Deer Browsing Pressure Research Articles

The presence of wolves leads to spatial differentiation in deer browsing pressure on forest regeneration

With the recent return of large carnivores to forest ecosystems, an important issue for forest owners and managers is how large predators influence the behaviour of their natural prey and, consequently, cervid browsing pressure on forest regeneration. To investigate this issue, we analysed deer pressure on Scots pine and European beech plantations in northern Poland's ecosystems with and without permanent wolf populations. Two characteristics were used to describe deer browsing patterns in plantations: distance from the forest edge (spatial pattern of browsing) and number of saplings browsed (browsing intensity). Beech saplings were more intensively browsed by deer compared to pine saplings. In a forest ecosystem not inhabited by wolves, spatial variation in browsing patterns on small-sized beech plantations was the same between the edge and the center. In contrast, browsing pressure by deer was greater at the edges on large-sized pine plantations. The presence of wolves reduced deer browsing on beech and increased browsing on pine saplings. In addition, deer foraging behaviour changed in large-sized pine plantations, and browsing pressure increased only in the central areas of the plantations. We assume that the presence of wolves in a forest landscape is an important factor that alters browsing pressure on the youngest stands and their spatial pattern, and that this may be a major factor in stand regeneration, especially in small forest patches.

Impact of white-tailed deer on northern white cedar recruitment beyond the browsing vulnerability zone: A broad-scale study

By selectively feeding on the new vegetation from forest regeneration, cervids influence the abundance of understory species. This can lead to recruitment failure for the consumed species. The impact of cervids on regeneration is generally studied at fine scales, making it difficult to generalize about the species distribution scale for co-occurring cervid and tree species. This is especially problematic for species such as northern white cedar (Thuja occidentalis L.), which is susceptible to browsing and has been declining throughout much of its range. In our study, we aimed to determine the effect of the spatio-temporal variations in white-tailed deer (Odocoileus virginianus Zimmerman) abundance on northern white cedar recruitment over a broad spatial scale. We combined distinct forest and wildlife monitoring databases for two Canadian provinces, Québec and New Brunswick. We also surveyed cedar regeneration and deer browsing pressure in a subset of plots located in Québec. In both provinces, we detected a negative effect of deer abundance and deeryard proximity on the recruitment of cedar saplings. In Québec, local deer browsing pressure also had a negative effect on cedar seedling abundance. As deer browsing pressure can vary considerably over a short distance, we recommend the inclusion of browsing indicators in national forest surveys to precisely assess the impact of browsing on regeneration and recruitment. This would allow managers to identify areas that are highly vulnerable to browsing and adapt forest management accordingly to reduce browsing-induced regeneration failures.

Reduction in local white-tailed deer abundance allows the positive response of northern white cedar regeneration to gap dynamics

Gap dynamics play a crucial role in forest regeneration by creating favourable regeneration and survival niches for some plant species. Nonetheless, potentially overriding factors, such as ungulate browsing, could limit or eliminate this gap dynamic-related regeneration. The deleterious effects of browsing may be exacerbated for slow-growing species such as northern white cedar ( Thuja occidentalis L.), a tree highly selected by white-tailed deer ( Odocoileus virginianus Zimmerman). We therefore aimed to understand how deer browsing and gap dynamics interact to affect cedar regeneration and hypothesized that cedar regeneration benefits from natural gaps but that deer browsing could override this effect. We inventoried natural canopy gaps along a spatiotemporal gradient of deer habitat use. We evaluated cedar regeneration abundance, tree height, and various gap, stand, and competition metrics. We found that deer browsing pressure greatly limited cedar regeneration; however, when deer populations decreased, cedar regeneration abundance increased within a decade, even after prolonged browsing pressure, and increased further over time. Our study illustrates that cedar regeneration can be favoured by gap creation and deer population control.

