Germination requirements and seedling responses to water availability and soil type in four eucalypt species

Acta Oecologica 23 (2002) 23–30 www.elsevier.com/locate/actao

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 availability 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 (Scheffer, 1998). Moisture requirements in early life stages, i.e., for germination and seedling growth, may therefore play an

* Corresponding author. Ökologiezentrum Kiel, Fachabteilung Landschaftsökologie, Christian-Albrechts-Universität, Schauenburgerstrasse 112, D-24118 Kiel, Germany. E-mail address: [email protected] (W. Schütz). © 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

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

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soil water potentials. We conducted four experiments: (i) germination at low and high temperatures in light and darkness; (ii) germination at different soil water potentials; (iii) seedling growth at different soil water potentials; and (iv) seedling survival in two contrasting unwatered soils with different initial water contents. We hypothesised that the species from sandy soils would germinate faster and make better use of soil water for root elongation than those from loam (Milberg and Lamont 1997).

2. Materials and methods 2.1. The species The eucalypt species studied occur exclusively in southwestern Australia and are part of the overstory vegetation in scrub-heath and open woodland (Chippendale and Wolf, 1981; Beard, 1990). Eucalyptus tetragona is a shrub or small tree common in deep sandy soils in scrub-heath communities and occurs in areas receiving 330–690 mm rainfall per year. E. loxophleba is a common mallee or tree in open woodland that occurs on loam or sandy loam with clayey subsoil that receives 250–480 mm rainfall. The relatively widespread tree E. wandoo grows on loamy lateritic soils with clayey subsoil in an area receiving 380–890 mm rainfall. E. macrocarpa, a large mallee shrub, occurs in open scrub-heath on deep sands or sand over laterite with annual rainfall of 380–500 mm (Chippendale and Wolf, 1981; Beard, 1990). The seeds of E. wandoo are small and almost round, those of E. loxophleba broadly crescent-shaped and small (Boland et al., 1989). E. tetragona seeds are irregularly pyramidal-shaped, with narrow hyaline wings around the edge. E. macrocarpa seeds are similar in shape, irregular, but more rounded and have somewhat wider wings. 2.2. Collection of seeds Fruits of the four species were collected between the end of February and beginning of April 1997 north and east of Perth, Western Australia (Table 1). The fruits were dried in an oven at 42 °C for three days, when they had opened and seeds were released. The chaff was removed by sieving and the seeds were stored dry at room temperature (20–30 °C) until used. Table 1 Site and seed characteristics of the four Eucalyptus species.

2.3. Germination response to light and temperature Germination was tested at 10° and 28 °C, in light and darkness. Batches of c. 50 seeds of each species were placed in 5-cm or 9-cm Petri dishes on filter paper (no. 595, Schleicher & Schüll, Germany) and wetted with deionized water. There were three replicates per treatment. Petri dishes intended for dark treatments were immediately wrapped in a double layer of aluminium foil. Germination tests were carried out in incubators (Rubarth Apparatebau, Laatzen, Germany) equipped with warm white fluorescent light providing 20–30 µmol m–2s–1 at seed level. Germinated seeds were counted and removed every second day in the light treatments and every two weeks in dark treatments under dim green light. The experiment was terminated after six weeks. Petri dishes were inspected daily until the first seeds had germinated to identify the onset of germination. Seeds were scored as germinated when at least 2 mm of the radicle was visible. The viability of ungerminated seeds was determined by a subsequent exposure to light at 28 °C and, if seeds did not germinate, by a cut test. Seeds with a white and firm embryo were scored as viable (Baskin and Baskin, 1998). Percentage germination was calculated excluding dead seeds. 2.4. Germination and seedling growth at different soil water potentials Experiments examining the effects of different soil water potentials were carried out with fine quartz sand (herafter referred to as ‘standard soil’) collected from an aeolian sandy soil near Perth, Western Australia. Based on grain size analysis (Table 2), the expected amounts of water to achieve soil-water potentials (W) of –0.1 –0.5, –1.0 and –1.5 MPa were calculated (Bodenkunde, 1982) and added to several small soil samples (50 mL) in thick-walled, tight-closing plastic bags (cf. 10). After four days with frequent mixing of the soils within the plastic bags to allow for equilibration, W was measured with a SC-10A thermocouple psychrometer sample changer connected to an NT-3 nanovoltmeter (Decagon Devices, Pullmann, Washington, USA). If the measured W deviated from the required one, water or soil was added to the samples, and the mixing and measuring procedure was repeated until the measured W matched that required. The required amount of water was then added to 1000 mL of soil and treated in the same way as before. These soil samples were subsequently used for the germination and the seedling growth experiments.

