The effect of outcrossing rate on the growth of selected families of Eucalyptus grandis

Seed collections were made in natural Eucalyptus grandis forests in the region of Coffs Harbour, New South Wales, following a year of good flowering and seed set. Outcrossing rates were determined for individual parent trees by isozyme analysis of progeny arrays. For each of 10 local provenances, 2 parent trees having the highest outcrossing rates and 2 having the lowest outcrossing rates were selected, and seedlings raised for field planting in a designed experiment with 4 replicates of 5-tree plots for each of the 40 selected families, on an effluent-irrigated planting site at Dubbo, New South Wales. Height and diameter were assessed 2.5 years after planting out. Preliminary analysis showed there was substantial variation across the planting site, so mixed-model analyses were carried out on plot mean values of height and diameter to recover family information from the incomplete blocking structures used and generate family means for these variates. The effects of family outcrossing rates and provenance were then examined using parallel line regression. Outcrossing rate strongly influenced all variates while differences between provenances were not significant. Families with higher outcrossing rates generally displayed faster growth and better survival. The interaction between outcrossing rates and provenance was significant for height and diameter. These results support the view that in this species, variations in outcrossing between parent trees have a large influence on differences in growth rate among natural provenances and progenies within provenances.