SEEDLING GROWTH AND SURVIVAL OF RED AND WHITE FIR IN A SIERRA NEVADA ECOTONE

Abstract

We hypothesized that differential seedling establishment was responsible for the transition from white fir‐ to red fir‐dominated forest. Nearly 2,000 seeds of each species were sown within six fenced gardens that spanned the ecotone (1,902–2,073 m). Germination and seedling mortality were monitored through the first growing season. Red fir germination percentage exceeded that of white fir in all gardens in sites dominated by red fir saplings and overstory trees, and a similar pattern was true for white fir. Survival at the end of the first growing season was low and statistically similar for both species, as was the average length of life for seedlings and the causes of mortality. Relative demographic performances (an integrated measure of germination and survivorship over time) suggest that: 1) open/xeric microhabitats within the ecotone favor white fir over red fir; 2) both open/mesic and shaded/xeric microhabitats favor red fir over white fir; and 3) shaded/mesic microhabitats are equally favorable for both species. Seedlings of red and white fir were grown for 6 mo in a growth chamber that imitated early summer temperature and soil moisture conditions. Red fir seedlings were consistently heavier than white fir seedlings, in part a result of the 2.5 × greater caloric content of red fir seed. Relative growth rates were similar for both. Allocation of photosynthate to roots was greater for red fir than white fir, a result contrary to the expected pattern, given that white fir occurs at more xeric, lower elevations than red fir. No consistent differences between the species occurred in root growth or morphology.