Rare Plant Research in the Sipes' Lab

 

 

Current research involving rare plants:

Broad spectrum herbicides such as hexazinone are sometimes applied after forest fires to increase growth and survival of conifer seedlings by reducing competing vegetation. This practice removes not only undesirable alien plant competitors, but also native species. This effect may be of special concern for rare native species such as Iliamna bakeri (Malvaceae), a sensitive plant of northern California. This plant is adapted to follow fire in early successional stages, and therefore, may be especially vulnerable to post-fire herbicide application. Currently, USDA Forest Service land managers have no basis for evaluating this risk because little is known of the biology of I. bakeri. I and my collaborators are currently examining the effects of hexazinone application on the demography, reproduction, and population genetic structure of I. bakeri. Based on the findings, site-specific recommendations will be developed regarding hexazinone application, to achieve necessary levels of conifer growth while supporting the reproduction of this sensitive plant species. Additionally, we will be examining the pollination relationships of I. bakeri. Preliminary observations (S. Smith, USDA) indicate that a specialist bee, Diadasia nitidifrons, may be the primary pollinator of I. bakeri. Specialized ecological interactions such as this must be understood, and management decisions must address the needs of the pollinator as well as those of the rare plant species. The relationship of I. bakeri and D. nitidifrons also presents an optimal opportunity to examine the efficacy of specialist bees as pollinators of their host plants. Photo credit Charles Webber, California Academy of Sciences.

Collaborators:

Vincent J. Tepedino, USDA/ARS Bee Biology and Systematics Laboratory, Logan UT.

Sheri L. Smith, USDA Forest Service, Susanville CA.


 

Spiranthes diluvialis is a terrestrial riparian orchid that occurs in widely scattered populations in the western mountains of the U.S. The species was initially believed to be quite rare and declining in abundance, and in 1992 it was listed as threatened under the U.S. Endangered Species Act. Since that time, an heroic surveying effort by various land management agencies and private interests has shown it to be more widespread than initially supposed. Nevertheless, this species' continued survival remains threatened by human modification of its riparian habitiat (dam building, flood control). The orchid is an early to intermediate successional species and depends largely on natural flood disturbance to maintain suitable habitat. I have been studying the reproductive ecology of S. diluvialis since 1992, with field studies still ongoing. My research has focused on:

1) Breeding system and pollination. By experimental hand-pollinations in the field, I determined that, although S. diluvialis is physiologically capable of self-pollination, it is predominantly outcrossing due to the acropetal development of the inflorescence, and the functional protandry of the flowers. Long-tongued bees of the genera Bombus and Anthophora (the primary pollinators) tend to forage from the bottom up on vertical spike inflorescences, thus the bees most often transfer pollinia from the top, younger male-phase flowers of one plant to the bottom, older,female-phase flowers of the next plant visited. However, there is considerable overlap in the sexual phases of flowers, and the specific characteristics of this overlap make self-pollinations especially likely under conditions of low pollinator visitation. Thus, there is a threshold pollinator abundance below which the outcrossing mechanism of S. diluvialis fails to perform. Land managers must strive to protect this rare plants pollinators, and maintain appropriate habitat for the bees.

2) Long-term studies of the correlates of male and female reproductive success. I and my collaborators have measured spatial and temporal variations in reproductive success over a period of 9 years. Because S. diluvialis pollen occurs in a single discrete packet (a pollinarium), the presence or absence of pollinaria serves as an estimate of male reproductive success. Thus we have been able to estimate both male reproductive success (pollinarium removal) and female reproductive success (fruit and seed set).

3) Experimental transplantation of pollinators. Our long-term reproductive studies showed that a disjuct population of S. diluvialis in Daggett co. UT, consistently suffered extremely low rates of bee visitation, pollinaria removal, and fruit set; these data strongly suggest pollinator-limited reproduction. In an attempt to augment pollination levels, my collaborators Vince Tepedino and Kim Pierson and I, working with federal land managers, have trapped and transplanted a native, solitary, twig-nesting bee pollinator (Anthophora terminalis) from sites in Utah co. to the Daggett co. sites. It is too early to know if these actions will significantly increase S. diluvialis reproduction in the long run, but preliminary data indicate a slight reproductive increase in the year following the release of transplanted bees. This novel conservation approach represents the first time native solitary bees have been transplanted in the management of a rare plant species.

Collaborators:

Publications and Research Reports:

 

Some of my past research involving rare plants:

As a side project to my M.S. research, I examined the pollination, breeding system, and population structure of Jones cycladenia, Cycladenia humilis var. jonesii, a federally-listed threatened, edaphically-restricted species occurring in just a few locations in Utah and Arizona. This taxon exhibits extremely low fruit and seed set even with hand-pollination, and no seedling recruitment has been observed. However, the taxon does reproduce clonally. Population genetic structure inferred from isozymes revealed unusually high levels of genetic variation and little evidence of inbreeding, and confirmed that populations consist of numerous interdigitated clones. I hypothesize that this taxon may be a paleoendemic surviving in the absence of its original pollinator through clonal growth. The high genetic variability may suggest that high genetic load has accumulated in very old clones, which in turn may also play a part in its current low levels of sexual reproduction. In an effort to further understand the restricted distribution of this species, I collaborated with soil scientist Janis Boettinger (USU) to study the edaphic relationships of C. h. var. jonesii. This species occurs only in certain members of the Chinle, Cutler, and Summerville formations in southern Utah and Northern Arizona. Because these soils are often gypsum rich, previous reports had classified Jones' cycladenia as an obligate gypsophile. However, we concluded that the physical, rather than chemical, characteristics of the plant's soil substrate are associated with its restriction to only certain geological members. Jones cycladenia sometimes occurred in very low-gypsum soils, but all soils in which it occurred shared numerous common physical attributes (i.e. shallow soils derived from blocky-fracturing mudstones with high % of rock fragments), and these characteristics were lacking in adjacent soils where Jones cycladenia did not occur. These studies have provided important information for land managers: sexual reproduction is rare and thus does not represent a means to augment population size or number. Additionally, the edaphic restrictions limit potential sites for new populations, so existing populations of this taxon must be protected. Photo: Jones cycladenia flower bud marked for monitoring, photo credit Sedonia Sipes.

Collaborators:

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Pendland's beardtongue (Penstemon penlandii) is a very rare species in the snapdragon family (Scrophulariaceae) that occurs in only one county in central Colorado. The species is listed as endangered under the U.S. Endangered Species Act. Its continued existence depends critically on being able to produce sufficient fruit and seed so that new individuals will be available in the future to replace adults in current populations when they die. We studied the reproductive biology and pollinators of Penland's beardtongue to determine if pollinators are required for fruit set, and to learn what those pollinators might be. We learned that plants reproduce only minimally in the absence of pollinators, and that the important pollinators are numerous species of megachilid bees in the genus Osmia, and bumblebees (Bombus spp.). Interestingly, the pollinator guild included over a dozen species of Osmia, which occur sympatrically within P. penlandii's habitat. However, in any given year, only a few Osmia species might be abundant. If Penland's beardtongue is to be preserved then its pollinators must also be encouraged to thrive. Because of the complexity of the pollination system in which it is embedded, this plant is best preserved by applying principles of ecosystem conservation. Photo copyright B. Jennings.

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Last updated: 18-Jan-02 / sds