Woodland Clearings Research Articles

The Alluvial Geoarchaeology of the Upper River Kennet in the Avebury Landscape: a Monumental Transformation of a Stable Landscape

Geoarchaeological research as part of the AHRC funded Living with Monuments (LwM) project investigated the upper Kennet river system across the Avebury World Heritage landscape. The results demonstrate that in the early–mid-Holocene (c. 9500–1000 bc) there was very low erosion of disturbed soils into the floodplain, with floodplain deposits confined to a naturally forming bedload fluvial deposit aggrading in a shallow channel of inter-linked deeper pools. At the time of the Neolithic monument building in the 4th–early 3rd millennium bc, the river was wide and shallow with areas of presumed braid plain. Between c. 4000 and 1000 bc, a human induced signature of soil erosion became a minor component of fluvial sedimentation in the Kennet palaeo-channel but it was small scale and localised. This strongly suggests that there is little evidence of widespread woodland removal associated with Neolithic farming and monument building, despite the evidently large timber requirements for Neolithic sites like the West Kennet palisade enclosures. Consequently, there was relatively light human disturbance of the hinterland and valley slopes over the longue durée until the later Bronze Age/Early Iron Age, with a predominance of pasture over arable land. Rather than large Neolithic monument complexes being constructed within woodland clearings, representing ancestral and sacred spaces, the substantially much more open landscape provided a suitable landscape with areas of sarsen spreads potentially easily visible. During the period c. 3000–1000 bc, the sediment load within the channel slowly increased with alluvial deposition of increasingly humic silty clays across the valley floor. However, this only represents small-scale landscape disturbance. It is from the Late Bronze Age–Early Iron Age when the anthropogenic signal of human driven alluviation becomes dominant and overtakes the bedload fluvial signal across the floodplain, with localised colluvial deposits on the floodplain margins. Subsequently, the alluvial archive describes more extensive human impact across this landscape, including the disturbance of loessic-rich soils in the catchment. The deposition of floodplain wide alluvium continues throughout the Roman, medieval, and post-medieval periods, correlating with the development of a low-flow, single channel, with alluvial sediments describing a decreasing energy in the depositional environment.

Grasslands of cleared woodlands have lower invertebrate diversity and different assemblages to remnant woodlands in grazed landscapes of eastern Australia

Clearing of woodlands is used by graziers to promote pasture production, even though understanding of impacts of clearing on native fauna is lacking. We evaluate impacts of clearing on biodiversity by comparing invertebrates associated with ground-layer vegetation of cleared woodlands (grasslands) to that of nearby uncleared woodlands. Two replicates of grasslands consisting of pastures dominated by introduced grasses were compared with two woodlands at each of four locations. The adjacent riparian forest to each grassland and woodland site allowed evaluation of the effect of woodland clearing on the adjacent riparian zone. All habitats were grazed. Invertebrates of ground-layer vegetation were sampled using three suction subsamples of 1m2 at each site. Grasslands had significantly lower order richness and abundance of herbivores, pollinators and macroinvertebrates (food for birds) than the woodlands, whereas the riparian forests closely resembled each other in all metrics. Invertebrate assemblages of grasslands also differed significantly from those of the woodlands. BEST analysis showed that groundcover and leaf-litter percentage cover correlated strongest with invertebrate composition. This study has demonstrated that grazing management relying on clearing of fertile grassy woodlands of the rangelands of Central Queensland alters invertebrate diversity and assemblage. Thus, tree clearing not only leads to biodiversity losses in the canopy layer, but also in the ground-layer vegetation.Implications for insect conservationPastoralists have the capacity to improve outcomes for invertebrate biodiversity by maintaining groundcover (ground-layer vegetation and litter cover) above 80%, by encouraging native pastures over introduced species such as Buffel Grass and by retaining native woodlands.

