The tree species matrix,influence on the level of herbivore browsing in mixed forest stands in southwest Sweden†

Herbivory pressure on a forest stand and each plant individual may be affected by the spatial distribution of conspecific and allospecific plants on the site; whether the plants are standing in solitary or groupwise settings; or by the differences in the preference of plants in relation to each other. The study was conducted in southwest Sweden, where 120 plots (1508?m²) were visited and 1280 individual woody plants evaluated and measured. We tested the hypothesis that preferred woody plants can protect unpreferred conspecific and allospecific woody species against herbivory in a system with one dominant, generalist herbivore, the fallow deer (Dama L.), and that the effectiveness of this protection varies depending on the relative preference of the neighboring conspecific and allospecific woody species placed in conspecific groups or standing solitary. Our results support the hypothesis that preferred woody plants can protect unpreferred conspecific and allospecific woody species against herbivory in the case of Picea abies dominating stands.     

The effect of herbivory on seedling survival of the invasive exotic species Pinus radiata and Eucalyptus globulus in a Mediterranean ecosystem of Central Chile

Herbivory may be an important factor affecting seedling survival of exotic species invading new habitats. We evaluated the effect of vertebrate herbivory on the seedling survival of two widely planted and invasive tree species (Pinus radiata and Eucalyptus globulus), in a Mediterranean-type ecosystem of central Chile. An important role of herbivory on seedling survival of these two species in their introduced ranges has previously been documented. However, this has mainly been evaluated in forest plantations where habitat and vegetation conditions differ from wild habitats in which invasion occurs. We planted seedlings with and without protection against vertebrate herbivores in different aspects (a mesic south-facing slope and a xeric north-facing slope) and vegetation cover (open sites and sites with patchy tree cover). We found that regardless of aspect or vegetation cover, herbivory, in this case mainly caused by exotic vertebrates, significantly and negatively affected seedling survival of both species. However, while the effect of herbivory on P. radiata was significant in every vegetation and habitat condition, for E. globulus, the effect of herbivory was significant only for open sites in the mesic habitat. These results suggest that, as observed in forestry plantations, vertebrate herbivory may constrain seedling establishment of these two exotic trees and potentially impede the invasion. However, the importance of herbivory in controlling exotic species may vary depending on the vegetation and habitat conditions in some species such as E. globulus.     

Predicting intensity of white-tailed deer herbivory in the Central Appalachian Mountains

In eastern North America, white-tailed deer(Odocoileus virginianus) can have profound influences on forest biodiversity and forest successional processes.Moderate to high deer populations in the central Appalachians have resulted in lower forest biodiversity.Legacy effects in some areas persist even following deer population reductions or declines. This has prompted managers to consider deer population management goals in light of policies designed to support conservation of biodiversity and forest regeneration while continuing to support ample recreational hunting opportunities. However,despite known relationships between herbivory intensity and biodiversity impact, little information exists on the predictability of herbivory intensity across the varied and spatially diverse habitat conditions of the central Appalachians. We examined the predictability of browsing rates across central Appalachian landscapes at four environmental scales: vegetative community characteristics, physical environment, habitat configuration, and local human and deer population demographics. In an information-theoretic approach, we found that a model fitting the number of stems browsed relative to local vegetation characteristics received most(62%) of the overall support of all tested models assessing herbivory impact. Our data suggest that deer herbivory responded most predictably to differences in vegetation quantity and type. No other spatial factors or demographic factors consistently affected browsing intensity. Because herbivory, vegetation communities, and productivity vary spatially, we suggest that effective broad-scale herbivory impact assessment should include spatially-balanced vegetation monitoring that accounts for regional differences in deer forage preference.Effective monitoring is necessary to avoid biodiversity impacts and deleterious changes in vegetation community composition that are difficult to reverse and/or may not be detected using traditional deer-density based management goals.     

Deer prevent western redcedar (Thuya plicata) regeneration in old-growth forests of Haida Gwaii: Is there a potential for recovery?

