Tree Leaves

What’s happening: The snow brought moisture, and the crust responded; moss in places last Sunday; looks like snow also loosened some rocks which came down into the arroyo; new basal growth on snapdragons Thursday.

What’s still green: Moss, juniper and other evergreens, Apache plume, yuccas, Japanese honeysuckle, pyracantha, grape hyacinth, large-leaved soapwort, sea pink, hollyhock, cheese, oriental poppy, blue flax, yellow and pink evening primroses, vinca, sweet pea, gypsum phacelia, tumble mustard, snakeweed, dandelion, anthemis, coreopsis, chrysanthemum, heath and strap leaf aster leaves; pampas and cheat grasses; rose stems and young chamisa branches.

What’s grey, blue-grey or grey-green: Piñon, four-winged salt bush, buddleia, pinks, snow-in-summer, loco weed, yellow alyssum, stick leaf, western stickseed, winterfat, golden hairy aster leaves.

What’s red/turning red: Cholla, prickly pear, small-leaved soapwort, beards tongues, coral bells leaves.

What’s yellow/turning yellow: Arborvitae; globe and weeping willow branches.

What’s blooming inside: Christmas cactus, aptenia, asparagus fern; chaste tree leaves dead.

Animal sightings: Robin in cottonwood Thursday; rabbit about.

Weather: Ground wet when I got up Wednesday morning, rain Thursday night turned to frost the next morning; 9:45 hours of daylight today.

Weekly update: A week ago Friday, one of the men I work with drove in from Pecos with a great, gaping hole in his windshield where a cottonwood branch had come down during the night. Snow and small branches still rode on the roof.

Cottonwoods have relatively soft, weak wood that doesn’t tolerate bending or compression. Ron Smith, of the North Dakota state extension service, calls them "free kindling," because they drop their branches so easily. There currently is a scatter of small, maybe half-inch thick, grey-white branches on the shoulder under a male cottonwood down the road.

One of the things that’s always puzzled me is why some local trees retain their leaves into winter and why those leaves don’t cause more problems for branches when they get wet or collect snow. I remember in Michigan the ice storms of January and February were always accompanied by broken limbs and downed power lines.

When the snow fell last week, my black locusts still had half their leaves, while the catalpa had some on its lowest branch and the cottonwood had scattered leaves near the bottom. Some cottonwoods growing near arroyos kept most of their leaves, while those near the river were bare.

This year the coming of winter in the Española valley has been marked by discrete, widely spaced events. We had our first hard freeze, when temperatures dropped into the high 20's, on October 26. Before that, the leaves on all three species were turning yellow in an orderly manner, but neither the locust nor the catalpa were prepared. The leaves on the one exotic turned dead green, the others brown. The native cottonwoods continued to yellow.

Night temperatures got decidedly colder, 20 on my front porch, on November 10. The cottonwood leaves turned brown.

The first snow moved through on November 16. When I woke the temperature was hovering around 32 and water was condensed on many leaves. I don’t know if we actually got snow, or only very cold moisture. The cottonwoods and catalpas dropped many of their leaves, but the locusts remained the same.

Another storm moved through on November 28, leaving snow in the mountains. High winds stripped my locusts of many of their leaves, especially toward the top. Three days later the morning temperature fell to 10 on my front porch. The remaining locust leaves turned brown.

Last week, the snow started falling just before dark on Thursday and stopped by the time blizzard conditions moved through the open lands between Santa Fé and Albuquerque that night. Pecos was already buried. The temperature was only 32 when I got home Friday night and little had changed except the roads were dry.

Saturday the sun never really broke through the clouds, but the temperature rose into the 40's. Snow either sank into the ground or evaporated into the air. Sunday morning was foggy from the heavy moisture load in the air. By mid afternoon, it was 50 on my front porch and most of the snow was gone.

Ice and leaves behaved the way I expected. On Saturday, evaporating water condensed on my metal roof and fell towards the roses’ drip line where it cooled as it dropped. When it landed on a vertical stem, it simply slid to the ground. When water landed on a leaf on a horizontal branch, it first encased the leaf in ice, then formed icicles which coalesced into elaborate, clear structures. Many rose leaves are now turning brown.

The immediate effect of cold damp wasn’t something I’d ever noticed before. Leaves on many non-woody plants turned black, especially the baptisia, which was never covered, and the purple coneflowers, which were buried. When I walked out towards the arroyo Sunday, I had a sense things were darker, but couldn’t identify any particular plant: it could have been the general wetness or it could have been leaves and stems darkening on some, but not all, individuals of plants like golden hairy asters.

The impact of the snow was less obvious. It tended to collect on lower, horizontal branches facing the storm. Leaves happened to be present on some of those branches that collected snow in my yard, but it was probably because both the leaves and the snow responded to similar wind patterns. The trunks of the trees must deflect the winds which keep leaves in place and allow snow to accumulate.

The weight of the snow did break the final connection between the leaves and the black locusts. By Saturday afternoon, there was a great mess of leaflets lying atop the snow. On the prairie, it looks like one cottonwood branch snapped, one that still retained its leaves. All the other downed wood looked old, long ago bleached and stripped of any signs of life.

The reason the leaves survived so long seems to be related to location as much as species. Botanists tell us plants respond to water stress and lower fall temperatures by slowing photosynthesis and producing abscisic acid, a hormone that seals the joints between leaves and stems so dead leaves can fall. If those biochemical messages are absent or contradictory, the plant is less likely to adapt to the changing season.

The cottonwoods growing on the relatively dry land back from the river near the city had no leaves this week, and the ground below was littered with long dead branches no one had cleared. Nothing looked like a new fall.

The cottonwood that lost its branch was growing in a gully cut by water leaving the end of an irrigation ditch for the arroyo. The tree that still has its leaves near the village sprouted in the run off of another irrigation ditch which has since dug a trench around the tree, exposing its roots. The arroyos kept running after the natural environment began to dry last fall, apparently countermanding any stress messages.

In my yard, the cottonwood is protected by a wooden fence on two sides, north and east. The surviving leaves are below the top of the fence towards the center of the tree. The locusts are close enough to the house, that the building may slow the wind near the ground where leaves still exist, while the leafy end of catalpa branch is less than 2' above the warmer ground.

The effect of piles of snow seems less important to trees than either damp or ice, which combines the weight of snow with the cold moisture. However, species seems more important than location. The locust shrugs off loosened leaves, while the cottonwood breaks from too much pressure.

Notes:
Smith, Ron. "Questions on Cottonwood," North Dakota State University extension website.

Taylor, Jennifer L. "Populus deltoides," 2001, in United States Forest Service, on-line Fire Effects Information System.

Photograph: Cottonwood growing in a gully carved by an irrigation ditch, with a leaf laden broken branch, 24 December 2010.