Inbreeding depression in three generations of selfed families of silver birch (Betula pendula)

Inbreeding depression in survival and growth vigor was investigated through three generations of selfing on the basis of a 13-year-old field trial in silver birch (Betula pendula Roth). Survival showed significant decline fro m outcrossed (S-0) to first-generation selfed trees (S-1), and some recover y from second- (S-2) to third-generation selfed trees (S-3), indicating the rapid purging of lethal, sublethal, or highly deleterious alleles by selfi ng. The continuous decline of growth vigor with increased inbreeding sugges ts a low efficiency in purging mildly deleterious alleles affecting quantit ative traits and a low possibility of recovery in growth with further inbre eding. Linear effects of inbreeding on survival and growth vigor (taken as height, diameter at breast height, and stem. volume) explained the major va riation caused by inbreeding and indicates that the additively combined dom inant effects at different loci is the major source of inbreeding depressio n in this species. A weak and significant nonlinear effect of inbreeding on height and stem volume was also observed, suggesting the existence of epis tatic effects.