Introduced red deer reduce tree regeneration in Pureora Forest, central North Island, New Zealand

This study uses data from repeatedly measured forest monitoring plots (20 × 20 m) (n = 32) and nine ungulate exclosures (paired fenced and unfenced plots; 20 × 20 m) to show the effects of introduced ungulates on tree regeneration in Pureora Forest Park, central North Island, between 974 and 2002. Results show that introduced ungulates, particularly red deer (Cervus elaphus scoticus), have suppressed the regeneration of hardwood species such as Coprosma grandifolia, Elaeocarpus dentatus, Griselinia littoralis, Melicytus ramiflorus, Schefflera digitata and Weinmannia racemosa. These species were only common as saplings and small trees in the complete absence of ungulate browsing. The results of this study suggest that red deer will need to be culled to low densities to assure regeneration of palatable tree species in Pureora Forest.     

Fallow deer impacts on Wakatipu beech forest

This study uses data from forty-nine 20 m × 20 m permanent plots measured in 1976, 1982, 1989 and 1997-2002 in Wakatipu Forest, western Otago. We relate changes in red (Nothofagus fusca), silver (Nothofagus menziesii) and mountain beech (Nothofagus solandri var. cliffortioides) forest vegetation to the presence of fallow deer (Dama dama). Vegetation composition is likely to have been altered prior to plot establishment, and results show that there was little change in vegetation composition during the study. There are some signs of fallow deer reducing silver beech seedling abundance at some plots, but most beech stands appear to be in the late phase of development and undergoing self thinning, so are probably not immediately vulnerable to suppression of canopy regeneration. In the future, widespread disturbance is likely to release plants in the understorey from competition for light and nutrients, and at that stage fallow deer browsing of beech species may be able to alter successional pathways more than has occurred over the past two decades. Seedling and sapling density of the palatable Griselinia littoralis (broadleaf) was probably insufficient to ensure successful regeneration. Comparisons of seedling densities with other mixed beech forests throughout New Zealand suggest that fallow deer density will need to be nearly zero in Wakatipu Forest before regeneration of all palatable subcanopy hardwood species is assured.     

Suppression of Regeneration in New Zealand Mountain Beech Forests is Dependent on Species of Introduced Deer

We compared the impacts on forest regeneration of introduced sika (Cervus nippon) and red (Cervus elaphus) deer in New Zealand. Plot data were used to compare mountain beech (Nothofagus solandri var. cliffortioides) regeneration between a region with sika deer, and four regions without sika deer. All regions surveyed had red deer present. In the region where sika deer had been present for more than a decade, there was evidence of poor mountain beech seedling regeneration. In the four regions without sika deer, there was evidence of a strong regenerative response at stands with low occupancy by trees. When compared to larger deer species, sika deer have a digestive morphology allowing greater dietary versatility, which may result in them impeding forest regeneration where red deer do not. In contrast to mountain beech, some small-leaved shrub species may have been competitively advantaged by intensive browsing from sika deer. This is contra to a current view that small-leaved shrub species with interlacing branches were able to tolerate browsing from extinct ratite birds, but not introduced deer. Sika deer have been introduced into countries where other deer species are indigenous, such as Canada, Denmark, Great Britain, Czech Republic, Ireland and the United States of America. Because of their dietary advantage, sika deer may have a greater potential to impede forest regeneration and competitively exclude larger deer species, particularly at low basal area sites where impacts on tree regeneration are likely to be greatest.     

Influence of ungulates on the vegetation composition and diversity of mixed deciduous and coniferous mountain forest in Austria

The often highly elevated stocks of ungulates (red and roe deer and chamois) in the Alps shape the composition of the woody vegetation. The aim of this study was to investigate the influence of ungulates on the mixed deciduous and coniferous mountain forest in the district of Reutte, which boasts the highest density of ungulates in Tyrol (Austria), with a special focus on the effect of browsing by ungulates on plant diversity of the herb layer, different shrub layers. and the tree layer. Our results showed that within the fenced ungulate exclosures, (1) the composition of trees shifted towards fir (Abies alba) and various deciduous trees, whereas outside the fences, spruce became the dominant species; (2) the cover of dwarf shrubs and upper and lower shrub layers (1.3–5.0 and 0.5–1.3 m, respectively) increased significantly; (3) the cover of grasses decreased significantly and (4) the diversity decreased as an increase in the diversity of the tree and shrub layer was overcompensated by a significant decrease in the diversity of the undergrowth vegetation. Browsing by ungulates benefited grass species in the understory and altered the relative abundance of tree species in the lower layer which could, over time, result in compositional shifts in the canopy.     

