Eriogonum pelinophilum

Low rounded heavily branched pulvinate subshrubs 5-10 cm high and 8-15 cm across; lower stems light brown, woody, bark exfoliating in long loose strips or wide plates; leafless, upper branches herbaceous, slender, floccose to glabrous; leaves solitary, scattered along entire length of herbaceous stems, except for the last 5-10 mm, somewhat closely placed and congested to widely spaced, leaf blades oblanceolate, 5-12 mm long, 1-1.5 mm wide, densely white-tomentose below, midveins totally obscured by the tomentum, subglabrous to glabrous and green above, margins entire, revolute and completely enclosing the lower surface, apices and bases acute, leaves persistent, petioles 1 mm long, light yellowish-brown to tan and thinly pubescent when young, becoming glabrous; flowering stems slender, 5-10 mm long, floccose to glabrous; inflorescence cymose, +/- compact and congested, 1-2 cm long and wide, trichotomous, rays 2-5 mm long, floccose to glabrous without, thinly tormentose within, connate at base; peduncles, when present, 1-1.5 mm long, floccose to glabrous, erect; involucres solitary, narrowly turbinate, 3-3.5 mm long, 1-1.5 mm wide, floccose to glabrous without, glabrous within, 5 acute lobes 0.3-0.4 mm long, bractlets oblanceolate, 1.8-2.5 mm long, minutely fringed with capitate gland-shaped cells, pedicels 2.5-4.5 mm long, glabrous; flowers white with reddish-brown midribs and brownish-red bases, 3-3.5 mm long, glabrous within and without except for microscopic glands along the midribs within, tepals similar; achenes 3-3.5 mm long (Peterson 1982).

Eriogonum pelinophilum differs from the similar E. clavellatum in that the latter is larger (10-20 cm vs. 5-10 cm high) and has glabrous stems with involucres 4.0-4.5 mm long vs. floccose to glabrous stems and involucres 3.0-3.5 mm long (Weber 1987).

Eriogonum pelinophilum is found in substrates derived from the Mancos Formation shales. The entire area is typified by rolling adobe (clay) hills and flats. Generally, the plants are found in a sharply defined soil microhabitat with shadscale (Atriplex confertifolia), on mid to lower slopes of the hills. The soil types are part of the Billings Series, known for its fine texture and weak and unstable structure. These soils are calcareous throughout and in some places have visible accumulations of calcium carbonate or calcium sulfate (Cline et al. 1967). Steeper barren slopes (badlands) are above, with flatlands below dominated by mat saltbush (A. corrugata) (Cline et al. 1967, U.S. Fish and Wildlife Service 1988). The change from the lower slope soils to the flatland soils is characterized by a jump in soil sulfate level (from <100 ppm to 1650 ppm) and sodium level (Potter et. al. 1985). Clay soils have a high water holding capacity, but this moisture is not readily available to plants (Barbour et. al. 1980, cit. in O'Kane 1985). Rainfall in the E. pelinophilum habitat averages 7-10 inches annually, further contributing to the low moisture availability (Colorado Climate Center 1984). Eriogonum pelinophilum generally prefers swales and bottoms where useable moisture is more available (O'Kane 1985). Because of the low moisture availability, communities in which E. pelinophilum occur are characterized by low species diversity, low productivity and minimal canopy cover. Eriogonum pelinophilum is codominant with other xerophytic shrubs or subshrubs such as shadscale, the rare Penstemon retrorsus, Castle Valley clover (Atriplex cuneata), mat saltbush, black sagebrush (Artemisia nova) and Xylorhiza venusta (Neely 1985, O'Kane 1985). The communities are apparently stable, climax associations, judging from the lack of invading species capable of dominating the sites. Field observations indicate that the species is most abundant where biological soil crust cover is not extensive (Ferguson 2007).

Results from the Colorado Natural Areas Program (CNAP) monitoring program on Eriogonum pelinophilum (1987, 1988) provide previously unknown information about Eriogonum pelinophilum biology and ecology. The program is in its 3rd of 12 years, so data collected thus far may not accurately portray the species' life characteristics over time. Preliminary conclusions are: i) Eriogonum pelinophilum is a long-lived perennial with a probable population turnover rate of approximately 20-50 years, ii) flowering, and therefore reproduction, does not occur until an individual plant reaches a critical size of approximately 100 cm2, iii) plant density does not appear to be limiting the success of Eriogonum pelinophilum (i.e. plants with close neighbors have life characteristics comparable to solitary plants), iv) Eriogonum pelinophilum appears to be distributed randomly among its associated species, v) after 2 years, the average mortality rate per year was 2.7 percent, and the average recruitment rate per year was 4.6 percent.

Unless otherwise noted, the following is from O'Kane (1985). Densities of Eriogonum pelinophilum range from 75-500 individuals per acre (180 per acre average). Because the plants prefer swales and lower slopes, those smaller areas have a considerably higher density than the per acre densities calculated above. Pollination agents are not precisely known, though ants have been observed pollinating other Eriogonum species. Flowers are proandrous, with the androecium maturing 1-2 days before the stigma is receptive. Seed dispersal is usually passive, either being consumed or carried by animals, windblown, or moved by gravity or water. Flowering occurs in June-July, fruiting in late June-early August. All Eriogonum species studied thus far have seeds that require a cold period to break dormancy (not necessarily a freeze), and some Eriogonum species have seeds with a 5 year shelf life (Reveal undated, cit. in O'Kane 1985).

Flowers are protandrous. Each individual flower is short-lived (about 30-42 hours), but the bloom period at the plant and population level is relatively long: individual plants have open flowers for 3-6 weeks, and one large population near Montrose, Colorado had open flowers from late May through early September (Bowlin et al. 1993).

E. pelinophilum requires an insect pollinator in order to set seed (Bowlin et al. 1993). Experimental study showed that the species is self-compatible (sets viable seed when pollen is transferred between flowers on the same plant); the authors believe that "pollinators moving from male-stage to female-stage flowers on the same plant will occasionally effect pollination" (Bowlin et al. 1993). Pollinators also frequently move between plants, resulting in a mixed breeding system with some pollination from other flowers on the same plant and some from flowers on different plants (Bowlin et al. 1993).

A wide variety of insects visit and probably pollinate the flowers. Over 50 species of insects, about half of them native bees, were recorded foraging on E. pelinophilium in one large population near Montrose, Colorado (Bowlin et al. 1993). 18 species of ants were observed foraging; most were either harvester ants (Pogonomyrmex occidentalis) or aphid-tending ants (Formica spp., most often F. obtusipilosa), with the aphid-tending ants generally carrying pollen more often and further than the harvester ants, especially early in the season (Bowlin et al. 1993). Both ants and flying insects appear to be effective pollinators, with a field experiment showing no significant difference in seed set among flowers visited only by ants, only by flying insects, or by both groups (Bowlin et al. 1993).

Most seed is dispersed locally (Bowlin et al. 1993).