Drought stress affects the germination of four co-occurring Eucalyptus species from north-west New South Wales

Abstract

The interactive effect of seasonal temperatures and water potential (Ψ) were examined on the germination of four co-occurring Eucalyptus species (Eucalyptus melanophloia F.Muell., E. pilligaensis Maiden, E. populnea subsp. bimbil L.A.S.Johnson & K.D.Hill and E. viridis R.T.Baker) from the Brigalow Belt South bioregion of New South Wales. These eucalypts are important canopy species, and are a key inclusion for restoring native vegetation to areas that have had significant landscape modification. Final germination, time to first germination, and time to 50% germination were recorded at three alternating temperature regimes for winter (day/night, 19/5°C), spring–autumn (27/12°C) and summer (34/20°C), and eight different Ψ treatments between saturation point (0MPa) and permanent wilting point (–1.5MPa) under controlled laboratory conditions. Final germination was highest in all species when seeds were incubated at winter conditions, but spring–autumn and summer temperatures had mixed effects on final germination, depending on the species. Final germination consistently declined with lower Ψ across all temperatures for E. pilliagaensis, E. populnea subsp. bimbil and E. viridis, whereas E. melanophloia displayed a mixed response to temperature and Ψ. No germination occurred when Ψ was lower than –0.25MPa. Time to first germination and time to reach 50% germination (t50) were lowest at summer temperatures for all species, and all species except E. pilligaensis experienced higher time to first germination and t50 (i.e. slower germination rates) as Ψ decreased from 0 to –0.25MPa. Our results indicate that all four eucalypts should be treated as drought-sensitive species with a higher probability of field recruitment during cooler and wet conditions.