Species

Site

Soil type

Seed mass (mg ± SE)

Table 2 Grain size distribution (%) of the soil substrates used for the experiments.

E. loxophleba E. wandoo E. macrocarpa E. tetragona .

Wundowie, 69 km E Perth Clackline, 80 km E Perth 20 km S Eneabba 20 km S Eneabba

Lateritic loam Lateritic loam Sand Sand

0.18 ± 0.01 0.23 ± 0.02 5.0 ± 0.9* 6.5 ± 2.0*

Grain size (mm)

* Milberg et al., 1998.

> 0.2

Standard soil 35.4 Lateritic loam 68.2 Sand 91.4 .

0.2–0.063

0.063–0.002

< 0.002

Organic matter

40.1 10.4 5.7

10.3 14.9 3.7

14.2 6.6 0

2.1 1.2 0.5

W. Schütz et al. / Acta Oecologica 23 (2002) 23–30

2.5. Germination experiment Petri dishes (diameter 90 mm) were filled with standard soil of known W (–0.1, –0.5, –1.0 or –1.5 MPa) to c. 3 mm below the rim. About four weeks after collection, 25 seeds of each species were placed separately in one quarter of the area on the substrate surface in each Petri dish. The dishes were then filled to the brim thus covering the seeds with a thin layer of soil. A transparent PE-sheet was placed on the soil surface to avoid condensation on the lid. The lid was sealed with parafilm, slightly compressing the soil in the dishes. Four replicates of the four different W treatments were prepared. Each dish contained c. 80 g of soil plus the requisite amount of water (see 2.4 above). The dishes were placed in an incubator at 21/17 °C (12/12 h). The germinated and ungerminated seeds were extracted and counted after 20 days (Boland et al., 1989). 2.6. Seedling growth experiment Seeds were germinated in darkness in Petri dishes on filter paper moistened with deionized water. Immediately after the radicle had ruptured the seed coat, the germinants were transferred to transparent 100 mL polyethylene vials (50 × 50 × 65 mm) containing 50 g of the soil and the requisite amount of water to adjust to five W values (–0.1, –0.5, –1.0, –1.5, –2.5 MPa) and covered with c. 3 mm of soil. Five replicate vials, each containing eight germinants, were prepared for each species. The soil surface was covered with a small sheet of plastic foil to minimize condensation on the walls of the vials. The vials were closed with a lid and sealed with parafilm to avoid water loss to the surrounding atmosphere by evaporation and then placed in a growth chamber (Conviron) equipped with a bank of neon tubes and bulbs providing PAR at 100–120 µmol m–2 s–1 inside the vials. Light was provided for 12 hours per day, coinciding with a temperature of 19 °C. Temperature during darkness was 14 °C. After 14 days, the seedlings were harvested and rinsed with tap water. Root and hypocotyl lengths and fresh weight were measured.

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experiment was run, as limiting nutrients are supplied by the seed (Milberg and Lamont, 1997). Therefore, nutrient levels were not measured (the values should lie close to those obtained for a sand and a loam used in a previous experiment (Milberg and Lamont, 1997). Plastic pots (80 mm diameter, 150 mm height) with drainage holes in their bottom were filled with 700 mL of soil material, on top of a 20-mm layer of exploded silica (Perlite). The pots were watered only once, either with 250 mL tap water, which resulted in water saturation in both soil types (‘full watering’), or with 125 mL (‘low watering’), which was below field capacity. Immediately after watering, eight germinants of E. wandoo, E. loxophleba and E. tetragona and five of E. macrocarpa were planted in each pot. There were nine replicates for each of the four treatments (sand and loamy lateritic soil in combination with full and low watering). The pots were placed on tables in an air-conditioned greenhouse and were initially covered with a transparent plastic sheet to avoid water loss by evaporation during establishment of the germinants (four days). Germinants that had died during this early phase of the experiment were replaced by freshly germinated ones to obtain a sufficient number of seedlings per pot. Average daily maximum and minimum temperatures (SD) were 29.6 (2.5) °C and 18.2 (2.3) °C, respectively, during the two months pots were under observation. Mortality was recorded every day until the last seedling had died (53 days from planting). Soil W, as a measure of soil moisture availability, was recorded daily in two pots per treatment and at two soil depths with psychrometric sensors connected to a HR-33T dew-point microvoltmeter (Wescor, Narrabri, Australia). Two sensors were fixed in the center of the pots, at 5 and 80 mm depth, and an average was calculated per treatment and soil depth. The water content of the air-dry soils at the end of the experiment (assessed by drying 50 g samples at 105 °C for 24 h) was 2% for the loamy lateritic soil and 0.3% for the sand.