The browsing ratio as an index of the impact of deer browsing on tree seedlings

The possibility of maintaining forested vegetation under deer browsing depends on the success of regeneration processes. Several indices have been proposed to estimate the pressure of deer browsing based on the incidence of browsing on woody plant seedlings. The maintenance of seedling populations is critically dependent on managing deer populations under the threshold levels. For two years we monitored the survival and growth of broad-leaved seedlings and the incidence of deer browsing on these seedlings to examine the effectiveness of the browsing ratio (i.e., the ratio of browsed seedlings per year to the total number of available seedlings, regardless of browse species preferences) as an index of deer browsing pressure, along with snow depth, dwarf bamboo Sasa spp. abundance, and light availability in nineteen 100 m2 plots located in natural forests in Hokkaido, northern Japan. The abundance of deer as indicated by the number of deer observed via spotlight count surveys was associated with the abundance of Sasa spp. and the snow depth. The browsing ratio was associated with snow depth, Sasa spp. abundance, light availability, and deer abundance. This result suggests that the impact of deer on seedlings differs depending on the local environment, even under similar deer abundance. Sasa spp. abundance had a negative effect on seedling density and survival. Sasa spp. are an important food source for wintering deer and deer reduce their biomass, whereas dense Sasa cover had a protective effect on small seedlings. Thus, Sasa spp. affects seedlings in various ways. Changes in both the number and mean height growth of seedlings in each plot were explained with the browsing ratio alone. Thus, the browsing ratio directly summarizes the effect of deer on seedlings. The estimated threshold browsing ratio was 0.27 for changes in seedling density and 0.51 for height growth of seedlings.

The long‐term impacts of deer herbivory in determining temperate forest stand and canopy structural complexity

Abstract Ungulates place immense consumptive pressure on forest vegetation globally, leaving legacies of reduced biodiversity and simplified vegetative structure. However, what remains unresolved is whether browse‐induced changes occurring early in succession ultimately manifest themselves in the developed forest canopy. Understanding the development and persistence of these legacies is critical as canopy structure is an important determinant of forest ecosystem functions such as carbon sequestration and wildlife habitat. We measured how white‐tailed deer Odocoileus virginianus browse during stand initiation affected canopy structure, tree species richness, diversity, stem density, and basal area on Pennsylvania's Allegheny Plateau using a portable canopy LiDAR system. We capitalized on an historic deer enclosure experiment where forests were subjected to four deer densities (4, 8, 15, and 25 deer/km2) for 10 years following stand initiation. Deer browsing impacts on the forest canopy are apparent nearly four decades since stand initiation. The highest deer density treatment experienced a significant reduction in tree species diversity, density, and basal area with stands becoming dominated by black cherry Prunus serotina. Reductions in overstorey diversity and tree density resulted in a more open canopy with low leaf area and high horizontal leaf variability. Canopies were tallest at the lowest and highest deer densities. Synthesis and applications. Using a portable canopy LiDAR system and a former deer enclosure experiment, we show that high deer browsing pressure during stand initiation can have a decades‐long impact on stand and canopy structure. High deer densities led to stands with lower species diversity and tree density, which resulted in canopies that were taller and less dense. As remote sensing of the canopy becomes more prevalent, considering the legacy of ungulate herbivory on canopy structure may inform both land management and our understanding of ecological function, such as forest carbon sequestration, maintenance of diverse understory communities, and creation of wildlife habitat

The protective effect of dwarf bamboo on broad-leaved seedlings against deer browsing

The forest understory is often densely covered by dwarf bamboo Sasa spp. in Hokkaido, northern Japan. Dwarf bamboo is an important winter forage for sika deer Cervus nippon, and dwarf bamboo inhibits forest regeneration by preventing seedling establishment. Although percentages of browsed seedlings are a useful index of deer browsing pressure, dwarf bamboo is also expected to affect browsing on seedlings by altering physical accessibility to seedlings. We hypothesized that the relationship between seedling height and browsing incidence depends on the coverage of dwarf bamboo because smaller seedlings were protected by dwarf bamboo. The incidence of browsing was recorded for broad-leaved seedlings ≥ 0.2 m in height and < 1 cm in diameter at breast height in 19 plots (40–200 m2 each) located in natural forests. Coverage of dwarf bamboo varied from 3% to 74%, except for one plot where dwarf bamboo was absent. The mean height of the dwarf bamboo in each plot was 0.44–1.17 m. The product of seedling height and coverage of dwarf bamboo was selected as an explanatory variable in the generalized linear mixed model for the incidence of browsing on each seedling. This suggests that smaller seedlings were protected by dwarf bamboo and browsing incidence is low for smaller seedlings within plots with high coverage of dwarf bamboo. This can be an example of associational defense against deer. The probability of browsing incidence was low for seedlings 0.2–0.5 m tall in sites with dense dwarf bamboo, while no significant differences were detected between seedlings 0.5–0.8 m tall and those ≥ 0.8 m. Even though seedlings under dwarf bamboo cover tend to escape from deer browsing, dwarf bamboo directly affects seedling survival and growth negatively. Therefore, it is necessary to manage both the deer population and dwarf bamboo to enable forest regeneration. The percentages of seedlings with browsing incidence on seedlings ≥ 0.5 m tall can approximate the browsing level and is useful for management decisions.

