Delayed Seed Germination in Whitebark Pine and Regeneration Patterns Following the Yellowstone Fires

Abstract

Whitebark pine (Pinus albicaulis) seeds are dispersed by Clark's Nutcracker (Nucifraga columbiana), a bird that makes caches under 2–3 cm of soil. Cached seeds may delay germination for one or more years in part because of underdeveloped embryos at the time of seed dispersal. Consequently, whitebark pine may show a soil seed bank strategy that is unique among pines (Pinaceae, Pinus). From 1990 to 1995 we studied natural whitebark pine regeneration following the 1988 Yellowstone fires to determine: (1) whether whitebark pine typically exhibits delayed seed germination and, if so, (2) how this affects patterns of regeneration over time, and (3) whether germination is the result of seed maturation or is stimulated by high levels of moisture availability. We established 275 permanent plots, each 20 m2 in area, divided between Henderson Mountain, Gallatin National Forest, Montana, and Mt. Washburn, Yellowstone National Park. In the Henderson Mountain study area, the ecological conditions or “treatments” included: dry, burned; moist, burned; dry, unburned; and moist, unburned. In the Mt. Washburn study area, the ecological treatments were dry, burned; moist, burned; and moist, moderately burned. Synchronous delayed seed germination occurred throughout both study areas. The greatest densities of new seedlings appeared in the summers of 1991 and 1993, but the greatest cone crops were produced in 1989 and 1991. Most germination followed two winters of seed dormancy. Regeneration densities were consistently highest on the Mt. Washburn moist treatments. High correlation between weighted means for new regeneration and March‐plus‐April precipitation, as well as the results of residual and multiple regression analyses, suggests that cone production two years prior and March‐plus‐April precipitation together account for the regeneration patterns in the Mt. Washburn study area. The role of precipitation requires further study. Delayed seed germination, producing a soil seed bank, may be an ecological strategy in whitebark pine that is the product of selection. Although underdeveloped embryos may be a consequence of both a short growing season and premature seed dispersal by nutcrackers, seed caching may further select for slow embryo maturation, as well as moisture‐resistant seed coats to reduce nonadaptive germination.

Influence of Site Conditions, Shelter Objects, and Ectomycorrhizal Inoculation on the Early Survival of Whitebark Pine Seedlings Planted in Waterton Lakes National Park

“…For example, 7 years after the 1988 Yellowstone fires, natural WBP seed germination rates were similar on burned and unburned sites, although germination was lowest on the moist, unburned sites (Tomback et al 2001b). However, the beneficial effect of fire in killing competing trees also should be taken into account.…”

“…The survival of limber pine seedlings (another five-needle pine) was also higher when planted in burns in WLNP (Smith et al 2011b). Seedlings may benefit from being planted in burns because mineral nutrition is at least temporarily improved and warmer soil temperatures in essence lengthen the growing season (Tomback et al 2001b). Soils typically experience a pulse of higher mineral concentrations following fire, and higher ammonium, nitrate, and phosphorus levels could benefit developing seedlings (Certini 2005).…”

Autumn Snowfall Controls the Annual Radial Growth of Centenarian Whitebark Pine (Pinus albicaulis) in the Southern Coast Mountains, British Columbia, Canada

“…Germination is generally delayed. Within three years, about 67% of the seeds typically have germinated (McCaughey and Tomback 2001) and germination of the remaining seeds lasts until about five years (Tomback et al 2001a). Thus, the median time to germination is three years.…”

“…Following 1-3 or more overwinter cycles (McCaughey 1994), germination then appears to occur when moisture conditions are favourable (Tomback et al 2001a). This strategy evens out the effects of masting (pulsed seed production) so that germination of a seed cohort occurs over several years and when conditions are favourable, thus enabling a continuity of regeneration in the absence of seed production.…”

Community Structure, Biodiversity, and Ecosystem Services in Treeline Whitebark Pine Communities: Potential Impacts from a Non-Native Pathogen

“…Foundation and keystone functions previously identified for whitebark pine are described in Table 1. Many of these functions result from highly effective seed dispersal by nutcrackers in combination with exceptionally hardy seedlings that rapidly develop deep tap roots and tolerate exposed, droughty sites [22,24,28]. Whitebark pine communities also provide ecosystem services, defined as ecosystem functions that benefit humans (e.g., [33]).…”