Vegetation impacts and early Neolithic monumentality: A palaeoenvironmental case study from south-west Ireland

New insights into prehistoric vegetation and human activities around a megalithic monument were inferred from a palaeoecological study at Arderrawinny, south-west Ireland. An early Neolithic portal tomb located c.500 m north-east of a small mire was investigated for pollen and loss on ignition analyses. This investigation provides a detailed reconstruction of the local vegetation development for the area surrounding the megalithic tomb, supported by 12 AMS 14C measurements and Bayesian modelling of palaeoecological and archaeological data. The palaeoecological record indicates a degree of ‘openness’ which was unusual in pre-Neolithic contexts in Ireland. Bayesian modelling of palaeoecological and archaeological chronologies intimates that evidence for woodland clearings preceded the construction of the megalith and suggests spatial separation between settlement, cultivation and ‘ritual’ activity in the early Neolithic. Despite low evidence for Neolithic agricultural, a potential correlation between human activity, as indicated by the presence of the nearby early Neolithic portal tomb, and the onset of the mid-Holocene ‘Elm Decline’ in the region is suggested.

A comparative review of soil charcoal data: Spatiotemporal patterns of origin and long-term dynamics of Western European nutrient-poor grasslands

The nutrient-poor grasslands of Western Europe are of major conservation concern because land use changes threaten their high biodiversity. Studies assessing their characteristics show that their past and on-going dynamics are strongly related to human activities. Yet, the initial development patterns of this specific ecosystem remain unclear. Here, we examine findings from previous paleoecological investigations performed at local level on European grassland areas ranging from several hundred square meters to several square kilometers. Comparing data from these locally relevant studies at a regional scale, we investigate these grasslands’ spatiotemporal patterns of origin and long-term dynamics. The study is based on taxonomic identification and radiocarbon AMS dating of charcoal pieces from soil/soil sediment archives of nutrient-poor grasslands in Mediterranean and temperate Western Europe (La Crau plain, Mont Lozère, Grands Causses, Vosges Mountains, Franconian Alb, and Upper-Normandy region). We address the following questions: (1) What are the key determinants of the establishment of these nutrient-poor grasslands? (2) What temporal synchronicities might there be? and (3) What is the spatial scale of these grasslands’ past dynamics? The nutrient-poor grasslands in temperate Western Europe are found to result from the first anthropogenic woodland clearings during the late Neolithic, revealed by fire events in mesophilious mature forests. In contrast, the sites with Mediterranean affinities appear to have developed at earlier plant successional stages (pine forest, matorral), established before the first human impacts in the same period. However, no general pattern of establishment and dynamics of the nutrient-poor grasslands could be identified. Local mechanisms appear to be the key determinants of the dynamics of these ecosystems. Nevertheless, this paleoecological synthesis provides insights into past climate or human impacts on present-day vegetation.

Forest clearance and land use by early farmers in Europe: Insights from north Greek oral history

European prehistorians have long debated whether Neolithic farmers, especially in temperate central and northern Europe, exploited cleared woodland for short-term ‘slash-and-burn’ crop husbandry or for cultivation of ‘permanent’ gardens/fields. Because empirical evidence was scarce and rather indirect, circumstantial arguments based on the relative costs and benefits of the two rival regimes have hitherto dominated this debate, but more direct archaeobotanical proxies for Neolithic crop husbandry (based on weed ecology and crop isotopes) now favour the predominance of intensively cultivated permanent plots. At the same time, it is argued from ongoing experimental cultivation at Forchtenberg in southern Germany that short-term slash-and-burn husbandry offers much better returns on both land and labour, at least on poor soils marginal for Neolithic cereals. This paper presents oral historical evidence for mid-twentieth century clearance and subsequent long-term cultivation of mixed deciduous woodland in the Pieria region of northern Greece. These data extend the sparse European ethnographic and historical record of manual clearance for crop husbandry to deciduous woodland of the northern Mediterranean, while also providing comparative material against which to evaluate experimental results. Pierian farmers achieved good cereal yields for many years on cleared woodland, with very limited use of fire, demonstrating the viability of a permanent-cultivation regime for early farmers in the north Mediterranean, but suitable land was scarce in Pieria and informants had no experience of the alternative slash-and-burn regime. Conversely, the Forchtenberg experiments have effectively tested only the slash-and-burn regime under soil pH conditions hostile to successful long-term cultivation. Evaluation of these experiments in the light of the Pierian data suggests that long-term cultivation of cleared woodland would have offered Neolithic famers better returns than slash and burn under good or neutral soil conditions. Comparison of the likely returns of the two rival regimes thus offers no grounds for questioning archaeobotanical indications of Neolithic permanent cultivation. The Pierian and Forchtenberg data agree, however, in suggesting that the time elapsed between clearance and reliable grain harvests, under either regime, may have been long enough to pose a significant challenge to the survival of pioneer Neolithic farmers, especially if these were colonists settling beyond ‘commuting’ distance from their previous homes.