Complex drivers of sugar maple (Acer saccharum) regeneration reveal challenges to long-term sustainability of managed northern hardwood forests

Single-tree selection silviculture management of northern hardwood forests relies on natural tree regeneration for long-term sustainability, yet current trends in tree regeneration and recruitment elicit concern. Low densities of economically valuable sugar maple (Acer saccharum Marsh.) in understories are often common, likely driven by many factors, including deer browsing, management-dictated stand structure, and site fertility/moisture regimes. However, landscape sugar maple regeneration patterns and relationships with underlying factors are largely unknown. We quantified associations of spatially varying factors with sugar maple regeneration using detailed vegetation and white-tailed deer winter fecal pellet surveys from 141 northern hardwood stands in Michigan, managed for decades with single-tree selection silviculture. We developed models of plot-level sugar maple regeneration counts for three key size classes as a function of plot- and stand-level predictors, including deer use, forest structure, and site quality. Among our 141 stands, sugar maple seedlings (<50 cm tall) were consistently abundant, averaging 69,000 stems ha−1 and present in 76% of plots per stand, on average (25 plots per stand, each 2 m2). In contrast, small (50–137 cm tall) and large (>137 cm tall and < 5 cm DBH) sugar maple sapling densities were much lower, averaging 2,300 and 1,100 stems ha−1, respectively, and occurring, on average, in 31% and 32% of plots per stand (25 plots per stand, each 12.6 m2). Under a wide range of potential sugar maple stocking criteria, most stands are understocked in saplings; e.g., only 29% of stands had > 2,500 small sugar maple sapling stems ha−1 and 35% of stands had > 1,000 large sapling stems ha−1. Based on our models, sapling densities negatively associated with deer use and were more abundant on medium than high quality sites. Across all size classes, negative associations with subcanopy trees and/or shrub densities suggest light limitation, whereas positive associations with sugar maple canopy trees > 25 cm DBH suggest persistent seed limitations. Overall, our study supports need for alternative forest and/or deer management strategies over much of the range of northern hardwood forests in Michigan to promote higher densities of large sugar maple regeneration for canopy recruitment. Medium quality sites with abundant large sugar maple canopy trees and low deer browsing pressure (for example, the deep snow region in the north western Upper Peninsula) are the exception; high sugar maple sapling densities suggest forests in this region are thriving under single-tree selection management.

Deer browsing and shrub competition set sapling recruitment height and interact with light to shape recruitment niches for temperate forest tree species

For temperate forests, Michigan, USA, we asked: (1) does evidence exist for a height-dependent sapling recruitment bottleneck caused by shrub layer competition and white-tailed deer (Odocoileus virginianus) browsing and, if so, (2) how do these factors interact with light availability in shaping species-specific sapling recruitment niches? The combination of deer browsing effects on tree population height structure and the effects of deer and shrubs on tree seedling height and mortality indicate a shrub-deer bottleneck for sapling recruitment. The maximum height of deer browsing and shrub canopies as well as height dependent seedling mortality rates revealed a critical height threshold of approximately 2 m, above which trees escape from shrub and deer effects (i.e. sapling recruitment). Species’ sapling recruitment niches differed across a harvest gap size (i.e. light availability) gradient, over which tree seedling height growth potential and shrub competition increased with gap size, but deer browsing probability decreased. Specific to forests with high deer populations, niche differences were related to species growth and survival properties, with species generally falling into three groups: High Light, Broad, and Nowhere. High Light species incur high mortality from shade, shrub competition, and deer browsing. However, recruitment can occur in recently formed large gaps, where seedlings growing within developing shrub canopies avoid competition via rapid height growth and are partially protected from browsing deer. Broadspecies have lower mortality in shade and grow slower thanHigh Lightspecies and, compared to other groups, exhibit height growth and mortality rates that are less-sensitive to deer browsing pressure.These properties allowssapling recruitment ofBroadspecies over a wide range of gap sizes, including larger gaps, where, via advance regeneration, they can avoid competition with developing shrub layers following canopy disturbance.Nowherespecies are similar toBroadspecies, except their height growth and mortality are sensitive to deer browsing pressure. Heavy browsing pressure in smaller gaps and an inability to maintain taller advance regeneration to contend with shrub competition when large gaps form leaveNowherespecies without a sapling recruitment niche. In conclusion, we provide evidence that shrub competition, ungulate browsing, harvest gap size and their interactions constrain height-specific sapling recruitment among tree species in the temperate forests of Michigan. Similar mechanisms may operate in other forests with high ungulate populations. Given these drivers and current management regimes, increasing tree species diversity and resilience to future disturbances may hinge on changing and integrating forest and ungulate management paradigms.

