Competition effects in breeding value prediction of forest trees

Competition is a concern for tree breeding because of its potential to reduce the genetic gain. Competition, if not accounted for in the analytical model, can potentially introduce a source of bias in genetic parameter estimation and breeding value prediction. This study modelled competition between trees in 20 Swedish progeny trials of Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and lodgepole pine (Pinus contorta Douglas ex Loudon). The competition model assumed a tree has a direct additive genetic effect, which affects the tree's own phenotype, and an indirect additive effect, which affects the phenotypes of its neighbours. Genetic parameters were estimated via a factor analytic structure in which separate indirect effects were considered for each neighbour, or via a combined indirect effect approach. We analysed diameter, as it is the trait that can be expected to be affected most by competition. Competition at the genetic level was detected in 17 of the 20 trials analysed. In most cases, the ratio of indirect to direct additive variance was less than 20% and no major changes in ranking resulted. At this stage, there is little incentive to incorporate indirect effects into program-wide genetic evaluation models. The added complexity is not commensurate with the benefit that would be gained.