Trees of BFL: Cottonwoods and Willows

Cottonwood trees and willows are similar in many ways. They germinate through wind dispersion and colonize moist muddy areas exposed to full sun. Both are present in Brackenridge Field Lab, and the cottonwoods in particular have a close connection to the history of the field lab.

COTTONWOODS

Cottonwoods (Populus deltoides) are one of the fastest growing trees in North America, and can grow five feet per year. This tree is typically found growing near rivers.

The oldest group at Brackenridge Field Lab used to be more common along the Colorado River, but out of this group, there is only one tree left. It’s quite likely this old group of trees got their start right after the breaking of the Austin dam in 1900, which caused big silt with direct exposure to the sun. That would make this single remaining tree around 120 years old. 

The next wave of cottonwoods that followed were established during the construction of the ponds at BFL in 1967 as this created open wet spots prime for germination. Cottonwoods do not germinate in shaded areas dominated by other vegetation. These trees are around 50 years old. Other temporary ponds that were created after 1967 also have cottonwoods. This pattern of cottonwood establishment shows what the area had to have been like, particularly disturbances that would have exposed the soil to moisture, in order for the trees to have germinated at all. In that way, these cottonwoods are very much connected to the history of BFL’s change and growth.

The cottonwood is dioecious, with the flowers (catkins) produced on single-sex trees in early spring. Catkins are slim cylindrical flower clusters with very tiny or completely absent petals. The term “catkin” is Middle Dutch and refers to a kitten’s tail. Catkins tend to be produced by plants that are wind-pollinated. Bees will certainly visit to collect pollen from them, but wind remains the primary way grains are dispersed through the air. Cottonwoods in particular are a major source of allergies for some people, particularly in the early spring. 

Central Texas is on the edge of the native range of this species. There are some growing in West Texas along banks of rivers. These West Texas trees have tended to live longer than ones in areas with other competing growth, primarily because they are not in proximity to other trees that can spread disease.

This tree is notable for several characteristics. One is how its leaves shimmer with a small breeze. This happens because of the flat stalk of the leaf. It’s also notable for the tufted seeds the female trees release from ripe capsules. These look a lot like cotton and travel through the air.

Additionally, the scales on cottonwood leaf buds are sticky, producing a resinous exudate. In the early spring, bees collect this resin which they use to make propolis, a substance that they in turn use to construct, protect, and repair their hives. 

The trees are a larval host for Mourning Cloak butterfly (Nymphalis antiopa), Red-spotted Purple butterfly (Limenitis arthemis), Viceroy Swallowtail butterfly (Limenitis archippus) and Western Tiger Swallowtail butterflies (Papilio rutulus).

The wood is weak, and used only for pulp, plywood, and crates.

The tree became an official state tree of Kansas in 1937, in Wyoming in 1947, and Nebraska in 1972.

BLACK WILLOWS

The Black Willow tree (Salix nigra) is the largest of the New World willows. It has one of the most extensive ranges in the US, including most of Texas. Like the cottonwood, it grows in wet soil, such as near the margins of streams, lakes, and ponds. It’s also commonly known as Gulf Black Willow or Swamp Willow.

It’s also a fast-growing tree, reaching heights from 35-100 feet tall. The largest example of a tree still living has a circumference of 32 feet and is 63 feet tall. The bark of the tree is ebony in color, thus how it has earned its common name. Sticks cut from them that are thrust into soil will take root and create a “living fence.”

Black Willows are dioecious, producing either all male or all female catkins on the same tree. These flowers are one of the first available in spring to provide honey bees with much needed nectar and pollen after they emerge from a long winter.

The roots of the tree have been used as a substitute for quinine, a medication used to treat malaria. Native Americans traditionally have used the tree to treat fever, headaches, and coughs. The bark of the tree contains salicylic acid which is a chemical compound similar to that of aspirin, acetylsalicylic acid.

The wood is one of the lightest of eastern hardwoods, fairly soft and doesn’t splinter easily. This makes it good for toys, barn floors, and shipping containers. The wood has also been used to make artificial limbs. During the American Revolution, the wood was made into a fine charcoal that was used for gunpowder. Young stems are flexible and can be used in basket making.

The tree is very beneficial for soil binding, meaning the fibrous roots keep the soil from washing away. Birds eat the catkins, and other animals like deer, rabbits, and beaver eat the tender twigs and buds. The tree is the larval host of these butterflies: Acadian Hairstreak (Satyrium acadica), Mourning Cloak (Nymphalis antiopa) and Viceroy (Limenitis archippus).

Thanks to George Yatskievych, Curator in the Billie L. Turner Plant Resources Center, and Larry Gilbert, Director of the Brackenridge Field Lab