Tree Regeneration Structure Following Beech Bark Disease-Motivated Harvests: Factors Associated with Patterns and Management Implications

In the northern Appalachian region of North America, mortality of mature American beech (Fagus grandifolia Ehrh.) via the introduced beech bark disease (BBD) can result in dense thickets of beech saplings that inhibit the regeneration of other species. It is unknown if similar structures characterize more recently infested managed forests in the Great Lakes region. If these dense beech sapling layers do exist, management would be aided by knowing which site/regional factors they are associated with and by identifying particular sapling structures that may threaten the sustainability of these forests under current management paradigms. To examine these patterns, we used a natural experiment with sample plots in 69 unevenly aged, selection silviculture-managed, maple (Acer spp.)-dominated northern hardwood stands. Our stands were dispersed across northern Michigan, USA and had undergone BBD-motivated partial harvests favoring beech removal (mean = 5.5 years before measurement). In each stand, we quantified tree regeneration structure in relation to winter deer use (fecal pellet count density), site quality (habitat type), geographic region (Eastern Upper Peninsula and Northern Lower Peninsula), and multiple measures of overstory stand density. We also examined the density effects of taller regeneration strata on subordinate strata. Across sites, the small sapling recruit class (i.e., &gt;137 cm tall and &lt;5 cm diameter at 137 cm tall) was dominated by beech and was often dense (44% of subplots &gt; 2000 stems ha−1 and 16% of subplots &gt; 5000 ha−1) but never exceeded the &gt; 10,000 stems ha−1 reported in the northern Appalachian region. Beech sapling density was higher in the Northern Lower Peninsula, on lower quality sites, at lower postharvest overstory densities, and on sites with higher densities of preharvest overstory beech. In contrast to the beech-dominated small sapling recruit class, seedlings (i.e., &lt;25 cm tall) were generally more species diverse than sapling strata and were dominated by maple species. Although generally dense, seedling density was negatively related to small sapling recruit density, suggesting that saplings may suppress the seedling stratum. The general pattern for the small sapling recruit layer of browsing-insensitive beech (and ironwood, Ostrya virginiana Mill. K. Koch) dominance and low representation of browsing-sensitive species (e.g., Acer spp.) circumstantially supports the notion that regeneration structure is heavily influenced by deer. However, current deer use was generally low in our stands, and relationships with tree regeneration structure were weak. Instead, regeneration structure is likely shaped by a combination of factors operating at long time scales (i.e., legacies of deer browsing pressure, selection silviculture (given beech and ironwood are shade tolerant), overstory composition, and site quality) and by those effects that are more proximal, such as postharvest overstory density. Minimum stocking criteria for species considered desirable for management (e.g., sugar maple and Acer saccharum Marshall) suggest many stands are inadequately stocked in the sapling recruit classes. Although future regeneration dynamics are unclear, current patterns suggest that many stands with high beech/ironwood small sapling recruit densities may require management intervention to overcome insufficient recruitment of species targeted for management.

