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Seed Size Effects on Seedling Emergence of Desert
Psammophytes in China

Kazuo Tobe a; Liping Zhang b; Kenji Omasa caNational Institute for Environmental Studies, Tsukuba, Japanb
Cold and Arid Regions Environmental and Engineering Research Institute,
Chinese Academy of Sciences, Lanzhou, Chinac
Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo,
Japan
Online Publication Date: 01 July 2007
To cite this Article: Tobe, Kazuo, Zhang, Liping and Omasa, Kenji (2007) 'Seed Size
Effects on Seedling Emergence of Desert Psammophytes in China', Arid Land Research and Management, 21:3, 181 192
To link to this article: DOI: 10.1080/15324980701429342
URL: http://dx.doi.org/10.1080/15324980701429342

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Arid Land Research and Management, 21:181–192, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1532-4982 print/1532-4990 online
DOI: 10.1080/15324980701429342

Seed Size Effects on Seedling Emergenceof Desert Psammophytes in China
Kazuo Tobe
National Institute for Environmental Studies, Tsukuba, Japan

Liping Zhang
Cold and Arid Regions Environmental and Engineering ResearchInstitute, Chinese Academy of Sciences, Lanzhou, China

Kenji Omasa
Graduate School of Agricultural and Life Sciences,
The University of Tokyo, Tokyo, Japan
To investigate the effects of seed size on the behavior of seeds and seedlings in sand,seedling emergence from sand was examined in Caragana korshinskii (Fabaceae;
mean seed weight 65 mg) and Atraphaxis bracteata (Polygonaceae; mean seed
weight 5.7 mg), two shrubs with nonphotoblastic seeds that grow on desert
sand dunes in China. Seeds were sown at depths of 0 to 50 mm in sand-filled pots,
and the pots were irrigated under different regimes. The results suggest that for both
species, a high proportion of the seeds buried fairly deep germinate during the summer rainy season with or without producing emergent seedlings, whereas at shallow
depths, where moisture is more likely to be lost by evaporation, some proportion of
the seeds remain ungerminated and viable. Comparison of results in this study withthose on several smaller-seeded species used in our previous studies shows that larger
seeds did not always give rise to seedlings capable of emerging from deeper depths insand. We suggested that for nonphotoblastic seeds, seed size is not a crucial determinant of the proportion of seeds that produce emergent seedlings in field conditions.
Keywords Atraphaxis bracteata, Caragana korshinskii, irrigation, sand dunes,
seed burial depth, seed fate, seed weight, water availability

Seed size varies greatly among species within communities (Leishman et al., 2000).
In the desert dunes of north central China, an annual Eragrostis poaeoides (meanseed weight 0.1 mg) and a shrub C. korshinskii (65 mg) have the smallest and largest
seeds, respectively. In previous studies, we examined seed germination and seedlingemergence of several species growing in desert sand dunes in China. Among them,four species with small seeds (E. poaeoides (unpublished results) and three Artemisia
semi-shrubs [mean seed weight 0.27–0.76 mg; Tobe et al., 2006]) required light for
germination. We suggested that the probability of seed germination of these species
Received 2 January 2007; accepted 12 March 2007.
We thank Dr. Qiushi Wei for providing the seeds of Atraphaxis bracteata.
Address correspondence to Dr. Kazuo Tobe, National Institute for Environmental
Studies, 16–2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan. E-mail: tobe@nies.go.jp

