Penstemon debilis

Penstemon debilis is a member of the figwort family (Schrophulariaceae). It is an herbaceous perennial which often forms mats or trails where the stems may root at the node. The flowers are white to lavender. There are no basal leaves and the cauline leaves are sessile, opposite, succulent and blue-glaucous (O'Kane and Anderson 1987).

Penstemon debilis is endemic to western Colorado. The only other Penstemon species on the western slope of Colorado that completely lacks aerial stems is P. acaulis var. yampensis. P. acaulis var. yampensis differs from P. debilis in its habitat and its green, non-succulent leaves (Weber and Wittmann 1996). Penstemon caespitosus is also mat forming, but has smaller blue flowers and pubescent leaves (McMullen 1998 personal communication). P. debilis is easily identified by its weak stems, soboliferous habit, succulent, glabrous, and glaucous leaves, and its white to pale lavender corollas (O'Kane and Anderson 1987).

This species grows on sparsely vegetated, south-facing, steep, white shale talus of the Parachute Creek Member of the Green River Formation. Soils are a mixture of thin shale fragments and clay (Spackman et al. 1997). Associated species include: Agropyron spicatum, Cercocarpus montanus, Chrysothamnus viscidiflorus, Cymopterus hendersonii, Eriogonum lonchophyllum, Galium coloradoense, Holodiscus dumosus, Machaeranthera grindeliodes, Monardella odoratissima, Astragalus lutosus, Festuca dasyclada, and Thalictrum heliophilum (O'Kane and Anderson 1987).

Flowers from mid-June to mid-July (Scheck 1994) and fruits in mid July-August (CNHP 1998).

Amy McMullen at Utah State University is currently researching the biology of P. debilis. The following was compiled from her 1997 field season findings, unless otherwise stated:

According to pollination biology studies conducted in 1997, pollinator rarity, chemical or physical properties of the substrate, or seed predation is not responsible for the restriction of P. debilis populations. Seed predation at the known locations is low. The habitat substrate is unusual, however, there is low survival of seeds in greenhouse conditions, either in shale soil or potting soil. P. debilis requires several months of moist chilling to break dormancy.

Bees are responsible for pollination, with Osmia brevis the primary pollinator. This species is thought to be a ground nester west of the Rocky Mountains. Little is known about the biology of this species, however, it has been documented that this bee is responsible for pollination of two other rare Penstemon species in Colorado (P. harringtonii and P. penlandii). Pollinators are also shared with two associated Penstemon species: P. caespitosus and P. mensarum.

The soils of the five known P. debilis locations are of similar physical characteristics but differ in the soil chemistry and plant tissue levels, especially in concentrations of arsenic and selenium. Of the two largest populations, the site with high concentrations of toxic elements had significantly lower densities between P. debilis plants and other species of neighbors. This may reflect reduced competitive ability or survival rates of P. debilis on a substrate with high levels of arsenic, selenium, and mercury.

Breeding system experiments found low fruit production. It is thought that this may be caused by: fragility of the species to disturbance and handling; need for multiple pollinations over several days to set fruit; or outbreeding depression.

Due to the fact that Penstemon debilis does not have any nearby close relatives, O'Kane and Anderson (1987) hypothesize that it is a relictual species. The isolation provided by barren outcrops of oil shale have allowed the species to persist without competition. P. debilis may be a poor competitor and limited to inhospitable sites through competitive exclusion. This explains the habitat specificity, however, there still appears to be more open potential habitat which is unoccupied. This species does seem to be well adapted to its habitat, producing long rhizomes in response to a shifting substrate.

Penstemon debilis flowers from mid-June to mid-July (Scheck 1994) and fruits in mid July-August (CNHP 1998). The plants are bee pollinated and Osmia brevis is the primary pollinator.