Large-scale model-based assessment of deer-vehicle collision risk

Ungulates, in particular the Central European roe deer Capreolus capreolus and the North American white-tailed deer Odocoileus virginianus, are economically and ecologically important. The two species are risk factors for deer-vehicle collisions and as browsers of palatable trees have implications for forest regeneration. However, no large-scale management systems for ungulates have been implemented, mainly because of the high efforts and costs associated with attempts to estimate population sizes of free-living ungulates living in a complex landscape. Attempts to directly estimate population sizes of deer are problematic owing to poor data quality and lack of spatial representation on larger scales. We used data on >74,000 deer-vehicle collisions observed in 2006 and 2009 in Bavaria, Germany, to model the local risk of deer-vehicle collisions and to investigate the relationship between deer-vehicle collisions and both environmental conditions and browsing intensities. An innovative modelling approach for the number of deer-vehicle collisions, which allows nonlinear environment-deer relationships and assessment of spatial heterogeneity, was the basis for estimating the local risk of collisions for specific road types on the scale of Bavarian municipalities. Based on this risk model, we propose a new "deer-vehicle collision index" for deer management. We show that the risk of deer-vehicle collisions is positively correlated to browsing intensity and to harvest numbers. Overall, our results demonstrate that the number of deer-vehicle collisions can be predicted with high precision on the scale of municipalities. In the densely populated and intensively used landscapes of Central Europe and North America, a model-based risk assessment for deer-vehicle collisions provides a cost-efficient instrument for deer management on the landscape scale. The measures derived from our model provide valuable information for planning road protection and defining hunting quota. Open-source software implementing the model can be used to transfer our modelling approach to wildlife-vehicle collisions elsewhere.     

Development of Tree Regeneration in Fern-dominated Forest Understories After Reduction of Deer Browsing

Dennstaedtia punctilobula (hay‐scented fern) can act as a native invasive species in forests in eastern North America where prolonged deer browsing occurs in stands with partially open overstory canopies. Ferns dominate the understory with a 60‐cm tall canopy, with little regeneration of native tree species. It has been hypothesized that, once established, ferns may continue to inhibit tree regeneration after deer browsing has been reduced. To test this hypothesis, we documented the pattern of recovery of the tree seedling understory in plantations of Pinus strobus (white pine) and Pinus resinosa (red pine) on the Quabbin Reservation watershed protection forest in central Massachusetts, where after 40 years of intensive deer browsing the deer herd was rapidly reduced through controlled hunting. Dense fern understories occur on nearly 4,000 ha of the predominantly oak–pine forest. Three years after deer herd reduction, stands with the highest density fern cover (77% of plots with>90% cover) had significantly fewer seedlings at least 30 cm in height, compared with stands with lower fern density, and those seedlings consisted almost entirely of Betula lenta (black birch) and white pine. Height growth analysis showed that black birch and white pine grew above the height of the fern canopy in 3 and 6 years, respectively. In contrast, two common species, Fraxinus americana (white ash) and Quercus rubra (red oak), grew beneath the dense fern cover for 5 years with height growth less than 5 cm/yr after the first year. A study of spring phenology indicated that the ability of black birch to grow through the fern canopy might have been due to its early leaf development in spring before the fern canopy was formed, in contrast to oak and ash with delayed leaf development. Thus, the ferns showed differential interference among species with seedling development after reduction of deer browse.     

