Sunday, February 20, 2011

Strapleaf Aster

What’s happening: New leaves on golden spur columbine; Russian olive, juniper and pyracantha berries persist.

What’s still green: Moss, evergreen, yucca, grape hyacinth, Jupiter’s beard, phacelia, pink evening primrose, broom senecio leaves; some grasses.

What’s grey, blue-grey or grey-green: Piñon, four-winged salt bush, yellow alyssum and winterfat leaves.

What’s red/turning red: Cholla, Madonna lily, small-leaved soapwort, beardstongue, yellow evening primrose, creeping mahonia leaves; rose and young tamarix stems.

What’s yellow/turning yellow: Globe and weeping willow branches.

What’s blooming inside: Zonal geraniums on enclosed porch; aptenia in house.

Animal sightings: A mouse came into the house in December when it was cold, but nothing invaded during the past cold spell.

Weather: Clouds all week at daybreak, winds yesterday; last snow 2/4/11; 10:30 hours of daylight today.

Weekly update: When I was a child in the 1950's, I was told composites were the youngest plant family, with the implication that everything had been formed.

Much of the early evidence for evolution had come from fossils laying in chronologically created rock strata. Plant families were seen as life forms that had emerged in particular past climates, the evergreens in the Carboniferous, the ferns in the Triassic, the grasses in the Oligocene. Each was thought to have flourished in its time, with only the most adaptable surviving the subsequent changes in environment.

Composites may have diverged in the Eocene, some 36 to 42 million years ago, but they have survived the tropical Eocene, the dry Oligocene, and the frozen Pleistocene to become the ones most actively adapting to the geobotanical present.

Some argue the reason the family has proliferated is its reproductive strategy matches modern conditions. Instead of a single flower or cluster, numerous small individual florets are held in clumps in a single receptacle, often with the large, outer ray flowers serving no other purpose that attracting insects to the central disk flowers. The seeds are dispersed by wind-driven parachutes.

Since I was a child, botanists have renamed the family the Asteraceae, and turned from fossils to DNA for evidence. They’ve exchanged the absolute "the" and superlative "est" for the safer "a" and "er" as they’ve come to recognize evolution is not just history.

Strapleafed spiny asters have traditional composite flowers, with a single ring of pollen producing rays surrounding yellow, bisexual disks. The long, narrow rays open mid-morning from early or mid June to early November and close each afternoon.

The perennials range from the prairie provinces of Canada to the silver producing states of México. Within that wide belt of plains and dry grasslands, the four pairs of chromosomes are permutating with clear patterns of selection appearing only in areas where populations have become isolated. In a few areas, the number of chromosomes has doubled to 16.

Whenever the separation is destroyed, subspecies begin interbreeding with other populations. Plants in the nodes have taken many forms, which in turn has led to at least ten generic names. The areas between the clearly formed species are filled with gradations that have led to the current preferred term, the Xanthisma spinulosum complex.

In our area the woody stems of the spinulosum subspecies branch almost immediately, to send radiating stems out six or eight inches along the ground before they curve up to hold single, terminal flower heads. When they first emerge, the alternate leaves resemble herringbone evergreens with sharp points.

As the stems elongate, the distance between the leaves expands, while the leaves lengthen with short lobes along central midribs that give them the name tansy aster. Up close, the leaves are bright green, but from a distance appear gray from fine hairs.

Down the Rio Grande in south Texas, the austrotexanum subspecies has an erect, unbranched habit with less deeply incised leaves and no glandular hairs. To the west in Arizona, the gooddingii subspecies has shiny leaves, tall, erect stems, and larger flowers.

Sleepy daisies seem to be particularly sensitive to variations in environment. At the Sevilleta National Wildlife Refuge north of Socorro, the plants prefer the black grama steppes to the blue grama grasslands. The first have rockier, coarser soils than the second, and tend to favor plants like these with long taproots.

During the 1930's, drought along the Missouri River in Iowa, Nebraska and Kansas killed many of the grasses and forbs. Strapleaf asters were one of the few not only able to survive in the drier western areas, but to spread east to colonize newly barren areas when the high winds of the period transported their yellowish-brown seed vessels with the loosened soil.

The plants are often seen in disturbed or open areas: they were first collected by Lewis and Clark on September 15, 1804, the day they passed the mouth of the White River in South Dakota. However, it may not be the tiny seeds need disturbed soil to germinate so much as the plants disappear when competition develops.

They grew in my yard between 1997 and 2002, but never in the same place. Then, a single plant bloomed for two years under the black locust, before disappearing. With the changing population of grasses, they behaved more like annuals than perennials.

Last summer I noticed them in the prairie growing in bare ground left by four-wheeled all terrain vehicles about five to ten feet away from the drop into the arroyo. They appeared there again this past summer.

The Sevilleta research team believes one reason non-grasses like strapleaf asters can coexist with black grama, but not with blue, is that the first begins growth later in the season after the plants have developed, while blue grama emerges earlier and competes with forbs for resources.

Here the pointed yellow rays seem particular well adapted to the cold: both years they continued to appear after morning temperatures had fallen below freezing. Soon after temperatures reached 20 degrees last November, the roots produced new basal growth. After the snow and zero degree cold of late December, those leaves turned dark green and may have died. However, new basal growth has appeared on a few, below the bare woody stems from last season.

Botanists have come to recognize new species result from mutations and genetic experimentation by nature, and that, over time, those species begin to solidify through continued genetic selection. What they’ve found with strapleaf asters is a species that is simultaneously proliferating and winnowing in an evolutionary present.

Barkley, Theodore M., Luc Brouillet and John L. Strother "Asteraceae Martinov," on eFloras’ Flora of North America website, summarizes current knowledge on plant family.

Kröel-Dulay, György, Péter Ódor, Debra P.C. Peters and Tamara Hochstrasser. "Distribution of Plant Species at a Biome Transition Zone in New Mexico," Journal of Vegetation Science 15: 531-538, 2004; identifies Machaeranthera pinnatifida.

McDougall, W. B. "Lessons in Botany," National Park Service Region III Quarterly 3:7-10: October 1941, summarizes the received wisdom of my mother’s generation.

Peterson Field Guide. Southwestern and Texas Wildflowers, by Theodore F. Niehaus with illustrations by Charles L. Ripper and Virginia Savage; uses the term strapleaf spine aster for Machaeranthera spinulosa.

Turner, B. L. "Xanthisma spinulosum var. austrotexanum (Asteraceae: Astereae), An Endemic of Southernmost Texas," Phytologia 89:349-352:2007.

_____ and Guy L. Nesom. "Taxonomic Review of the Xanthisma spinulosum Complex (Asteraceae: Astereae)," Phytologia 89:371-389:2007.

Weaver, J. E. and F. W. Albertson. "Deterioration of Grassland From Stability to Denudation with Decrease in Soil Moisture," Botanical Gazette 101:598-624:1940; identifies
Sideranthus spinulosus.

Photograph: Young leaves, some probably killed by cold, at base of prairie strapleaf aster with seed (achene), 13 February 2011.

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