The current increase in deer populations in many forests has fostered a growing concern about their impact on forest ecology. Sitka black-tailed deer (Odocoileus hemionus sitchensis) were introduced to Haida Gwaii (British Columbia, Canada) in the late 19th century, and they have dramatically affected the regeneration of woody species in both old- and second-growth forests since then. The lack of recruitment in western redcedar (Thuja plicata) in old-growth forests has been attributed to deer. The objectives of this study were to (1) experimentally confirm that deer browsing causes a lack of western redcedar recruitment and (2) assess the potential for and speed of recovery after a prolonged exclusion of deer. We installed a set of 20 enclosures and monitored them over a period of 8 years from 1997 to 2005. We compared temporal changes in redcedar cover and in the survival and growth of marked seedlings in plots that were or were not accessible to deer. Redcedar cover in the vegetation layer accessible to deer was generally low (from 3 to 5%) but higher inside the enclosures (an average difference of 2.3%). Protected seedlings survived better, were higher, presented more leafed shoots, and had less stems than unprotected individuals, features that suggest that deer were key to the lack of regeneration. However, growth was very slow (protected marked seedlings grew 2.5 cm on average in 8 years) and, under the current conditions, the time required for a protected seedling to escape deer would probably take over two decades. This very slow growth rate under closed canopy conditions probably reflects a gap-phase regeneration strategy and/or sensitivity to competition with other woody species. The combination of a very slow growth with a high palatability and a lack of physical defences, in contrast to the other dominant conifers in this ecosystem, probably explain why redcedar regeneration can be eliminated from old-growth forest by abundant deer populations.     

Habitat selection by ungulates in relation to predation risk by wolves and humans in the Białowieża Forest, Poland

Predation risk is an important factor influencing the distribution of ungulates and their impact on forest structure. However, simultaneous predation risk by wolves and humans is rarely considered in the analyses of habitat selection by ungulates. We counted ungulate pellets on transects to analyse the influence of wolves and humans on ungulate density distribution in the Białowieża Forest, Poland. We assessed whether (1) forest exploitation influenced ungulate habitat selection, (2) ungulate density was higher in areas without human hunting, (3) ungulates avoided the surroundings of a major road, (4) prey density was higher in the strife zone between home ranges of wolf packs both in the presence and absence of human hunting, (5) ungulates avoided areas selected by wolves, and (6) wolf kill sites were in high prey density areas. Red deer (Cervus elaphus) and wild boar (Sus scrofa) selected unexploited over exploited forests and areas without hunting, whereas roe deer (Capreolus capreolus) preferred exploited forests. Wild boar and European bison (Bison bonasus) avoided the area within 300 m of a major road, whereas we could not detect any avoidance by red or roe deer. Prey density was not higher in wolf strife zones, regardless of human hunting. Ungulates did not avoid areas selected by wolves. Wolves killed red deer in areas with prey density of about 4 red deer/km², regardless of whether the average red deer density in those areas was higher or lower. We conclude that habitat alteration by forest exploitation and hunting by humans influenced the density distribution of ungulates more than predation risk by wolves.     

Population growth rate of a common understory herb decreases non-linearly across a gradient of deer herbivory

Overabundant white-tailed deer (Odocoileus virginianus) are a significant management problem in North America that exerts unprecedented herbivory pressure on native understory forest communities. Conserving understory plant populations requires quantifying a sustainable level of deer herbivory. To date, most population projection models consider only deer presence and absence. To estimate population growth rate along a gradient of herbivory, we focused on Trillium grandiflorum because it is a common understory species and a bellwether of deer effects and forest decline. We used matrix population models, and employed both prospective and retrospective analyses using a regression life table response experiment (LTRE).     

