Seedbanks and germination ecology of bitter cherry (Prunus emarginata) on southern Vancouver Island

Background/Question/Methods

Bitter cherry (Prunus emarginata) is a shade-intolerant tree that sometimes colonizes in abundance following disturbance of low-elevation west-side forests in Oregon, Washington and southern British Columbia. Although cherry can reduce the growth of planted conifers, it also provides habitat diversity in a conifer-dominated landscape and can be a source of appearance-grade wood. Cherry regenerates largely via long-lived seed from a soil seedbank following disturbance; however, its seed ecology is little-studied. We determined (1) cherry seed populations in forest soil, (2) effects of light and temperature on germination in controlled environments, and (3) effects of canopy cover on emergence and survival under field conditions. In (1), seeds were sampled from soil in 15 stands, counted, and related to stand age. In (2), freshly-collected seed were initially cold-stratified (0 or 70 days), followed by temperature (10, 17, or 25C) and photoperiod (0, 14 hours) treatments, which were interrupted with a second cold period. In (3), seeds from the same collection were planted in fall 2007. Treatments were canopy cover (closed, forest edge; open clearcut), seedbed (mineral soil, organic), and with or without cages to deter herbivores. Emergence, survival, seedbed temperature and moisture were measured for 3 years.

Results/Conclusions

Population densities of filled seed were greatest in 51-75 year-old stands. In controlled environments, germination began during the second cold stratification, suggesting alternating warm and cold periods were needed to overcome seed dormancy. Germination was greatest at 17C, but only after the first cold stratification and in the light. In field plots, emergence occurred only in years 2 (79%) and 3 (21%). Of all emergent seedlings through year 3, 81% occurred in the open, 19% in the edge, and none under the canopy. Emergence in the open was greatest in caged, mineral soil plots. Survival of emerged seedlings was 18% in the open, but 0% for edge plots. Most seedlings emerged in April (e.g., 89% in year 2). Midday seedbed temperatures during emergence were consistent with optimal temperatures for germination in the controlled environment. However, greater temperature amplitude related to loss of canopy cover may be a more reliable cue for germination. Disturbance is necessary to maintaining cherry in the conifer-dominated landscape. The observed seed numbers, emergence and early survival rates are consistent with stem densities observed following clearcutting. The long-lived seedbank is a critical component of the life history of bitter cherry, allowing persistence between disturbances.