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in the field is very limited because the light requirement restricts seed germination toshallow sand layers, where moisture is lost rapidly due to evaporation. On the other
hand, species with medium-sized seeds (three annuals [Agriophyllum squarrosum,
Bassia dasyphylla, and Aristida adscensionis; mean seed weight 1.8, 1.0, and1.3 mg, respectively; Tobe et al., 2005b] and two Fabaceae herbs [Astragalus adsurgens and Melilotus albus; mean seed weight 1.7 and 2.8 mg, respectively; Tobe andGao, 2007]) did not require light for germination and showed high percentages ofseedling emergence, when the seeds were buried at appropriate sand depths. If seedling emergence from sand depends on seed size, the responses of larger-seeded species
might differ from those of smaller-seeded species examined in our previous studies.
To better understand the behavior of seeds and seedlings in desert sand dunes in
China, we need to investigate how seed size affects the responses of seeds to water
availability and burial depth.
Many studies (e.g., van der Valk, 1974; Buckley, 1982; Maun and Lapierre,
1986; Jurado and Westoby, 1992; Martı́nez et al., 1992; Bond et al., 1999) have indicated that larger nonphotoblastic seeds, which have more seed reserves than smaller
ones, had the advantage of giving rise to taller seedlings that could emerge from deeper sand. Bond et al. (1999) reported that maximum seedling emergence depth is proportional to the cube root of seed weight. On the other hand, the effects of seed sizeon the response of seedling emergence to water availability have been less studied.
Bansal et al. (1980) examined water absorption by seeds of 40 species growing in cultivated fields of an arid zone in India. They reported that seeds that absorbed lesswater during germination were more likely to be stimulated to germinate by lighter
precipitation. Precipitation is usually the most crucial factor to seedling emergence inarid and semi-arid regions (Sala et al., 1992; Lauenroth et al., 1994), and water
availability in sand depends on both precipitation and sand depth (Tobe and Gao,2007). Thus, size-dependent responses of seeds to water availability can result insize-dependent differences in seedling emergence and seed bank behavior in desert
sand dunes.
In the present study, we examined seedling emergence of Caragana korshinskii
Kom. (Fabaceae) and Atraphaxis bracteata A. Los. (Polygonaceae), two deciduous
shrubs that grow on desert sand dunes in China and produce nonphotoblastic
seeds (unpublished data). A. bracteata is found on mobile sand dunes, whereas
C. korshinskii is generally found on stable sand dunes. Seed weight of these species
is considerably larger than that of species examined in our previous studies, and themean seed weight of C. korshinskii (65 mg) is more than 11 times that of A. bracteata(5.7 mg). The amount of water absorbed by seeds before radicle protrusion begins is
ca. 84 mg=seed for C. korshinskii and ca. 6.7 mg=seed for A. bracteata (estimatedfrom data in Tobe et al. (2001)) versus ca. 0.75 mg=seed for A. squarrosum (unpublished data). Because seed germination of these two species, as well as that of the fivespecies with non photoblastic medium-sized seeds examined in our previous studies(Tobe et al., 2005b; Tobe and Gao, 2007), does not depend greatly on temperature(Tobe et al., 2001), water availability and seed burial depth are crucial for emergenceof their seedlings in the field.
Thus, the purpose of the present study was to understand how seed size affects
seedling emergence and seed fates of species growing on desert sand dunes in China.
We investigated seedling emergence of C. korshinskii and A. bracteata for seeds
sown at different depths in sand and irrigated under different regimes. In addition,
we tried to determine the fates of seeds that did not give rise to emergent seedlings

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after irrigation. For C. korshinskii, we also investigated the effect of intraspecificdifferences in seed size on seedling emergence. The results were compared with thoseof several smaller-seeded species (especially with five species whose seeds arenonphotoblastic and medium-sized) used in our previous studies.