Gap-, stand-, and landscape-scale factors contribute to poor sugar maple regeneration after timber harvest

Natural regeneration in canopy gaps is a key process affecting long-term dynamics of many forests, including northern hardwood forests. The density and composition of regenerating trees are often highly variable, reflecting sensitivity to a suite of driving factors operating at different scales (e.g., harvest gap to regional landscape), including production of seeds, physical characteristics of gaps and stands, competition with non-tree vegetation, and browsing by animals. Multivariate analyses over broad geographic areas provide insights into the relative effects of these factors and permit exploration of spatial patterns in regeneration. We examined the effects of gap-, stand-, and landscape-scale factors on densities of tree seedlings (<1 m tall) and saplings (1-2 m tall) in 59 selection-harvested northern hardwood stands located across a 4500 km² region of Michigan's Upper Peninsula. We used Bayesian multilevel modeling to account for the hierarchical structure of the data and assess uncertainty in parameter estimates. Sugar maple (Acer saccharum) saplings were absent from 61% of 154 m² plots centered in harvest gaps (n = 347) despite its high shade tolerance and overstory dominance, but densities were high in other gaps. Densities of sugar maple seedlings and/or saplings were negatively associated with a combination of greater stand-scale densities of white-tailed deer (Odocoileus virginianus), greater gap-scale cover of non-tree vegetation, and lower gap-scale light availability, with deer density having the greatest effect. Densities of unpalatable and commercially less valuable ironwood (Ostrya virginiana), the second most common regeneration species, were positively related to gap-scale seed-production potential but were unrelated to factors affecting sugar maple. Ironwood tended to replace sugar maple saplings in areas with high deer density. At the landscape scale, densities of sugar maple seedlings and saplings decreased with decreasing latitude and snow depth and increasing winter deer densities. These inverse spatial patterns suggest that deer herbivory can lead to landscape-scale variation in regeneration success. However, the spatial distribution of habitat types (a proxy for soil moisture and nutrient conditions) confound this observation, with higher densities of sugar maple generally located on stands with less nutrient-rich habitat types. Results demonstrate that combinations of factors operating at different scales, and with different relative magnitudes of impact, contribute to high variation in regeneration composition and density following timber harvest. Selection silvicultural practices, as currently applied, do not ensure regeneration of desirable species; practices might require modifications in general (e.g., increasing gap size) and to match them to regionally varying factors like deer density.     

Deer Browsing and Impact on Forest Development

Abstract

Browsing by large herbivores can cause serious costs in forest production. The most important costs related to browsing damage are insufficient stand regeneration, volume losses and timber quality impairment. In Sweden, moose (Alces alces)and roe deer (Capreolus capreolus) are the most abundant deer species and therefore cause the majority of browsing damage. Balancing costs and benefits related to deer density is difficult since few data about the final losses in timber volume or quality impairment is available. Simulations of browsing and the impact on stand growth suggest that a minor or moderate browsing intensity has little effect on final harvest volume. The reason is that trees seem to recover some of the lost growth as they grow out of reach of the animals. Balancing between different uses of the forestry resource is complicated by our limited capability to predict the impact of different management regimes.     

Tree mortality and habitat shifts in the regeneration trajectory underneath canopy of an old-growth subalpine forest

To better understand tree regeneration trajectories and the resultant coexistence of Abies with co-dominants, Picea jezoensis var. hondoensis, Tsuga diversifolia and Betula ermanii, in an old-growth subalpine forest, we investigated spatial mortality patterns during the regeneration of Abies mariesii and A. veitchii, which are abundant in the understory reflecting their shade tolerance. Regeneration of these Abies spp. from shaded understory to canopy status is affected by other canopy co-dominants. Snags of understory Abies spp. were common, suggesting that the primary mortality agent is suppression by the overstory. Although live, small Abies trees in the understory were positively associated with a Picea canopy, the long-term survival was reduced among Abies trees close to the canopy, suggesting that shading by large Picea in the overstory negatively affects understory Abies plants. The existence of shade-intolerant canopy co-dominants such as Picea and also Tsuga, which are larger and longer lived than the shade-tolerant Abies, may play an important role in preventing the Abies spp. from competitively displacing these other tree species, which are much rarer in the understory, though common in the canopy. Moreover, in spite of the fact that Betula canopies fostered recruitment and growth of Abies saplings, Abies showed no association with Betula canopy and their survival at later-stage was rather reduced near or beneath Betula canopies at the subsequent understory small tree stage. Based on spatially significant events related to tree death, this study detected such “habitat shifts” in the trajectory of tree regeneration. Accordingly, it can be concluded that careful consideration of the regeneration habitat is required for a fuller understanding of ecological processes in spatially complex old-growth forest systems.     