Different facets of tree sapling diversity influence browsing intensity by deer dependent on spatial scale

Browsing of tree saplings by deer hampers forest regeneration in mixed forests across Europe and North America. It is well known that tree species are differentially affected by deer browsing, but little is known about how different facets of diversity, such as species richness, identity, and composition, affect browsing intensity at different spatial scales. Using forest inventory data from the Hainich National Park, a mixed deciduous forest in central Germany, we applied a hierarchical approach to model the browsing probability of patches (regional scale) as well as the species‐specific proportion of saplings browsed within patches (patch scale). We found that, at the regional scale, the probability that a patch was browsed increased with certain species composition, namely with low abundance of European beech (Fagus sylvatica L.) and high abundance of European ash (Fraxinus excelsior L.), whereas at the patch scale, the proportion of saplings browsed per species was mainly determined by the species’ identity, providing a “preference ranking” of the 11 tree species under study. Interestingly, at the regional scale, species‐rich patches were more likely to be browsed; however, at the patch scale, species‐rich patches showed a lower proportion of saplings per species browsed. Presumably, diverse patches attract deer, but satisfy nutritional needs faster, such that fewer saplings need to be browsed. Some forest stand parameters, such as more open canopies, increased the browsing intensity at either scale. By showing the effects that various facets of diversity, as well as environmental parameters, exerted on browsing intensity at the regional as well as patch scale, our study advances the understanding of mammalian herbivore–plant interactions across scales. Our results also indicate which regeneration patches and species are (least) prone to browsing and show the importance of different facets of diversity for the prediction and management of browsing intensity and regeneration dynamics.     

Seed dispersal by ungulates in the point calimere wildlife sanctuary: A scientific and perspective analysis

Exotic woody weed plants are a very serious threat to seed dispersed by ungulate in the tropical forest of Asia. The ungulates in Point Calimere Wildlife Sanctuary (PCWS) are a significant role in native indigenous seed dispersal. The exotic woody weed tree Prosopis juliflora prevalence distributed in the PCWS and they might potentially alter the native medicinal plant species. In the present investigation, we have assessed the seed dispersal by ungulates in PCWS from January to March 2017. Four different ungulate species were selected to understand their seed dispersal rate of different plant species in selected sanctuary. This investigation was planned to confirm the seed dispersal by ungulates of blackbuck, spotted deer, wild boar and feral horse. Among the four different ungulates tested, the maximum numbers of pellets collected from blackbuck and no seed found in their pellets. The low quantities of pellets were collected from wild boar and this study has recorded medium-sized ungulates which dispersed variety of plant. However, the dispersal of the seed of medicinal plants were not considerably high and relatively moderate percentage of seeds dispersal occurred in medium-sized ungulates like wild boar and spotted deer. P. juliflora had 100% seed germination rate were observed from the faecal samples of wild boar and feral horse. The control seed achieved maximum seedling rate than the ungulates seeds.     

Ratite and ungulate preferences for woody New Zealand plants: influence of chemical and physical traits

Ratites (ostriches Struthio camelus) and ungulates (red deer Cervus elaphus scoticus and goats Capra hircus) were presented with 14 indigenous shrubs in cafeteria-style trials. The shrubs represented the spectrum of woody plant architecture, ranging from broadleaved monopodial species through to small-leaved highly branched divaricates. Trials were stopped when almost all shoots of the plant expected to be most preferred had been consumed. There were considerable differences between the herbivores in their selection of certain plant species. Aristotelia fruticosa was avoided by deer, neutrally selected by goats, and preferred by ostriches. All herbivores strongly avoided two species, Pseudopanax crassifolius and Coprosma rugosa. Analysis of relative offtake (proportion of biomass consumed from each species, relative to total biomass consumed) showed that all three herbivores ate less of species with small leaves. Consumption by all herbivores was reduced by structural plant traits (i.e. divarication and related attributes) more than by chemical plant traits. The immediate impact of browsing on the plants, measured as the rank of proportion of shoots eaten, was broadly similar across the herbivores. The broadleaved species (e.g. Myrsine australis, Aristotelia serrata) experienced the greatest browsing, while divaricate (e.g. Coprosma rugosa) and conifer species (e.g. Podocarpus hallii) were generally least browsed. Although cafeteria-style experiments have limitations, our results for deer broadly correspond to those of field-based diet preference studies.     