Rethinking Northern Hardwood Forest Management Paradigms with Silvicultural Systems Research: Research–Management Partnerships Ensure Relevance and Application

AbstractWell-designed, robust partnerships are important for addressing forest resource problems that intersect diverse ownerships, management, and research expertise. Here, we describe a case study of a research–management partnership. First, we describe the rationale and design of a large-scale, long-term experiment focused on increasing the sustainability and resilience of managed northern hardwoods. With the experiment as context, we next discuss details of the partnership contributing to its success, as well as some challenges. The experiment is a Michigan-wide evaluation of silvicultural alternatives to the current paradigm of selection silviculture. Our goal is to identify combinations of silvicultural treatments that increase the diversity and density of sapling recruits over forested landscapes where deer-browsing pressure and other limiting factors are at play. Pretreatment data from our 140-site study bolstered justification for the research, with sapling and pole classes generally lacking adequate stocking and diversity of desirable species over large geographic extents. We anticipate that the wildlife and forestry research–management model used to develop this project will be useful for other rapidly developing challenges that cannot wait decades to be addressed without potentially catastrophic losses to forest function. Key beneficial elements of our partnership include: (1) sensitivity to information needs at all management levels (i.e., policy to practice), (2) capitalizing on differences in strengths and reward systems of managers and scientists, (3) structured, frequent scientist–manager meetings to maintain focus on research and management application outcomes, (4) long-term relations and support that build trust, and (5) explicit wildlife and forestry collaboration on problems requiring joint expertise. Challenges to our partnership generally revolved around communication, workload, and implementation issues, and were partially alleviated by planning.

Red oak seedlings as indicators of deer browse pressure: Gauging the outcome of different white-tailed deer management approaches.

After decades of high deer populations, North American forests have lost much of their previous biodiversity. Any landscape‐level recovery requires substantial reductions in deer herds, but modern societies and wildlife management agencies appear unable to devise appropriate solutions to this chronic ecological and human health crisis. We evaluated the effectiveness of fertility control and hunting in reducing deer impacts at Cornell University. We estimated spring deer populations and planted Quercus rubra seedlings to assess deer browse pressure, rodent attack, and other factors compromising seedling performance. Oak seedlings protected in cages grew well, but deer annually browsed ≥60% of unprotected seedlings. Despite female sterilization rates of >90%, the deer population remained stable. Neither sterilization nor recreational hunting reduced deer browse rates and neither appears able to achieve reductions in deer populations or their impacts. We eliminated deer sterilization and recreational hunting in a core management area in favor of allowing volunteer archers to shoot deer over bait, including at night. This resulted in a substantial reduction in the deer population and a linear decline in browse rates as a function of spring deer abundance. Public trust stewardship of North American landscapes will require a fundamental overhaul in deer management to provide for a brighter future, and oak seedlings may be a promising metric to assess success. These changes will require intense public debate and may require new approaches such as regulated commercial hunting, natural dispersal, or intentional release of important deer predators (e.g., wolves and mountain lions). Such drastic changes in deer management will be highly controversial, and at present, likely difficult to implement in North America. However, the future of our forest ecosystems and their associated biodiversity will depend on evidence to guide change in landscape management and stewardship.

Competition and tolerance of low soil water favor Carex dominance over establishing Acer seedlings in managed temperate mesic forests

Carex pensylvanica (Pennsylvania sedge) dominance of managed temperate mesic forest understories is associated with high deer browsing pressure, frequent partial timber harvests, invasive earthworms, and low seedling-to-sapling recruitment rates of canopy-dominant Acer saccharum (sugar maple). There is concern that Carex dominance may be difficult to reverse because Carex may decrease soil resource availability and tolerate these conditions better than Acer seedlings. However, evaluation of this notion and of management aimed at reducing possible Carex impacts on Acer seedlings is lacking. Here, we examine the effects of (1) removing Carex on soil water and the growth and survival of naturally-established Acer seedlings over a forest understory light gradient, (2) drought on growth, survival, water relations, and soil mineral N pools of potted monocultures and mixtures of Carex and Acer seedlings, and (3) summer vs. autumn glyphosate herbicide treatments on Carex and Acer seedling dynamics in group-selection harvest openings. Carex removal increased soil water and growth of naturally established Acer seedlings but only in harvest gaps >8% light. Carex exhibited greater tolerance of drought than Acer by closing stomates at lower pre-dawn water potentials (−4.2 MPa vs. −3.0 MPa), and incurring lower post-drought mortality (~0% vs. 50%). In well-watered Carex-Acer mixtures, Carex reduced soil NH4+ but not Acer growth or survival relative to Acer monocultures. Glyphosate treatments reduced Carex by >90%, but possible benefits to young Acer seedlings may have been offset by greater deer herbivory on Acer in glyphosate treatments. Our results indicate that Carex reinforces its dominance of mesic hardwood forest understories via water competition and drought tolerance mechanisms that slow the establishment and recruitment of canopy dominant Acer. These effects could be exacerbated by increased intensity and frequency of droughts in the future, such that Carex dominance of understories could accelerate the xerophication of these systems in the face of climate change. Herbicides can be used to reduce Carex but may need to be combined with deer population reductions to promote adequate Acer seedling to sapling recruitment for sustainable management by selection silviculture. The sustainability of other temperate mesic forests globally characterized by high ungulate populations and associated high graminoid density may be similarly challenged.