Deer-proof fence prevents regeneration of Picea jezoensis var. hondoensis through seed predation by increased woodmouse populations

The indirect consequence of sika deer (Cervus nippon) grazing on the regeneration of Picea jezoensis var. hondoensis through its effect on the small rodent population was assessed by evaluating seed predation by mice inside and outside the deer-proof fence on Mt Ohdaigahara, central Japan. Following construction of the deer-proof fence, Sasa nipponica, which covers the forest floor and is the main forage of deer, grew thicker inside the fence due to the absence of deer grazing than outside of the fence. Higher numbers of two species of woodmice (Apodemus speciosus and A. argenteus) were found inside the fence, possibly because the thick S. nipponica growth provides better cover. Inside the fence, more seeds of P. jezoensis var. hondoensis were eaten by the mice than outside the fence. These results suggest that the mouse populations inside the fence increased due to changes in S. nipponica cover, which subsequently increased seed predation by the mice, possibly preventing regeneration of P. jezoensis var. hondoensis.     

Long-term changes of structure and tree species composition in Dinaric uneven-aged forests: are red deer an important factor?

Uneven-aged silver fir-European beech forest stands were studied to (1) analyse the dynamics of diameter structure and tree species composition in the past two centuries and (2) determine the impact of red deer on the regeneration and recruitment of silver fir. The study used current data on forest stands, archival data from old forest management plans for the period 1789–2004, and red deer harvesting records for the period 1907–2006. During the observation period, the silver fir population aged and silver fir and European beech alternated in dominance. The study revealed a strong impact of red deer on the composition and recruitment of tree regeneration, especially on silver fir regeneration. The drastic changes in red deer density (from extermination up to 5.8 animals km⁻²) and past forest management practices were apparently the main factors driving the population dynamics of silver fir (regeneration, recruitment, and diameter structure) in the study area during the past two centuries.     

Can spines deter deer browsing?: a field experiment using a shrub <Emphasis Type="Italic">Damnacanthus indicus</Emphasis>

We examined the defensive function of spines of Damnacanthus indicus against deer herbivory by experimentally removing spines in the field where deer density is high. Individuals of D. indicus whose spines had been removed had a higher probability of being browsed by deer than control individuals. In addition, plant height was significantly lower for individuals that were browsed than those that were not. These results support the notion that spines of D. indicus have a function to deter deer browsing. This study is the first to demonstrate experimentally the defensive function of spines against large mammals in temperate areas.     

Testing prescribed fire as a tool to promote Scots pine <Emphasis Type="Italic">Pinus sylvestris</Emphasis> regeneration

Techniques for encouraging natural tree regeneration are of increasing interest to managers of Scots pine Pinus sylvestris forests. We tested prescribed fire at a management scale, with deer present or excluded, as a means of increasing rates of Scots pine forest expansion on heathland. At a semi-natural pinewood in Scotland, ten experimental blocks were established, within range of pine seed-fall. Each block comprised four, 100 m² plots. Two plots at each site were burnt and two fenced, allowing the effects of burning on pine regeneration to be measured at two levels of deer abundance. We monitored pine seedlings, seed-fall, deer dung and vegetation for 5 years following treatment. Differences in seedling detection rates between treatments were quantified using dummy seedlings, and analyses corrected accordingly. Mean new pine seedling establishment was 9.8 times higher on burnt ground than unburnt ground (confidence intervals 3.2–30). Differences were even more pronounced in a year of high seed-fall, and following fires with low rates of spread. Establishment rates varied strongly between experimental blocks. Exclusion of deer increased establishment rates, but only in the first 2 years after fire. There was evidence that both seedling survival, and cumulative recruitment of older (over 12 months) seedlings, were improved by prescribed burning. Our results support the use of prescribed fire as a tool to promote increased Scots pine seedling establishment. This technique is likely to give strong fine-scale variation in seedling densities, and so would most suit areas where a variable spatial pattern of regeneration is sought, for landscape or naturalness reasons. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Expansion of dwarf bamboo, Sasa nipponica, grassland under feeding pressure of sika deer, Cervus nippon, on subalpine coniferous forest in central Japan