3. Results

2.7. Seedling survival in unwatered soils

3.1. Germination response to light and temperature

To investigate the survival of seedlings of the four eucalypts under different soil and water conditions, a greenhouse experiment was carried out with two soil types and two starting levels of water availability. The soil material, encompassing the upper layer to 5 cm depth of a loamy lateritic and a sandy soil, were collected at two sites where at least one of the investigated species occurred. Dead and live parts of plants were removed and the soils were sieved (2 mm mesh size) before usage. Their grain size distribution is described in Table 2. We did not expect any differential effect on growth due to different nutrient contents of the two soils within the two weeks that the

Seeds of all species showed almost complete germination in all four environments, the exception being the two small-seeded species whose germination attained 27.9% (SE ± 3.0) in E. wandoo and 29.5% (SE ± 1.9) in E. loxophleba at 28 °C in darkness. Germination commenced in all species the first day after sowing at 28 °C, and on day 11 at 10 °C except for seeds of E. tetragona which started after nine days. The large-seeded species germinated faster than the small-seeded ones: 50% of final germination percentage was attained at 28 °C after five days in E. tetragona and after three days in the other species; at 10 °C after 17 and 13 days in E. macrocarpa and E. tetragona,

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W. Schütz et al. / Acta Oecologica 23 (2002) 23–30 Table 3 Average seedling mass (fresh weight) and root length of the four Eucalyptus species after two weeks at different soil water potentials. Seedling mass (mg)

Root length (mm)

MPa

–0.1

–0.5

–1.0

–0.1

–0.5

–1.0

E. loxophleba E. wandoo E. macrocarpa E. tetragona .

2.3 3.6 35.7 26.3

1.9 3.0 20.5 22.6

0.4 0.5 6.6 5.4

10.3 11.4 31.3 28.8

12.1 13.1 23.5 31.9

0.3 0.3 3.5 5.9

Table 4 ANOVA of data on seedling mass and root length of four Eucalyptus species when kept for 14 days at three soil water potentials. Fig. 1. Germination of four Eucalyptus species at different soil water potentials. Bars indicate ± SE.

respectively, and after 19 and 21 days in E. loxophleba and E. wandoo. 3.2. Germination at different soil water potentials Almost all viable seeds of the four species germinated at –0.1 MPa while germination was slightly reduced in three species at –0.5 MPa (Fig. 1). Only a few seeds, all of E. tetragona, germinated at –1.0, and none at –1.5 MPa. An inspection of the ungerminated seeds of E. macrocarpa revealed that most of them were non-viable in the –0.1 MPa treatment, perhaps due to an infection or detrimental effect of the treatment. By contrast, seeds of E. macrocarpa were viable at the end of the experiment at –0.5 and –1.0 MPa. 3.3. Seedling growth at different soil water potentials ANOVAs (log-transformed data) revealed highly significant differences among species and W, but there was no significant interaction effect on seedling mass, and only a slightly significant one on root length (Table 4). The latter effect was due to a decline in root length of E. macrocarpa by –0.5 MPa (Table 3, Fig. 2). Average root length after 14 days varied between 30 mm in the large-seeded and c. 12 mm in the small-seeded species at –0.1 MPa, and increased slightly in three species at –0.5 MPa, followed by a sharp decline at –1.0 MPa. At –1.0 MPa root elongation occurred in 86% of the E. tetragona and in 64% of the E. macrocarpa seedlings. By contrast, 45 and 42% of the E. loxophleba and E. wandoo seedlings, respectively, showed some root elongation, but in almost all cases root growth was arrested at c. 1 mm length. Hypocotyl length decreased from –0.1 to –0.5 MPa and hypocotyl emergence was arrested in almost all seedlings in all species at –1.0 MPa (data not shown). Seedling mass was more than ten times greater in E. macrocarpa and E. tetragona than in E. loxophleba and E. wandoo at each soil moisture level (Table 3), probably owing to the smaller seed mass of the two latter species (Table 1).