Materials and Methods
Seeds and Habitat Climate
Seeds of C. korshinskii were collected in June 2001 from a sand dune in Shapotou,
China (37260 N, 104570 E, annual precipitation 188 mm, annual mean temperature
10.5C), and seeds of A. bracteata were collected in August 2000 from a field botanical garden in Minqin, China (38340 N, 102580 E, annual precipitation 120 mm,
annual mean temperature 7.8C). Seeds were transported to Japan and stored at
ca. 0C in a refrigerator until used in experiments. Seed size of C. korshinskii varied
more than that of A. bracteata; thus, we classified the C. korshinskii seeds as large(60–90 mg; ca. 67% of the total) or small (30–60 mg; ca. 33% of the total). The small
C. korshinskii seeds were used only in one experiment. All experiments were carriedout from May 2002 to March 2004. During the storage period in Japan, we found no
indication of seed dormancy nor any reduction in seed germinability.
Mean winter temperature in Chinese deserts is below 0C, and winter is an unfavorable season for seed germination and plant growth. In the eastern and central
parts of deserts in China, where the two study species, as well as those used in ourprevious studies (Tobe et al., 2005b, 2006; Tobe and Gao, 2007), are distributed, precipitation occurs primarily in summer. For example, in Shapotou, which is a typical
habitat for the two species investigated in this study, as well as for many of the otherspecies investigated in our previous studies, daily precipitation during the summer
rainy season often exceeds 16 mm, and it occurs at relatively short intervals. In contrast, daily precipitation in other seasons is usually less than 8 mm, and periods
between precipitation events are usually separated by long dry periods (Tobe et al.,2005b).
Experimental Methods
All experiments were carried out in incubators maintained at 12-h 25C=12-h 15C,
which is representative of temperatures in the habitats of these species in early summer, when seeds are expected to germinate. Seed germination and radicle growth inPetri dishes were examined in the dark following the method described in Tobe et al.
(2005b).
To examine seedling emergence from sand, replicates of 10 or 16 seeds weresown on=in sand-filled cylindrical plastic pots (inside diameter 80 mm and sand surface area ca. 50 cm2) with a height of 50 mm (short pots) or 100 mm (tall pots). There
was a circular drainage opening (diameter: 12 mm) at the bottom of each pot thatwas covered with cloth. Dry fine or coarse sand was used to fill the pots to a depthof 45 mm (short pots) or 90 mm (tall pots). In most experiments, we used fine sand
(91% wt. was 63–250 mm), which had a particle-size distribution similar to thatof sand dunes in China (Tobe et al., 2005b). However, in one experiment weused coarse sand (91% wt. was 250–600 mm) to examine the effects of particle sizeon seedling emergence. The pots were exposed to light in the 25C part of the dailyDownloaded By: [Tobe, Kazuo] At: 16:39 18 September 2007

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thermoperiod. During the 12-h light period, the pots were illuminated withfluorescent lamps (photosynthetic photon flux density at the sand surface was
80–130 mmol m 2 s 1). Relative humidity in the incubator was generally 40–70%
during the light period and 90–95% during the dark period. The pots were irrigatedat the beginning of the light period by slowly and evenly dripping deionized water
onto the sand with a pipette. They were observed daily, and emerging seedlings ineach pot were counted. A seedling was considered to have emerged when its height
above the sand surface exceeded 3 mm. For moisture condition in sand for each irrigation regime, refer to Tobe and Gao (2007).
Seedling Emergence from Seeds Sown on Surface of, or at a Depth of 10 mm in,
Fine Sand and Irrigated Daily
Replicates of 16 seeds (large C. korshinskii seeds or A. bracteata seeds) were sown on
the surface of, or at a depth of 10 mm in, fine sand in the short pots. The pots were
initially irrigated with 16 mm P of water (where x mm P denotes irrigation equivalent
to x mm precipitation) and were subsequently given 3 mm P of water at 1-d intervals(the [16 þ 1=1] treatment) for 14 d. From pots in which seeds were sown on the sandsurface, seeds were later transferred into Petri dishes, and germination was observed
daily for an additional 10 d to determine whether seeds ungerminated after 14 d ofirrigation were still viable.
Seedling Emergence from Seeds Buried at Different Depths in Fine or Coarse
Sand and Irrigated under Different Regimes
Replicates of 10 seeds (large C. korshinskii seeds or A. bracteata seeds) were sown at
depths of 10, 20, 30, 40, or 50 mm in fine or coarse sand in the tall pots. The pots
were initially irrigated with 32 mm P of water and subsequently with 3 mm P of waterat 1-d intervals (the [32 þ 1=1] treatment) or at 4-d intervals (the [32 þ 1=4] treatment) for 20 d (C. korshinskii seeds buried at 10–40 mm) or 30 d (all other seeds).
To examine whether seeds that were buried deeply and that did not give rise to
emerging seedlings had germinated, seeds buried at 50 mm were excavated fromthe sand after 30 d of irrigation in the pots and inspected for germination.
Seedling Emergence from Seeds Buried at a Depth of 10 mm in Fine Sand andIrrigated Once with Different Amounts of Water
To investigate the effect of a single irrigation on seedling emergence, replicates of 10
seeds (large C. korshinskii seeds or A. bracteata seeds) were sown at a depth of 10 mm
in fine sand in tall pots, and the pots were initially irrigated with 8 or 16 mm P ofwater and subsequently were not irrigated at all for 14 d. For the treatment in which
initial irrigation was 8 mm P, we determined whether seeds that did not give rise to
emerging seedlings were viable by irrigating each pot with 8 mm P of water on
day 14 after the initial irrigation and subsequently with 3 mm P of water at 2-d intervals (the [8 þ 1=2] treatment) for an additional 12 d.
Seedling Emergence from Small C. korshinskii Seeds Buried at a Depth of 10 or
30 mm in Fine Sand and Irrigated Once with Different Amounts of Water
Replicates of 10 small C. korshinskii seeds were sown at a depth of 10 or 30 mm infine sand in the tall pots. The pots were initially irrigated with 8, 12, 16, 24, or 32 mm
P of water and subsequently were not irrigated at all for 16 d. For treatments inwhich the initial irrigation was 8 mm P, we determined whether seeds that did notDownloaded By: [Tobe, Kazuo] At: 16:39 18 September 2007

