Quantitative genetic evidence that the timing of vegetative phase change in Eucalyptus globulus ssp. globulus is an adaptive trait

Abstract

The adaptive significance of the timing of the abrupt change in leaf form in Eucalyptus globulus Labill. spp. globulus was investigated using quantitative genetic analysis of several field trials containing open-pollinated progenies. Five large trials contained progeny from across the whole geographic range of this taxon. On this broad scale, early phase change appears to promote growth on two sites but not the other three, implying differential selection for the timing of phase change. The timing of vegetative phase change varied markedly between broad geographic regions, consistent with either adaptation to broad scale variation or historical differentiation. Data from one small trial demonstrated a genetic basis to a steep local cline in habit, in the size of plants flowering and in the height of the change in foliage type. In this trial, the progeny from an exposed coastal cliff top had markedly slower growth, earlier vegetative phase change and first flowering than the progeny from 1.5 km inland. This genetically determined combination of slow growth, early phase change and precocious flowering appears to be maintained in exposed coastal environments by current selection, and contrasts with more complex patterns of broad scale geographic variation. The genetic association of the timing of vegetative phase change with growth rate, a fitness surrogate, ranged from positive to negative at different sites. Early phase change may, for example, be favoured in warm, wet environments to reduce damage by leaf fungi, but may also be favoured on exposed dry sites to increase xeromorphy. The patterns of genetic variation in nature may thus result from multiple causes (both biotic and abiotic) and their interpretation will be complex.