related to connectivity with remnant forest. Cover of native wind‐dispersed species was best explained by higher percentage forest cover surrounding a site within a 1000‐m buffer, whereas cover of native water‐dispersed species was higher closer to the river. Neither patch size nor time since restoration explained a significant amount of native or exotic species richness or cover. Synthesis and applications. Local factors explained more of the variance in understorey plant communities, but much of the variance remained unexplained. Our results provide weak support for the predictions of island biogeography theory and the importance of landscape‐scale variables. These theories did not have strong predictive power in this applied restoration context at this temporal scale. Given limited resources, efforts to restore understorey plant communities in this highly fragmented system should focus on local‐scale restoration methodologies, such as increasing cover of native overstorey species and reducing cover of exotic plants.

Influences of deer browsing, coppice history, and standard trees on the growth and development of vegetation structure in coppiced woods in lowland England

In parts of England areas of woodland previously coppiced are being brought back into active coppice management at a time when deer populations are increasing, and after decades with an absence of management. This paper examines the effects of overstorey canopy cover, coppice restoration, and deer browsing on the growth, and structure of the coppice and other understorey vegetation. At 11 coppice woodlands in lowland England, coppice compartments were categorised based on their coppicing history (restored versus continuous), low and high overstorey canopy cover, and low and high deer browsing levels. Gross vegetation structure in the years following harvesting was also examined. Deer browsing was assessed from signs of structural browsing damage and presence of deer. Deer browsing and overstorey cover reduced the density of the understorey. There was also a weak effect of coppice history with restored coppice exhibiting less vigorous vegetation than continuous coppice. Deer browsing damage was more pronounced in compartments with a low overstorey canopy cover, possibly due to the more luxuriant understorey coppice re-growth. Browsing damage is consistent with that expected at low to moderate levels of potential browsing intensity. Future restoration of coppice and growth of stools after coppicing is likely to be successful and worthwhile only if active measures are taken to reduce deer browsing damage and possibly deer populations as a whole.

Corrigendum to: Assessing woody vegetation cover change in north-west Australian savanna using aerial photography

Models to calibrate tree and shrub cover assessed from aerial photography with field measurements were developed for a range of vegetation types in north-western Australia. The models verify previous studies indicating that woody cover can be successfully determined from aerial photography. The calibration models were applied to estimates of woody vegetation cover determined for 279 randomly located sample areas in the Ord–Victoria Rivers region using aerial photography from 1948 to 1950 and 1988 to 1997. Overstorey cover increased from a regional average of 11.5% to 13.5% and understorey cover increased from 1.3% to 2.0%. Downs, Limestone Hills and Alluvia land-types showed the most substantial increases in overstorey cover while overstorey cover in the Limestone plains land-type decreased. Relatively open structured vegetation is most susceptible to thickening. Rainfall records reveal an extreme multi-year rainfall deficit in the study area in the 1930s and relatively wet times in the 1970s and 1980s. Interpretation of a limited set of aerial photographs taken between 1964 and 1972 suggests that most of the increases in cover have occurred since this time. The study highlights the possibility that the average trend of vegetation thickening represents recovery during the relatively wet times after the 1970s. There was no relationship between structural change and a grazing intensity surrogate (distance of sample points to stock watering-points). However, the causes of structural change are undoubtedly multi-factored and the relative contributions of climate, fire and grazing vary for different landscapes and tree species.

Translocation of an imperilled woodrat population: integrating spatial and habitat patterns

AbstractMany species have strong habitat preferences that directly influence population viability. For successful reintroduction of threatened populations that rely on habitat structures, the correct placement of artificial structures is also important to population persistence. In this paper, we present a hierarchical approach to the problem of translocating animals that rely on permanent habitat structures, in which we first use population dynamics data to identify areas of suitable habitat, and then identify optimal configuration for habitat structures. We use data collected from a non‐endangered, conspecific population of the endangered riparian woodrat (Neotoma fuscipes riparia) to examine the degree to which the distribution of dens in translocation sites might influence the likelihood that animals persist in their new environment. We characterize the habitats in which dens occur, analyze their spatial clustering, and compare them to temporal changes in population status for sex and age classes. We compare the potential efficacy of translocation efforts using spatial analysis versus solely habitat‐based approaches and identify the optimal spatial configuration of dens that should be considered in this translocation effort. We found that patterns of habitat use were positively correlated with overstorey cover, and animal weight was positively correlated with understorey cover. Woodrats appear to select den locations on the basis of understorey cover, but also benefit from dense overstorey cover and distance to nearest tree. Our results suggest that in translocation efforts, artificial dens should be placed in clusters within a radius of 15 m, as values above and below this value showed negative correlations with body mass. Translocations should occur after reproductive events, which occur in April and August for woodrats in southern California. Our analyses provide practical guidelines in determining appropriate timing and spacing for translocation events in the context of animal condition, minimizing disease transmission, and reproduction.