Source Seedling mass A: Species B: Soil water potential A*B Seedling root length A: Species B: Soil water potential A*B .

df

MS

F

P

3 2 6

4.595 2.888 0.022

466.5 293.2 2.3

< 0.0001 < 0.0001 0.0561

3 2 6

0.840 4.394 0.052

39.1 204.4 2.4

< 0.0001 < 0.0001 0.0446

3.4. Seedling survival in unwatered soils Differences in soil moisture content between the full (F) and low (L) initial water treatments (Fig. 3) became apparent after 2–3 weeks when the upper soil layer (0.5 cm) started to dry out, especially when W fell below the permanent wilting point (PWP; 9.5 µV –1.5 MPa), first for the low water treatments in both sand (Sa) and loamy laterite (La) after 16 d, and after 19 d in FSa and 22 d in FLa. Temporal differences were smaller at 8 cm depth: FSa and LSa dropped below the PWP after 35 d, and 25 d (LLa) and 30 d (FLa). Seedlings started to die first in the low water treatments, except for E. macrocarpa, which showed a high mortality in FLa as well. About 7 d later, mortality increased also in the full water treatments in all species. Seedlings did not respond by a sudden increase in mortality when PWP was attained, neither at 0.5 cm nor at 8 cm depth. An exception might be E. tetragona in LLa where most seedlings died within 10 d after PWP had been reached at 8 cm depth, and in E. loxophleba in LSa when the upper soil layer dried up quickly after 20 days. Rather uniform (moderate) slopes of most species survival-curves (Fig. 3) may indicate individual variation in drought resistance due to a more or less extended root system. A three-factorial ANOVA (Table 5) revealed significant effects of soil type, water treatment, and species at t50 (the time to reach 50% mortality), but only one weakly significant interaction effect (species*soil). This species*soil interaction was further investigated in a post hoc test (Tukey HSD) by making contrasts of the t50 (i) for the different soils and (ii) for the two large-seeded species with the two small-seeded. These six contrasts revealed that the largeseeded species, both occurring on sandy soils in the field, survived longer on sand than on lateritic loam (P = 0.0046)

W. Schütz et al. / Acta Oecologica 23 (2002) 23–30

Fig. 2. Seedling characteristics after 14 days in soil with different water potentials. a) Relative seedling biomass; b) Relative root length. Bars indicate ± SE.

while no other contrast was significant (P > 0.1). Seedlings of E. tetragona survived best irrespective of treatment, and the last one died after 53 days.

4. Discussion 4.1. Germination responses Dormancy is a common trait in freshly matured seeds of many species growing in mediterranean environments, which prevents seeds from germinating in the hot, dry season (Thompson, 1973; Bell et al., 1995; Schütz, 1999). The four eucalypts in our study, however, did not show any sign of a marked seed dormancy mechanism. Their weak dormancy may be coupled with aerial seed storage (se

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Fig. 3. Seedling survival and ψ (measured as electrical charge generated) in two soils with two different initial amounts of water added.

Table 5 ANOVA of median time to 50% survival when seedlings of four eucalypts were left unwatered in two soil types at two levels of initial water availability.

A: Soil B: Water C: Species A*B A*C B*C A*B*C .