Firewood, food and human niche construction: the potential role of Mesolithic hunter–gatherers in actively structuring Scotland's woodlands

Over the past few decades the potential role of Mesolithic hunter–gatherers in actively constructing their own niches, through the management of wild plants, has frequently been discussed. It is probable that Mesolithic hunter–gatherers systematically exploited specific woodland resources for food and fuel and influenced the ‘natural’ abundance or distribution of particular species within Mesolithic environments. Though there has been considerable discussion of the pollen evidence for potential small-scale human-woodland manipulation in Mesolithic Scotland, the archaeobotanical evidence for anthropogenic firewood and food selection has not been discussed in this context. This paper assesses the evidence for the active role of Mesolithic hunter–gatherer communities in systematically exploiting and managing woodlands for food and fuel in Scotland. While taphonomic factors may have impacted on the frequency of specific species in archaeobotanical assemblages, it is suggested that hunter–gatherers in Mesolithic Scotland were systematically using woodland plants, and in particular hazel and oak, for food and fuel. It is argued that the pollen evidence for woodland management is equivocal, but hints at the role of hunter–gatherers in shaping the structure of their environments, through the maintenance or creation of woodland clearings for settlement or as part of vegetation management strategies. It is proposed that Mesolithic hunter–gatherers may have actively contributed to niche construction and that the systematic use of hazel and oak as a fuel may reflect the deliberate pruning of hazel trees to increase nut-yields and the inadvertent – or perhaps deliberate – coppicing of hazel and oak during greenwood collection.

Survival of cold-adapted species in isolated mountains: the population genetics of the Sudeten ringlet, Erebia sudetica sudetica, in the Jeseník Mts., Czech Republic

Relic populations of cold-adapted species, trapped in isolated mountain pockets within the temperate zone, are predicted to suffer considerably due to ongoing climate warming. The butterfly Erebia sudetica sudetica is an example restricted to the Eastern Sudety Mts. Here, the butterfly forms permanent populations on subalpine tall-herb grasslands, but also occupies woodland clearings and hay meadows at lower altitudes. We assume differences among the genetic diversities of the populations due to differences in the temporal continuity of these habitats. Therefore, 17 allozyme loci were analysed for 276 individuals from 13 different localities (six tall-herb stands, two meadows, five forest clearings) in the Jesenik Mts. with a maximum distance of 20 km among them. We obtained a significantly higher genetic diversity for the subalpine populations than for the forest clearing populations. The genetic differentiation among the forest clearing populations was higher than among the subalpine ones. They also showed a significant isolation-by-distance system. These findings support the idea that the lower-elevation populations might have been founded by more than one dispersal event from the subalpine sites, but also secondary colonisations and gene flow in the forest belt. Due to founder effects and possibly further subsequent bottlenecks, these forest clearing populations did not harbour the entire genetic diversity of the taxon. Therefore, conservation actions should focus on the subalpine tall-herb formation.

Host plants, biology and distribution of Acraea terpsicore (Linnaeus, 1758) (Lepidoptera: Nymphalidae): a new butterfly for northern Australia with potential invasive status