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185

give rise to emerging seedlings were viable by irrigating the pots with 24 mm P ofwater on day 16 after the initial irrigation and observing seedling emergence foran additional 16 d.

Statistical Analysis
Student’s t-test was used to test the significance of differences between two means.
Two-way ANOVA or Tukey’s test was used for comparison of multiple means. Percentage values were arcsine square-root transformed prior to statistical analysis.

Results
Seed Germination in Petri Dishes
Large C. korshinskii seeds moistened in Petri dishes in the dark germinated earlier than those of A. bracteata by ca. 1d ( symbols in Figure 1). Germination
percentages 10 d after moistening were 81  1.2% (mean  standard error, n ¼ 4)
for C. korshinskii and 66  2.0% for A. bracteata.

Figure 1. Changes over time in percentages of seedling emergence from seeds sown at different
depths in fine sand in the [32 þ 1=4] irrigation treatment. A: C. korshinskii, B: A. bracteata.
Seed burial depth: : 10 mm, &: 20 mm, 4: 30 mm, .: 40 mm, &: 50 mm. n ¼ 6 for A. bracteata and n ¼ 7 for C. korshinskii. The change over time in the percentage germination of seeds
moistened in Petri dishes in the dark is also shown by  symbols connected with a broken line(n ¼ 4).

