Ungulate impacts on herbaceous‐layer plant communities in even‐aged and uneven‐aged managed forests

Abstract

Abstract. Forest management and ungulate herbivory are extant drivers of herbaceous-layer community composition and diversity. We conducted a white-tailed deer (Odocoileus virginianus) exclosure experiment across a managed landscape to determine how deer impacts interact with the type of forest management system in influencing herb-layer (all vascular plants < 0.5 m tall) species richness and composition. Our study took place 3 yr after harvest in a deciduous forest landscape being managed through even-aged (~4.1 ha openings) and uneven-aged (~1.4 ha openings) silvicultural systems. We expected the severity of deer impacts on herb layer species richness and composition to vary according to opening position, opening size, and the spatial scale of inference. At forest stand and landscape scales, species richness within silvicultural openings was greater outside compared to inside deer exclosures, and did not differ according to deer access in edges or the forest matrix. However, greater levels of species richness associated with deer access were driven by infrequently occurring forbs, and overall species composition did not differ. Notably, these species were not exotics or ferns. Deer reduced the density of large saplings and blackberry (Rubus spp.) shrubs in the smaller openings characteristic of uneven-aged management stands, but had no effect on sapling density in the larger openings characteristic of even-aged management stands. This result extends the forage maturation hypothesis to silvicultural systems, and is consistent with predictions that plant tolerance and avoidance of herbivory increase with resource availability. Deer may have facilitated the establishment of forbs in recently created silvicultural openings by temporarily slowing sapling regeneration, creating establishment sites through physical disturbance, and seed dispersal via epizoochory and endozoochory. This outcome is contingent upon declining deer visitation rates as woody vegetation matures as well as distance from source populations of exotic species. We conclude that ecological context, such as local ungulate abundance, disturbance, and landscape factors, influence how ungulates interact with forest management systems.

Ungulates increase forest plant species richness to the benefit of non‐forest specialists

“…At the lowest wild boar population densities (index <10), there was no difference in the proportion of bare soil between exclosures and control plots, whereas at higher population densities, bare soil developed outside the exclosures(Figure 4c).4 | DISCUSSION4.1 | Deer browsing moderates the development of the shrub layer which reverberates on the herbaceous layerExcluding ungulates for 10 years resulted in higher cover and species richness of low and high shrub layers inside the exclosure. The increasing species richness of the shrub layers inside exclosures, relative to outside, suggests that deer browsing limits the growth of woody and semi-woody species, thus slowing their recruitment in higher layers(Kuijper et al, 2010;Murray, Webster, Jenkins, Saunders, & Haulton, 2016;Tanentzap et al, 2009). Our results also indicated a delayed response of the two shrub layers, the effect of exclosure on both vegetation cover and species richness being significant after 5 years for the low shrub layer, then after 10 years for the high shrub layer.…”

“…It is striking that deer emerged as a moderator for the plant cover of this species group, not only in other parts of Europe (Kirby & Thomas, 2000;Perrin, Mitchell, & Kelly, 2011), but also for Rubus spp. species in the United States (Faison et al, 2016;Horsley, Stout, & deCalesta, 2003;Murray et al, 2016;Royo et al, 2010 (Faison et al, 2016;Murray et al, 2016;Nishizawa et al, 2016;Perrin et al, 2011). After 10 years of fencing, the lower species richness of the herbaceous layer inside the exclosures of our 82-site network, along with the gradual response to deer abundances provide a large-scale support for this grazing curve.…”

“…The classic grazing response curve, based on the intermediate disturbance hypothesis, predicting an increase in plant species richness from low to moderate browsing pressure, has been already locally documented (Faison et al., ; Murray et al., ; Nishizawa et al., ; Perrin et al., ). After 10 years of fencing, the lower species richness of the herbaceous layer inside the exclosures of our 82‐site network, along with the gradual response to deer abundances provide a large‐scale support for this grazing curve.…”

“…Excluding ungulates for 10 years resulted in higher cover and species richness of low and high shrub layers inside the exclosure. The increasing species richness of the shrub layers inside exclosures, relative to outside, suggests that deer browsing limits the growth of woody and semi‐woody species, thus slowing their recruitment in higher layers (Kuijper et al., ; Murray, Webster, Jenkins, Saunders, & Haulton, ; Tanentzap et al., ). Our results also indicated a delayed response of the two shrub layers, the effect of exclosure on both vegetation cover and species richness being significant after 5 years for the low shrub layer, then after 10 years for the high shrub layer.…”

“…It is striking that deer emerged as a moderator for the plant cover of this species group, not only in other parts of Europe (Kirby & Thomas, ; Perrin, Mitchell, & Kelly, ), but also for Rubus spp. species in the United States (Faison et al., ; Horsley, Stout, & deCalesta, ; Murray et al., ; Royo et al., ). Experimental results by Van Uytvanck and Hoffmann () showed that dense thickets of R. fruticosus agg .…”