Sunday, March 09, 2008

Rio Grande Cottonwood

What’s green: Conifers, rose stems, yuccas, rockrose, coral bell, sea pink, sea lavender, snapdragon, Saint John’s wort, Mount Atlas daisy, chrysanthemum, anthemis, some grasses

What’s gray or gray-green: Salt bush, winterfat, snow-in-summer, some pinks.

What’s red: Cholla, some pinks, small-leaved soapwort, coral and purple beardtongues, purple aster.

What’s blooming inside: Aptenia, zonal geranium, kalanchoë, Christmas cactus.

Animal sightings: White geese in a village farm yard; quail tracks in the snow.

Weather: Heavy snow Wednesday night still covered the ground Friday when temperatures fell to 10 degrees on my front porch; most of the 8" has melted or sunk back into the just dried layer of mud above the freeze line. 11:49 hours of daylight today are not enough to justify daylight savings time.

Weekly update: When snows fall and leave behind mud and clumps of brown hugging the ground, cottonwoods silhouetted against the gray sky are all that attract the eye.

The oldest trees line roads at the far end of the village, maimed by age, relics of a land now gone. At some time, that area had to have been wetter than it is now because cottonwoods only germinate if the ground is moist when the seed falls in mid-summer. The seed has little viability, and if it doesn’t settle within two weeks, it’s lost. To survive, the shallow roots have to reach the water table.

During the depression, the Interior department reported Rio Arriba county had the highest malaria rate of any county in the country. The Federal Emergency Relief Administration, the precursor of the WPA, hired some 30 local men to drain wetlands east of the river between 1934 and 1935 to control the mosquito population.

The general population of Rio Grande cottonwoods has been declining ever since, partly from droughts of the 1930's and 1950's, and partly from projects like the 1935 El Vado dam, the 1963 Abique dam, and the 1971 Heron dam, all on the Chama tributary, which eliminated floods that deposited the silts that nurtured seeds.

The short-lived trees were always endangered. Wood was still the primary source for heat for many when I moved here in 1991. My closest neighbors still have wood hauled in every season from the mountains, even though natural gas has been available since 2000. Some of these trees may have begun life as green fence posts that put down roots.

Still, until the wetlands disappeared, the trees apparently were able to maintain themselves. Our wislizeni subspecies of the eastern cottonwood, Populus deltoides, adapts to subtle changes in environment. In the middle 1990's, Diane Rowland found 120 genotypes from four river locations that varied from one another in how they handled light through photosynthesis and moisture through transpiration. Ours bloom late in spring so the seed ripens just as the monsoons return.

She and Nancy Johnson also found that while the ratio of male to female trees did not change during droughts, the females stopped producing their fluffy catkins to conserve resources. Cottonwoods also drop their triangular leaves and smaller branches under stress, paring the winter structure.

Cottonwoods do still sprout, especially in the low channels down from roads near arroyos, where water collects during summer rains beyond the reach of mowing machines. Unlike the tall sentinels, younger members of the willow family still have many limbs branching a few feet from the ground and fully rounded heads of twigs that are indistinguishable from other trees in the band of dark grays on the winter horizon.

Since the trees don’t tolerate shade, each limb has to grow outward to survive. The straight-grained light-weight wood is notoriously weak, and, with time, the branches fall from their own weight, often during high winds or heavy snows like we had Thursday morning. Every one of the older village trees has had at least one limb amputated, often back to the main trunk.

Their silhouettes do more than soothe the spirit on dreary days. The shadows towering above one-story homes speak of resilience in the face of life-threatening changes that transform my boredom with continuing dreary weather into a hope that all this late winter moisture will help native plants recover from the recent dry seasons.

Rowland, Diane L. "Diversity in Physiological and Morphological Characteristics of Four Cottonwood (Populus deltoides var. wislizenii) Populations in New Mexico: Evidence for a Genetic Component of Variation," Canadian Journal of Forest Research 31: 845–853:2001.

_____ and N. C. Johnson. "Sexual Demographics of Riparian Populations of Populus deltoides: Can Mortality Be Predicted from a Change in Reproductive Status?," Canadian Journal of Botany, 79:702-710:2001.

US Dept of Interior. Tewa Basin Study, volume 2, 1935, reprinted by Marta Weigle as Hispanic Villages of Northern New Mexico, 1975.

Photograph: Village cottonwoods, 8 March 2008.

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