Modelling white-tailed deer impacts on forest regeneration to inform deer management options at landscape scales

Long-term continuity of forest cover in eastern North America may be threatened in part by white-tailed deer (Odocoileus virginianus), but effectively managing that threat requires greater understanding of the multi-scale nature of deer browsing pressure and other forces affecting forest regeneration. We isolated the effects of white-tailed deer on the regeneration of ten ecologically and commercially important tree species across the state of New York by fitting forest regeneration models with variables representing fine-scale plant competition, stand-scale seed dispersal, and region-wide patterns of climate, land-use, and deer abundance. Deer effects were not consistent across species or space. Increasing deer abundance was associated with declining seedling abundance for six species (Acer rubrum, Picea rubens, Pinus strobus, Tsuga canadensis, Prunus serotina, and Abies balsamea), and nonlinear changes (peaking at intermediate deer abundance) for four species (Acer saccharum, Betula alleghaniensis, Fagus grandifolia, Fraxinus americana). Models further indicated: (1) in many areas, meaningful increases in tree seedling abundance may not be achievable by reducing deer abundance alone; (2) in some areas, modest reductions in deer abundance may improve forest regeneration—these would be of high deer management priority; and (3) in some areas, the magnitude of deer population reductions required to achieve meaningful improvement in forest regeneration may be too large to be practical. Predicting forest regeneration across heterogeneous landscapes is complex, and isolating the effects of deer requires greater understanding of multiple potentially limiting and interacting factors (e.g., land-use, climate, forest characteristics). Our approach, based on readily available spatial data, may help prioritize areas, define management goals, and lay a foundation for adaptive management of deer to improve forest regeneration.

Natural regeneration of Pinus pinaster facilitates Quercus ilex survival and growth under severe deer browsing pressure

Maritime pine (Pinus pinaster) stands in Italy have recently been invaded by maritime pine bast scale (Matsucoccus feytaudi), resulting in widespread tree mortality. Clearcutting has been applied to progressively remove infected maritime pine, concurrent with planting of holm oak (Quercus ilex) seedlings. Establishment of holm oak regeneration has, however, been severely constrained by fallow deer (Dama dama) browsing pressure. We hypothesized that the ability of planted holm oak to grow above the browse line was closely correlated with protective cover provided by natural regeneration. This study was carried out in a coastal protected area in central Italy, where overall performance of planted holm oak was assessed under browsing pressure in relation to the relative distance of seedlings to surrounding vegetation. To interpret the results, four categories of holm oak were identified: (i) dead plants, (ii) sprouts from stump, (iii) repeatedly browsed plants without growth, and (iv) plants that were able to develop at least one tall shoot safe from browsing. The occurrence of the latter was positively related to increasing canopy cover and plant height of surrounding vegetation, especially maritime pine. Density, height, canopy cover, and crown radius of maritime pine, as well as its relative distance from holm oak all promoted the survival and growth of target plants. Complete protection around holm oak was not required for facilitation, but the presence of crown and branches occupying the space above the target plants was crucial. Maritime pine saplings primarily served as a mechanical barrier to browsing and thereby favored the development of holm oak shoots. Site-specific, long-term monitoring studies on competition from and facilitation by surrounding vegetation are required, before implementing under-planting strategies.

