Bog bird’s-foot trefoil (Lotus pinnatus) COSEWIC assessment and status report: chapter 8
Biology
Lotus pinnatus is a monoclinous (having the stamens and pistils in the same flower), perennial plant, flowering from May to July, and withering over the winter months (Hitchcock et al. 1961). Though little information regarding breeding systems and pollination biology is available on Lotus pinnatus specifically, some of the information about the genus in general may apply to this species.
Zandstra and Grant (1968) studied the biosystematics of native and introduced Lotus species in Canada and reported that although many of the Old World species are polyploids, none of the North American species examined exhibited the same trait. North American Lotus species have a base chromosome number of 6 or 7. Of the species native to Canada, L. pinnatus Hook. and L. formosissimus are considered to be the most closely related, based on morphological, habitat, and cytological characteristics. A chemotaxonomic study using thin-layer chromatography further supported the general relationships among Canadian Lotus species with L. formosissimus and L. pinnatus demonstrating the greatest degree of similarity (Grant and Zandstra 1968). These species differed from other native Canadian species in that both were perennial outcrossers with large flowers on long pedunculate umbels. The other three native species were self-fertile annuals with small flowers. Evolution in angiosperm reproduction has frequently occurred with a decrease in basic chromosome number, a shift from an outcrossing (cross-pollinating) to an inbreeding (self-pollinating) reproductive system and a shift from a perennial to an annual habit (Stebbins, 1957). On this basis, Zandstra and Grant (1968) considered L. pinnatus and L. formosissimus to be more primitive than the annual species.
Lotus pinnatus germinates and begins growth in late winter – early spring as a result of increasing daylength and the warm, moist conditions found at this time of the year. Since the surrounding vegetation, especially grasses, are shorter during this period, the herbs also benefit from greater light levels and space to grow. There is no specific information on the effect of photoperiod on L. pinnatus, but L. corniculatus has been reported to require a minimum daylength of 14.0 to 14.5 h for flowering (Joff 1958, McKee 1963). Vegetative growth in L. pinnatus occurs in early spring and each plant sends up 3 to 6 shoots from its taproot, on average. Though the specific timing of events will depend on local habitat conditions and may vary from year to year, flowering in L. pinnatus generally occurs between May and the end of June. The pea-like, bisexual flowers are borne at the end of a long peduncle arising from the leaf axil, and the umbellate inflorescences are produced sequentially throughout the growing season. As the flowers senesce, some become progressively more reddish (Isely 1981). Fruiting begins before summer moisture deficits reduce the plant’s productivity. Each ramet is likely to have flower buds, flowers, and immature fruits in various stages of development, at any time during the growing season. During July, when drought conditions are prevalent, seed maturation and dispersal begins, followed by die-back of the stems to the rootstock, which is located several centimetres below the soil surface. The plants become senescent and remain dormant until the following year.
Young and Young (1986) reported that seeds of many native North Amercian Lotus species are deeply dormant, with multiple forms of complex dormancy, and require pre-treatment to induce germination. Although a hot water treatment may be used to enhance germination, high germination rates are not expected with this technique (Young and Young 1986). Like other members of the Fabaceae, the seeds of L. pinnatus have a hard seed coat and may require cold stratification as germination occurs mainly in the spring. Although the specific germination requirements of L. pinnatus have not been documented in the literature, Hitchcock et al. (1961) indicated that the three perennial species, Lotus pinnatus, L. crassifolius and L. formosissumus, were easily grown from seed. Zandstra and Grant (1968) reported that they grew L. pinnatus from seed to provide live material for their investigations, although the cultivation methods were not described.
Seeds of Lotus pinnatus, collected from Harewood Plains were used in an applied research program initiated in 1996 to determine the utility of native Vancouver Island grasses and legumes in the restoration of disturbed areas. About 80-90% of untreated seed sown in 1998 at a Seed Increase Nursery in Duncan, B.C., and left to over winter in the greenhouse, germinated early the next spring. Approximately 60 emergent seedlings were transplanted to a test plot during moist conditions in April, 1999. As of July 2003, only 10 plants have survived. Although the addition of legumes to seeding mixtures has the potential to improve soils through fixation of nitrogen, continuous flowering and pod shattering in Lotus pinnatus renders mechanical seed harvesting impractical. Lotus pinnatus was eliminated as a realistic reclamation candidate from a large-scale seeding perspective, as the costs currently associated with production exceed fair market value (Vaartnou, pers. comm., 2003).
The tendency of seed pods to shatter is common to all known varieties of L. corniculatus, and is responsible for substantial seed losses in this widely grown forage crop. Though efforts to reduce shattering through recurrent selection have been unsuccessful, attempts to transfer the indehiscent trait from related indehiscent species via interspecific hybridization and by interspecific somatic hybridization, have shown some promise (Grant 1996, 1999).
A review of the literature indicates that no morphological comparisons or DNA studies have been conducted to address the issue of whether L. pinnatus populations in British Columbia are distinct from those in the southern part of its distribution.
Like other perennial species in genus Lotus, Zandstra and Grant (1968) suggested that the flowers of Lotus pinnatus required cross-pollination to set viable seed. MacDonald (1944) showed that twice as many flowers of L. corniculatus were fertilized with pollen from other plants of the same species as with pollen of the same plant. When pollinators were excluded, no fertilized ovules were produced. Though specific pollinators have not yet been reported in the literature for L. pinnatus, honey bees (Apis melifera), bumblebees (Bombus spp.) (Jones et al. 1986) and leaf-cutting bees (Megachile rotundata) have been described as effective pollinators of the related perennial species, L. corniculatus (Beuselinck and McGraw 1988). It is not clear how frequently pollen is exchanged in Lotus pinnatus and over what distances but Morse (1958) reported that 12 to 15 visits per flower were required for maximum seed set in L. corniculatus.
