The results of a study conducted in North America show that large carnivores can have strong effects on prey populations by reducing their density and changing their behaviour, which can lead to a chain of changes at different trophic levels, i.e., cascade events39,40. In our study, we hypothesised that the presence of wolves in the forest ecosystem would reduce the browsing intensity of deer in forest plantations regardless of their size. However, the results obtained partially contradicted this prediction. The presence of wolves increased the browsing pressure of deer on pine saplings planted in large forest plantations (1–5 ha), while a decrease in browsing was observed in small beech plantations (0.1 ha). It was also noted that the presence of predators altered the spatial pattern of deer foraging. In large pine plantations, deer tended to consume more of the pine saplings in the central zone. In small beech plantations, we observed a decrease in browsing at the forest edge. These results contrast with observations in Yellowstone National Park, where 15 years after the return of wolves, a decline in the deer population was accompanied by a significant decrease in the number of young trees damaged and an increase in young tree survival41 (but see Kauffman et al.42). Our results also differ from those of a study from Scandinavia, where the presence of wolves had no effect on damage caused by moose in pine plantations30, but they are consistent with the results of other studies in which moose browsing on pines was higher in wolf territories28,29. However, as has been highlighted in North America, predation on large prey is sometimes wishful thinking, while a trophic cascade may be weaker than claimed and strongly dependent on adequate sampling41.
So the obvious question arises: why does the presence of wolves increase the grazing pressure of deer on saplings? The most intuitive answer is that deer browsing pressure in wolf-inhabited landscapes may be concentrated in places with high food availability and good visibility, such as young and large forest plantations. Such behaviour helps animals minimise foraging time and easily detect danger (predators). Some confirmation of the above explanation comes from the results of studies conducted in North America showing that deer minimize the risk of being preyed upon by coursing predators by relying on early detection, which is facilitated by the use of large, open forest plantations44,45. Although dense vegetation cover near the forest edge may provide safety to deer by reducing detection34, it may obstruct visibility and escape routes, increasing predation risk from apex predators25. This would explain why distance from the forest edge was not a statistically significant factor in the case of the beech plantations - these gaps were too small to provide sufficient distance for early detection of predators by deer, so they likely felt equally safe (or unsafe) within the entire plantations. We confirmed our second hypothesis that deer foraging behaviour varies spatially, but only under specific spatial conditions. However, sometimes the forest edge is the safest location34 or simply the most attractive location where a tradeoff can be made to provide both cover for safety and open space for grazing at safer times46. Considering the strength of range and wolf effects, it appears that even in human-dominated landscapes such as commercial forests, where human activities (e.g., recreation, hunting) influence deer grazing behaviour at the stand and landscape level28,31,47 and fear is triggered by anthropogenic disturbance rather than the presence of large predators31,48, wolf risk is still an important factor creating a landscape of fear and influencing deer foraging behaviour.
One might wonder whether the results obtained were affected by differences in the densities of deer within the study areas, especially since the available data indicate a strong relationship between the density of deer and foraging intensity7,49,50,51. However, in the previously mentioned studies, significant differences in foraging intensity were found for deer densities ranging from a dozen to several dozen individuals/km2. In contrast, density varied at most in the presented study by only a few individuals/km2. Therefore, in our opinion, the effects of deer densities on levels of browsing damage were negligible.
This study illustrates well how apex predators alter deer browsing intensity and young tree survival in anthropogenic forest ecosystems through spatial differentiation in the landscape of fear. Resurgent wolf populations may increase browsing pressure by deer in large-scale forest plantations and decrease it in small-scale plantations. However, we must remember that the earliest stand stages in commercial forests represent only a part of the total ecosystem and deer browsing pressure may be lower at other stages of forest development. Although the threat posed by human hunters is thought to be the most important determinant of cervid responses in commercial forests during the day through the hunting season31,52, we have observed a clear pattern of changes in habitat selection by deer in the face of cursorial predators. This is consistent with the results of a study conducted in Scandinavia, which found that clearcuts and young forests were the safest places for moose to be to reduce predation by wolves52.
Our study suggests that the presence of wolves significantly affects forest regeneration by influencing the foraging behaviour and browsing of wild herbivores. However, the inconsistent results when combining research studies on this topic highlight the need for further research on the cascading effects of large predator populations and human activities on forest regeneration (in both commercial and protected forests), as this could benefit forest management and support sustainable management of wildlife populations53.