Browsing Pressure Research Articles

Enhancing Garry Oak Seedling Performance in a Semiarid Environment

Successful establishment of desired species is crucial for accelerating the development of functioning ecosystems. In the Pacific Northwest, Garry oak (Quercus garryana) woodlands are of significant conservation value as many have been lost or degraded due to anthropogenic factors. Restoration plantings are one strategy for reversing this loss and enhancing the resilience of wildlife habitats confronting climate change. We tested the effects of seedling age (1- vs. 3-years old at planting), plastic mulch, tree shelters, and first-year irrigation on the performance of Garry oak seedlings for three years after outplanting in the semiarid East Cascades of Washington state. Net survival was 33% after three years. Growth was minimal as basal area increment and height growth averaged 21 mm2 and 5 cm, respectively, over this period. Most mortality occurred early in the first growing season after planting and was related, in part, to stock quality; this obscured the effects seedling age might have had on performance. Survival and growth were enhanced primarily by plastic mulch and to a lesser extent by tree shelters. First-year irrigation enhanced survival but not growth. We highly recommend the use of plastic mulch, which is easily installed and inexpensive, when planting Garry oak in semiarid environments. Irrigation, while beneficial, is not feasible for many projects. Tree shelters were of limited utility, likely because browse pressure was low. Attention to stock quality and the use of post-planting treatments appropriate to local climatic and site conditions are essential for restoration plantings to be ecologically and economically effective.

Shrubs protect oak seedlings against ungulate browsing in temperate broadleaved forests of conservation interest: A field experiment

In many oak-rich temperate broadleaved forests of conservation value, high ungulate browsing pressure restricts oak regeneration. We examined the protection of oak ( Quercus sp.) seedlings from browsing provided by naturally occurring shrubs in 10 forests across southern Sweden over 3 years. We planted oak seedlings in four plots in each forest; two with naturally regenerated shrubs and two with no shrubs. Ungulate browsers were excluded from two plots at each site, one with and one without shrubs. Fencing provided the best protection against ungulate browsers for the seedlings. The probability of a seedling being browsed (browsing frequency) was approximately 20% units lower for individuals growing among shrubs than for individuals growing in the absence of shrubs. When browsing did occur, the intensity (measured as a reduction in height growth) was significantly lower for seedlings in shrubs. Regression analyses showed that browsing frequency increased on seedlings in tall shrubs, and decreased on seedlings that had been browsed previously. Browsing intensity decreased if the seedling grew in tall and dense shrubs. Browsing frequency and intensity increased on oak seedlings that over topped the shrub canopy. Increased abundance of the prickled Rubus idaeus and Rubus fruticosus coll. in plots with shrubs did not affect browsing frequency and intensity. Two and a half years after planting, oak seedling mortality increased by the presence of shrubs. Although shrubs restricted oak seedling growth, we conclude that shrubs initially facilitated oak regeneration by concealment, and subsequently by numeric dilution. Shrubs may be used to reduce browsing damages if long-term evaluation indicates a net positive outcome for oak survival and growth.

Techniques to Promote Garry Oak Seedling Growth and Survival in Areas with High Levels of Herbivory and Competition

Though regeneration of Garry oak (Quercus garryana) seedlings within Garry oak ecosystems is important to maintain this threatened habitat, seedling regeneration is often poor. Two independent studies were initiated in the early 2000s to examine questions surrounding Garry oak recruitment: one at the Crow's Nest Ecological Research Area (CNERA) on Salt Spring Island, BC, and another at the Pacific Rim Institute for Environmental Stewardship (PRI) on Whidbey Island, WA. In both areas, Garry oak seedlings were caged and monitored for growth and relative health. Over 8 years at CNERA, growth of caged plants outpaced growth of controls, which was indicative of high browsing pressure and relatively high palatability of Garry oak seedlings to black-tailed deer and feral sheep. The PRI oaks grew more slowly and at four PRI sites survival was 30–50% over 5–6 years as compared to 90% survival at CNERA over 8 years. The mortality at PRI was likely due to a combination of dry summer conditions and large numbers of voles, which took advantage of the cages for protection from predators; furthermore, lack of mycorrhizal hosts could have inhibited seedling establishment. Weed block inside exclosure cages provided ideal nesting conditions for voles. This study demonstrated that the caging of Garry oak seedlings, although labor-intensive and requiring frequent maintenance, provides valuable protection from large ungulate browsers and can be maintained as a relatively long-term measure, but vigilance is required to protect young seedlings from other threats such as voles or competing vegetation.

