Relationships between seed fates and seedling establishment in an alpine ecosystem.
Interactions of seeds with their environments determine seedling establishment patterns and influence the structure of both plant populations and communities. Although there is increasing interest in the topic of seed fates, relatively little is known about how relationships between seed morphology and soil surface characteristics affect the microsites in which seeds come to rest. Even less is known about the consequences of these relationships for seedling establishment. In general, seeds that arrive on exposed surfaces can remain trapped in place where they initially land, they can move horizontally over the surface, or they can move vertically through the soil column (Chambers and MacMahon 1994). The type and distance of movement depends on characteristics of the surface or surfaces to which the seed is exposed (Harper et al. 1965. Sheldon 1974. Peart 1981, Reichman 1984, Chambers et al. 1991. Johnson and Fryer 1992), the abiotic or biotic forces that act upon the seed. and the interactions of the seed with its environment (Sheldon 1974, Peart 1979, Peart and Clifford 1987). The characteristics of the microsite in which the seed arrives determine the probabilities of seed survival, seed germination, and seedling emergence and survival (Chambers and MacMahon 1994). Factors that influence seeds often differ from those that affect seedlings' and the microsites in which seeds come to rest may or may not be conducive to seedling establishment (Schupp 1995).
Wind is an important dispersal vector in both arid and tundra environments, and seeds that arrive on exposed soils are often blown along until they encounter a barrier or crevice that holds them in place (Nelson and Chew 1977, Reichman 1981. Eckert et al. 1986, Chambers et al. 1990). An earlier study showed that relationships between seed morphology and soil particle size determine both the horizontal and vertical movement of seeds in exposed alpine soils (Chambers et al. 1991). At small particle sizes, small seeds and seeds with mucilaginous seed coats were trapped, but most large seeds moved horizontally across the soil surface and were not trapped. At large particle sizes, high numbers of large seeds were trapped and more seeds moved vertically through the soil column. Seeds with morphological adaptations for primary or secondary dispersal may exhibit seed entrapment patterns that differ from those in the above study. Many species. including those with adaptations for primary or secondary dispersal, have seed morphologies that facilitate incorporation of seeds into soils. The pappuses of some Asteraceae species collapse irreversibly or collapse and expand in response to humidity, thus placing the seed in closer contact with the soil surface (Sheldon 1974), hygroscopic awns twist and untwist in response to humidity and cause diaspores to move across the soil surface (Pears 1979. Peart and Clifford 1987), and mucilaginous seed coats or hairs adhere to the soil surface upon wetting (Gutterman et al. 1967, Chambers et al. 1991). Such adaptations may result in higher seed entrapment and seedling establishment over a broader range of soil surfaces than in seeds without these adaptations.
The degree to which patterns of seedling establishment actually correspond to patterns of entrapment on different soil surfaces or for seeds with varying morphologies is unknown. Although seed entrapment and retention in exposed soils largely depend upon relationships between seeds and soil surfaces, seed germination and seedling survival are determined by the biological requirements of the different life stages. In tundra environments, seed germination is species specific and is influenced by soil temperature regime, exposure to light. and exposure to cold, wet conditions that can result in stratification (Chambers 1995). Seedling growth and survival are influenced by growing season conditions. soil temperature regimes. plant and soil water relations and nutrient dynamics (Chambers 1995). Differences in seed morphology may influence seed entrapment and seedling establishment in potentially opposing ways. Small seeds exhibit high entrapment over a broader range of soil surfaces than large seeds (Chambers et al. 1991), but have lower nutrient reserves that may restrict establishment to favorable microsites (Venable and Brown 1988). In contrast, larger seeds have higher nutrient reserves that may facilitate establishment in a wider variety of microsites.
This study examined seed and seedling fates of alpine species with varying seed sizes and adaptations for primary or secondary dispersal that were sown over uniform- and mixed-particle-size soils. The questions addressed were: (1) How do relationships between seed morphology and soil particle size affect seed and seedling fates? (2) Are patterns of seedling emergence and survival related to patterns of seed entrapment?
The study was conducted on the Beartooth Plateau. Montana. USA (45 [degrees] 00' N. 109 [degrees] 30' E) above tree line in an area from which gravel had been removed [approximate] 40 yr ago. The soils were sandy loams and sparse ([is less than] 25%) vegetation cover characterized the site. During the growing season mean wind velocity measured with a three-cup anemometer (R. M. Young Company. Traverse City, Michigan) at 40 cm above the soil surface was 3.4 m/s with gusts up to 23 m/s. Growing season precipitation on the study site averaged 131 mm. Additional information about the study area is in Chambers et al. (1990).