The long-term changes of the area of Sasa nipponica grassland, bark stripping damage to trees by sika deer, Cervus nippon, and sika deer densities on Mt. Ohdaigahara were assessed with aerial photographs, information from previous studies, and field experiments. The grasslands expanded year after year. The expansion rate was highest from 1982 to 1992, and lowest from 1992 to 1997. Bark-stripping damage on coniferous trees was heavier than that on broadleaf trees in both the early 1980s and 1998. The deer density in the eastern part of the area had been high (14.4–64.3 km⁻²) from 1984 until 1998. We presume that rich growth of S. nipponica and heavy feeding pressure by sika deer over the long term caused expansion of S. nipponica grasslands by killing canopy trees by bark stripping and preventing forest regeneration.     

Establishment and growth of coniferous seedlings in an altered forest floor after long-term exclusion of deer

We evaluated differences between the forest floors and the establishment and growth of coniferous seedlings in fenced (13 years) and unfenced plots on Mt Ohdaigahara where the sika deer (Cervus nippon) population density is high. Large coniferous seedlings (height > 0.05 m) were less abundant in the unfenced plot, as a result of deer browsing. Small coniferous seedlings (height < 0.05 m), however, were more abundant in the unfenced plot, where most seedlings of Abies homolepis were found on bare ground and those of Picea jezoensis var. hondoensis were found on buttresses and fallen logs. The large area of bare ground in the unfenced plot was caused by deer browsing. Deer therefore have an indirect effect on the emergence and growth of small coniferous seedlings by modifying the forest floor.     

Effects of deer and mice on seedling survival in a temperate mixed forest of Japan

To detect the magnitude of indirect positive effects of deer and mice on seedling survival of some woody species in a Japanese temperate forest, we analyzed the data from an earlier field experiment using a hierarchical Bayesian approach. The forest studied was inhabited by sika deer (Cervus nippon) and mice (Apodemus spp.), and the floor was covered with dwarf bamboo, Sasa nipponica, which negatively affected tree seedlings and was in turn negatively affected by deer and mice. The field experiment was designed as the combination of exclusion or removal of these factors: deer, mice and dwarf bamboo. A hierarchical Bayesian model was constructed and the parameters were estimated by the Markov chain Monte Carlo method. The model successfully showed the indirect positive effects of deer and mice, which improved the survival rate of five cohorts of the seedlings of three tree species by ameliorating the direct negative effect of dwarf bamboo. The cohorts studied were formed by the seedlings of Abies homolepis that emerged in 1997 and 2002, those of Fraxinus lanuginosa f. serrata that emerged in 1998 and 2002, and those of Fagus crenata that emerged in 1999. The positive indirect effect was especially large in F. crenata, which is known to be severely affected by dwarf bamboo. The total effect of deer was shown to be positive for all cohorts except the A. homolepis cohort that emerged in 1997, at which time the dwarf bamboo had not yet fully recovered from the browsing pressure of deer. The total effect of mice was shown to be positive for all of the cohorts. We conclude that these positive effects were due to the large negative effect of dwarf bamboo on the seedlings.     