Opposing deer and caterpillar foraging preferences may prevent reductions in songbird prey biomass in historically overbrowsed forests

Overbrowsing by ungulates decimates plant populations and reduces diversity in a variety of ecosystems, but the mechanisms by which changes to plant community composition influence other trophic levels are poorly understood. In addition to removal of avian nesting habitat, browsing is hypothesized to reduce bird density and diversity through reduction of insect prey on browse‐tolerant hosts left behind by deer. In this study, we excluded birds from branches of six tree species to quantify differences in songbird prey removal across trees that vary in deer browse preference. Early in the breeding season, birds preyed on caterpillars at levels proportional to their abundance on each host. Combining these data with tree species composition data from stands exposed to experimentally controlled deer densities over 30 years ago, we tested whether overbrowsing by white‐tailed deer reduces prey biomass long after deer densities are reduced. Our analysis predicts total prey availability in the canopy of regenerating forests is fairly robust to historic exposure to high deer densities, though distribution of prey available from host species changes dramatically. This predicted compensatory effect was unexpected and is driven by high prey abundance on a single host tree species avoided by browsing deer, Prunus serotina. Thus, while we confirm that prey abundance on host trees can act as a reliable predictor for relative prey availability, this study shows that quantifying prey abundance across host trees is essential to understanding how changes in tree species composition interact with ungulate browse preference to determine prey availability for songbirds.     

Big game or big loss? High deer densities are threatening woody plant diversity and vegetation dynamics

Land-use change and current game management have favored an increasing population of wild ungulates (especially deer) in many regions of the Northern Hemisphere. Here, we assess the impact of high deer densities (>30 ind km^?2) on the highly diverse woody vegetation of Mediterranean ecosystems, where big game have been favored for the last decades. We examine whether prolonged deer browsing (over 30 years) affected plant composition, diversity and dynamics of the original (non-browsed) vegetation. Deer browsing led to an average decrease of 30.4 % in woody plant diversity (species richness), due to a lack of regeneration for the most preferred plant species. Species belonging to early stages of succession (mostly Labiatae and Cistaceae) were non-preferred by deer. Conversely, the most preferred species belonged mainly to late stages of plant succession. Deer impact on Mediterranean shrublands is causing biotic homogenization of plant communities and is threatening vegetation dynamics by forcing it to return to earlier succession stages. Strict deer population control favouring larger trophies but lower offspring numbers together with an adequate habitat management (increasing grass and acorn availability) would be the most efficient measures to reverse this diversity loss. Restoration work seems only appropriate for the most vulnerable species. We highlight the need of sampling deer-free areas with low or null historical browsing to assess the real impact of deer on woody plant diversity and vegetation dynamics.     

Herbivory on planted oak seedlings across a habitat edge created by timber harvest

Edge habitats create environmental gradients that affect plant community composition and herbivore behavior. Silvicultural disturbance creates edge habitat with direct (via changes in light) and indirect (via changes in herbivore behavior) consequences for the growth and survival of tree seedlings, and thus, the composition of the future forest stands. Herbivores, particularly ungulates, can be a major limiting factor in oak regeneration, and silvicultural disturbance may alter the abundance or behavior of herbivores following harvest. We measured the severity of herbivory on experimentally planted white (Quercus alba) and black oak (Quercus velutina) seedlings by white-tailed deer (Odocoileus virginianus) and eastern cottontail rabbits (Sylvilagus floridanus), as well as foliar damage from insects, across gradients created by clearcuts in a deciduous forest in Indiana, USA. Overall browse pressure on oaks was low in our study. Nonetheless, spatial variation in herbivory depended on herbivore taxa; herbivory by rabbits was highest inside harvest openings, whereas foliar damage by insects peaked in the forest. Intensity of deer herbivory was constant across the edge. In addition, we observed indirect interactions among herbivore species mediated by a seedling’s browsing history. Herbivore damage by deer was positively related to past browsing by rabbits, and foliar damage from insects was positively related to past browsing by both deer and rabbits. Increasing woody plant competition reduced herbivory on seedlings by both deer and rabbits. Given the lack of spatial variability in deer herbivory and low overall herbivory by rabbits, we suspect that interactions between timber harvesting and herbivory did not have a strong impact on oak seedlings at our study sites.     

Use of an ungulate‐specific feed structure as a potential tool for controlling feral goats in Australian forest ecosystems

The feral Goat (Capra hircus) has successfully exploited a range of landscapes around the world with occurrences of overabundance resulting in significant damage to ecological values. In forested ecosystems in Australia, there are currently limited means to control the species when compared to the range of management techniques available for other pest animals. To redress this deficiency, we designed a feed structure combined with commercially available salt blocks to attract goats to set locations in a forested study area. Structures that exploited differences in the pedal morphology (foot size and shape) of native herbivores (kangaroos and wallabies) and ungulates (feral goats and deer) were found to be highly target‐specific, with feral goats freely able to access salt blocks, whilst nontarget native species were effectively excluded. Other introduced ungulate species, Fallow Deer (Dama dama) and Red Deer (Cervus elaphus), successfully accessed salt blocks in feed structures but at a considerably lower rate than feral goats. The capacity to present a range of bait types within a target‐specific feed structure, once matched with a humane toxicant, could provide land managers with an additional cost‐effective lethal control tool for future management of feral ungulates, particularly goats.     