In the experiment, seeds of eight alpine species with different seed morphological attributes were sown over six uniform-particle-size soils and three mixed-particle-size soils. Six replications of each soil were used (n = 54). Soils were collected on the Beartooth Plateau, returned to the laboratory, and sieved to obtain the proper particle sizes. The uniform particle sizes were: [is less than] 0.5, 0.5 1.0. 1.0 2.0. 2.0-4.0, 4.0-8.0, and 8.0 16.0 mm. The three mixed particle sizes were predominantly fine, coarse, or intermediate and were based on volume (Table 1). A 50 x 50 m grid was established within a homogeneous area on the study area. Sample locations were randomly positioned 5 m apart within the grid and assigned one of the nine soil types. At each sample location a seed trap was installed that consisted of a 20 cm diameter polyvinyl chloride pipe (5 cm in height) with a screen bottom. The traps were buried flush with the soil surface with minimal disturbance to the surrounding area, a paper filter was inserted, and the traps were filled with soil of the appropriate particle-size class.
Table 1. Percentage composition of the six different particle size ranges for the mixed particle size soils.
Particle size range (mm) Mixed particle 0.5- 1.0- 2.0- 4.0- 8.0 sizes [is less than]0.5 1.0 2.0 4.0 8.0 16.0 Fine 50 20 10 10 5 5 Intermediate 5 10 35 35 10 5 Coarse 5 5 10 10 20 50
Because fall seeding maximizes germination and establishment in this environment (Chambers 1995), the study was initiated in mid-September 1991. Seeds were collected on or adjacent to the site during fall 1990 and tested for viability in August 1991 using a standard tetrazolium test (Association for Official Seed Analysts 1981). Seed viability ranged from 50 to 91%. The equivalent of 10 viable seeds of each species was sown over the soils within each seed trap (e.g., if viability equaled 50%, 20 seeds were sown; if viability was 91%, 11 seeds were sown). For each particle-size class, 60 viable seeds of each species were sown (10 viable seeds x 6 replications). Inflorescences of all plants within a 2 m radius of each trap were removed to minimize additional seed input. Seeds of most low-statured plants seldom move far from the parent plant (Sheldon and Burrows 1973, Willson 1992), and this distance was considered adequate. The original soils were low in nutrients (Chambers et al. 1990). and a complete nutrient fertilizer (Miracle-Gro) was applied in dry form at a rate of 8 g/[m.sup.2] nitrogen.
The study species occurred on the study area or in adjacent Geum turf vegetation and represented a range of seed morphologies. They included forbs that have achenes or seed without appendages (Silence acaulis L., Potentilla diversifolia Lehm., and Lupinus argenteus Pursch), grasses characterized by caryopses with lemmas that have bent or twisted awns (Deschampsia cespitosa (L.) Beauv. and Agropyron scribneri Vasey). forbs that have achenes with pappuses (Antennaria lanata (Hook.) Greene and Agoseris glauca (Pursh) Raf.), and a forb with a mucilaginous seed (Polemonium viscosum Nutt). See masses, lengths, (with and without appendages), appendage lengths. and widths were quantified for each species (Table 2). An attempt was made to select a range of seed sizes within the different morphological types, but this was not always possible. Alpine species are characterized by high variability in seed viability among years (Chambers 1989). and seeds of many of the species collected in 1990 had viability too low for inclusion in the study.
Table 2. Seed morphological attributes of the study species. Mean seed masses were obtained from a composite sample of n = 100; individual lengths and widths are mean [+ or -] 1 SE; n = 20.
Length Mean with Seed morphology mass appendage species (mg) (mm) Round or oval Silene acaulis 0.034 1.1 [+ or -] 0.02 Potentilla diversifolia 0.067 1.7 [+ or -] 0.03 Lupinus argenteus 0.717 4.0 [+ or -] 0.13 Awns Deschampsia cespitosa 0.035 3.7 [+ or -] 0.07 Agropyron scribneri 0.156 23.7 [+ or -] 0.19 Pappuses Antennaria lanata 0.018 4.9 [+ or -] 0.13 Agoseris glauca 0.316 21.1 [+ or -] 0.66 Mucilaginous seed coat Polemonium viscosum 0.160 3.3 [+ or -] 0.07 Length without Maximum Seed morphology appendage width Species (mm) (mm) Round or oval Silene acaulis 1.1 [+ or -] 0.02 0.9 [+ or -] 0.03 Potentilla diversifolia 1.7 [+ or -] 0.03 1.3 [+ or -] 0.02 Lupinus argenteus 4.0 [+ or -] 0.13 3.3 [+ or -] 0.12 Awns Deschampsia cespitosa 3.2 [+ or -] 0.07 0.7 [+ or -] 0.03 Agropyron scribneri 7.7 [+ or -] 0.19 1.2 [+ or -] 0.06 Pappuses Antennaria lanata 1.2 [+ or -] 0.03 0.33 [+ or -] 0.02 Agoseris glauca 9.1 [+ or -] 0.32 1.0 [+ or -] 0.50 Mucilaginous seed coat Polemonium viscosum 3.3 [+ or -] 0.07 1.4 [+ or -] 0.00 Appendage Seed morphology length species (mm) Round or oval Silene acaulis ... Potentilla diversifolia ... Lupinus argenteus ... Awns Deschampsia cespitosa 3.9 [+ or -] 0.14 Agropyron scribneri 19.4 [+ or -] 0.70 Pappuses Antennaria lanata 3.7 [+ or -] 0.12 Agoseris glauca 12.0 [+ or -] 0.50 Mucilaginous seed coat Polemonium viscosum ...