The role of large predators in maintaining riparian plant communities and river morphology

Studies assessing the potential for large predators to affect, via trophic cascades, the dynamics of riparian plant communities and the morphology of river channels have been largely absent in the scientific literature. Herein, we consider the results of recent studies involving three national parks in the western United States: Yellowstone, Olympic, and Zion. Within each park, key large predators were extirpated or displaced in the early 1900s and subsequent browsing pressure by native ungulates initiated long-term declines in recruitment (i.e., growth of seedlings/sprouts into tall saplings and trees) of palatable woody species and impairment of other resources. Channel responses to browsing-suppressed riparian vegetation included increased widths of active channels via accelerated bank erosion, erosion of floodplains and erraces, increased area of unvegetated alluvium, channel incision, and increased braiding. A reduced frequency of overbank flows indicated these rivers have become increasingly disconnected from historical floodplains because of channel widening/incision. Results from Zion National Park also identified major biodiversity affects (e.g., reduced abundance of plant and animal species). Although these studies were conducted in national parks, results may have implications concerning riparian plant communities, biodiversity, and channel morphology for streams and rivers draining other public lands in the western US. It is on these lands that native and introduced ungulates have often heavily utilized riparian areas, largely in the absence of key predators, with significant consequences to plant communities and channels.

Short‐term Effects of Single Species Browsing Release by Different‐sized Herbivores on Sand Forest Vegetation Community, South Africa

ABSTRACTManipulations of herbivores in protected areas may have profound effects on ecosystems. We examine short‐term effects on tree species assemblages and resource utilization by a mesoherbivore and small‐size herbivores (ungulates <20 kg) in Sand Forest, after browsing release from a megaherbivore (elephant), or both a mega‐ and mesoherbivore (nyala), respectively. Effects were experimentally separated using replicated exclosures where all trees were counted, identified to species and browsing events recorded. Tree species assemblages were impacted by both elephant and nyala, and by each herbivore species individually. Tree turnover rates were higher where both herbivore species were present than in their combined absence. Diet was segregated among elephant, nyala and small‐size herbivores. Both resource specificity and browsing pressure by nyala increased in absence of elephant; small‐size herbivores increased resource specificity in absence of elephant, and increased browsing pressure in absence of both elephant and nyala. This implies interference competition with competitive release. The indirect effect of the manipulation of herbivore populations, through the removal of one or two herbivore species, caused a shift in tree species composition and diet of smaller‐size herbivores. These indirect effects, especially on tree species composition, can become critical as they affect vegetation dynamics, biodiversity and ecosystem processes. Therefore, in order to conserve habitats and biodiversity across all trophic levels, conservation managers should consider the effects of: (1) the full herbivore assemblage present; and (2) any effects of altering the relative and absolute abundance of different herbivore species on other herbivore species and vegetation.

Effects of supplemental feeding and density of white‐tailed deer on rodents

AbstractSpatial distribution, population density, and reproductive success of many wildlife species may be altered by changes in vegetation composition, habitat structure, and availability of food. Altered distributions of key herbivores such as white‐tailed deer (Odocoileus virginianus) may impact all of these factors. Our objective was to determine the direct and indirect effects of supplemental feeding of deer on rodent populations in south Texas. We modeled effects of supplemental feeding and habitat change due to deer browsing through surveys of rodents. Rodents have a short generation time and populations respond quickly to change, so they are a suitable indicator of changes in habitat structure brought about by deer browsing pressure. We sampled rodent populations near to and far from deer feeders within twelve 81‐ha enclosures containing three different densities of deer with and without supplemental feed. The three deer densities were low (8.1 ha/deer), medium (3.2 ha/deer), and high (2 ha/deer). We conducted rodent trapping during March and April of 2007 and 2008. Abundance of rodents was much higher (P < 0.001) in 2008 than in the previous year due to an increase in rainfall. However, we found little effect of deer density, supplemental feeding of deer, or distance from deer feeders on rodent populations. Thus we conclude that supplemental feeding of deer and deer density had little influence on rodent communities in this environment. Rodent species native to semi‐arid environments are probably adapted to large changes in vegetative productivity brought about by the highly variable annual rainfall patterns, therefore they can adapt to the less abrupt habitat changes resulting from changing densities of deer. Conservation concerns that providing supplemental feed to deer in semi‐arid rangeland will disrupt the ecology of the land through changes in rodent populations were not supported. © 2011 The Wildlife Society.

