Shrubs are woody, perennial plants with multiple stems branching from or near the ground, forming the forest’s middle layer, known as the understory. This layer rarely exceeds 20 feet in height but provides an abundance of leaves, soft shoots, and fruits. The consumption of this woody vegetation is called browsing, a feeding behavior distinct from grazing on grasses. Browsing animals constantly prune and shape the plant life beneath the main canopy, introducing a dynamic force into the ecosystem.
Large Mammals: The Primary Browsers
Large herbivorous mammals, particularly ungulates, are the most impactful consumers of forest shrubs due to the sheer volume they consume. White-tailed deer, elk, and moose are the primary drivers of shrub consumption in many temperate and boreal forests. Their size allows them to reach the upper parts of the shrub layer, accessing forage unavailable to smaller animals.
These large browsers are highly selective feeders, choosing plants based on palatability, nutrient content, and defense mechanisms like thorns or chemical compounds. Deer, for example, prefer a more digestible diet, making them more selective than elk, which have a greater digestive capacity for diverse plant matter. This selective pressure often results in the overconsumption of preferred species, allowing less palatable shrubs to proliferate. Evidence of their feeding is often visible as a “browse line,” a distinct horizontal line where all vegetation up to the animal’s maximum reach has been consumed.
Ungulates also damage shrubs and young trees through non-feeding behaviors. Male deer, for instance, rub their antlers on stems to remove velvet or mark territory. This rubbing can inadvertently strip the bark, effectively girdling the plant. The combination of high-volume consumption and incidental damage makes large mammals the greatest factor in determining the density and composition of the forest understory.
Smaller Herbivores and Ground-Level Feeders
A different class of herbivores operates closer to the forest floor, focusing on the most vulnerable parts of the shrub. Lagomorphs (rabbits and hares) and various rodents (voles and mice) target seedlings, low stems, and the bark of mature shrubs. This feeding becomes particularly concentrated during winter when snow cover limits access to other food sources.
These ground-level feeders often engage in girdling, a destructive pattern where they gnaw the bark completely around the circumference of a stem. Girdling removes the phloem, the vascular tissue transporting sugars from the leaves to the roots. If the girdling is complete, the roots starve, causing mortality for young or thin-barked shrubs. Voles and mice often work beneath the snow cover, targeting the bark at the base, while rabbits may reach higher by standing atop crusted snow.
Insect and Avian Consumers
Shrubs are also consumed at a smaller, more specialized scale by numerous insect and avian species. Leaf-chewing insects, such as caterpillars and beetles, are folivores that can cause significant, localized defoliation. These insects typically target the softest tissues, including newly emerging buds and leaves, impacting the shrub’s growth potential.
Birds play a dual role as both consumers and protectors of shrubs. Frugivorous birds consume the berries and seeds, a mutualistic relationship that aids in seed dispersal. Other birds are insectivores that forage directly on the shrubs, consuming the herbivorous insects and arthropods that damage the foliage.
How Shrub Consumption Shapes the Forest
The collective pressure from all these consumers fundamentally dictates the structure and future of the forest ecosystem. Browsing animals determine the species composition of the understory by eliminating preferred, palatable plants and allowing less-preferred, browse-resistant species to dominate. Intense browsing can reduce the diversity and density of the shrub layer, creating a more open understory.
This consumption pressure also directly impacts forest regeneration by removing the seedlings and saplings of canopy trees before they grow out of the reach of browsers. This selective removal can alter the course of forest succession, potentially preventing certain tree species from establishing themselves for decades. The resulting landscape reflects the feeding choices of the herbivores that inhabit the area.