Seedling emergence and survival within each of the seed traps were recorded by species in mid-July, mid-August, and mid-September 1992. At the end of the experiment, soils were collected from the 0-1 cm depth interval within the seed traps. Previous work in this system by my colleagues and myself showed that seed "survival" in a soil column is the result of processes that move seeds either horizontally or vertically (Chambers et al. 1991). Total seeds retraining in a soil column are equivalent to seeds that do not leave the soil surface by horizontal processes. Seeds contained in the 0-1 cm depth interval are those seeds that do not leave the surface by horizontal processes or the 01 cm depth by vertical processes. The 0-1 cm depth is the approximate depth at which seed germination and emergence can occur for many alpine species. In this study, the 0-1 cm depth was sampled because of its importance for germination and emergence. and because of its relevance to our past work.
In the laboratory, seeds remaining in the particle-size classes were extracted using a high-density salt solution (Malone 1967). Seeds were then separated from any organic matter, identified to species, and counted under a dissecting microscope. Intact, filed seeds were recorded as viable, other seeds were recorded as nonviable. The accuracy and precision of the extraction method had been tested previously for the same particle-size classes (Chambers et al. 1991). Percent recovery for all species combined in the earlier extraction test was 98.5% for the 0.5.1.0 mm particle size and 98.9% for the 1.0-2.0 mm particle size.
Differences in seed fates among species over the range of particle sizes and among particle sizes for individual species were examined with ANOVA. The variables evaluated included numbers of viable and nonviable seeds remaining in the particle-size classes, seedlings that emerged. seedlings alive or dead at the end of the growing season, and total seeds and seedlings accounted for among species and particle sizes. Prior to analyses, numbers of viable and nonviable seeds were adjusted to account for differences in initial viability among species. Data for all of the variables were expressed as a percentage of the total. Normality of the data was verified from univariate analyses (box and leaf diagrams and probability plots). Mean comparisons were made using Fisher's protected least significant difference (P [is less than] 0.05) (Steel and Torrie 1980).
Relationships between seedling emergence and survival and the total number of seeds and seedlings at the end of the study (the equivalent of seed entrapment and retention within the 0-1 cm depth interval) were examined with linear regression.
Seed fates in uniform-particle-size soils
Both the number of seeds remaining in the particle-size classes and the number of seedlings that emerged were particle-size dependent. The particle-size class with the highest number of remaining seeds increased as seed size increased (Fig. 1). The highest number of seeds remaining in the particle-size classes was in the 2 4 mm particle size for S. acaulis and P. diversifolia. In contrast, most of the larger L. argenteus seeds were in the 4-8 mm particle size. For all of the species combined, the lowest seed numbers occurred in the smallest and largest particle-size classes (ANOVA, P [is less than or equal to] 0.001; Fisher's protected LSD. P [is less than] 0.05). There were no significant differences in the number of viable relative to nonviable seeds among particle-size classes for any of the study species (ANOVA, P [is less than or equal to] 0.001).
[Figure 1 ILLUSTRATION OMITTED]
Seedling emergence over the growing season was low (3-15%), with most emergence occurring primarily in particle sizes of [is less than] 0.5 to 1-2 mm (Fig. 1). Seedlings that emerged in particle sizes of 2-4 mm or larger died. The number of individuals alive at the end of the growing season did not differ significantly among particle sizes of [is less than] 0.5 to 1-2 mm.
The total number of seeds and seedlings was dependent on seed size (mass and length): L. argenteus (large seeds) [is less than] S. acaulis (small seeds) [is less than] P. diversifolia (intermediate seeds) (ANOVA, P [is less than or equal to] 0.001; Fisher's protected LSD, P [is less than] 0.05). For most of the study species, the total number of seeds accounted for (seeds plus seedlings) reflected the number of seeds remaining because of low seedling emergence.