Patterns of woody plant abundance,recruitment, mortality,and growth in a 65 year chronosequence of old-fields

We surveyed vegetation along forest margins in a 65-year chronosequence of old-fields at the Cedar Creek Natural History Area in east-central Minnesota, USA, to identify successional patterns of woody plants and to determine if these were correlated with soil nitrogen. We predicted that shrub and tree abundance, size, and distance of occurrence from the forest edge would be correlated with field age or soil nitrogen. Instead we did not find successional trends in the abundance or composition of woody species. Even in the oldest field the abundance of trees and shrubs was low and concentrated in areas close to the forest. Though trees were larger and present further from the forest edges in older fields, average tree height was less than 126 cm in all fields.Since we did not find successional trends we looked at various local factors (local seed sources, deer browsing, and forest edge aspect) and their relation to recruitment, mortality, or growth to explain variation among fields in abundance of trees or shrubs. The three most common tree species (Quercus rubra, Q. macrocarpa,and Populus tremuloides) all had a higher relative abundance of seedlings, and two (Q. rubra and Q. macrocarpa) had a higher relative abundance of large trees adjacent to forests with a high abundance of conspecific adults. Most trees taller than 20 cm were browsed by deer and were shorter in 1995 than they were in 1993. Mortality was higher for trees less than 30 cm indicating that mortality was size-dependent. Forest edge aspect did not significantly influence the abundance or demography of any species. Our results suggest that the patterns of seedling recruitment were largely determined by the proximity of seed sources and that these patterns may persist so that tree communities in old-fields resemble adjacent forests. Deer may be a significant factor in the suppression of tree populations in old-fields through repeated browsing which reduces tree growth and elevates tree mortality by prolonging the period of time trees remain susceptible to size-dependent mortality.     

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.     

Moose in our neighborhood: Does perceived hunting risk have cascading effects on tree performance in vicinity of roads and houses?

Like large carnivores, hunters both kill and scare ungulates, and thus might indirectly affect plant performance through trophic cascades. In this study, we hypothesized that intensive hunting and enduring fear of humans have caused moose and other forest ungulates to partly avoid areas near human infrastructure (perceived hunting risk), with positive cascading effects on recruitment of trees. Using data from the Norwegian forest inventory, we found decreasing browsing pressure and increasing tree recruitment in areas close to roads and houses, where ungulates are more likely to encounter humans. However, although browsing and recruitment were negatively related, reduced browsing was only responsible for a small proportion of the higher tree recruitment near human infrastructure. We suggest that the apparently weak cascading effect occurs because the recorded browsing pressure only partly reflects the long‐term browsing intensity close to humans. Accordingly, tree recruitment was also related to the density of small trees 5–10 years earlier, which was higher close to human infrastructure. Hence, if small tree density is a product of the browsing pressure in the past, the cascading effect is probably stronger than our estimates suggest. Reduced browsing near roads and houses is most in line with risk avoidance driven by fear of humans (behaviorally mediated), and not because of excessive hunting and local reduction in ungulate density (density mediated).     

The regeneration of pioneer tree species under browsing pressure of Sika deer in an evergreen oak forest

The effects of Sika deer (Cervus nippon) browsing on the regeneration of pioneer species were studied in relation to canopy gaps in a warm temperate evergreen oak forest in Kasugayama, Nara City. Four study sites, three in canopy gaps and one under a closed canopy, were selected and each divided into fenced and unfenced plots. Under the closed canopy, seedlings of all the pioneer species died irrespective of browsing pressure. However, in the canopy gap sites, seedlings of the pioneer species could establish and grow well. The seedling survival ratio in the fenced plots in the canopy gaps was >60% 1 year after germination. However, in the unfenced plots, only <20% of the seedlings survived 1 year, with all dying within 3 years after germination. Thus, the regeneration of pioneer trees in this forest was strongly inhibited by deer browsing. Successful regeneration of a pioneer,Zanthoxylum ailanthoides, occurred for several years even after two major wind disturbances during the past 90 years. This may be due to less browsing pressure from the deer.     

