Sandhill cranes. Credit: Larry Lamsa
A recently published field guide lists more than 500 different species of animals living in New Mexico’s bosque. While many of the animals are difficult to see — such as bats due to their nocturnal nature — others are frequently encountered on walks through the woods, including desert cottontails, rock squirrels, muskrats, porcupines and coyotes. The American beaver is another bosque resident. This aquatic rodent’s diet consists primarily of the cambial tissue under the bark of cottonwood and willow growing in the woodland. Walking through the bosque, gnawed tree trunks and stripped bark provide clear evidence of beaver activity. Along the river, beaver tracks can be found in the mud and burrows in the bank.
Hedderig explains that the bosque also is an excellent migration route, providing food, shelter and water for large numbers of ducks, geese, sandhill cranes and a host of other migratory birds. Many of these birds, like herons and egrets, are wading birds and not typically found in forests.
The cottonwood trees, with heart- or triangular-shaped leaves, are sometimes referred to as the heart of the bosque, as they provide critical habitat for many of the birds, mammals, insects, spiders and crustaceans of the riparian ecosystem. Resident birds of the bosque include Cooper’s hawk, red-tailed hawk, American kestrel, roadrunner and a variety of hummingbirds, woodpeckers and owls. Porcupines rest high in the branches of cottonwood trees, and toads seek shelter in the leaf litter on the forest floor.
The Importance of Cottonwood
Rio Grande cottonwoods have been growing in the bosque for more than a million years and are heavily dependent on a reliable water supply for germination and survival. Rio Grande cottonwoods reproduce by seeding. Cottonwood seeds, carried by downy white tufts, are easily dispersed long distances by wind or water. For germination, they need bare soil, moisture and plenty of sunshine. Cottonwoods flower in the spring before their leaves come out, around the same time that spring rains cause the river to flood over its banks, which creates open, muddy flats where seeds can germinate and establish seedlings.
Today, the cottonwoods and the bosque ecosystem that depends on them face serious threats to their survival. Farming, river-control projects and urban development have dramatically decreased the water supply, seriously compromising the health of the bosque. Beginning in the 1950s, thousands of jetty jacks — large, steel, cross-like structures — were installed by the U.S. Army Corps of Engineers to trap sediments and stabilize riverbanks in times of flooding. These jacks, along with conveyance channels, levees and dams, changed the Rio Grande from a meandering, free-flowing stream to a highly modified water storage and transportation system. Several irrigation ditches, along with water gates and frames, are visible from the walking paths in the Rio Grande bosque.
With these changes to the river came dramatic changes for the entire forest and ecosystem. Deprived of the floods that spread their seeds, the cottonwood trees cannot to regenerate naturally. As older cottonwoods die, holes are left in the canopy, and the ecosystem has no new trees to fill them.