Genetic variation in competitive ability of some shade-tolerant and shade-intolerant Pacific Coast (USA) conifers

Abstract

Variability in growth response to stocking density and neighbor composition was compared among half-sibling families of four tree species representing two pairs of shade-tolerant and shade-intolerant species, each pair from the same life zone. The hypothesis was that the wider regeneration niche of shade-tolerant species, which establish both in disturbed areas and within closed forests, would produce greater genetic variability among families than occurs in shade-intolerant species, which establish primarily in disturbed areas. Families of western hemlock ( Tsuga heterophylla (Raf.) Sarg.), coast Douglas fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii) and Pacific silver fir ( Abies amabilis (Dougl.) Forbes) varied in their responses to stocking density, but those of noble fir ( A. procera Rehd.) did not. As hypothesized, shade-tolerant species were significantly more variable than shade-intolerant ones, a result consistent with electrophoretic studies of shade-tolerant and -intolerant plants and supportive of the hypothesis that at least a portion of genetic variability contained within populations represents adaptation to variable environments.