Forest regeneration following ungulate removal in a montane Hawaiian wet forest

Most Hawaiian forests lack resiliency following disturbance due to the presence of non‐native and invasive plant and animal species. The montane wet forest within Hakalau Forest National Wildlife Refuge on Hawai'i island has a long history of ungulate disturbance but portions of the refuge were fenced and most ungulates excluded by the early 1990s. We examined patterns of regeneration within two 100 ha study sites in this forest following the removal of ungulates and in the absence of invasive woody tree species to determine, in part, if passive restoration techniques can be successful under these conditions. We characterized growth, mortality, and basal area (BA) changes for approximately 7,100 marked individuals of all native tree species present in two surveys over a 17–18‐year period within two hundred 30 m diameter forest plots. Considerable recruitment within plots of new trees of all species significantly changed size class distributions and erased deficits in small‐sized trees observed during the first survey, particularly for the codominant canopy tree, koa (Acacia koa). Overall, growth of established dominant 'ōhi'a trees (Metrosideros polymorpha) and recruitment of mid‐canopy trees contributed to increases in BA while high levels of mortality for large A. koa trees contributed to decreased BA. This resulted in a slight increase in BA between the two surveys (+1.9%). This study demonstrates that fencing and ungulate removal may have rescued the A. koa population by facilitating the first real pulse in recruitment in over a century, and that passive restoration can be a successful management strategy in this forest.     

Non‐trophic plant–animal interactions mediate positive density dependence among conspecific saplings

Trophic plant–animal interactions (e.g. browsing by ungulates, insect attack) are an important and well‐studied source of mortality in many tree populations. Non‐trophic tree–animal interactions (e.g. deer antler rubbing) also frequently lead to tree death, and thus have significant effects on forest ecosystem functioning, but they are much less well studied than trophic interactions are. As deer populations have increased in recent decades in the Northern Hemisphere, their impact on tree populations via browsing and antler rubbing will increase. The aim of the study was to illustrate the potential ability of non‐trophic plant–animal interactions to regulate the dynamics of a natural forest. Specifically, we wanted to determine whether and how density and distance‐dependent processes affect sapling mortality caused by an antler rubbing by red deer Cervus elaphus. We used a spatially explicit approach to examine density and distance‐dependent mortality effects in almost two thousand Picea abies saplings over 20 years, based on a fully mapped permanent 14.4 ha plot in a natural subalpine old‐growth spruce forest. Antler rubbing by deer was the main identified cause of sapling mortality, and it showed a strong spatial pattern: positive density dependence of survival among spruce saplings. Deer selectively killed spruce saplings that were isolated from conspecifics. In consequence, non‐trophic plant–deer interactions were a major driver of the spatial pattern of P. abies sapling survival. The other mortality causes (e.g. breaking, overturning) did not show density‐dependent patterns or their effects were much weaker. In the medium and long term, the density‐dependent pattern of sapling mortality due to antler rubbing can alter the tree stand structure. Our results highlight the ecological relevance of non‐trophic plant–animal interactions for forest ecosystem functioning.     

Positive interactions between herbivores and plant diversity shape forest regeneration

The effects of herbivores and diversity on plant communities have been studied separately but rarely in combination. We conducted two concurrent experiments over 3 years to examine how tree seedling diversity, density and herbivory affected forest regeneration. One experiment factorially manipulated plant diversity (one versus 15 species) and the presence/absence of deer (Odocoileus virginianus). We found that mixtures outperformed monocultures only in the presence of deer. Selective browsing on competitive dominants and associational protection from less palatable species appear responsible for this herbivore-driven diversity effect. The other experiment manipulated monospecific plant density and found little evidence for negative density dependence. Combined, these experiments suggest that the higher performance in mixture was owing to the acquisition of positive interspecific interactions rather than the loss of negative intraspecific interactions. Overall, we emphasize that realistic predictions about the consequences of changing biodiversity will require a deeper understanding of the interaction between plant diversity and higher trophic levels. If we had manipulated only plant diversity, we would have missed an important positive interaction across trophic levels: diverse seedling communities better resist herbivores, and herbivores help to maintain seedling diversity.