AbstractThe nymphalid butterfly Acraea terpsicore has recently become established in northern Australia where it is spreading rapidly. The immature stages are briefly described, illustrated and compared with those of A. andromacha (Fabricius, 1775) from Australia, and information on its larval food plant specificity, biology and distribution is presented. The species occurs in highly modified open areas, including suburban roadsides, disturbed grassland (i.e. cleared woodland that is regularly slashed or graded) and degraded savannah woodland where it breeds primarily on Hybanthus enneaspermus (L.) F.Muell. (Violaceae), a widespread native herb that is also used as a natural larval food plant in India, and to a lesser extent on Passiflora foetida L. (Passifloraceae), an introduced vine also used as a food plant in Indo‐China and Indonesia. Extensive laboratory larval feeding trials on Violaceae, Passifloraceae and Cucurbitaceae revealed limited ability of the Australian population to consume Passifloraceae and lack of ability to feed on Cucurbitaceae, indicating that A. terpsicore is unlikely to become a pest of vegetable and horticultural crops. It is hypothesised that the narrower host range in Australia is attributed to the founder effect of a limited gene pool. Within 14 months of colonisation, A. terpsicore has occupied much of the western half of the ‘Top End’ of the Northern Territory and spread to the eastern Kimberley of Western Australia, with an estimated extent of occurrence of 112 000 km2 and average dispersal rate of 315 ± 56 (SE) km/year. Because the butterfly has the ability to exploit (injure) Violaceae, and potentially outcompete A. andromacha, it may qualify as an invasive species.

Propagule pressure, not fire or cattle grazing, promotes invasion of buffel grass Cenchrus ciliaris

Summary The invasion of exotic grasses into savannas is occurring globally. Despite threats to ecosystem processes and biodiversity, few studies have investigated factors that facilitate invasion success. The abundance of the exotic perennial Cenchrus ciliaris buffel grass was assessed across woodland clearings at 46 sites in Queensland, Australia. The influence of fire and grazing on the invasion success were also examined experimentally within 18 sampling stations (including cattle grazed and ungrazed treatment pairs) with unburnt, low‐intensity and high‐intensity fire treatments. Mixed‐effects logistic regressions suggest that the probability of buffel grass occurring in remnants was predominantly a function of the abundance of buffel grass in adjoining paddocks. The abundance of buffel grass in remnants was greater where the boundaries with clearings faced the prevailing wind adjacent to paddocks with high buffel grass abundance. There was a significant positive association between buffel grass and tree canopies at approximately half of the sites, and the strength of this association was positively related to the amount of coarse sand in the surface soil. This association was interpreted as nutrient enrichment from litter fall on sandy infertile soils being a requirement for invasion. The invasion of buffel grass at the experimental site accelerated with above‐average rainfall after a prolonged drought, and invasion continued even after imposing all combinations of fire and grazing. Cattle grazing modestly enhanced invasion, relative to the absence of grazing, but this difference was only significant without burning. The occurrence of fire in both the survey and experiment was not associated with enhanced buffel grass invasion. Invasion of buffel grass may be inevitable over large areas of savanna, especially when abundant rainfall follows periods of drought. However, invasion will be minimized in areas where buffel grass has not been planted, areas without cattle grazing and in areas with low soil fertility. Synthesis and applications. The invasion of buffel grass accelerates during abundant rainfall after drought and is enhanced by propagule pressure, dispersal trajectories and under trees, probably as a result of nutrient enrichment. Disturbance had little influence on invasion, and there is no support for fire‐promoted invasion as predicted by the grass–fire cycle theory.

Ecology and long-term land-use, palaeoecology and archaeology – the usefulness of interdisciplinary studies for knowledge-based conservation and management of cultural landscapes

Natural and cultural heritage management is dependent on knowledge about present species and habitats and presence of cultural heritage sites, respectively. Knowledge about long-term development helps to identify factors explaining both biodiversity and cultural heritage and to predict future changes based on changes in, for example, climate or grazing regimes. In the present interdisciplinary study, vegetation surveys, pollen analysis from a bog and soil profiles within archaeological localities, archaeological test-excavations and historical data have been combined to elucidate cultural landscape development in an upland landscape of Erdalen in Stryn, western Norway. Activity in what is now the summer farm area started in the Bronze Age, followed by clearance and grazing in the Early Iron Age. After a period of reduced activity, most of the valley seems to have been utilised for summer farming in the Late Iron Age and first part of the Medieval Period. A regression period, probably related to the Black Death, is followed by new activity with extensive woodland clearings from the seventeenth century, whereas recent changes have resulted in increased tree cover. The investigations show a close relationship between land-use practices, presence of grassland species and vascular plant species richness. They also show the importance of continued management for existence of semi-natural habitats in the future, and for the visibility of archaeological sites.