The Lotus flower is constructed so that the anthers of the longer fused stamens are enclosed within the beak of the keel petals. Pollen is released into the beak of the keel petals prior to flowering. Pressure exerted by visiting insects on the keel causes the five dilated staminal filaments to act as a piston, pushing pollen out of the tip onto the ventral surface of the insect’s thorax (Kirkbride 1999). In L. corniculatus, the protrusion of the pistil beyond the stamens places the pistil in closer contact with pollen deposited on the body of visiting insects than with the pollen of its own flower and is believed to be an adaptation to ensure cross-pollination (Morse 1958).
Given the bright yellow appearance of the flower and the wide keel that serves as a suitable landing platform, flowers of L. pinnatus also appear to be adapted to bee pollination. Although the importance of the bumblebee to the flower (and vice versa) is yet to be specifically determined, bumblebees were observed visiting flowers of L. pinnatus at Harewood Plains during field observations made in 2003 (personal observation). Although bees are capable of dispersing pollen over short distances, cross-pollination between most populations in Canada is unlikely in view of the distances involved.
Asexual reproduction has not been studied in L. pinnatus, but shoots of L. corniculatus have been successfully regenerated from callus or suspension cultures derived from leaves, cotyledons, hypocotyls and roots. Excised root segments cultured on a hormone-free medium also gave rise to shoots with high frequency (Morris et al. 1999).
There is no documentation of the average life span of an individual genet and survivorship curves have not been developed for this species, but L. pinnatus is a perennial species that may survive for several years, under favorable conditions. The species germinates in the late winter or early spring when soil moisture and temperature conditions are most favorable and seedling survival appears to be dependent on continuous surface moisture. Established populations of L. pinnatus, at most sites, consist of both perennial plants and first year seedlings (personal observation 2003).
Field observations at all sites during in 2003 did not reveal any significant cases of adult mortality from natural causes; however, a patch of plants in a small wetland area at Harewood Plains had been crushed by off-road vehicles entering the wetland and in some areas the soil has been rutted down to the bedrock. This particular wetland is heavily used by off-road vehicles and anecdotal reports suggest that the population of L. pinnatus has declined from about 500 individuals observed in previous years to only 40 plants observed in 2003 (Thurkill pers. comm., 2003).
The fruits (pods) of L. pinnatus are linear legumes, 3–6 cm in length, derived from a superior ovary, which holds a single carpel. Development of the pod is accompanied by progressive changes in color from a waxy dark green to deep brown (Isely, 1981). In L. corniculatus, the pods mature in approximately three weeks. At maturity, the two valves of the pod twist spirally in a quick snapping motion and the seed is ejected when the pod dehisces along the ventral and dorsal sutures of the carpel margins (Grant 1996).
The glabrous, dark brown seeds in L. pinnatus are oval to spherical and number approximately 332 per gram (Piggott, pers. comm., 2003). In an examination of ten pods, the average number of seeds per pod was ten (personal observation 2003).
The seeds lack any strong adaptations for long-distance dispersal by wind or animal vectors. Most seeds are gravity-dispersed and generally land in the immediate vicinity of the parent plant. The plant’s habitat along stream channels may permit the legumes and seeds to be transferred by water during times of seasonal flooding. However, germination and seedling survival appear to depend upon continuous surface moisture and the species does not appear to be a strong competitor with native shrub species or with invasive exotic plants. The chances of a healthy population returning to Canada if local populations become extirpated are highly unlikely. The nearest population in Washington State, from which collections were made in 1940, is 240 km away in Bremerton. Whether this population is extant is unknown.
Like many other legumes, L. pinnatus appears to be associated with nitrogen-fixing Rhizobium bacteria that occupy root nodules and provide the plants with a source of reduced nitrogen in exchange for a supply of carbon and other nutrients. Specific strains of Rhizobium bacteria are required for effective nodulation of Lotus species grown as forage crops, such as L. corniculatus and L. tenuis. To maximize establishment in areas that have never produced Lotus, inoculation of seeds with the appropriate rhizobia is necessary (Blumenthal and McGraw 1999).
An examination of root nodules collected from L. pinnatus at Harewood Plains indicated the presence of bacteria, most likely of genus Rhizobium (Berch pers. comm., 2003).
Cyanogenic glucosides are chemical compounds that are widely distributed in legumes. These compounds release hydrogen cyanide (HCN) if the leaf tissues are damaged and are presumed to be a deterrent against herbivory, though many animals possess the ability to detoxify cyanide (Vetter 2000). Though cyanogenic glucosides are commonly found in Old World Lotusspecies, Grant and Sidhu (1967) found the reverse to be true for North American species. Of the fifteen Lotus pinnatus plants that were tested for the presence of hydrogen cyanide, all were negative.
At Harewood Plains, small plants have been observed growing in the moisture retained in tire ruts, suggesting that this species can tolerate some disturbance, provided subsurface drainage and other critical factors are not altered. However, the thin soils where L. pinnatus grows take a long time to form and are highly sensitive to disturbance. All-terrain vehicle use is inappropriate in these habitats as they damage fragile soils and vegetation (McPhee et al. 2000). As Lotus pinnatus does not appear to be a strong competitor, ecological succession following disturbance and invasion by non-native species has the potential to alter the structure and composition of habitats occupied by Lotus pinnatus, possibly preventing their re-establishment.
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