Forests for the BATS Part IV: Riparian Forest Buffers for Bats
Our streams need trees. The very best thing we can do for water quality is to protect and increase the amount of streamside (or, if you’re inclined to speak Latin, riparian) forest cover. In the conservation world we call these strips of recently planted streamside trees riparian forest buffers; they are protecting our water bodies from pollutant-laden runoff that would otherwise flow from our upland farms and communities. These critical forested areas are also vital for wildlife, including bats, our magnificent yet imperiled mammalian friends.
Riparian forests are important to bats for a few reasons. First, they are valuable foraging habitat; bats following a stream can fly in a more or less linear fashion to efficiently hunt insects. Bats need to drink water while flying, so hunting near a source of open water is all the better. Forested streams tend to be much wider than ones surrounded by herbaceous vegetation; fibrous grass roots will rapidly collect fine sediments and make stream channels narrower but deeper. Conversely, the shade of a forest doesn’t allow for dense grassy vegetation along the stream channel, and large woody roots anchor the streambank instead, allowing the stream to widen and the bank to taper on a grade that is less susceptible to eroding away in times of high water.
The forested streams that make for better bat flyways and easier drinking on the wing also provide more habitat for benthic macroinvertebrates, bottom-dwelling aquatic bugs that are primary food sources for predators both within the streams, like trout, but outside as well, like songbirds, bats, and numerous other critters. Many of these species are aquatic as larvae and emerge, sometimes in large synchronized flushes called hatches, as aerial adults to reproduce. These invertebrates, which are relied upon by both aquatic and terrestrial communities, rely in turn on forest cover to survive. They need water that is clean, cool, and full of organic input (like leaves, sticks, and dissolved carbon) that only comes from forests. Without riparian forests, the macroinvertebrates would find themselves: 1) in water that wasn’t being protected from upland runoff, 2) that had no shade from canopy trees and is thus was too warm to survive in, 3) that had no food from surrounding trees, and 4) that had no stream bottom surface to cling to because of rampant erosion, which smothers the rocks and gravel that the bugs need to hide under and between. Macroinvertebrates are not only vital components of the aquatic food web, but through their filter-feeding have been shown to substantially reduce pollutant loads in water!
Beyond foraging habitat, bats need roosting sites, where they can rest during the day and raise their pups safe from predators all summer. A majority of bat species in eastern forests roost under the bark of trees. Some species, in particular white oak (Quercus alba) and shagbark hickory (Carya ovata), have ample space beneath the shaggy bark of mature trees for bats to roost beneath. Dead trees, which are called snags by biologists, are also invaluable roosting habitat for bats: the bark that slowly sloughs off snags provides perfect protection for bats to squeeze beneath. Snags are all the better as roosting sites for raising pups because the trunks aren’t shaded by leaves and are thus nice and warm, helping the young bats to put less energy into body heat and more energy into growing.
Riparian forests tend to have more snags than upland areas do, partly due to human management and partly due to natural forces. Timbering pressure is often lower in wet areas near streams, so trees can grow longer there to die of old age. Additionally, there are generally less buildings constructed in floodplains, so people don’t feel the need to remove trees that die for safety or aesthetic purposes. Streams are dynamic systems, occasionally violently so, and phenomena like floods and ice scour kill trees at higher rates than they do upland trees. Finally, fire, which historically ran through our forests at intervals as regularly as every 3-25 years in many areas, would consume snags in upland areas but be less likely to permeate our wet riparian forests, leaving more dead trees standing for longer near streams. All of these factors combine to make riparian forests home to more large maternity colonies of bats than any other habitat in eastern landscapes.
We need riparian forests, if we are to have clean water and if we are to support bat populations. Bats numbers been severely reduced in the past decade (up to 98% loss for some species) due to white-nose syndrome, an introduced fungus. This fungus is exceedingly difficult to control, so we need to create as much breeding habitat as possible in the hopes of keeping bats, an incredibly important group of mammals, on our landscape. If you have a stream on your property that isn’t currently surrounded by at least 100 feet of forest cover, consider planting more trees to help out our beleaguered bats! If you are interested but don’t know where to start, reach out and we will help direct you: firstname.lastname@example.org.
To learn more about riparian forest buffers, visit chesapeakeforestbuffers.net. To read more about bats and managing habitat for them, check out our past Forests for the Bats articles, Forests for the Bats Part I, Part II (on roosting and hibernating habitat), and Part III (on foraging habitat).