Characterization of American hazelnut (Corylus americana) accessions and Corylus americana × Corylus avellana hybrids using microsatellite markers

The American hazelnut (Corylus americana Marshall) is native to the eastern United States and Southern Canada. In the early decades of the last century, breeding work by several individuals attempted to combine the cold hardiness and disease resistance of the American hazelnut with the larger nut size of the European hazelnut (C. avellana L.). Many hybrid selections grown today trace back to these early efforts. Over the past three decades, representatives of C. americana were collected and are preserved in Corvallis, Oregon. Seeds were collected along roadsides, in hedgerows, in woodland clearings, and in gardens. The seeds were germinated and the best of the resulting seedlings were preserved for later use. In this study, genetic diversity was studied in 87 American and 67 hybrid hazelnut selections using 21 microsatellite loci. The total number of alleles was 266 in the 162 accessions examined, of which 229 were present in C. americana accessions, but only 168 in the Arbor Day Farm hybrids, and 116 in the hybrids of C. americana ‘Rush’. In the American accessions, a high level of genetic diversity was observed (H e = 0.74, H o = 0.68). A genetic similarity matrix of the American and hybrid accessions, and a few European cultivars, was constructed and the resulting dendrogram revealed seven major groups: European cultivars and ‘Rush’ hybrids, two groups of Arbor Day Farm hybrids, three groups of C. americana accessions, and a mixed group of hybrid and American accessions. Analysis confirmed the reported parentage of 10 hybrids. The genetic diversity among the American hazelnut accessions, untapped to date, will be useful in breeding hybrids that will allow expansion of hazelnut production into eastern North America.

Root Infections May Challenge Management of Invasive Phytophthora spp. in U.K. Woodlands.

Because sporulation of Phytophthora ramorum and P. kernoviae on Rhododendron ponticum, an invasive plant, serves as primary inoculum for trunk infections on trees, R. ponticum clearance from pathogen-infested woodlands is pivotal to inoculum management. The efficacy of clearance for long-term disease management is unknown, in part due to lack of knowledge of pathogen persistence in roots and emerging seedlings. The main objectives of this work were to (i) investigate whether both pathogens infect R. ponticum roots, (ii) determine the potential for residual inoculum of P. kernoviae to infect R. ponticum seedlings in cleared woodlands, and (iii) assess potential for R. ponticum roots to support survival and transmission of P. kernoviae. Roots of R. ponticum were collected from both unmanaged and cleared woodlands and assessed for pathogen recovery. Both P. ramorum and P. kernoviae were recovered from asymptomatic roots of R. ponticum in unmanaged woodlands, and P. kernoviae was recovered from asymptomatic roots from seedlings in cleared woodland. Oospore production of P. kernoviae was observed in naturally infected R. ponticum foliage and in inoculated roots. Roots also supported P. kernoviae sporangia production. The results of this study suggest that post-clearance management of R. ponticum regrowth is necessary for long-term inoculum management in invaded woodlands.

Late Mesolithic environmental change at Black Heath, south Pennines, UK: a test of Mesolithic woodland management models using pollen, charcoal and non-pollen palynomorph data

The recognition of Mesolithic impacts in mid Holocene pollen diagrams of the British Isles has led to the development of models describing sophisticated woodland management, particularly through the use of fire, by Mesolithic populations. However, the significance of human agency in creating mid Holocene woodland disturbances is unclear, with natural and human-induced clearings arguably indistinguishable in the pollen record. Analysis of non-pollen palynomorphs (NPPs) should aid the identification of events and processes occurring within these woodland disturbances and provide more precise palaeoecological data. In this paper we present pollen, charcoal and NPP analyses from a potentially critical location in the Mesolithic impacts debate. NPP types aid significantly in the reconstructions, suggesting periods of dead wood, grazing, local burning and wetter ground conditions. The results indicate that between 7700 and 6800 cal b.p., a predominantly wooded environment periodically gave way to phases of more open woodland, with inconsistent evidence for animal grazing. From 6800 cal b.p., a phase of open woodland associated with high charcoal concentrations and indicators of grazing was observed. This probably represents the deliberate firing of vegetation to improve grazing and browse resources, although it remains unclear whether fire was responsible for initially creating the woodland opening, or if it was part of an opportunistic use of naturally occurring woodland clearings.

