Do large herbivores maintain open habitats in temperate forests?

In recent decades, grazing by wild and domestic ungulates has become a strategy for conservation management to restore or maintain open landscapes. One of the species playing an increasing role in ecological restoration is the European bison – the largest terrestrial mammal in Europe. We studied the impact of this large herbivore and other ungulates (moose, red deer, and roe deer) on tree encroachment in open habitats in the Białowieża Primeval Forest (Poland). On 30 study plots located in meadows, we measured crown volume and the density of woody vegetation and monitored visitation and behavior of ungulates with the use of camera traps. The mean visitation rate to meadows by European bison was 0.11 ind./day/plot, and 0.22 ind./day/plot by other ungulates. The duration of foraging was significantly higher in European bison (55.8 s) than in other ungulates (16.3 s). The density of woody vegetation on meadows varied from 13 to 6213ind./ha and the crown volume from 0.6 to 1145 m³/ha. We found that increased visitation by European bison resulted in a significant reduction in the density and volume of woody vegetation in meadows. The reducing effect on woody vegetation was over eight times higher in frequently visited plots when compared to unvisited plots – the density of woody vegetation decreased from 879 to 101 saplings/ha, while the crown volume declined from 295 to 35 m³/ha. In addition, the density of woody vegetation was related to the level of meadow openness. Less open (smaller) meadows had a significantly higher density of woody vegetation than meadows characterized by high openness. Combined visitation by other ungulates did not affect either the volume or density of woody vegetation. The most plausible mechanism of observed patterns can be a remarkably higher foraging activity in meadows by bison in comparison to other ungulates. As a consequence, European bison, being adapted to open habitats, can effectively reduce the growth of tree seedlings and limit tree encroachment at the initial stages of forest succession. Thus, populations of this wild herbivore can play a role in the restoration or maintenance of open habitats and woody pastures that serve as an important foraging ground for bison in suboptimal forests, where populations of these herbivores were restored.