Rhus glabra response to season and intensity of fire in tallgrass prairie

Altered fire regimes play a key role in shrub expansion in grasslands worldwide. We assessed how season and type or intensity of fire affected the growth and demography of Rhus glabra, a common woody invader in North American mesic grasslands. Fire during any season killed 99% of ramets but stimulated new ramet recruitment from belowground buds, resulting in a near-complete turnover of ramet populations. During the first 2 years following fire, populations on spring-burned sites had the greatest post-fire ramet densities and population growth rates, and winter- and spring-burned populations showed the highest resprouting rates. However, after 10 years, R. glabra cover on summer-burned sites was 3.5 times greater than on autumn- or winter-burned treatments. Thus, short-term post-fire responses may not be good predictors of long-term changes in abundance. Low-intensity spring backfires resulted in the highest ramet population growth rates, whereas high-intensity headfires in any season resulted in slower growth, and populations burned with low-intensity winter fires declined. In addition, season of fire influenced browsing pressure, suggesting that plant responses may be partially a result of indirect effects of fire on rates of herbivory. Overall, our results demonstrate that the application of frequent autumn or winter backfires is an effective management tool for limiting R. glabra expansion in grasslands, and that long-term data are critical for management decision-making, particularly in systems characterised by high interannual climate variability.

The ecology of Canada Yew (Taxus canadensisMarsh.): A review

Canada yew (Taxus canadensis Marsh.) is a shade-tolerant evergreen shrub native to the understories of the boreal and deciduous forests of northeastern North America. Canada yew has a relatively unique growth form, with low sprawling branches capable of forming dense clusters of stems. Historic accounts suggest that before Euro-American settlement, Canada yew was a common plant across its range in many forest types and that it was locally abundant or dominant in 5%–20% of forest stands in the northern part of its range. Canada yew’s range has declined during the last century as a result of browsing by native ungulates, fire, intensive forest management, and clearing of land for agriculture and other development. It is considered uncommon throughout most of its present North American range, except for often isolated populations in areas that remain free from heavy browsing pressure by moose and deer (e.g., islands, areas with heavy snow cover, or areas naturally low in ungulate abundance). Its decline has likely resulted in changes to abiotic and biotic conditions, including structure and composition of understory vegetation and concomitant effects on understory vertebrates. Increasing white-tailed deer populations and reduced snowfall as a result of climate change in eastern North America threaten to extirpate this species from additional parts of its range in the next century. Suggestions for future research are discussed.

Deer density effects on vegetation in aspen forest understories over site productivity and stand age gradients

Studies examining the interacting effects of ungulate herbivore pressure and site productivity on vegetation are mostly on grassland–grazing systems and have shown conflicting patterns. Here we examine the effects of deer density (>30 years differences in density between two landowners), site productivity (site index, SI) and stand age on subcanopy vegetation characteristics in 60 closed canopy, clear-cut origin Populus tremuloides dominated stands, Michigan, USA. Stand age effects were included because age varied among stands and can affect subcanopy vegetation patterns. Compared with fewer deer, stands with more deer had greater total forest floor vegetation mass, and its major components bracken fern (Pteridium aquilinum), sedge (mostly Carex pensylvanica) and trees/shrubs <0.25m tall, but lower forb mass and lower forest floor vegetation species richness and diversity. Deer density and SI had strong interacting effects on total forest floor mass, forb mass, and species richness. Forb mass increased with SI, but only in stands with fewer deer, whilst total vegetation mass was greater in stands with more deer at lower SI and declined with SI more sharply than for stands with fewer deer. Species richness increased with SI but more so at lower than higher deer density. Deer density and age had interacting effects on mass of trees/shrubs <0.25m tall and sedge. Compared with fewer deer, stands with more deer had greater sedge and tree/shrub mass, and sedge mass decreased and tree/shrub mass increased more sharply with age. In lower deer stands there was a dense subcanopy tree and shrub strata within and beyond the reach of deer 0.9–10m tall whereas in higher deer stands this vegetation layer was nearly absent. We conclude that higher deer browse pressure in early successional Populus stands (1) strongly limits the recruitment of woody stems to larger (>0.9m tall) size classes, which could affect long-term successional trajectories, and (2) diminishes forb density and species richness, especially at higher site productivity, but increases total forest floor vegetation mass (mostly bracken fern and sedge), especially at lower site productivity. Given associations of bracken fern and sedge with poorer and/or more open sites and assuming high palatability of forbs, this pattern may result from the combination of selective herbivory and higher light availability caused by limited recruitment of trees and shrubs to taller strata.