Managing successional species: Modelling the dependence of heath fritillary populations on the spatial distribution of woodland management

Species that persist on a shifting mosaic of successional habitat offer particular challenges to conservation, to monitoring methods, and to population dynamic modelling. The conservation of the heath fritillary butterfly ( Melitaea athalia) in woodland in England, for example, depends on the creation of woodland clearings by coppicing (rotational cutting). We have developed a simulation model to assist in the conservation of such populations, called MANAGE. We have parameterised the model for the M. athalia population in the Blean Woods in Kent, and used it to answer several management questions. We find that: (1) simulations predict that the observed rates of coppicing will not be enough to meet existing conservation (Biodiversity Action Plan) targets, except when the most generous modelling assumptions are made; (2) the greatest uncertainty in the model outcome arises from uncertainty in the colonisation parameters; (3) in the worst case scenario (using the most pessimistic model assumptions), a population would require 2.3% of the Blean Woods to be coppiced each year, which is around double the currently-observed rate; (4) the four management units of the Blean where coppicing is practised are not independent metapopulations– they support each other; and (5) to sustain a population in a smaller landscape would require less coppicing overall, but more as a percentage of the landscape. This modelling approach may prove useful in the development of conservation management plans for other species that inhabit successional habitats.

What is a patch in a dynamic metapopulation? Mobility of an endangered woodland butterfly, Euphydryas maturna

In metapopulations of butterflies inhabiting transient forest openings, the dynamic character of biotopes prevents unequivocal delimitation of habitat patches, complicating analyses of inter‐patch dispersal. We analyse mobility of one such metapopulation, using mark‐recapture data on an endangered butterfly, the scarce fritillary Euphydryas maturna, at its last site in the Czech Republic. The butterfly inhabits woodland clearings, its population numbered 190 individuals in 2002. We modelled movements in eight habitat patchworks delimited according to different criteria and superimposed over the forest, using the Virtual Migration model. Resulting mobility parameters were generally robust against patchwork definitions, the main distinction was between patchworks based on presence of adults and consisting of small patches and patchworks based on distributions of larval nests and consisting of large patches. Emigration constant decreased towards the latter, whereas males suffered higher migration mortality than females in the former. Patchworks delimited according to presence of adults performed better than patchworks based on distribution of larval nests. A comparison of our parameters with those found for the species in Finland showed that the Czech butterflies were more prone to leave individual patches and suffered higher migration mortality, which combined into considerably shorter survival. We conclude that transferring dispersal parameters among different regions and populations is more risky than using less suitable definitions of habitat patches within single region.

An Overview of Weed Management in the Wild Lowbush Blueberry—Past and Present

SUMMARY The wild lowbush blueberry (Vaccinium angustifolium Ait.) is an important successional species of cleared woodland and abandoned farmland of northeastern North America where commercial, managed blueberry fields have been developed. Unlike other fruit crops, the weed flora is unique and consists mainly of a broad range of native herbaceous and woody perennial species that thrive under the two-year cropping system. Traditionally, weedy vegetation was controlled or suppressed by burning, cutting, and roguing, and regenerating woody and herbaceous species were the major weed problems. The introduction of phenoxyalkanoic herbicides in the late 1940s lead to the early development by innovative growers of selective roller/wiper applicators that could control the taller, weedy overstory. Several selective preemergence herbicides (terbacil and diuron) were introduced in the 1970s to control grasses and some broadleaved weeds, and hexazinone was approved in Canada in 1982 and in Maine in 1983. This soil-app...

White-Tailed Deer Use of Clearings Relative to Forage Availability

Use of woodland clearings by white-tailed deer (Odocoileus virginianus) may be in response to increased availability of forage within open patches or increased ability of deer to locate predators. We tested predictions, based upon the forage-maturation hypothesis, that white-tailed deer used areas with the greatest availability of high-quality forage, and that habitat use changed seasonally depending upon availability and quality of major types of forage in their diet. We tested those predictions in subtropical thorn woodland in South Texas, United States. Treatments included (1) areas with continuous woody cover as controls, (2) clearings with high availability of forbs and shrub sprouts, (3) clearings with low availability of shrub sprouts, (4) clearings with low availability of forbs, and (5) clearings with low availability of forbs and shrub sprouts. Intensity of use by deer during summer and autumn increased with increasing biomass of shrub sprouts and then declined with increasing shrub biomass as areas became dominated by mature shrubs with less accessible, usable forage. During spring, intensity of deer use increased in clearings with increases in forage availability and quality, indicated by an index to carrying capacity, then declined as vegetation matured. Responses of white-tailed deer to clearings supported the forage-maturation hypothesis in which herbivore responses to clearings resulted, in part, from the presence of shrub sprouts of high nutritional quality, particularly during summer and autumn when forage availability was low. Maintenance of clearings that are interspersed in a woodland matrix and maintaining high levels of immature shrub sprouts may alter the spatial distribution of white-tailed deer on the landscape.

