Pink sand-verbena (Abronia umbellata) COSEWIC assessment and status report: chapter 6

Biology

General

Abronia umbellata is a perennial that grows on ephemeral sandy beach habitats. It only reproduces by seeds that undergo a period of forced dormancy since the tough fruit coat must first be abraded in order to allow seeds to germinate. Studies indicate that when seeds are treated by various stratification methods a high proportion germinate. In nature, however, germination success appears to be low. Long and short-term survival appears precarious since the ephemeral shoreline habitat fluctuates and, as well, plant numbers tend to be low and fluctuate greatly on a yearly basis.

Reproduction

Reproduction of A. umbellata is only by seed; a high proportion of the seeds collected by Kaye (1999) in 1991 and 1992 were shown to be viable. Germination tests were conducted by Kaye (1999) using a variety of treatments. These ranged from placing whole fruits on moist sand through placing dry stored, individual seeds on wet paper, wet sand or KNO₃ soaked paper. These treatments were also conducted with stratified seeds. Results from the germination experiments indicated that when fruits were left intact seeds germinated at less than a 1% rate. These experiments suggest that, in the natural environment, Abronia umbellata fruits must be sand-abraded to expose the seeds and subsequently germinate. Stratified treatments involving the naked seeds had germination rates of 52 % to 74%. Another germination test with seed collected in 1996 from the same site, however, did show that stratification significantly improved germination from an average of 32% to 81% (df=10, t=2.23, P<0.0001).

Abronia umbellata seed production in the field in British Columbia was good with virtually all fruits collected in 2000 having a viable seed. The two plants growing at Clo-oose Bay had a total of about 300 flower heads with 15 to 20 seeds in each head for a total of approximately 5200 seeds (Douglas 2001).

Survival

The short-term survival of A. umbellata plants is precarious due to the ephemeral, sandy beach habitat that occurs within the extreme high-tide zone. Its long-term survival, however, is enhanced by the extreme toughness of the fruits of the plants. Experiments have shown that, until the fruit is abraded and the seed exposed, germination rates are low (Kaye 1999). However, even when the seeds are exposed, the germination rates in the field remain low (Kaye et al 1998). The latter researchers reported that only 1 in 1000 untreated seeds germinated after scattering on natural beaches.

Long-term survival is also highly variable. At Otter Point, Oregon, numbers ranged from nine to 15 between 1993 and 1996, increased to between 136 and 177 from 1997 to 1999, declined to zero in 2000, then increased to three in 2001 (Kaye et al. 1998, Kaye 2002). In British Columbia the sporadic appearance of the plant over long periods of time indicates that the seeds either are able to persist for years in the seed bank or are able to germinate successfully after traveling on ocean currents from Oregon.

Physiology

Unknown.

Genetics

Since most populations of A. umbellata contain very few plants it might be expected that there is a potential for the population to be vulnerable to demographic and environmental variation and loss of genetic variability. In some cases, small populations are at risk of inbreeding depression, genetic drift and loss of fitness (Primack 1998).

McGlaughlin et al. (2002 quantified genetic variation in four reintroduced populations of Abronia umbellata ssp. breviflora in Oregon ranging in size from 18 to 4,111 individuals. Genetic variation was also quantified in the natural seed-source population. Two of the reintroduced populations maintained approximately 90% of the genetic variation in the source population. Based on these results, it is predicted that reintroduced populations of A. umbellata that have at least 1,000 individuals should maintain 90% of the genetic variation of the source population.

Movements/dispersal

Abronia umbellata does not reproduce from cuttings or pieces of the plants (Kaye 1998) thus dispersal of the species is dependent on seeds. Local dispersal is probably aided by winds blowing the winged fruits into and out of the driftwood zone. As previously noted, the tough fruits of this species provide the potential to persist in the seed bank or travel on ocean currents.

Nutrition and interspecific interactions

Dune and beach sands are nutrient poor soils. Since there is no accumulation of organic matter, nutrient status is so low it cannot be effectively measured (Wiedemann 1984). Because of the high rainfall and rapid drainage, salinity is not an important factor on the sand beaches (Wiedemann 1984).

Behaviour/adaptability