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Seedling Emergence from Seeds Sown on Surface of, or at a Depthof 10 mm, in Fine Sand and Irrigated Daily
No seeds sown on the sand surface in the [16 þ 1=1] treatment germinated in either
species, but 10 d after they were transferred to Petri dishes, the total germination was
91  2.5% for C. korshinskii and 61  5.0% for A. bracteata (mean  standard error,
n ¼ 5). On the other hand, for seeds sown at a depth of 10 mm in the [16 þ 1=1] treatment, seedlings of both species began to emerge 6 d after irrigation, and final percentages of seedling emergence were 86  14.6% for C. korshinskii and 76  6.4% for
A. bracteata (mean  standard error, n ¼ 5).
Seedling Emergence from Seeds Buried at Different Depths in Fineor Coarse Sand and Irrigated under Different Regimes
In both species and under both irrigation regimes, delay of seedling emergenceincreased with increase in seed burial depth (e.g., Figure 1). Generally, seedlings
emerged 1–2 d earlier in C. korshinskii than in A. bracteata. Final percentages ofseedling emergence decreased with increasing seed burial depth in both species,
and they were generally higher in the [32 þ 1=4] irrigation treatment than in the[32 þ 1=1] irrigation treatment (Figure 2). There was no conspicuous difference
between the two species in maximum seedling emergence depth (Figure 2). Twoway ANOVA (seed burial depth  irrigation regime) indicated that in both species
final seedling emergence percentage depended significantly (P ¼ 0.0005) on bothseed burial depth and irrigation regime. The percentage of seedling emergence of
C. korshinskii seeds buried at 40 and 50 mm was higher in coarse sand than in finesand in the [32 þ 1=1] irrigation treatment (Figure 2).
Fairly high percentages of seeds of both species buried at 50 mm and excavatedfrom the sand 30 d after irrigation had germinated, but few or none had reached thesand surface (Figure 2); lengths of seedlings that did not emerge were 20–150 mm(C. korshinskii) and 2–30 mm (A. bracteata). Since fairly high percentages ofA. bracteata seeds buried at 50 mm did not germinate, the ungerminated seeds andgerminated seeds with short radicles of A. bracteata were transferred to Petri dishes
and moistened to examine their viability. No ungerminated seeds germinated, andthere was no further growth of radicles that had emerged from seeds.
Seedling Emergence from Seeds Buried at a Depth of 10 or 30 mm in Fine
Sand and Irrigated Once with Different Amounts of Water
For large C. korshinskii seeds and for A. bracteata seeds, a single irrigation of 8 mm P
did not result in seedling emergence, whereas subsequent reirrigation with the[8 þ 1=2] irrigation treatment resulted in a high percentage of seedling emergence (Table 1). A single irrigation of 16 mm P of water resulted in no seedlingemergence from the large C. korshinskii seeds, whereas a fairly high percentage of
seedlings emerged from small C. korshinskii seeds and A. bracteata seeds.
Percentage seedling emergence from small C. korshinskii seeds after a single irrigation was generally higher at a seed burial depth of 30 mm than at a depth of 10 mm(Table 2), but the difference was significant only for the 12 mm P treatment.
Although a single 8 mm P irrigation did not result in seedling emergence from small
C. korshinskii seeds, subsequent re-irrigation with 24 mm P of water 16 d after the

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Figure 2. Final percentages of seedling emergence from seeds sown at different depths in fineor coarse sand and irrigated under different regimes (.: fine sand, [32 þ 1=1], &: fine sand,[32 þ 1=4], ~: coarse sand, [32 þ 1=1]). A: C. korshinskii, B: A. bracteata. Final percentagesof germinating seeds are also shown for seeds sown at 50 mm (: fine sand, [32 þ 1=1]; &: finesand, [32 þ 1=4]; 4: coarse sand, [32 þ 1=1]). Coarse sand treatment was carried out only for
C. korshinskii. n ¼ 6 for A. bracteata and n ¼ 7 for C. korshinskii; bars indicate standard
errors, which are shown only where they were larger than the symbol.

Table 1. Final percentages of seedling emergence of seeds buried at a depth of 10 mm
in fine sand and treated with a single irrigation of 8 or 16 mm P of water. Since noseedlings emerged after a single 8 mm P irrigation, the pots were subsequently
reirrigated with a single 24 mm P (small C. korshinskii seeds) or [8 þ 1=2] (theother seeds)
Final seedling emergence (%)
C. korshinskii
Initial Irrigation

Large seeds

Small seeds

A. bracteata

8 mm P
16 mm P0 (0)!68 (8.3)
0 (0)0 (0)!30 (5.8)
47 (9.9)0 (0)!74 (6.5)
59 (8.3)

Percentages of seedling emergence after reirrigation are shown at right of arrows. n ¼ 6 forA. bracteata and n ¼ 7 for C. korshinskii; standard errors are presented in parentheses.