Assessing woody vegetation cover change in north-west Australian savanna using aerial photography

Models to calibrate tree and shrub cover assessed from aerial photography with field measurements were developed for a range of vegetation types in north-western Australia. The models verify previous studies indicating that woody cover can be successfully determined from aerial photography. The calibration models were applied to estimates of woody vegetation cover determined for 279 randomly located sample areas in the Ord–Victoria Rivers region using aerial photography from 1948 to 1950 and 1988 to 1997. Overstorey cover increased from a regional average of 11.5% to 13.5% and understorey cover increased from 1.3% to 2.0%. Downs, Limestone Hills and Alluvia land-types showed the most substantial increases in overstorey cover while overstorey cover in the Limestone plains land-type decreased. Relatively open structured vegetation is most susceptible to thickening. Rainfall records reveal an extreme multi-year rainfall deficit in the study area in the 1930s and relatively wet times in the 1970s and 1980s. Interpretation of a limited set of aerial photographs taken between 1964 and 1972 suggests that most of the increases in cover have occurred since this time. The study highlights the possibility that the average trend of vegetation thickening represents recovery during the relatively wet times after the 1970s. There was no relationship between structural change and a grazing intensity surrogate (distance of sample points to stock watering-points). However, the causes of structural change are undoubtedly multi-factored and the relative contributions of climate, fire and grazing vary for different landscapes and tree species.

Quantitative assessment of vegetation structural attributes from aerial photography

Cover of vegetation understorey and overstorey was determined from aerial photography at 1:25 000 and 1:40 000 scales by a grid sampling technique. Models were developed relating values of aerial cover to field cover as determined by intensive field measurement. The influence of photo-scale, photo colour, the angle of the image, shadow, the hiatus between aerial and field sampling, crown width, crown height, proportion of dead trees, drought prior to aerial sampling, land type, previously cleared vegetation and incline on explanatory models was also examined. The only variables that could be clearly interpreted as influencing the models were vegetation height, photo-scale and land type. Only the latter two variables are useful for predictive models. The smaller the scale of photography the greater the exaggeration of the aerial image of tree crowns. This probable result of photo graininess would be most significant when tree crowns are small, an inverse surrogate of tree height. Two-phase models were developed for predicting basal area and biomass from aerial cover. In most instances models were successful for predicting overstorey and understorey cover and for predicting total basal area and biomass. The technique offers a powerful and cost-effective method of assessing vegetation change over long time periods in a way that no other technique can duplicate.

Small Mammals in Farm Woodlands: The Effects of Habitat, Isolation and Surrounding Land-Use Patterns

1. The results of this study, which surveyed 38 farm woods ranging in size from 0.05 to 4.45 ha, demonstrated clearly that the dynamics of wood mice Apodemus sylvaticus and bank vole Clethrionomys glareolus populations in small woods are strongly influenced by the landscape around the woods, for example the distance to neighbouring woods, abundance of hedges and types of crops grown, as well as the habitat within the wood. 2. In terms of the habitat features of the woods, the maturity of the wood and the amount of understorey and overstorey cover were important for both species, while the availability of herbaceous plants influenced bank vole abundance. 3. Wood mice, in particular, were influenced by the types of crops grown in neighbouring fields. Their abundance in spring, the proportion of juveniles in the autumn and the extent to which the population increased from spring to autumn were all influenced by the proportion of adjoining fields that were planted with wheat and/or oilseed rape. 4. Isolation variables, such as the distance to the nearest large wood and the extent of adjoining hedgerows, influenced both wood mice and bank vole populations. Populations of both species increased more from spring to autumn in more isolated woods. This was probably because of reduced opportunities for dispersal and the shortage of alternative woodland habitat for animals moving off the fields after harvest. Woods well connected with hedges supported higher densities of both wood mice and bank voles. 5. Within the size range of woods examined here, no effect of woodland size was found on the probability of catching wood mice or bank voles or their abundance in either spring or autumn. The data suggest that even very small farm woods are important as refuges for small mammals, particularly after harvest when arable fields provide little cover.