Spatio‐temporal feeding selection of red deer in a mountainous landscape

AbstractUnderstanding the dietary consumption and selection of wild populations of generalist herbivores is hampered by the complex array of factors. Here, we determine the influence of habitat, season, and animal density, sex, and age on the diet consumption and selection of 426 red deer (Cervus elaphus scoticus) culled in Fiordland National Park, New Zealand. Our site differs from studies elsewhere both in habitat (evergreen angiosperm‐dominated forests) and the intensity of hunting pressures. We predicted that deer would not consume forage in proportion to its relative availability, and that dietary consumption would change among and within years in response to hunting pressures that would also limit opportunities for age and sex segregation. Using canonical correspondence analysis, we evaluated the relative importance of different drivers of variation in diet consumption assessed from gut content and related these to available forage in the environment. We found that altitude explained the largest proportion of variation in diet consumption, reflecting the ability of deer to alter their consumption and selection in relation to their foraging grounds. Grasses formed a high proportion of the diet consumption, even for deer culled several kilometres from the alpine grasslands. In the winter months, when the alpine grasslands were largely inaccessible, less grass was eaten and deer resorted to woody plants that were avoided in the summer months. Surprisingly, there were no significant dietary differences between adults and juveniles and only subtle differences between the sexes. Sex‐based differences in diet consumption are commonly observed in ungulate species and we suggest that they may have been reduced in our study area owing to decreased heterogeneity in available forage as the diversity of palatable species decreased under high deer browsing pressures, or by intense hunting pressure.

Defensive Plant-Ants Stabilize Megaherbivore-Driven Landscape Change in an African Savanna

Tree cover in savanna ecosystems is usually regarded as unstable, varying with rainfall, fire, and herbivory. In sub-Saharan Africa, elephants (Loxodonta africana) suppress tree cover, thereby maintaining landscape heterogeneity by promoting tree-grass coexistence. In the absence of elephants, tree encroachment may convert savannas into closed-canopy woodlands; when elephants increase in abundance, intensified browsing pressure can transform savannas into open grasslands. We show that symbiotic ants stabilize tree cover across landscapes in Kenya by protecting a dominant tree from elephants. In feeding trials, elephants avoided plants with ants and did not distinguish between a myrmecophyte (the whistling-thorn tree [Acacia drepanolobium]) from which ants had been removed and a highly palatable, nonmyrmecophytic congener. In field experiments, elephants inflicted severe damage on whistling-thorn trees from which ants had been removed. Across two properties on which elephants increased between 2003 and 2008, cover of whistling-thorn did not change significantly inside versus outside large-scale elephant exclusion fences; over the same period of time, cover of nonmyrmecophytes differed profoundly inside versus outside exclusion fences. These results highlight the powerful role that symbioses and plant defense play in driving tree growth and survival in savannas, ecosystems of global economic and ecological importance.

Deer-mediated expansion of a rare plant species

Numerous plant colonizations have been putatively attributed to deer, based on plant species traits, fur brushing or dung analyses. But, in woodlands, direct links between the expansion of zoochorous plant species and ungulate presence have seldom been reported. Based on coupled floristic and browsing surveys, repeated in time, we analysed the causes of the spatio-temporal progression of the epizoochorous species Cynoglossum germanicum over 30 years in a network covering an 11000 ha forested area in north-eastern France. In this area, deer populations reached a peak in the 1970s, then were reduced in order to meet forest management requirements. Although initially rare and protected locally, C. germanicum has displayed an unexpected fast colonization rate during the last few decades but only in the northern part of the forest, which previously had the highest animal populations. Absent in the initial 1976 survey, C. germanicum occurred in 8% of the plots in 1981, then 46% in 2006. Logistic regression models revealed that the probability of occurrence of C. germanicum in 2006 increased not only with light indicator values, in accordance with its ecological requirements, but also with past deer browsing pressure. This result provides direct evidence of long-lasting impacts of deer populations on plant species distribution. Combining two complementary traits, animal transport and herbivory avoidance, C. germanicum benefited from epizoochorous dispersal and, once established, was protected from deer browsing by the presence of toxic proteins in its tissues. Due to the triggering role of ungulates, this species switched from the status of rare to that of colonizer within only a few decades.

