THE EFFECTIVENESS OF PLUS-TREE SELECTION FOR YIELD

In response to recent suggestions in the literature that plus-tree sel ection for yield in forest trees might in general be ineffective, the empirical information contained in 24 published reports was examined. It was found that progeny of plus-trees selected for yield traits tend to be faster growing than their controls. It is argued that this obse rved tendency is likely to be due at least in part to a response to se lection. The reported values indicate that genetic gains of up to 15% in height and diameter growth, and up to 35% in volume per unit area, can readily be achieved through plus-tree selection. This is consisten t with theoretical expectations. However, it should be emphasised that the amount of gain from any particular plus-tree selection system dep ends on the values of the parameters that determine the response to se lection (selection intensity, genetic variance, heritability). In unfa vourable situations, gain could be close to zero.