Population fragmentation may reduce fertility to zero in Banksia goodii - a demonstration of the Allee effect.

All individuals of all known populations of Banksia goodii were assessed for seed production. Small populations produced no or only a few seeds per unit canopy area. Effects of population size on seed production per unit area and seed production per plant were present over the whole range of population sizes, indicating that even in large populations seed production may still not be at its maximum. Resource differences could not explain this disproportionate decrease in seed production with decline in population size, because there were no differences in soil properties and understorey or overstorey cover between the small and large populations. Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations. This was not because plants in small populations produced fewer cones but because the fraction of these cones that was fertile was much lower. Five of the nine smallest populations (<200 m2) produced no fertile cones over the last 10 years. The number of seeds per fertile cone did not depend on population size. The results are discussed in relation to pollination biology.

Species Richness of Sclerophyll (Heathy) Plant Communities in Australia ̵2 the Influence of Overstorey Cover

The species richness (number of vascular-plant species per unit area) of sclerophyll (heathy) plant communities is examined from south-east Queensland to south-west Western Australia. The species richness of communities of heathy open forest, heathy open scrub, dry heathland and wet heathland is consist- ently similar throughout southern Australia and decreases from dry heathland (on laterite, coastal and inland localities) to heathy open forest, heathy open scrub and wet heathland. Investigation of related microcommunities at Cooloola, Stradbroke Island, Ku-ring-gai Chase and Wilsons Promontory indicates that species richness decreases linearly as overstorey cover increases. In post-fire succession on Stradbroke Island heathy woodland and Dark Island heathland, species richness declines linearly as overstorey cover increases during the regeneration of the community. The appli- cation of limiting fertiliser to Stradbroke Island heathy woodland and Dark Island heathland increases the rate of development of overstorey cover, with a simultaneous decrease in species richness. Species richness of the understorey strata of plant communities appears to be inversely related to the rate of development of foliage projective cover in the overstorey. If an environmental or biotic factor inhibits or retards the development of overstorey cover, the understorey increases in species richness. Conversely, if any environmental or biotic factor accelerates the development of overstorey cover, the understorey species show a reduction in species richness.

Recovery of vegetation cover following logging and regeneration in the southern forest of Western Australia and the short term hydrologic implications

Summary A survey was conducted in 1986 in a series of stands of different ages in the lower rainfall jarrah (Eucalyptus marginata) forest ( 1100 mm) and karri (E. diversicolor) forest (> 1000 mm), in the southern forest of Western Australia, in order to assess the recovery of vegetation cover as a function of time since regeneration. The data were then used to estimate the time required for the water balance of regenerating stands to return to the pre-logging state. Overstorey cover and total cover were assessed. In both types of jarrah forest, both overstorey cover and total cover exceeded 80 per cent of the value for unlogged stands within five years of regeneration, and had exceeded 90 per cent within 10 years. They were very close to the value of unlogged stands in about 20 years, and thereafter remained at that value. In karri forest, total cover reached the value for unlogged stands five years after the beginning of regeneration, rose fo...

The balance between the foliage projective covers of overstorey and understorey strata in Australian vegetation

Foliage Projective Cover of the overstorey (canopy) of a‘climax’community appears to reach an equilibrium value determined largely by the prevailing climate. Overstorey FPC decreases in‘climax’communities in a graded series from humid to arid regions. Understorey cover (of all strata below the canopy) in‘climax’communities attains a balance with overstorey FPC. Disturbance (gaps, microhabitats, fire, overgrazing, invasion of woody weeds, etc.) may reduce the overstorey cover which will be compensated by an increase in understorey cover. Secondary succession back to the‘climax’structure will follow a path maintaining an inverse linear relationship between understorey cover and overstorey cover. At the same time, species diversity appears to decrease as overstorey cover increases.