Sunday, November 13, 2011

Cottonwood Copse

Weather: Morning temperatures settled into low 20's, last rain 11/05/11; 10:20 hours of daylight today.

What’s blooming: Pansies, sweet alyssum, other plants in sheltered positions may have one or two flowers left.

What’s still green: Juniper, arborvitae and other evergreens, prickly pear, yuccas, grape hyacinth, red hot pokers, privet, Japanese honeysuckle, oriental poppy, golden-spur columbine, purple and coral beards tongue, sea pinks, coral bells, Saint Johns wort, oxalis, hollyhocks, winecup, sweet pea, alfalfa, clovers, bindweed, yellow evening primrose, vinca, gypsum phacelia, tansy, Hopi teas, anthemis, blanket flowers, coreopsis, strapleaf and purple asters, June, cheat and other grasses.

Pepper plant dropped its drying red pod when the stem holding it was killed by the cold.

What’s red/turning red: Cholla, leaves on purple leaved plum and sand cherry, leathery Bradford pear, raspberry, privet, Japanese barberry, Husker’s beard tongue.

What’s blue or grey: Piñon, leaves on four-winged saltbush, California poppy, loco, catmints, snow-in-summer, pinks, baby’s breath, blue flax, stickleaf, winterfat, chocolate flower, creamtips, hairy golden and heath asters.

What’s yellow-green/turning yellow: Leaves on weeping willow, Siberian elm, Apache plume, rugosa rose, snakeweed, perky Sue.

What’s blooming inside: Aptenia, asparagus fern, zonal geranium.

Animal sightings: Berries have disappeared from my Russian olive along with the leaves that covered them. Berries are also gone from the pyracantha near the village. With the drought leaving less food for local animals, those passing through seem to have taken more that I can see than usual.

Weekly update: Cottonwoods have become endangered in this area because their habitat has been altered, partly from draining malaria incubating wetlands, party by dams that have stopped tributaries from flooding the banks of the Rio Grande, and partly by generally drying weather the past decades.

A few weeks ago I walked out in the far arroyo and discovered a cottonwood copse in the process of forming. There were mature trees near the point where the arroyo and the county road intersect. Trees about three feet tall were growing back by a high sand and clay bank. Much younger seedlings were growing in raised areas in the arroyo bottom itself.

Cottonwoods don’t easily reproduce themselves. The seeds are only viable for a couple weeks after they mature, which is in the spring, but they germinate within 24 hours if they land on bare, moist soil.

They then need to stay moist at the level of the young roots, but the leaves need full sunlight to feed the roots, a difficult balance in this environment. During the time the roots are reaching down to the water table, seedlings can tolerate very wet conditions. However, young plants don’t always survive heavy water flows.

In the wild, good conditions for developing new trees occur periodically, maybe every five to ten years. The existence of three generations of trees so close to each other means these ideal conditions have been met at least twice in the past decade.

The mature trees are growing where water would have collected or washed from the river ford.

The dirt road through the arroyo has been there for more than 60 years; it shows on the USGS map from the early 1950's. It dropped from relatively high, maybe 15' high, banks. Work must have been done by the county to keep the banks stable and provide a slightly sloped route in and out. I saw the remains of one protecting drain pipe that had emptied water upstream.

The upstream trees lie in the path of water that flows along the arroyo bank from the point where local acequia water enters the arroyo. That area gets lots of water during the summer, but the rate is probably slow and constant so the water supply is fairly reliable. Still it has cut a channel around an island which also supports a tamarix and some chamisa.

Trees could have germinated there any time. The fact they have not is probably indicative of the difficult balance of water and sun young trees require.

One set of young trees are growing between these saplings and the ford, very close to the island bank. I’ve noticed in other parts of the arroyo, those short banks seem to retain water later than the bottom itself.

The other set is in the wide bottom itself on the other side of the island, still closer to the ford. They look about the same age, but were battered by the scouring water that poured through earlier this fall.

I’m guessing these may all have germinated in the spring of 2010 when we had a cold, wet winter followed by a wet spring that lasted long enough for the seedlings to get established. The drought began that summer and lasted until this fall.

Their survival may be helped by changes made to the road this summer. They finally built a bridge over the arroyo. This narrows the water channels and creates a need for a larger area for water to back into while it waits to flow through. Those areas may become pools if the surface is rough enough to prevent the lowest level of water from moving when the flow drops and islands may develop around the area where the young trees are growing.

Braatne, Jeffrey H., Stewart B. Rood and Paul E. Heilman. “Life History, Ecology and Conservation of Riparian Cottonwoods in North America” in R. F. Stettler, H. D. Bradshaw, P. E. Heilman, and T. M. Hinckley, Biology of Populus.

Photograph: Cottonwoods taken in the far arroyo, 25 October 2011.

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