David Giblin Center for Urban Horticulture University of Washington 3501 NE 41st Street Seattle, Washington 98195 Aster Curtus: Current Knowledge of Its Biology and Threats to Its Survival David Giblin Abstract Aster curtus Cronq. , a taxon in the family Asteraceae, is a rhizomatous, herbaceous perennial endemic to the remnant prairies of Oregon, Washington and Vancouver Island, British Columbia. The current population center is located on Fort Lewis Military Base in Pierce and Thurston counties of Washington State. A “species of concern” under the Endangered Species Act, its current listing in Washington is “Sensitive”. Aster curtus cover is highest on prairies with low exotic species invasion and competes poorly with associated native and exotic species at the seedling stage. Under field conditions, recruitment by seed appears to be low, and persistence of this species is achieved primarily through clonal growth. Aster curtus is self-compatible, though putative out crossing appears to result in more filled seeds. Industrial, residential and agricultural developments are the largest threats to its survival. Considering the degree of habitat degradation that has already occurred throughout the range of A. curtus, the long-term survival of this species will depend on how well we understand its autecology and how well we protect its habitat. This paper summarizes past and present information that can be applied to the conservation efforts of this species. Introduction range, as evidenced by the lack of current records for populations between Aster curtus is endemic to the Pierce/Thurston counties, Washington Willamette-Puget lowlands of the Pacific and Vancouver Island, British Columbia. Northwest. Its present range extends Additionally, no populations are from lower Vancouver Island, British presently known to exist between these Columbia to Lane County, Oregon, with two Washington counties and Clackamas the largest populations found on Fort County, Oregon (Gamon and Salstrom; Lewis Military Base in Pierce and 1992). The reasons for this patchy Thurston counties, Washington (Gamon distribution are unknown, but conversion and Salstrom; 1992). Over the past 20 of grasslands to pasture and commercial/ years, more than 70 new populations of residential development have likely been A. curtus have been reported, however major factors. Despite much attention these discoveries have largely occurred give to A. curtus over the past 15 years in counties where the species was (Alverson, 1983; Clampitt, 1984; Giblin, already known to be extant (Gamon and 1997) many questions remain regarding Salstrom; 1992). The distribution of A. the reasons for its rarity. curtus is not continuous throughout its D. Giblin Aster curtus 93 Taxonomic Status As a federal level, A. curtus is listed as a Aster curtus Cronq., was originally species of concern (USFWS, 1996). This described as Sericocarpus rigidus classification is given to those species Lindl.in 1834 on the basis of collections for which more information is needed made by Scouler along the Columbia before formal listing as endangered or River. Cronquist changed S. rigidus to threatened (Thomas and Carey, 1996). Aster curtus (Hitchcock et al., 1955), The Washington Natural Heritage and more contemporary taxonomic work Program downgraded A. curtus from its has placed Sericocarpus as a subgenus original listings as “Threatened” to of Aster (Jones, 1980; Semple and “Sensitive” (Washington Natural Brouillet, 1980). This subgenus is Heritage Program, 1990), undoubtedly comprised of the western U.S. species A. the result of more populations having curtus and A. oregonensis, the eastern been located since earlier versions of the U.S. species A. paternus, A. list. The state of Oregon lists A. curtus as solidagineus, and A. tortifolius, and the “Threatened”, and this guarantees legal Asian species A. baccharoides. Only A. protection for this species on all state- curtus is a grassland native with the owned or state-leased property (Oregon remaining species typically inhabiting Natural Heritage Plan, 1993). British the forest under story. The common Columbia lists A. curtus as “critically name of A. curtus, white-topped aster, imperiled” because of extreme rarity derives from the effect produced by the (Argus and Pryer, 1990). pappus which expands just prior to seed dispersal. Habitat Characteristics The ranges of A. curtus and A. A. curtus inhabits the glacial outwash oregonensis overlap from southern prairies and grasslands of the Puget- Washington through central Oregon, Willamette lowlands that were formed however the two species are separated during the Pleistocene glaciations on the basis of habitat preference as well 15,000 years ago. Historically, high as reproductive and vegetative