Dense-flowered lupine (Lupinus densiflorus) COSEWIC assessment and status report: chapter 8
Biology
Lupinus densiflorus is a winter annual; a large proportion of its seeds germinate during the fall and seedlings overwinter. No information is currently available on its biology and ecology in B.C.
Information on pollination in Lupinus densiflorus is contradictory. Dunn and Gillett (1966) claimed that the Canadian population is maintained by obligate self-pollination and Riggins (pers. comm.) describes the species as cleistogamous. Direct observation of plants at Trial Island and Macaulay Point suggest a different breeding system. The petals are incompletely fused and bumblebees were observed foraging on the plants. The bees moved from plant to plant, only visiting fully-developed, fresh flowers. They appeared to probe the flowers carefully. This suggests that the Canadian populations of Lupinus densiflorus are chasmogamous and outcross regularly.
Surveys of British Columbia populations from September 2001 to February 2002 revealed that seed set was prolific, which is consistent with an annual species that has a seed bank as part of its lifecycle.
Some seeds germinate in the fall, as early as November in the Victoria area. During early February 2002, non-germinated seeds were observed on the soil surface at most plots. Germination may recommence in the spring, these seeds may enter the soil seed bank, or they may be lost through predation and other factors.
Germination requirements appeared to vary greatly among species of lupine. Seeds may remain dormant for long periods if the hard seed coat requires either decomposition or abrasion prior to germination. Neilson (1964) found that seeds of L. densiflorus remain viable for up to four years. Scarification may greatly improve germination --Neilson (1964) observed 100% germination of scarified seeds (n=10) in Lupinus densiflorus in his study of seeds from California populations. Some species of lupine germinate best in moist, loose soil when the temperature is relatively low, near freezing at night Dunn (1956). Under controlled conditions and adequate moisture, California collections of L. densiflorus germinated best at temperatures between 13 and 27°C (Neilson 1964).
Fall seedlings overwintered in the cotyledon stage or with a few emergent primary leaves. Neilson (1964) suggested that outgrowths of the cotyledons of Lupinus densiflorus form an effective enclosure over the leaf primordial, offering one of the best mechanisms in the genus for protecting seedlings. The cotyledons are persistent --collections of seedlings from Victoria bearing cotyledons were made as late as March and April (see RBCM accession numbers 142029 and 40414 respectively).
Observations were made from plots in populations one and two during the autumn of 2001 in order to track seedling survival during the winter. Seedlings were already established in the late autumn (as early as November) and overwintered with enlarged cotyledons and a few primary leaves. Large numbers of seedlings persisted throughout early and mid-winter, although seedling densities decreased in most plots as the winter progressed.
General observations in the winter of 2001/02 showed that seedlings germinate on a variety of substrates including gravel, rotting wood and crevices in driftwood. Most seedlings did not survive on such substrates and adults had not been found occupying similar substrates during the previous summer. Regardless of the substrate, salt-water flooding during high autumn tides caused the greatest mortality among seedlings.
Seedling establishment was most successful on clay soils saturated by winter rains. These soils are often subject to micro-slumping,which exposes bare mineral materials with cracks which taproots may penetrate easily. Seedlings may establish on level marine clays, as was observed at population three. Lupinus densiflorus seedlings were notably absent from lenses of sand and sandy silt at population two.
Though Lupinus densiflorus seedlings may germinate in a variety of microhabitats, seedling and juvenile mortality were common. Conversely, adult plants seemed to be more restricted, likely due to either edaphic requirements or competitive exclusion, but field observations during the summer of 2001 did not reveal any significant cases of adult mortality
Some seed, flower and leaf herbivory was noted in most populations but was low and not considered a significant factor in population survival. At the seedling stage, some individuals may have large portions of the cotyledons consumed which could affect individual fitness. Inter-specificcompetition or chance events during this period probably dictate year-to-year population fluctuations.
Web references (e.g., Crescent Bloom) state that the seeds, pods, and foliage contain the toxic alkaloid anagyrine,which may limit herbivory on this species.
Little is known about the physiology of Lupinus densiflorus, although it may be symbiotic with nitrogen-fixing Rhizobium bacteria. Roots of young Lupinus densiflorus have abundant nodules that resemble those containing Rhizobium, and may also be a factor in survival and reproduction. The habitat of this species seems to offer low nutrient and low water availability during the summer and its competitive advantage may be enhanced through this symbiosis.
Lupinus densiflorus is physiologically adapted to winter conditions by germinating in the fall when water is available and maintaining itself in the cotyledon stage with a protective covering over the leaf primordia. Salt intolerance limits populations on the lower slopes that receive marine spray.
If some or all flowers are cleistogamous, there is no opportunity for pollen dispersal. Otherwise, pollen dispersal in Lupinus densiflorus may be limited by the foraging behaviour of pollinators. Some species of bees are capable of fairly long-distance pollen transfer but little evidence of pollination was observed in the populations. This may enable genetic transfer within sub-populations but not among the three Canadian populations or with the Washington State populations. It is more likely that pollination occurs among flowers on the same inflorescence due to the density of the flowers and assumed pollinators’foraging behaviour.
Most seeds are probably passively gravity-dispersed. Onshore winter winds buffeting coastal bluffs in the Victoria area are very strong and may blow the seeds over short distances despite the lack of adaptations for wind-dispersal. This would be exceptionally important if populations are otherwise threatened by gradual depletion of up-slope plants. Explosive release of seeds from mature capsules has been reported in some lupines (e.g., Dunn 1956, Neilson 1964) but this was not observed in the field by the writers.
The localized distribution of British Columbia populations that are seemingly below site carrying capacity suggest that dispersal and establishment are rare and there would be limited rescue effect even among the 3 populations in the Victoria area.
The potential interaction between Lupinus densiflorus and Rhizobium bacteria may have a significant effect on the survivorship of Lupinus densiflorus as a seedling or juvenile and may also potentially affect its ability to compete with other species. The ability of Rhizobium bacteria to fix nitrogen for its host legume plant in nitrogen-poor environments is well documented. As the other major interaction is a negative one between Lupinus densiflorus and other native or non-native plants, the presence of nitrogen-fixing Rhizobium bacteria is likely to confer some competitive advantage.
Germination of Lupinus densiflorus has been treated previously and adaptations such as high seed set and germination, seasonal phenology, and seed banks represent common adaptations of annual species to unpredictable environments.
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