Wolves, tree logs and tree regeneration:Combined effects of downed wood and wolves on the regeneration of palatable and less palatable tree species

The objective was to study via both descriptive studies and experiments how regenerating saplings can profit from the interplay between wolves and downed wood. We studied this in the Białowieża Primeval Forest, Poland, where wolves and deer co-occur for more than 100 years, and where downed wood covers the forest floor. Previous studies have shown that deer become more vigilant on sites they perceive as risky, such as in the core of a wolf territory and near downed wood that block view and escape possibilities. I was interested to see whether this change in deer behavior let to different browsing patterns and what this would mean for the regeneration of different tree species. We found that obstacles, like downed wood, enhance successful tree regeneration by physically obstructing access for deer, and because deer avoid such obstacles. Moreover, deer visit areas with a higher perceived predation risk less often, however this is attenuated on more productive sites. Therefore saplings have the highest chance to outgrow the 2 meter browsing line (after which their top shoot is safe for browsing) when surrounded by downed wood, or when associated with downed wood in areas actively used by wolves. Especially preferred tree species (Acer platanoides, Tilia cordata) profit from the reduction in browsing on risky places leading to a more diverse forest in the long-term. Therefore the presence of downed wood and a complete assemblage of ungulates and carnivores leads to a spatial, structural and compositional heterogeneous forest

Behavioral response of naïve and non-naïve deer to wolf urine

Large carnivores are recolonizing many regions in Europe, where their ungulate prey have lived without them for >150 years. Whether the returning large carnivores will modify ungulate behavior and indirectly affect lower trophic levels, depends on the ability of ungulates to recognize risk based on past encounters and cues indicating carnivore presence. In two case studies, we tested, by means of camera trapping, the behavioral response of deer to wolf urine. The first case study was in the Netherlands where deer (still) live in absence of wolves, and the second in Poland with long-term wolf presence. As controls we used water (no scent) and all-purpose soap (unfamiliar scent). Deer vigilance level on control plots was 20% in both case studies indicating that wolf occupancy per se does not lead to a consistent difference in behavior. Placing wolf urine did not significantly affect deer behavior in either the wolf-absent or the wolf-present area. More intense cues, or a combination of cues, are likely needed to affect deer behavior. Moreover, we found an unexpected reaction of deer towards all-purpose soap of reduced foraging (and tendency for increased vigilance) in the wolf-present area, whereas it did not affect deer behavior in the wolf-absent area. We hypothesize that deer associate all-purpose soap with human presence, causing no response in human-dominated landscapes (the Netherlands), but triggering a behavioral reaction in more remote areas (Poland). This illustrates attention should be paid to controls used in scent experiments as they may be associated differently than intended

Impediments affect deer foraging decisions and sapling performance

Impediments, such as tree logs, can prevent access to saplings for deer, and can increase perceived predation risk by blocking view and escape possibilities for deer in areas with large carnivores. Therefore, impediments can influence deer foraging decisions and the trade-off between safety and food of different quality indirectly influencing tree regeneration. The aim of our study was to test how the presence of an impediment affects deer foraging behavior and tree sapling performance of eight species that differ in preference by deer. We planted saplings without, nearby and inside impediments and followed their fate for three consecutive years in Białowieża forest, Poland. We constructed 1 m high impediments of 5 × 5 m that would still allow deer to enter the impediment and forage from the saplings planted inside. However, we never recorded deer inside the impediments. Near the impediments deer visitation rate and cumulative visitation time was reduced. As a result, browsing intensity of all tree species was lower nearby and especially inside the impediment. Deer did not select different tree species without, near or inside the impediment. Due to the overall lower browsing intensity, tree saplings increased in height near the impediment and heights increased significantly within the impediment. The palatable, but browsing intolerant, Acer platanoides benefited most from the impediment as this species was highly selected and heavily browsed without an impediment. Followed by the palatable, but more browse tolerant Tilia cordata and Pyrus pyraster. In comparison, the presence of an impediment had a smaller effect on the less preferred Alnus glutinosa, Picea abies and Pinus sylvestris which survived well without an impediment. Our study showed that impediments modified deer behavior as they visited these plots less frequently and thereby indirectly reduced the browsing impact on the preferred tree species. Therefore, the potential for successful recruitment of preferred tree species is higher near an impediment, and especially when surrounded by impediment structures. In the long-term, the presence of natural impediments like tree logs allows browsing intolerant tree species to escape browsing, ultimately leading to a more diverse forest composition

Wolves and tree logs:Landscape-scale and fine-scale risk factors interactively influence tree regeneration

Large carnivores can reduce ungulate numbers by predation and via induced risk effects alter ungulate behavior, indirectly affecting lower trophic levels. However, predator-induced risk effects probably act at different spatial scales, which have often been ignored in trophic cascade studies. We studied how a fine-scale risk factor (distance from tree logs) affects ungulate browsing intensity and how this is modified over a landscape-scale risk gradient (distance from human settlements to wolf core) in the Białowieża forest, Poland. We found that landscape-and fine-scale risk factors strongly interacted in determining the strength and magnitude of carnivore-induced risk effects on lower trophic levels. In low-risk areas, tree logs reduced browsing intensity in small patches (approx. 4–6 m from logs), whereas in high-risk areas browsing intensity was reduced up to at least 16 m from tree logs. Moreover, the magnitude of these effects changed, with the strongest reduction in browsing intensity around tree logs in high-risk areas (up to 37%) and the smallest in low-risk areas (< 20%). Overall, the results of this study indicate that perceived risk factors act at different spatial scales, where impediments (objects blocking view and escape routes) act as a risk factor at a fine scale and carnivore distribution shapes perceived risk at the landscape scale. Moreover, these risk factors strongly interact, thereby determining the functional role of large carnivores in affecting ecosystem processes. These interactive effects should be incorporated in predator-induced trophic cascade studies to understand patterns of tree regeneration in ecosystems where large carnivores and herbivores live together

Replication Data for: Wolves and tree logs: Landscape-scale and fine-scale risk factors interactively influence tree regeneration

This dataset contains all data used for the paper Van Ginkel et al, 2018, Ecosystems, and is organised in three zipped folders. The metadata file describes all abbreviations used in the data files: Van-Ginkel-etal_2018_Ecosystems_RAW_Data Van-Ginkel-etal_2018_Ecosystems_PROCESSED_Data Van-Ginkel-etal_2018_Ecosystems_txt-files_R-scripts Van-Ginkel-etal_2018_Ecosystems_Metadata.txt If you have questions regarding this dataset or are interested in using our data, please contact one of the authors indicated