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Table 2. Final percentages of seedling emergence for small C. korshinskii seeds
buried at a depth of 10 or 30 mm in fine sand and irrigated with different
amounts of water. Since no seedlings emerged after a single 8-mm P irrigation, thepots were subsequently reirrigated with 24 mm P of water
Final seedling emergence (%)
Seed burial depth
10 mm
30 mm

8 mm P

12 mm P

16 mm P

24 mm P

32 mm P0 (0) c!30 (5.8) 6 (3.8) bc 47 (9.9) abc 57 (6.8) ab 56 (5.3) ab
0 (0) c!4 (3.0) 63 (6.8) a 77 (6.4) a
67 (3.6) a 74 (3.0) a

Percentages of seedling emergence after the reirrigation are shown at right of arrows. Values
with the same letter are not significantly different from each other (P ¼ 0.05; Tukey’s test).
n ¼ 7; standard errors are presented in parentheses.

8 mm P irrigation caused 30% and 4% of the seeds to produce emergent seedlings
from the 10 and 30 mm depths, respectively. However, these percentages were considerably lower than those of seedling emergence after a single 24 mm P irrigation
at the beginning of the study period (57% and 67%) (Table 2), indicating that thesingle 8 mm P irrigation reduced the number of viable seeds. For C. korshinskii seeds
buried at 10 mm and reiirrigated after the single 8 mm P irrigation, the percentage of
seedling emergence (Table 1) was significantly higher (P ¼ 0.005) from large seeds(68%) than from small seeds (30%), indicating that small seeds were more likely
to lose viability by 8 mm P of irrigation. For small C. korshinskii seeds that werereirrigated after a single 8 mm P irrigation, percentage seedling emergence (Table 2)
was significantly lower (P ¼ 0.005) from seeds buried at 30 mm than from those buried at 10 mm (4% versus 30%).

Discussion
Dependence of Seedling Emergence on Seed Burial Depth
Comparison of maximum emergence depth among C. korshinskii (mean seed weight
65 mg), A. bracteata (5.7 mg), and the five species with medium-sized, nonphotoblastic seeds in our previous studies (Tobe et al., 2005b; Tobe and Gao, 2007; mean
seed weight 1.0–2.8 mg,) shows that larger seeds do not always improve seedlingemergence. Despite the large difference in seed weight between C. korshinskii and
A. bracteata, we found no conspicuous difference between the two species in the
maximum seedling emergence depth. Moreover, seedling emergence from seeds buried at 40 mm was only 1% in C. korshinskii and 2% in A. bracteata in the [32 þ 1=1]
irrigation treatment (this study) versus 30% in A. squarrosum (mean seed weight
1.8 mg) in the [16 þ 1=1] irrigation treatment (Tobe et al., 2005b). Many deeply buried seeds of both C. korshinskii and A. bracteata germinated, but seedling elongation
was inhibited. Thus, ability of seedlings to emerge from deep burial in sand did notappear to be determined by seed size. The fairly large maximum seedling emergence
depth in A. squarrosum seeds is because seedling elongation in this species was lessinhibited than that of the other six species in moist sand (Tobe et al., 2005b).
The fact that seedling emergence was inhibited by the more frequent irrigation
regime in C. korshinskii and A. bracteata (Figure 2) suggests that inhibition ofDownloaded By: [Tobe, Kazuo] At: 16:39 18 September 2007