Ungulate Browsing Limits Bird Diversity of the Central European Hardwood Floodplain Forests

Temperate hardwood floodplain forests along lowland rivers are considered important forest biodiversity refugia in the European cultural landscape. The absence of apex predators combined with an artificial feeding of herbivore populations in winter seasons has caused an increase in browsing pressure on hardwood trees, nearly preventing their regeneration in some localities. There are still important knowledge gaps in understanding the relationships between deer abundance (and browsing pressure) and the abundance (and diversity) of forest bird species in unmanaged hardwood forests. We have studied the red deer and fallow deer browsing pressure in Central European unmanaged hardwood floodplain forests using a novel method based on monitoring browsing pressure along transects combined with bird census data in the Litovelské Pomoraví Protected Landscape Area (Czech Republic). The monitoring data suggested a very high browsing pressure on hardwood trees, causing a strong reduction of the shrub layer and young tree layer (30–210 cm above ground surface). The bird census data from the study area were collected using the territory mapping method. Our results revealed a bird diversity decline in all study plots and the bush nesters guild was found to be completely absent. As bird species from the bush nesters guild are generally common (usually dominant) in hardwood floodplain forest ecosystems with a rich shrub and young tree layer and low browsing pressure, we conclude that intense browsing by large herbivores represents a limiting factor to the bird diversity (especially bush nesters) of hardwood floodplain forests.

The method of conserving herbaceous grassland specialists through silvicultural activities under deer browsing pressure

Early-successional herbs such as grassland specialists in semi-natural grasslands are decreasing because the intensity of using semi-natural grassland is decreasing for the decrease of human population or the usage of fossil fuel as energy source. Deer browsing also decreases grassland specialists. Silvicultural activities such as clearcutting, mowing of understory vegetation, and deer countermeasures may provide a suitable habitat for such grassland specialists. We aimed to clarify the method of conserving grassland specialists through silvicultural activities. We recorded all plant species that emerged in each quadrat, and measured their maximum heights within 80 quadrats that were established in four types of treatment sites in Yanagidaira, Japan. The four treatment sites were clearcut, fenced, and mowed (hereafter “fence site”); clearcut and mowed with tubed saplings (hereafter “tube site”); clearcut without deer countermeasures and mowing (hereafter “open site”); and not clearcut forest (hereafter “forest”). The total number of species and the number of species that occurred only in one treatment site of the four sites were highest in fence sites among the treatments. Those of tube and open sites were intermediate, whereas those of forest were the lowest. Grassland specialist forbs dominated in fence site, and grassland specialist graminoids and unpalatable grassland specialist forbs dominated in tube and open sites. The presence probabilities and maximum heights of grassland specialist forbs were significantly increased by deer exclusion, but those of grassland specialist graminoids were rarely affected by deer exclusion. Establishment of deer-proof fences in clearcut sites is effective in conserving grassland specialist forbs.

Ungulate browsing affects subsequent insect feeding on a shared food plant, bilberry (Vaccinium myrtillus)

Browsing by ungulates may induce plant responses and affect subsequent plant food quality for other animals. Populations of many deer species have increased to unprecedented levels in Europe and North America. In Norway, population densities of red deer (Cervus elaphus) have increased over the past decades, but little is known about how increased deer browsing pressure may change the palatability of key food plants for other taxa in the boreal ecosystem. We conducted a cafeteria experiment to assess if long-term deer-browsing intensity affected the palatability of bilberry (Vaccinium myrtillus) leaves for leaf-eating larvae (mainly Lepidoptera). We found that leaf-eating insect larvae preferred bilberry leaves from the lightly browsed bilberry plants; the larvae consumed twice as much leaf biomass from the lightly browsed plants than from the unbrowsed and moderately browsed ones, and four times more than from highly browsed plants. Larvae never selected leaves from highly browsed plants as their first choice. Our study suggests that browsing-induced changes in the quality of shared food plants may be important in mediating indirect interactions between browsers of widely separated taxa. Whereas low levels of long-term red deer browsing increases the palatability of bilberry leaves for leaf-eating larvae, high browsing pressure reduces food consumption. Whether changes in palatability lead to changes in population densities of leaf-eating larvae remains to be studied, but any such adverse effects could have cascading ecological consequences for insectivorous birds and mammals.

An indicator approach to capture impacts of white-tailed deer and other ungulates in the presence of multiple associated stressors.

