Germination requirements and seedling responses to water availability and soil type in four eucalypt species Wolfgang Schütz *, Per Milberg, Byron B. Lamont Department of Environmental Biology, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia Received 29 September 2000; received in revised form 5 November 2001; accepted 13 November 2001 Abstract We conducted experiments on seed germination, seedling survival and seedling growth of four Eucalyptus species to identify factors that might explain why they are restricted to the two major soil types in southwestern Australia, deep sands (E. macrocarpa, E. tetragona) and lateritic loam (E. loxophleba, E. wandoo). At high temperatures (28 °C), germination in darkness was lower for the two ‘loam species’ than for the ‘sand species’, while there were no differences in light or at low temperatures (10 °C). Germination commenced earlier, and was faster in the sand species than in the loam species, but was almost inhibited in all species by –1.0 MPa. E. tetragona proved the most drought-tolerant in terms of germination level and seedling survival. Seedlings of the sand species had much longer roots two weeks after germination in the absence of water stress, and the roots of more seedlings continued to elongate under moderate water stress (–1.0 MPa), than the two loam species. Roots were longer in all species, except E. macrocarpa, at –0.5 MPa than at –0.1 MPa, despite seedlings having a smaller mass and hypocotyl length. As water availability declined, there was a tendency for the sand species to survive longer on sand than on loam while soil type had no effect on the loam species. Pattern and duration of seedling survival of the loam species was similar to that of the sand species despite their smaller seeds. We conclude that seedlings from the large-seeded sand species are able to penetrate the soil profile faster and deeper, but that they are not less prone to drying soils than seedlings from the small-seeded loam species. Instead, seed size and germination speed are important prerequisites to cope successfully with unstable soil surfaces and to exploit the rapidly descending water in deep sands. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. Keywords: Eucalyptus; Lateritic loam; Sand; Seeds; Soil water potential 1. Introduction In arid and mediterranean environments water availabil- ity and temperature are the main factors regulating the timing of germination and the survival of seedlings (Went, 1953; Winter, 1974). Establishment of seedlings is the most critical life stage in dry environments (Thompson, 1973) and a lack of soil moisture is often a major reason for seedling mortality (Lamont et al., 1993). Germination and seedling survival may differ between soil types, since moisture availability may be a function of soil type (Schef- fer, 1998). Moisture requirements in early life stages, i.e., for germination and seedling growth, may therefore play an important role in determining the distribution patterns of species (Lamont et al., 1989; Guttermann, 1993; Mustart and Cowling 1993). Several adaptations may increase the seedling’s ability to cope with the consequences of drought: the ability to maintain viability at water potentials below the turgor loss point (Richards and Lamont, 1996), rapid root growth to exploit the temporally- and spatially-restricted soil water resources (Gutterman, 1993; Reader et al., 1993; Leishman and Westoby, 1994; Milberg and Lamont, 1997; Schütz, 1999), and a high root: shoot ratio (Grime, 1979; Osunkaya et al., 1994). In the mediterranean part of Western Australia, many eucalypt species show distinct distribution patterns with respect to climate, especially annual rainfall, and certain soil types (Beard, 1990). We chose four such species occurring on either sand or loamy lateritic soils and we expected their seed and early seedling stage to differ in their response to * Corresponding author. Ökologiezentrum Kiel, Fachabteilung Land- schaftsökologie, Christian-Albrechts-Universität, Schauenburgers- trasse 112, D-24118 Kiel, Germany. E-mail address: wschuetz@ecology.uni-kiel.de (W. Schütz). Acta Oecologica 23 (2002) 23–30 www.elsevier.com/locate/actao © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved. PII: S 1 1 4 6 - 6 0 9 X ( 0 1 ) 0 1 1 3 0 - 4