Seed Size and Seedling Emergence of Psammophytes189

seedling growth in deep sand resulted from a higher moisture content deeper in thesand, where moisture persists due to the low evaporation rate (Bowers, 1982; Jalota
and Prihar, 1986). A higher moisture content in the sand can result in an oxygen
deficiency that inhibits normal seedling growth (Crawford, 1992). Furthermore,moisture can cause hardening of sand (Hornbaker et al., 1997; Halsey and Levine,
1998), which can impede seedling growth (Collis-George and Yoganathan, 1985a,
1985b; Tobe et al., 2005a, 2005b). Thus, it would be reasonable to interpret the suppression of seedling emergence of the two species in deep sand as the result of an
oxygen deficiency and=or hardening of the sand caused by high moisture levels.
Improved seedling emergence from coarse sand compared with emergence from finesand could have resulted from a higher ratio of air to water and=or less hardening incoarse sand (Tobe et al., 2005a).
In the experiment in which seeds were buried deeply, the amount and frequency ofirrigation ([32 þ 1=1] and [32 þ 1=4]) were greater than the amount and frequency
of precipitation that occurs naturally in the field. Thus, it might be argued that conditions for seedling emergence from deep sand may be more favorable in the field thanunder our experimental conditions, because the burial depth from which seedlings canreach the surface depends on the sand moisture conditions. However, field observations indicated that although C. korshinskii seeds are buried in sand up to a depthof 200 mm, seedlings emerged only from those buried at depths of less than 50 mm(Wen, 1992 cited in Zheng et al., 2004).
On the other hand, in shallow sand rapid evaporation can cause a moisture
deficiency for seeds (Maun and Lapierre, 1986; Zhang and Maun, 1990b). Thus,in the present study suppression of germination of C. korshinskii and A. bracteata
seeds sown on the sand surface may be due primarily to moisture deficiency. Similarly, the lower percentage seedling emergence from small C. korshinskii seeds buried
at 10 mm than from those buried at 30 mm could have resulted from moisture
deficiency. In the field, where evaporation from the sand surface is higher than that
under our experimental conditions, seedling emergence from shallow depths in sandmay be lower than under our experimental conditions. Suppression of seedlingemergence of other species from shallow depths in sand has been observed in thefield (Maun and Lapierre, 1986; Zhang and Maun, 1990a, 1990b; Yanful and Maun,
1995; Huang and Gutterman, 1998). On the other hand, the fact that higher percentages of small C. korshinskii seeds appeared to have lost their viability after a single
8 mm P irrigation at a seed burial depth of 30 mm than at a depth of 10 mm indicatesthat seeds buried at shallow depths are more likely to remain viable after a precipitation event.
Dependence of Seedling Emergence on Irrigation
Larger C. korshinskii seeds evidently required a larger amount of water in a single
irrigation for seedling emergence than did smaller seeds, suggesting that seed size
can determine the fates of C. korshinskii seeds after a precipitation event. Also, when
we compared the amount of a single irrigation required for seedling emergence fromseeds buried at a depth of 10 mm among the two species in the present study and thefive medium-seeded species in our previous studies, we found a general tendency forlarger seeds to require more irrigation than for smaller seeds for seedling emergence.
For example, although seeds of C. korshinskii and A. bracteata required a single
irrigation of  16 mm P, seedlings of B. dasyphylla and A. adscensionis emergedDownloaded By: [Tobe, Kazuo] At: 16:39 18 September 2007