Spatial patterns of accumulated browsing and its relevance for management of red deer Cervus elaphus

AbstractThe substantial increase of deer populations in Europe and North America in recent decades has led to concerns regarding the long‐term sustainability of current management from an ecosystem perspective. Key questions to be answered are how herbivore density relates to available resource levels, e.g. food availability and browsing pressure, and how this relation should be monitored and included in management. In Norway, the harvest of red deer Cervus elaphus has increased from 1,479 to 35,700 during 1960‐2008. Current monitoring programmes focus on deer body mass and number of seen deer rather than on the state of the vegetation communities. In our study, we quantify browsing frequency on common shrubs and trees in two municipalities, Gloppen and Flora, in the county of Sogn and Fjordane, Norway, to document current browsing pressure levels and to evaluate the potential of using browsing frequency of indicator species as a tool in the monitoring of red deer populations. We found that several species were heavily browsed (median browsing frequency &gt; 60%). Due to their wide availability, we analysed the spatial patterns of accumulated browsing of the highly selected rowan Sorbus aucuparia compared to the less selected bilberry Vaccinium myrtillus and birch Betula sp. We predicted less spatial variation in browsing frequency of the highly selected rowan. However, we found that the best model predicting browsing on rowan (forest type, habitat productivity, canopy cover, aspect, slope and distance to arable land) based on habitat variables was more complicated than for birch (habitat productivity and altitude) and bilberry (forest type, habitat productivity, altitude and distance from the coast). This suggests large spatial variation in browsing frequency of rowan even though the average browsing frequency of rowan was higher than for bilberry and birch. Browsing frequency for all species was positively correlated with faeces counts, but only bilberry showed additional correlation with red deer (harvest) density at the local management unit scale. Due to its wide distribution and promising link to local red deer density, bilberry stands out as the most promising species to monitor among winter browse species. However, browsing frequency on different species was not always correlated. It remains to be determined to which extent browsing on bilberry can be used as an indicator for the browsing pressure on the whole vegetation community. The documentation of high browsing frequencies clearly suggests that further focus on these issues is warranted.

Long-term browsing impact around diversionary feeding stations for moose in Southern Norway

Diversionary winter feeding of browsing ungulates is an increasingly common management practice although evidence for its efficacy to reduce excessive browsing remains ambiguous. Moreover, comparative estimates on changes in browsing pressure (proportion of available shoots browsed) following long-term winter feeding are currently lacking. We quantified spatiotemporal changes in browsing pressure of moose ( Alces alces L.) on commercial and non-commercial tree species around 30 feeding stations after 15–20 years of winter feeding. The results were compared with browsing pressure indices recorded at the same feeding stations 10 years previously. We expected leader stem and lateral twig browsing to have increased over time at a fine spatial scale (≤200 m from feeding station) and to have increased in spatial extent away from feeding stations. Furthermore, we tested whether moose browsing patterns conformed to central-place foraging theory. Despite 2–3-fold higher faecal pellet group numbers in the vicinity of feeding stations, leader stem browsing increased only on the commercially valuable Norway spruce ( Picea abies (L.) Karst), a species normally avoided by moose. Lateral twig browsing largely decreased within 200 m of feeding stations while at a broader scale (≤1 km from feeding stations), leader stem browsing was high on most tree species (ca. 60% of available stems browsed) as expected, Peak browsing on lateral twigs of Scots pine ( Pinus sylvestris L.) occurred further from feeding stations after 15–20 years of winter feeding than 10 years earlier. Browsing pressure on active feeding stations ( n = 18) was comparable to that on inactive feeding stations ( n = 12), suggesting the occurrence of rebrowsing. The ability of central-place foraging theory to explain fine-scale browsing patterns around feeding stations decreased as diversionary feeding continued over time. Long-term diversionary winter feeding of moose in unproductive boreal forests has serious implications for the intensity of fine-scale browsing pressure, which may lead to resource depletion close to feeding stations followed by high browsing pressure at distances further away from feeding stations. Our study emphasizes the importance of considering the longevity of a winter feeding programme at its inception.

