Inbreeding in mid-rotation coastal Douglas-fir: implications for breeding

Abstract

The effects of inbreeding on growth traits have been studied and are fairly well understood in young conifers. However, in trees approaching mid-rotation, this information is not widely available at present. The aim of this study is to assess inbreeding depression in mid-rotation coastal Douglas-fir in growth traits, survival, and wood density. Several levels of inbreeding were created in coastal Douglas-fir, using a nine-clone founder population to produce 148 families with varying levels of inbreeding ranging from f = 0 (outcrossed) to f = 0.5 (selfed). The trees were planted in 1987 on two farm-field sites in coastal BC. Here, we report effects of inbreeding in height, diameter, tree volume, survival, and wood density in 26-year-old trees. Previous results obtained from this test population showed negative near linear effects with levels of inbreeding in seed production, nursery growth performance, and growth traits in the field assessed at age 10. At age 26, inbreeding depression was highest in survival, ranging from 20 to 80 % for f = 0.125 and f = 0.5, respectively. In contrast, the most severe inbreeding depression among the three levels of inbreeding was only 4 % for wood density at f = 0.5 (selfing). Inbreeding depression in height, diameter at breast height (dbh), and volume increased linearly from f = 0 to f = 0.25, then leveled off. Founder genotypes had varied responses to inbreeding as parental breeding values across inbreeding levels were inconsistent (in magnitude, sign, and trait). No differences in levels of inbreeding depression were found between full-sib matings and parent-offspring matings. These findings have implications for the testing of parents within sublines, where inbreeding is accumulated in sublines. However, since parents respond differently to levels of inbreeding, their performance may not be well correlated to their quality as outcrossing parents.