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following a single, 8 mm P irrigation (Tobe et al., 2005b). An increase in the amountof irrigation results in a higher moisture content in sand and persistence of moisture
for a longer period (Tobe and Gao, 2007). The requirement of a larger amount ofwater by larger rather than by smaller seeds for seedling emergence may have been
due to a larger amount of water absorbed by larger seeds, which resulted in moisture
deficiency in the sand adjacent to the seeds, thus preventing the seeds from imbibing
enough water to germinate.
However, although seed size varied greatly among C. korshinskii, A. bracteata,
and the five species with medium-sized seeds in our previous studies, there was noclear relationship among them between seed size and the amount of water required
for seedling emergence. For example, although A. squarrosum seeds are much smaller
than A. bracteata seeds and small C. korshinskii seeds, seeds in all three seed-size
categories required a 16-mm P irrigation for seedling emergence. This suggests thatinterspecific differences in seed traits other than size influenced the responses of seeds
to irrigation. In particular, rapid germination responses would favor germination insand by allowing the seeds to absorb water before it was decreased by evaporation.
The requirement of more water for seedling emergence in A. squarrosum than in theother four medium-seeded species (Tobe et al., 2005b; Tobe and Gao, 2007) may
have been due primarily to slower germination in A. squarrosum than in the otherfour species. Also, more rapid germination in C. korshinskii seeds than in those of
A. bracteata (Figure 1) may have counteracted the higher moisture requirement
for germination of C. korshinskii seeds. The results suggest that the amount of water
absorbed by seeds to germinate and rapidity of germination are crucial determinantsof the amount of irrigation required for seedling emergence from sand.
Seedling Emergence Expected in the Field Conditions
Seeds of C. korshinskii and A. bracteata tested in the present study and those of thefive species with medium-sized seeds in our previous studies germinated at temperatures that commonly occur between spring and autumn in many desert regions ofChina (Tobe et al., 2001, 2005b; Tobe and Gao, 2007). Moreover, seed dormancy
did not appear to have crucial effects on germination. Therefore, it appears that germination of these species is determined primarily by water availability, which isdetermined by the amount and frequency of precipitation and depth of seed burial.
Interspecific differences in the amount of water required for germination can resultin differences in seedling emergence.
In Shapotou (see Materials and Methods) and many other parts of deserts innorth-central China, precipitation occurs primarily in summer. Infrequent and small
amounts of precipitation from autumn to spring will not result in seedling establishment of these seven species, although some proportion of seeds (especially, those of
species with moisture-sensitive seeds) may germinate without resulting in surviving
seedlings. Seedling establishment of these species would occur only during thesummer rainy season.
The [8 þ 1=2] and [16 þ 1=1] irrigation treatments caused a high percentage of
seedlings to emerge, even from the large C. korshinskii seeds (for [8 þ 1=2], reirrigation in Table 1). Water content in sand in the summer rainy season in Shapotou
occasionally becomes more favorable for seed germination than that in sand in
the [16 þ 1=1] treatment. Thus, for C. korshinskii and A. bracteata, as well as forthe five medium-seeded species previously studied (Tobe et al., 2005b; Tobe andDownloaded By: [Tobe, Kazuo] At: 16:39 18 September 2007

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191

Gao, 2007), high percentages of seeds buried at appropriate sand depths are expected
to give rise to emergent seedlings. The large amounts of precipitation in the rainyseason will moisten sand to relatively deep layers, and moisture deep in the sand persists for longer than that in shallow sand (Bowers, 1982; Jalota and Prihar, 1986).
Thus, water availability in sand will not limit seed germination or seedling growthof these species except at shallow depths. When the nonphotoblastic seeds of theseseven species are buried deeply, most seeds of five species, including C. korshinskii
and A. bracteata, should germinate during this season without producing emergent
seedlings and thus be lost from the seed bank (Tobe et al., 2005b; Tobe and Gao,2007), whereas seeds of the other two species are expected to remain ungerminatedand viable (Tobe et al., 2005b). On the other hand, at shallow depths, where water
availability limits seed germination, some proportion of the seeds will remain ungerminated and viable, even during the rainy season and thus will germinate in the
future if they become buried more deeply. The proportion of shallowly buried seeds
that remain ungerminated depends on the amount of water they need to absorb to
germinate and on the rapidity of their germination.
Although there were some differences among the seven species in responses ofseeds and seedlings in sand, no clear seed size dependence is evident among these species in seedling emergence. For nonphotoblastic seeds, seed size does not appear to
be a strong determinant of the proportion that produces emergent seedlings under
the climatic conditions in Shapotou or in many other parts of deserts in north-central
China. On the other hand, many photoblastic seeds, for which the light requirement
for germination restricts their germination to dry shallow sand depths (Tobe et al.,2006), remain ungerminated and viable even after the rainy season. Thus, great differences in seedling emergence or seed bank behavior in desert dunes in China are
expected between species with small photoblastic seeds and those with larger nonphotoblastic seeds.

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