Establishment of native Eucalyptus pauciflora and exotic Eucalyptus nitens on former grazing land

The success of tree plantings in agricultural landscapes within dry and variable rainfall regions is often below expectations. The aims of this study were to investigate techniques for establishing a local native species (Eucalyptus pauciflora) and an exotic species (E. nitens) in the Midlands of Tasmania, a tree-denuded region that has been grazed for over 100 years in Australia. We established trials at the Sorrel Springs and Woodland Park properties. We compared six establishment techniques: control; understorey nurse planting; spot burn; mulch; tree-guard and; self-watering tree-guard. Three months, 1 and 6 years after planting we measured tree height, health index and mortality. At Sorrell Springs tree-guarded treatments had greater height than other treatments at year one, but there were no effects of treatment at year six on height, health or mortality. At Woodland Park mulch, burn and tree-guarded treatments had greater height 1 and 6 years after planting. At year six tree-guarded treatments had lower mortality than other treatments. At Sorrell Springs height of E. nitens was greater than E. pauciflora at year one but not at year six. At Woodland Park height of E. nitens was generally greater at year six but having almost double the incidence of mortality relative to E. pauciflora. We conclude that expensive tree-guarding is only warranted where high browse pressure is expected and that on such sites species with tough leaves that are resistant to browsing should be considered. Further that in environments marginal for growing trees, best practice establishment technique is generally as effective as additional treatments of understorey nurse planting, spot burning or mulching.

Effects of simulated browsing on growth and leaf chemical properties in Colophospermum mopane saplings

AbstractBrowsing intensity influences a plant’s response to herbivory. Plants face a trade‐off between investment in the production of secondary compounds and investment in growth. To elucidate this trade‐off, we simulated four browsing intensities (0%, 50%, 75% and 100%) on mopane saplings, Colophospermum mopane (J.Kirk ex Benth.) J.Léonard, in a greenhouse experiment. This showed that, with increasing defoliation intensity, plants change their investment strategy. At intermediate levels of defoliation (50%), mopane saplings increased the synthesis of condensed tannins, so that tannin concentrations followed a hump‐shaped relation with defoliation intensity, with significantly higher tannin concentration at intermediate defoliation levels. When defoliated heavily (75% and 100%), tannin concentrations dropped, and plants were carbon stressed as indicated by a reduced growth rate of the stem diameter, and leaf production and mean individual leaf mass were reduced. This suggests that, at intermediate defoliation intensity, the strategy of the plants is towards induced chemical defences. With increasing defoliation, the relative costs of the secondary metabolite synthesis become too high, and therefore, the plants change their growing strategy. Hence, browsers should be able to benefit from earlier browsing by either adopting a low or a relatively high browsing pressure.

Non-lethal strategies to reduce browse damage in eucalypt plantations

Browsing by mammalian herbivores is a major problem in plantation forestry worldwide. This has traditionally been controlled through a reduction in herbivore numbers achieved by lethal means, namely shooting and poisoning. In recent years, however, there has been increasing social and political pressure to reduce reliance on such lethal controls, and consequently research into non-lethal controls is becoming increasingly important. In order for non-lethal methods to be efficiently incorporated into management, however, we must first identify those that are most effective under operational conditions. Different methods are rarely tested concurrently, and many have only been proven effective with captive animals or in small scale field trials. This study therefore sought to consolidate past research by combining non-lethal methods, which were known to significantly reduce browsing damage, into a single trial to enable their relative effectiveness to be compared under operational conditions. Eucalypt seedlings with combinations of browsing resistance, nursery fertilizer, chemical repellent (Sen-Tree) and tree guard treatments were planted in replicated blocks spread around the perimeter of eight operational plantations across Tasmania, Australia. Seedlings were monitored regularly for browsing damage, with seedling height and characteristics of the surrounding vegetation being assessed periodically. We found that the most effective treatments at reducing the severity of browsing damage in the short-term (<6 months) were tree guards and a combination of chemical repellent and low nursery fertilizer; there was no effect of the latter after 12 months. These treatments are easily incorporated into existing regimes and, while unlikely to be effective in isolation in areas with high browsing pressure, can be incorporated into management plans to reduce the reliance on lethal controls. While initially less effective than the other treatments, the effects of genetic resistance were more persistent and showed promise for long-lasting protection from browsing.