Do seed predation and dispersal limit downslope movement of a semi‐desert grassland/oak woodland transition?

Abstract. Semi‐desert grasslands and dryland forests are typically arranged along elevation gradients, with low elevation grasslands and savannas separated from higher‐elevation woodlands by a diffuse boundary. Recent (< 200 yr) woody plant encroachment into adjacent semi‐desert grasslands appears to be a cosmopolitan phenomenon, and has been attributed to disturbance by humans (e.g., livestock grazing, fire suppression); however, little is known about rates, patterns and mechanisms. Are observations of increased woody plant abundance in semi‐arid grasslands indicative of a downslope shift in the boundary zone? Experimental plots were established in adjacent oak woodlands and semi‐desert grasslands in southeastern Arizona, to determine if mechanisms of acorn predation and dispersal would facilitate or limit downslope movement of oak woodlands. We recovered acorns that had been dispersed 50 m into adjacent grasslands, which suggests that they were cached by acorn predators. Acorns in grassland plots were more than twice as likely to escape predation than acorns that remained in woodland or cleared woodland plots. Contrary to previously published research, simulated perturbations (gap formation, understory and below‐ground vegetation removal) did not affect seed predation, suggesting that generalizations from mesic, deciduous oak forests may not be accurately extrapolated to xeric, evergreen oak forests. Recently published studies indicate that xeric conditions limit survival of oak seedlings across the boundary zone, suggesting that observed patterns of acorn dispersal and predation provide the potential for colonization of adjacent grassland only during infrequent years with cool, moist conditions during late summer months.

Effects of biotic factors on emergence and survival of Quercus emoryi at lower treeline,Arizona, U.S.A.

Woody plants have increased in abundance in many of the world’s grasslands during the last three centuries. In contrast, Quercus emoryi Torr. has not encroached into adjacent grasslands for several centuries. The objective of this study was to identify potential constraints on emergence and survival of Quercus emoryi at the boundary between oak woodland and semi-desert grassland. We established a large-scale field experiment at this boundary in southern Arizona, U.S.A. Emergence of Q. emoryi seedlings occurred only during years in which supplemental water was added to experimental plots (1993, 1995), and emergence was lower in grassland than within adjacent intact woodland or cleared woodland. Seedling survival of both cohorts was reduced by herbaceous interference; survival of the 1993 cohort was greater in cleared woodland than in intact woodland or grassland, and survival of the 1995 cohort was reduced by vertebrate herbivory. Seedling mortality was almost always attributed to desiccation. Biotic factors (e.g., herbaceous interference, vertebrate herbivory) can reduce seedling emergence and survival; however, their effects become important only after abiotic constraints are overcome. Thus, we conclude that abiotic factors exert primary control in structuring the boundary of these communities. In addition, our research supports the interpretation that the lower treeline in this system is currently static.

Open Corridors Appear to Facilitate Dispersal by Ringlet Butterflies (Aphantopus hyperantus) between Woodland Clearings

We studied the ringlet butterfly (Aphantopus hyperantus) in an area of woodland in eastern England. A. hyperantus occurs in open fields, rides (grassy tracks), and glades within the woodland. Mark‐recapture methods showed that exchange rates of adult A. hyperantus between fields and glades can be predicted better by distance‐via‐rides than by direct distance. Behavioral observations showed that A. hyperantus readily moved from glades into rides but rarely moved from glades into dense woodland. The rides are likely to be corridors that act as conduits between fields and glades. In the A. hyperantus system, connectivity could reduce local extinctions and increase rates of recolonization in the event of local extinction.