Rayless goldfields (Lasthenia glaberrima) COSEWIC assessment and status report 2000: chapter 6

Biology

Little has been written about the biology of Lasthenia glaberrima, apart from the taxonomic treatment Ornduff (1966) provides for the genus. The following information, unless otherwise cited, is based on the unpublished field observations of Matt Fairbarns.

Life cycle and reproduction

Lasthenia glaberrima is a short-lived annual. Germination appears to begin in April and end in early May. Despite numerous visits in 2003, 2004 and 2005, no seedlings were observed in the field later than May. Ornduff (1966) reports that in some species of Lasthenia all viable seed germinated following a thorough watering, and thus fail to produce a long-term seed bank.

Flowering begins in early May and peaks by mid-month. The species is self-fertilizing (Chan pers. com. 2005, Ornduff 1966). This is unusual within the genus and likely explains the reduction of the ray flowers. Seed dispersal begins in mid-May and most plants have shed their achenes by late June. Plants tend to produce several heads although some smaller plants produce as few as 1 or 2 heads. Most heads produce numerous ripe achenes at maturity.

Plants continue to grow until they succumb to summer drought. Mortality normally occurs in late May or June and summer rainfall events appear to be too rare to trigger renewed vegetative growth, flowering and fruiting.

Herbivory

There is no published information on herbivory, nor was any evidence of foliar herbivory observed during numerous visits to the Canadian population over a three-year period although flower heads were grazed in 2005 and 2006.

Physiology

Lasthenia glaberrima survives summer drought and winter cold by persisting as seeds. Like many other annual members of the genus, plants probably die by mid-summer even if watered continuously (Ornduff 1966).

Dispersal

The pappus of the achenes is unlikely to aid in wind-dispersal but probably helps attach the achenes to the fur of passing mammals (Ornduff 1966). This may only affect a small proportion of the achenes as evidenced by a study of Lasthenia californica which has a similar achene and pappus. Achenes of Lasthenia californica do not disperse very far; a gentle tap or current of air will cause its mature achenes to separate from the receptacle. Seeds either fall to the ground or get trapped in nearby tufts of grass (Rajakaruna and Bohm 1999). Nevertheless, Lasthenia glaberrima is one of the most widely distributed members of its genus and its awned pappus (as well as its self-compatible breeding system) may have been critical to its wide distribution.

Interspecific interactions

There are no known interspecific interactions that increase the susceptibility of Lasthenia glaberrima to extirpation in Canada.

Adaptability 

Lasthenia glaberrimais well-adapted to the strong seasonal fluctuations in moisture regime characteristic of rock-bound vernal pools. It is tolerant of the saturated conditions that prevail during its period of germination and early growth, conditions that limit the growth of competing perennial vegetation. It conducts its early growth as a submerged plant with short internodes and long, flaccid leaves. The lower stems may be lax and develop an aerenchymatous cortex in response the poorly aerated environment. Its tolerance of flooding allows it to grow rapidly and mature before the onset of the summer drought. As the pool dries, the aerial stems grow rapidly. They tend to have longer internodes and bear shorter, more rigid leaves. Lasthenia glaberrima survives the intense and prolonged summer drought characteristic of vernal pool habitats by persisting in the soil in a transient seed bank. Like many other vernal pool annuals, species of Lasthenia tend to vary considerably depending on fluctuations in precipitation, flowering and producing seed under a variety of circumstances. In a single locality, the plants may be small, few-leaved and few-flowered in a dry year and immense and highly branched in a more favourable year. Plants generally produce a single, terminal head early in the flowering season and additional heads are produced on lateral branches if moist conditions continue. Paradoxically, in dry years (when plants tend to be quite small) their population numbers may be very large even though biomass and seed production may be well below levels reached in moist years. Plant size is also heavily influenced by the degree of crowding (Ornduff 1966).

Transplanting has not been attempted. The species’ reliance on a taproot, the fact that it is an annual, and its specialized habitat requirements suggest that transplantation from horticultural sources would be unsuccessful.

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