df

MS

F

P

1 1 3 1 3 3 3

645.5 370.0 671.0 54.9 198.8 18.6 76.9

8.726 5.002 9.071 0.742 2.687 0.252 1.039

0.0037 0.0271 < 0.0000 0.3905 0.0493 0.8597 0.3776

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rotiny), exhibited by all four species (Lamont et al., 1991). Species with serotinous fruits, that do not open to release the seeds until a fire or other environmental cue, are usually able to germinate immediately. Seeds of many mediterranean-climate species which are non-dormant, or conditionally dormant at maturity, often have restricted temperature requirements, i.e., they germinate only at low temperatures commonly coinciding with those prevailing during the rainy season in their natural habitats. The majority of species inhabiting mediterraneantype environments in Western Australia and that have hitherto been investigated germinate well at low and medium temperatures but not at high ones (Bell et al., 1993, 1995). In contrast, all four eucalypts investigated in the present study germinated not only at low (10 °C) but also at high temperatures (28 °C), whereas many other southwestern Australian species germinate poorly (Bell et al., 1993, 1995). Eucalypts generally seem to have a wider temperature range for germination than other taxa, and germinate well at relatively high temperatures (Turnbull and Doran, 1987; Bell et al., 1995). For instance, E. tetragona germinated to high levels at temperatures between 10 and 35 °C in the study of Bellairs and Bell (1990). Differences among the four eucalypts were detectable in their light requirement, with germination in the smallseeded species reduced in darkness at 28 °C. A tendency to retard germination in darkness is observed in species with small seeds, which is explained as a mechanism to avoid fatal germination at a depth too great for the seedling to reach the surface (Milberg et al., 2000). Since the seeds of serotinous species are short-lived once released (Lamont and Barker, 1988), the adaptive significance of delaying germination in the absence of light seems small, except at high temperatures when it may function as a signal that the appropriate germination conditions have yet to commence. Seeds may become more readily and deeply buried in the deep sands where E. tetragona and E. macrocarpa occur as the surface is less stable, closer to the coast and the low vegetation is more wind-prone. Lamont et al. (1993) obtained mean burial depths of 5–13 mm for a range of microsites following post-fire wind movement of seeds where the two sand species occur, a depth that might exclude E. loxophleba and E. wandoo seeds from successful emergence. Another factor that may be important in regulating germination in drought-prone environments is the duration of the moist periods compared with the germination requirements of species (Gutterman, 1993; Schütz and Milberg, 1997; Pérez-Fernández et al., 2000). As sands dry out faster than loams near the surface (Scheffer, 1998) germination level and rate may be expected to be higher in species typical of sandy soils. Onset of germination was faster and the number of days to reach 50% final germination lower in the two eucalypts occurring on deep sands than those occurring on loam, at least at 10 °C. Hence, depth of burial, light requirements and germination speed appear more

important for regeneration niche separation in these four species than temperature requirements. In arid environments, drought can be the main factor regulating germination. Species occurring under the same climatic conditions may show variations in their germination responses to soil moisture (Evans and Etherington, 1990). Thus, germination of seeds of some species may be more or less restricted to certain habitats or soil types with a high water holding capacity. Such habitat-related differences have been shown between two eucalypts with mesic distributions and two others with more xeric distributions (Facelli and Ladd, 1996). In the xeric species, a majority of the seeds germinated at water potentials of –0.55 and –1.05 MPa, while germination of the mesic species was strongly reduced already by –0.55 MPa. We expected the large-seeded species to germinate at lower water potentials as the sandy soils dry out quicker after rain than loams. Our study, however, found only minor differences in germination capability between the four investigated species at the soil water potentials used. Only E. tetragona deviated to some extent, since some germination still occurred at –1.0 MPa (Fig. 1). 4.2. Seedling growth Root plasticity may be of critical importance during seedling establishment in drying soils (Reader et al., 1993). Root growth may be regarded as the most important factor in early seedling survival, since rapid extension of roots enables the seedling to exploit water from previously unexplored areas of soil. Milberg and Lamont (1997) showed that E. loxophleba elongated faster in its own loam than sand, and E. todtiana elongated faster (and much faster than E. loxophleba in loam) in its own sand than loam. Not surprisingly, species of xeric habitats (Enright and Lamont, 1992) and large-seeded species (Milberg et al., 1998) are thought to have a superior ability for root extension. Evans and Etherington (1991) showed a wide range of species growth responses to lower soil matric potentials mainly between, but also within, habitats. Generally, seedlings of species restricted to dry sites developed longer roots than those from wet sites at moderate matrix potentials. The existence of marked differences in ability to increase root growth at declining soil water potentials is also supported for two species of the Mediterranean basin occurring in contrasting habitats (Schütz, 1999). Therefore, we expected differences in growth response of seedlings to contribute to an explanation of soil preferences of our four eucalypt species. Surprisingly, relative changes in growth response to soil water potentials in root length and seedling mass were fairly similar between the four species. However, root length was greater and relatively less retarded under drought conditions (–1 MPa) in the standard soil for the large-seeded species. As sands will dry out at a faster rate near the surface than loams (Scheffer, 1998), this might indicate a greater capacity of E. tetragona and E. macrocarpa to keep pace

