Genetic differentiation between adjoining populations of Eucalyptus obliqua L'Herit

Genetic differentiation at the site level was investigated within Eucalyptus obliqua L'Herit., by comparing the progeny of phenotypically distinct populations from adjacent sites along ecological gradients. Seed was collected from 12 maternal trees within each of four sites in south-east Tasmania. The Forestier sites comprised a topographic sequence of a wet sclerophyll forest type within a moist gully (Gully population) adjoining an open, dry sclerophyll forest type along an exposed ridge (Ridge population). The Lune sites formed a sequence of a wet sclerophyll forest type from the mid-slope position of a broad ridge system (Mid population) adjoining an open woodland from the lower slopes and plain (Plain population). Studies of the progeny from each of the 48 maternal trees were undertaken in various laboratory experiments and in planting trials established within each of the four sites. The results provided evidence of genetic differentiation between and within the populations from the Forestier and Lune sites. Within the Lune populations, significant differences between progeny from the Mid and Plain were demonstrated for a number of attributes. The Mid population had higher germination energy, larger cotyledons, faster growth to age five years and lower susceptibility to leaf spotting fungi than the Plain population. Progeny from the Plain had higher frost resistance, higher persistence of coppice and a lower susceptibility of coppice shoots to browsing by native mammals. Within the Forestier populations, there were no differences between progeny from the Gully and Ridge for factors such as growth to age five years, leaf morphology, frost resistance or coppicing ability. However, significant differences were demonstrated for a range of other attributes. The Gully population had higher germination energy, lower production of lignotubers and lower susceptibility to infection by leaf spotting fungi and browsing by native mammals than the Ridge population. Many of the differences between adjacent populations could be regarded as evidence of adaptation to environmental factors operating at the local site level. The results indicated that differential selection forces may result in genetic differences between populations of E. obliqua over the scale of hundreds of metres, in addition to the tens or hundreds of kilometres normally associated with broad geographic variation at the ecotypic or provenance level. Variation at the site level has implications for the conservation of genetic diversity and for the probability of successful regeneration and long term adaptation and growth across heterogeneous sites within native forests.