Houghton's goldenrod (Solidago houghtonii) COSEWIC assessment and status report: chapter 6

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Biology

General

Houghton’s goldenrod is a perennial herb that reproduces asexually by rhizomes and sexually. It grows on alvar substrates where plants are subjected to flooding and dessication. Plants are pollinated by a variety of insects.

Reproduction

Solidago houghtonii is a perennial that flowers in August and early September. It is similar to other species of Solidago in terms of its reproductive biology and floral morphology (Jolls et al., in review). It has an outer whorl of pistillate ray flowers and inner bisexual disc flowers with outer flowers maturing earliest. Flowering appears to be related to plant size, and is less likely to occur in plants that had flowered the previous year (Jolls et al. in review). This indicates there is a metabolic cost for flowering, and is therefore most likely to occur in older larger robust plants. Flowering occurs in approximately 6 to 31% of shoots (Jolls et al., in review; Penskar, 1997). Flowers are insect pollinated, with a large range of potential pollinators including bees, moths and beetles. Experiments show that self-pollination is largely unsuccessful, and that reproduction is likely pollen-limited. Fruit set for open pollination was low (17%) (Jolls et al., in review).

Growth chamber experiments by Jolls et al. showed low germinability (56%). Germination does not appear to be affected by substrate type or moisture (Jolls et al., in review), though this does not exclude the possibility that other stages of the reproductive cycle might be affected. Seeds buried 1 cm did not germinate at all, indicating there is a light requirement as in other species of Solidago. Seeds also required a chilling period in order to germinate.

Solidago houghtonii can also reproduce vegetatively by means of underground rhizomes. Plants have between 2 to 12 ramets that rapidly disarticulate and become established as independent plants (Penskar, 1997).

Basal rosettes live 1-6 years without flowering and often persist vegetatively after successfully flowering and fruiting (Penskar, 1997).

Survival

Predation by aphids and other insects has been noted (Penskar, 1997, Jolls et al., in review). No information was found regarding offspring survival, population age structure and reproductive/recruitment rate.

Physiology

Houghton’s goldenrod is a perennial that dies back to its rhizomes and goes dormant in winter.

Movements/dispersal

Pollen is insect dispersed. Plants can spread locally by rhizomes and seeds are windborn. Some 50 – 60 sites occur in 9 counties of northern Michigan (NatureServe 2005; map of range in Michigan in DNR, 2005, web site); seeds from some of the closest populations in Michigan could potentially reach Cockburn Island, the site of the westernmost population in Ontario.

Nutrition and interspecific interactions

An arthropod survey by Jolls et al. (in review) found a number of insect species inhabiting flowers and plants. Flowers were visited by beetles (Phalacriae), bees (Halictidae) and moths (Phalacridae). Residents on seed heads included thrips, herbivorous mites, mould feeders (Melanopthalma sp.), and detritivores (Psocopera). Spiders that predate on these insects also occupied flowers.

Behaviour/adaptability

There is little information about the adaptability of this species to stress or disturbance. Its presence in alvar habitat indicates that it is likely to be tolerant of droughts and floods, and a poor competitor, though no studies have been published that test these.

Houghton’s goldenrod has been successfully transplanted from the wild and raised at the W.J. Beal Botanical Garden in Michigan (Chittenden, 1995). Transplants both flowered and produced rhizomes.