Friday, April 20, 2012

Dance of the Cottonwoods

Spring frenzy is here for the birds and the bees. But behind the hubbub, there's a slow dance playing in the background that continues year-round, year after year. It's the slow-motion dance of the cottonwoods (Populus spp.) Riverside cottonwood trees have an intimate relationship with water and ice that plays out across the generations.

In this dance, the river leads and the cottonwoods follows. For a cottonwood seed to grow into a sapling, it must sink its roots precisely in time and space. A misstep in the meandering floodplain means that the song ends for that young cottonwood.

Fall cottonwood leaf resting
beneath winter's textured ice
(c) John Ashley
A fall cottonwood leaf rests in the textured ice of a Montana spring.
Timing is everything.

For the frenzied birds, fertilization must occur immediately prior to egg laying. For cottonwoods to successfully reproduce, a big spring flood must take place just before seed release. Then the well-timed seedling will need the river to retreat slowly into summer. And for a young cottonwood to reach maturity (8-10 years), its "birth" must also land in a year that's followed by 2-3 years of relatively small spring floods, which gives it time to brace for the next major flood.

Mature cottonwood trees are dioecious, with separate male and female trees. The male and female flowers occur in catkins, near the treetops, and male catkins open just before the females. Flowering and pollination take place 1-2 weeks before leaf initiation (March-May), timed so that the seed release coincides with the river's "average" peak flow. And because spring arrives at different elevations at different times, the timing of seed release within any one cottonwood species can vary by more than two months.

Cottonwoods are prolific seed producers, begetting an estimated 25 million seeds per mature female tree. But the fuzzy seeds are unusually tiny, just 0.01 to 0.02 ounces (0.3 to 0.6 grams) each, because they contain little or no endosperm, or food stores.

Cottonwood seeds are only viable for 1-2 weeks, so they must land on moist, mineral soil that is free of competition for sunlight. Viable seeds that land in a good spot will sprout roots and leaves within 24 hours. The roots must maintain contact with water, and the leaves must convert sunlight into food. Seeds landing in unfavorable spots may sprout, but they will dry out (away from the riverbank) or run out of food (in a shaded location) in just a few days.

Of course, the best spots for cottonwood seeds to land are the sunny points and sandbars of a wild (undamed) river, which are swept clear of competing plants during a good, above-average spring flood.

Early cottonwood roots grow "collet hairs" that help anchor the plants in the flood-prone sandbars. The roots must grow about 1/4 inch (4-6mm) per day to keep pace with the receding water table. After one summer, the surviving seedlings are those that can sink their roots 30-60 inches (75-150 cm) and tap into the permanent groundwater (roots grow faster in sand than gravel). Due to capillary action, the water table below the stream bank is actually between 20-80 inches (0.5-2 meters) above the summer river level.

Young cottonwood trees are especially susceptible to drying out during their first summer, but the saplings that survive can handle subsequent inundation better than most plants. Mild flooding and silting in subsequent years are okay, unless the flooding includes major ice jams.

Occasionally, a sudden spring flood on a frozen river pushes massive blocks of broken ice downriver, scouring away most of the plants living along the banks. And while the grinding ice removes many of the young cottonwoods that landed too close to the river, it is also exactly the kind of flood that prepares the riverbanks for the next round of seeds.

Ice scars on the trunk of an old cottonwood
along the upper Missouri River
(c) John Ashley
Ice scar on an old cottonwood along the upper Missouri River.

The best river conditions for cottonwood seedling recruitment are periodic major floods and ice jams. In general, wild rivers tend to reach full bank about every other spring. Over-bank flooding tends to occur every 5-10 years, and major floods are 10-year to 50-year events.

Without periodic disturbance from floods or ice, the river banks fill in with other plants, and the yearly crop of cottonwood seeds can't compete. The great groves of shady cottonwood trees grow old and are slowly replaced with exotics, especially Russian olive (Elaeagnus angustifolia), whose seeds remain viable for up to three years. And that is exactly what is happening on many of Montana's rivers, especially where they've been tamed with flood-reducing dams.

In Montana, sections of the Milk, Marias and Missouri Rivers currently have low cottonwood recruitment rates, and younger bands of cottonwood trees are now missing from large sections of their banks.

On the Milk River, flood events that correlate with cottonwood recruitment occur every five years, on average. Below the Fresno Dam, reduced flooding and river movement have led to a "significantly lower" density of cottonwoods. Russian olive, on the other hand, was already present on 69 of 74 riverbank plots in one study, less than 50 years after its introduction to the area.

On the Marias River, below the Tiber Dam, Russian olive is getting even more help from the native beavers that apparently prefer native trees. Researchers on the Marias found that 77% of the cottonwood trees (of all age classes) were damaged by beavers, while only 22% of the Russian Olive trees were damaged. With beaver activity close by the river, and high flooding eliminated by the dam, the bands of cottonwood recruitment have been narrowed in some places and eliminated in others.

Along the upper Wild and Scenic portion of the Missouri River, it is cattle that are eating the young cottonwoods. Researchers core-dated the old cottonwoods and correlated their ages to the flood levels from the past 112 years. They found that 72% of the remaining cottonwoods were established in the year of, and up to two years after, a major flood level that occurred on average every 9.3 years. Almost all of these massive trees were more than 3 feet (1 meter) higher than the riverbank.

In other words, the conditions necessary for cottonwood seeds to survive to maturity only occurred once every 9-10 years (on average), or only 12 times in the past 112 years -- and that's on an upper reach of "wild" river above the Fort Peck Dam.

But change may be arriving on the spring winds. Further down the Missouri (below Fort Peck Dam), federal officials, ranchers and biologists are exploring the goal of restoring a higher spring flow to the river. Currently, the lower Missouri flows at less than one-half of the pre-dam levels of 70,000 cubic feet of water per second (cfs). To encourage cottonwood regeneration (and pallid sturgeon reproduction), managers are looking at increasing the flow to almost 50,000 cfs.

This political give-and-take may or may not be enough to grow enough cottonwoods to shade the riverbanks of our great-grandchildrens' generation. But because this is a slow dance, time is on our side if we choose to cut in.