Tree-grass coexistence in a flood-disturbed, semi-arid savanna system

The coexistence of trees and grasses in savanna ecosystems is a contentious phenomenon. Fire and herbivory disturbances are often cited as major structuring forces that create a sustainable tree–grass relationship. However, periodic flooding of savanna patches may also enable coexistence. The aim of this study was to investigate the effects of flood-disturbance on the recruitment patterns of Acacia karroo trees in a semi-arid savanna system in South Africa. We analysed the spatial coincidence of A. karroo seedlings with tussocks of the tall spiny grass Stipagrostis namaquensis in the riverbed and related herbivory intensity to spatial position. The data showed that A. karroo seedlings were significantly positively associated with S. namaquensis (Chi-square test, \( \chi_{1}^{2} = 4 5. 20 \), n = 118, P < 0.001); A. karroo seedlings growing inside of tussocks experienced less browsing pressure than those growing in the floodplain (Kruskal–Wallis test, H = 11.90, n = 118, P < 0.01); and recruitment success of A. karroo trees was spatially discrete (K–S test, D = 0.78, n = 196, P < 0.01). We suggest that floods create an enemy-free zone, which S. namaquensis colonises and then facilitates successful A. karroo establishment. High levels of A. karroo recruitment in the riverbed may replenish the woodlands fringing the river, which appear to be sink areas for A. karroo seedlings. Thus, the interaction between disturbances at different spatial and temporal scales (flooding versus herbivory) seems to maintain the inherently unstable coexistence of tree and grass species in this ecosystem. These findings also suggested that flood disturbances alter the tree–grass relationship.

Browser selectivity alters post-fire competition between Erica arborea and E. trimera in the sub-alpine heathlands of Ethiopia

Mammalian herbivores have the potential to alter the competitive relations of woody species, if consumption is unevenly distributed between species. At elevations above 3,500 m in the southern Ethiopian highlands, vegetation is dominated by Erica arborea and E. trimera. Both species can potentially grow into short trees, but are burnt on a rotation of 6–10 years, and regenerate by re-sprouting from belowground lignotubers. The regenerating scrub is heavily browsed by cattle. We set up browsing exclosures at three burnt sites to quantify the impact of browsing over a 3-year period. When protected from browsing, E. trimera had similar or better height growth than E. arborea, but in browsed vegetation, E. arborea instead grew taller. Browsing was more intense on E. trimera in the first years after fire, indicating a difference in palatability between the species. We checked if browse quality differed, by analysing shoot contents of acid detergent fibre (ADF), protein, phenolics and tannins. Contrary to expectations, the preferred E. trimera contained more ADF, less protein and had a higher tannin activity than E. arborea. Although the vegetative growth of E. arborea is favoured relative to E. trimera under high browsing pressure, rapid change in abundance would not be expected, since short-interval fire will repeatedly eradicate any gains in vegetative growth. However, within the typical fire return interval of less than 10 years, E. trimera barely reach a reproductive state, whereas E. arborea flower profusely. Under the current regime of fire and browsing, this may in the long run be more important than differences in height growth, leading to a gradual increase in the proportion of E. arborea.

The Condition and Survival ofPopulus tremulaand Other Deciduous Saplings in a Moose Winter-Foraging Area in Southern Finland

The current heavy browsing pressure by moose (Alces alces) affects and even prevents the regeneration of deciduous species in Fennoscandia. We studied the current establishment status of Populus tremula and other deciduous species in managed regeneration stands in a moose winter-foraging area in southern Finland. The regeneration of P. tremula was more abundant in height classes 40–100 cm. There was a lag in stem density for height class 20 cm and there were no P. tremula saplings taller than two meters. The majority of living P. tremula regeneration was healthy, despite the fact that many stems had been browsed repeatedly, particularly in stands with greater sapling densities. The majority of Sorbus aucuparia and Salix spp. stems were also healthy despite recurrent browsing in all stands, and their density decreased sharply for saplings taller than 100 cm. Betula pendula and Betula pubescens were the most abundant of the studied species. They had less bites per stem and a very high proportion of them wer...