Natural history, population ecology and conservation biology of Slim-leaf onion (Allium amplectens)
Date
2002
Authors
Hawryzki, Allan Ronald
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Abstract
Slim-leaf onion (Allium amplectens Torr.) is found in western North America from British Columbia to southern California. It is rare in British Columbia and is on the Blue List of the British Columbia Conservation Data Centre. Allium amp lee tens typically occurs on dry rocky bluffs and meadows of Garry oak ecosystems in southwestern British Columbia.
I carried out a field inventory of known and potential population locations of A. amplectens in B.C. I relocated eight previously known populations, and (with others) discovered 21 new populations, mainly on the Gulflslands, and around Nanaimo on Vancouver Island.
I also conducted experiments on the breeding system and reproductive biology of A. amplectens. Chromosome counts from 24 British Columbia populations showed them to be trip lo id (2n = 3X = 21) in all cases. Tests for agamospermy resulted in O seed set, in contrast to what might be expected in triploid plants. Tests for autogamy and self compatibility yielded low seed set (about 3%), one-quarter that of control (outcrossed) plants . These findings suggest that A. amplectens either has an unbalanced form of meiosis, or is apomictic with a requirement for cross-pollination. Seed germination rates were very low (5%); vegetative propagation via bulb offsets is apparently the major mode of reproduction in these populations.
Populations were monitored at six different sites for total numbers of adult (flowering) plants, and I also mapped plants of all stages within marked plots. All of the monitored sites were moist in spring but dry by midsummer. Data from the marked plots were used for Lefkovitch matrix (demographic) analyses. Matrix analyses indicated a decreasing growth rate for 3 of the annual transitions at the CFMETR (Nanoose Hill) location, suggesting these populations may be in gradual decline.
Rates of dormancy, as determined from monitoring of individual plants, were very high; 83 . 7% of the monitored marked plants exhibited dormancy at some point during the study. Dormancy in A. amplectens lasted most often for 1 year, but sometimes as long as 3 years. Yearly February-April precipitation levels were positively correlated with total plant numbers and inversely correlated with dormancy rates. Differences among habitats, especially with respect to substrate type, also influenced dormancy rates. Observed (above ground) population numbers of A. amplectens fluctuated less in rocky habitats than in those habitats with deeper soils.
However, high densities of nonflowering (vegetative) plants, rapid senescence, high dormancy rates, and bulb offsetting tended to make tracking of individual plants difficult. The monitoring of demographic changes and projecting of future population trajectories is especially challenging in these cases.
Although a number of new populations were discovered during the course of this study, they probably are not recently established, but rather may have existed for quite some time and have persisted because of their relative remoteness. The majority of populations that are no longer extant were located on southern Vancouver Island, where human impacts on habitat are extensive. This suggests that habitat loss is a major factor in the rarity of A. amplectens in B .C. Future efforts to conserve A. amplectens in B.C. should focus on the protection of known, extant populations.