Habitat use of large ungulates in northeastern Germany in relation to forest management

Estimating large herbivore density has been a major area of research in recent decades. Previous studies monitoring ungulate density, however, focused mostly on determining animal abundance, and did not interpret animal distribution in relation to habitat parameters. We surveyed large ungulates in the Biodiversity Exploratory Schorfheide-Chorin using faecal pellet group counts. This allowed us to explore the link between relative ungulate abundance, habitat use, and browsing damage on trees in a region with several types of forest, including unharvested and age-class beech forests, as well as age-class pine forests. Our results demonstrate that roe deer and fallow deer relative abundance is negatively correlated with large tree cover, and positively correlated with the cover of small shrubs (Rubus spec., Vaccinium spec.), and winter food supply. Habitat use of roe deer and fallow deer, as estimated by counting faecal pellet groups, revealed a preference for mature pine forests, and avoidance of deciduous forests. This differential habitat use is explained by different distributions of high quality food resources during winter. The response of deer to understory cover differed between roe deer and fallow deer at high cover percentages. The amount of browsing damage we observed on coniferous trees was not consistent with the relative deer abundance. Browsing damage was consistently higher on most deciduous trees, except for beech saplings which sustained less damage when roe deer density was low. Because roe deer is a highly selective feeder, it was reported to affect tree diversity by feeding only on trees with high nutritional value. Consequently, we propose that managing the number of all deer species by hunting is necessary to allow successful forest regeneration. Such an adjustment to deer numbers would need to account for both current tree diversity and alternative food resources. Our findings may be applicable to other forest landscapes in northeastern Germany including mature pine stands and differently harvested deciduous forests.     

Growth of <Emphasis Type="Italic">Euterpe edulis</Emphasis> Mart. (Arecaceae) under forest and agroforestry in southern Brazil

The palm Euterpe edulis has high ecological and economic importance in Brazil. Currently, this species is being cultivated and managed for spontaneous regeneration in banana plantations. However, there are no data comparing its plantation growth performance to its native forest growth. We evaluated growth and mortality (M) of individuals of E. edulis planted in secondary dense ombrophilous forest and in banana plantations, as well as their relationships with site variables (canopy opening, soil nutrient availability, density of existing E. edulis, and herbivory). Twelve banana plantation sites and 12 sites in secondary dense ombrophilous forests were selected. At each site, 25 young individuals of E. edulis were planted in 2003. Annually until 2008, morphometric, herbivory, and M of the individuals were evaluated. In 2008, canopy and soil variables were measured at each plot. E. edulis growth was five times higher in banana plots compared to forest plots; current annual increment on height reached 38.9 cm in banana plots, compared to 7.3 cm in forest plots. M was relatively low and similar at both sites, presenting an intraspecific density-dependence pattern. Significant correlations were found between morphometric variables, M, and herbivory of E. edulis and canopy and soil variables. Euterpe edulis presented plasticity that allows for its establishment in banana plantations, indicating high potential for management in agroforestry consortia. Such management may be a useful conservation strategy for this and other shade-tolerant species.     

Woodland salamander response to two prescribed fires in the central Appalachians

Using coverboard arrays, we monitored woodland salamanders on the Fernow Experimental Forest in the central Appalachian Mountains, West Virginia, USA prior to and following two prescribed fires in mixed oak (Quercus spp.) forest stands. Treatments were burn plots on upper slopes or lower slopes fenced to prevent white-tailed deer (Odocoileus virginianus) herbivory or control plots that were unfenced and unburned. Most of the 7 species we observed were the mountain dusky salamander (Desmognathus ocropheaus), red-backed salamander (Plethodon cinereus) and slimy salamander (Plethodon glutinosis). Significant population responses were difficult to interpret with numerous treatment and year interactions. Results largely were equivocal. We found no change in woodland salamander assemblage prior to burning or afterwards. There were few differences in adult to juvenile ratios of salamanders among treatments. Still, a priori contrasts of mountain dusky salamanders and red-backed salamander counts corrected for detection probability were greater under coverboards in the 2 years monitored after both prescribed fires had occurred than before burning or in unburned controls. This suggests that these species responded to the reduced leaf litter on the forest floor by utilizing coverboards more. Similarly, the three predominate species of salamanders also were more numerous under coverboards in plots subjected to deer herbivory with less subsequent forest floor vegetation as compared to those burned plots that were fenced. Our observations would suggest that woodland salamanders somewhat are tolerant of two prescribed fires within close temporal proximity. However, because woodland salamanders can be significantly reduced following timber harvest, continued research is needed to fully understand impacts of fire as a pre-harvest management tool in central Appalachian forests.