frequency, low intensity fires and morphology. Most notable, the periodically dry soils have kept the woodland species A. oregonensis has bunchgrass-dominated prairie system four ray flowers and over winters as a free from encroachment by forest stout caudex, whereas the grassland vegetation (Hansen, 1947). A. curtus is dweller A. curtus, typically has two ray typically found in open locations where flowers and is strongly rhizomatous cover of native grasses and forbs (Hitchcock et al., 1955). The other five exceeds 50% (Thomas and Carey, 1996), Aster species of the subgenus but will also grow in Quercus garryana Sericocarpus have a haploid (Garry Oak) woodlands where light chromosome number n=9, and this is the levels are high enough to support other presumed condition for A. curtus open prairie vegetation (personal (Clampitt, 1984; Jones, 1980; Semple observation). Native species strongly and Brouillet, 1980). associated with A. curtus include Festuca idahoensis (Idaho fescue), Legal Status Eriophyllum lanatum (Woolly daisy), D. Giblin Aster curtus 94 Hieracium cynoglossoides ( inflorescences during anthesis (pers. Houndstongue hawkweed), and obs.). A. curtus is self compatible Solidago spathulata var. neomexicana (Giblin, 1997), however pollinators (Thomas and Carey, 1996; Crawford et increase the amount of endosperm-filled al., 1994). seeds produced 4-fold (Giblin, 1997; Clampitt, 1984). The prairie soils of Pierce and Thurston counties, where the majority of A. curtus seed viability is low but not populations are found, typically belong atypically so for species Asteraceae to the Spanaway series (Anderson et al, (Clampitt, 1984). Clampitt (1984) found 1955; Ness, 1958). These are well- that 20% of the achenes produced per drained, gravelly sandy loams which plant were filled with endosperm. Of become excessively dry between July these filled seeds, only 50% proved to be and September. Low moisture viable through tetra-zolium testing availability during the summer months (Clampitt, 1984). Greenhouse studies results in the dormancy of many spring showed that seed germination was most flowering forbs. favorable after 5 weeks stratification at 5 degrees C, and high light/warm Life History temperature (30 degree C) conditions (Clampitt, 1984). A. curtus is a rhizomatous, herbaceous perennial which produces erect stems 1- Ecology 3 dm in height from slender rhizomes (Hitchcock et al, 1955). New shoots Rabinowitz (1981) developed a emerge in March and become prominent classification system of rarity on the in late spring (pers. obs.). Flowering basis of geographic distribution, habitat occurs from late July through early specificity, and population size. Within September with individual florets being this system, A. curtus is described as a strongly protandrous. Pollen is presented regional endemic due to its narrow in the morning and usually absent by distribution, narrow habitat specificity days’ end as a result of removal by bees, and large population sizes. Indeed, when disruption of the inflorescence by wind, encountered in the field, A. curtus is and perhaps desiccation. Stigmas appear observed to be locally abundant over a to be receptive for up to 3 days (pers. small geographical area. obs). On average there are 13 disk flowers and 3 ray flowers per capitulum Plants are divided into three groups on (Clampitt, 1984; Hitchcock et al, 1955). the basis of whether their survival Only 10-30% of the ramets within a strategies are ruderal, stress tolerant, or patch bear inflorescences (Gamon and competitive (Grime, 1977). Due to its Salstrom, 1992). Most seeds ripen and slow growth rate, ability to persist under are wind dispersed by late September. severe competition, low stature, and Wind dispersal is achieved via the bristly allocation of resources toward clonal pappus attached to the mature achene. expansion prior to flowering, A. curtus is considered to be a stress tolerant species Effective pollination is achieved by 3-4 (Clampitt, 1984). bee species, though butterflies do visit D. Giblin Aster curtus 95 On account of its rhizomatous nature, A. strongly rhizomatous Agrostis tenuis curtus forms patches where it occurs. (Bentgrass) and the fall germinating Patch size is extremely variable both in Hypochaeris radicata (Cat’s ear) are two area and number of ramets, with native exotic species which can quickly species abundance strongly influencing colonize microsites favorable for these factors. ON Fort Lewis prairies recruitment of A. curtus seedlings. where native species dominated, Thomas Carey (1996) found patch area to Current Research average 10m2, though they observed patches up to 40m2. They also report In the summer of 1996 I began ramet numbers ranging from 16 to researching the reproductive biology of