W. Schütz et al. / Acta Oecologica 23 (2002) 23–30

with gravimetric water movement through the profile, which would give them an adaptive advantage in deep sand (Enright and Lamont, 1992). 4.3. Seedling survival in drying soils The few observations on the fate of seedlings under drying conditions in field and laboratory experiments suggests the existence of species-specific differences to withstand drought. Lamont et al. (1989) provided evidence that water availability was the factor responsible for the distribution patterns of three Banksia species growing in close proximity along a soil moisture gradient. Seedlings of the Mediterranean stream bank species Nerium oleander were more susceptible to drought compared with seedlings of three species frequently co-occurring with Nerium but with wider ecological limits (Schütz, 1999). With these results in mind, we designed an experiment with seedling survival in drying soils. We chose two soil types which we expected to differ in water-holding capacity. This assumption, however, was not substantiated since the temporal course of soil moisture potentials, i.e., onset of drought conditions, showed a relatively small difference between soil types. Surprisingly, soil water availability declined somewhat earlier, and seedling mortality increased earlier too, in the loamy than in the sandy soil, both in the low and the full watering treatments (Fig. 3). However, this does not mean that the temporal course of water loss in the field is the same as in this pot experiment. It can be expected that water availability is not only higher in loams than in sand but that loams will dry out at a slower rate near the surface. In undisturbed soil profiles capillary rise through the pore system will provide a higher moisture level for a longer time near the surface in loamy soils rather than in deep sands (Scheffer, 1998). When selecting species for this study, we were unable to find eucalypts with comparable seed masses but contrasting soil affinity. Hence, the two species restricted to sand were large-seeded and those restricted to lateritic loam were small-seeded. Seedlings from large-seeded species were larger and had longer roots than the small-seeded and more seedlings emerged in large-seeded spp. at –1.0 MPa (Fig. 2). Hence, seedlings of large-seeded species were expected to stand a better chance of establishment in drying soils. Differences among large-seeded and small-seeded species were, however, inconsistent between soil types and watering regimes (Fig. 3; Table 5), indicating that each species responded individually and independent of seed size. This means that large-seeded species did not survive better than the small-seeded ones as the soils dried out. Thus, germinants from large seeds are not necessarily more droughttolerant (Leishman and Westoby, 1994). They are more likely to be drought-avoiders, relying on reaching and maintaining contact with soil water at depth (Milberg et al., 1998). Should this not succeed, they could be highly susceptible to severe drought.

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The advantages of seedlings from large seeds seems to be that they can produce extensive roots quickly under the poor nutrient conditions associated with sandy soils (Milberg and Lamont, 1997; Milberg et al., 1998). In contrast, seedlings of small-seeded species can take advantage of the extra nutrients available in more fertile soils (loam). Milberg and Lamont (1997) showed that growth rate and root extension of the small-seeded E. loxopleba was much greater in sand than loam, while growth rate was the same and root extension slower for the large-seeded E. todtiana (which may co-occur with E. macrocarpa and E. tetragona) in loam than in sand. These differences did not show up until eight weeks after germination, whereas our study was terminated after two weeks, clearly insufficient time for E. loxopleba and E. wandoo to take advantage of the extra nutrients in the loam. In conclusion, the sand species had much longer roots when plenty of water was available in the soil, and the roots of more seedlings continued to elongate under moderate water stress (–1.0 MPa), than the two species restricted to loam. The greatest ability to cope with dry conditions in terms of germination and seedling survival can be assigned to the sand species, E. tetragona. As water availability declined, there was a tendency for species to survive longer on their own soils. The results were confounded by seed size, although we chose representative and widespread species from these two major soil types. There is mounting evidence that seedlings of large-seeded species restricted to deep sands undergo much less summer drought stress than their small-seeded congeneric associates (Lamont et al., 1993; Richards and Lamont, 1996) and that this may be attributed to their deep root systems (Enright and Lamont, 1992; Richards and Lamont, 1996). Small-seeded species are not able to take advantage of their inherently higher relative growth rates in these nutrient-impoverished soils, so are not able to maintain contact with soil water at this time. As a consequence, small-seeded species are either restricted to the more fertile loams, or, if they occur in sand, are physiologically stress-tolerant (Richards and Lamont, 1996; Milberg et al., 1998).

Acknowledgements We are grateful to Neil Turner at CSIRO for allowing us to borrow equipment, Thomas Baumgartl who gave valuable advice in assessing soil parameters and Philip Groom who assisted in various ways.

References Baskin, C.C., Baskin, J.M., 1998. Seeds. Ecology, Biogeography, and Evolution of Dormancy and Germination. Academic Press, San Diego. Beard, J.S., 1990. Plant life of Western Australia. Kangaroo Press, Kenthurst, NSW.

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