Management of ungulates is contested ground that lacks stakeholder agreement on desirable population sizes and management approaches. Unfortunately, we often miss information about extent of local impacts, for example on plant communities, to guide management decisions. Typical vegetation impact assessments like the woody browse index do not assess herbaceous plants, and differences in browse severity can be a function of deer density, deer legacy effects, localized deer feeding preferences and/or differences in plant community composition. Furthermore, in heavily affected areas, few remnant plants may remain for assessments. We used a sentinel approach to assess impact of white-tailed deer (Odocoileus virginianus), rodent attack, invasive earthworms and three invasive plants on survival and growth of 3-month-old red oak (Quercus rubra) individuals. We planted cohorts in 2010 and 2011 into deer accessible and fenced 30 × 30 m plots at 12 forests in New York State. We found year and site-specific effects with high deer herbivory of unprotected individuals (70-90 % of oaks browsed by deer versus none in fenced areas) far exceeding importance of rodent attacks. Oaks planted at low earthworm density sites were at significantly higher risk of being browsed compared with oaks at high earthworm density sites, but there was no detectable negative effect of invasive plants. Surviving oaks grew (~2 cm per year) under forest canopy cover, but only when fenced. We consider planting of oak or other woody or herbaceous sentinels to assess deer browse pressure a promising method to provide quantifiable evidence for deer impacts and to gauge success of different management techniques. The strength of this approach is that typical problems associated with multiple stressor impacts can be avoided, areas devoid of forest floor vegetation but under heavy deer browse pressure can still be assessed and the method can be implemented by non-specialists. Implementation of regular assessments can guide ungulate management based on meaningful evidence.

Deer browsing promotes Norway spruce at the expense of silver fir in the forest regeneration phase

Deer populations have dramatically increased over the last decades in Western Europe and their browsing has affected forest vegetation, especially plant community composition. High deer browsing pressure may threaten forest ecosystems and needs to be assessed over the long run. However, few studies to date have addressed the long-term impact of deer on forest regeneration.During one regeneration phase, we assessed the impact of deer (Cervus elaphus L. and Capreolus capreolus L.) browsing on mixed silver fir (Abies alba Mill.)/Norway spruce (Picea abies L. H. Karst.) stands in the Vosges Mountains in North-eastern France. For both tree species, we measured seedling and sapling density, height and diameter in 28 paired fenced – unfenced plots distributed over two sites that had been fenced 27years ago, for 10 and 23years respectively.We demonstrated that the presence of deer decreased the density, height and diameter of silver fir saplings, while it increased the height of Norway spruce saplings. We also noticed that the effect of fencing was very different depending on fencing duration, and that even 10years of fencing led to changes that were still visible 17years later.Our results confirm that deer browsing has a long-term impact on the regeneration of silver fir. Furthermore, they suggest that deer browsing may cause a species substitution, with Norway spruce gradually replacing silver fir. Such a trend would have an impact on the ecological and economical value of these forests. How browsing impacts the forest’s ability to adapt to global warming should also be considered, since browsing might impede the implementation of management policies designed to address this issue.

Life history trait divergence among populations of a non‐palatable species reveals strong non‐trophic indirect effects of an abundant herbivore

When large herbivores exert selection on their prey plant species, co‐occurring, non‐prey species may experience selection through non‐trophic indirect effects. Such selection is likely common where herbivores are overabundant. Yet, empirical studies of non‐trophic indirect effects as drivers of non‐prey trait evolution are lacking. Here we test for adaptive shifts in life history traits in an unpalatable species, Arisaema triphyllum, a common forest perennial that is unique because it exhibits size‐dependent sex switching. We collected A. triphyllum from six sites that experience a gradient in abiotic stress caused by deer browse pressure on prey plant species that generate indirect effects. We grew A. triphyllum from these sites in a common garden for five years to evaluate life history predictions linking strong indirect effects and abiotic stress to changes in life history traits: flowering onset size threshold, female flowering size threshold, relative growth rate (RGR), biomass allocation, and asexual reproduction. Despite observed differences among phenotypes in the field, expression of flowering onset size threshold, biomass allocation, and asexual reproduction did not differ among the six populations in the garden, indicating common plastic responses. In contrast, A. triphyllum collected from sites experiencing the two highest deer impacts exhibited smaller female flowering size thresholds and the highest RGR. Responses in these traits support the predictions of adaptive divergence in response to indirect effects. Our results reinforce the idea that non‐trophic indirect effects of large herbivores can elicit evolutionary responses in some traits of non‐prey species. In general, life history traits of unpalatable species may be cryptically adapting to stressful indirect effects where large herbivores are overabundant.