Seeds with appendages
Seed fates of species with specialized appendages or seed coats depended on both seed size and morphological attributes and could not be predicted from seed size alone. For the awned species, the small-seeded grass, D. cespitosa, had the highest number of seeds remaining in the 2-4 mm particle size, while the larger seeded grass, A. scribneri, had the highest number of seeds in the 4-8 mm particle size (Fig. 2). Deschampsia cespitosa had the highest overall seedling emergence, followed by P. diversifolia (ANOVA, p [is less than or equal to] 0.001: Fisher's protected LSD. P [is less than] 0.05). Seedling emergence of D. cespitosa occurred in particle sizes of [is less than] 0.5 to 2-4 mm. but was highest in the 0.5-1 and 1-2 mm particle sizes (Fig. 2). Seedling emergence of A. scribneri was low (5-11%) and occurred only in particle sizes of [is less than] 0.5 to 1-2 mm. For D. cespitosa, no seedlings survived in the 2-4 mm particle size. There was no mortality recorded for A. scribneri. The total number of seeds accounted for reflected both the seeds remaining in the particle-size classes and seedlings emerged.
[Figure 2 ILLUSTRATION OMITTED]
The species with pappuses exhibited very different patterns of entrapment and emergence. Antennaria lanata had small seeds with fluffy pappuses, and most seeds were probably moved horizontally from the soil surface soon after sowing. Only a few seeds remained in the 0-1 cm depth interval of the [is less than] 0.5, 2-4, and 4-8 mm particle sizes (Fig. 2). Agoseris glauca was a large-seeded species with large, stiff pappuses. The highest number of seeds remaining were in the 4-8 mm particle size (Fig. 2). Seedling emergence was low for both species. For A. lanata, emergence occurred only in the [is less than] 0.5 and 1-2 mm particle sizes, while for A. glauca, seedling emergence occurred in the [is less than] 0.5. 1-2, and 2-4 mm particle sizes. Few seedlings of either species survived to the end of the growing season.
The large adhesive seeds of P. viscosum had moderately high seed retention (18-35%) in all particle sizes, and there were no significant differences in seed number among particle sizes (Fig. 2). The only seedling emergence that occurred was in the [is less than] 0.5 mm particle size, and it was low (3%).
Seed fates in mixed-particle-size soils
Seeds without appendages.--In mixed-particle-size classes, seed fates depended on seed morphology and the relative proportions of fine and coarse soil particle sizes. The results for species without appendages or mucilaginous seed coats were similar to those for the uniform particle sizes. The number of remaining S. acaulis seeds was moderately high (37-61%) for all three particle sizes (Fig. 3). In contrast, for P. diversifolia, fewer seeds remained in predominantly fine particle sizes than in intermediate or coarse particle sizes. The number of L. argenteus seeds was low regardless of particle size distribution. Seedling emergence of all three species was low, ranging from 5-12% for S. acaulis, to 2-3% for L. argenteus. No detectable differences in mortality existed among particle sizes.
[Figure 3 ILLUSTRATION OMITTED]
Seeds with appendages.--The seed fates of species with specialized appendages or seed coats were also largely predictable from the results for the uniform particle sizes. Deschampsia cespitosa had higher numbers of seeds remaining in predominantly intermediate and coarse particle sizes than in fine particle sizes (Fig. 4). Seedling emergence was highly variable, and predominantly coarse particle sizes had significantly lower emergence than fine particle sizes. Numbers of seedlings alive at the end of the growing season were highest in predominantly fine particle sizes. Higher total numbers of seeds were accounted for in intermediate particle sizes than in fine or coarse particle sizes (ANOVA, P = 0.014: Fisher's protected LSD, P [is less than or equal to] 0.05). For A. scribneri the highest number of seeds accounted for were found in coarse particle sizes (ANOVA. P = 0.054): Fisher's protected LSD, P [is less than] 0.05). Seedling emergence was moderately high (17-35%), and seedling death was higher in coarse and intermediate particle sizes than in fine particle sizes (Fig. 4).
[Figure 4 ILLUSTRATION OMITTED]
The light pappuses of A. lanata again resulted in significant horizontal movement over the soil surfaces, and seeds were found only in fine particle sizes (Fig. 4). No emergence occurred in any of the three particle size distributions. The results obtained for A. glauca were highly similar to those for A. scribneri. For A. glauca, higher numbers of seeds remained in predominantly coarse-particle-size soils than in fine or intermediate particle sizes. Moderate seedling emergence occurred (5-29%) although subsequent mortality was high. Coarse particle sizes had higher total numbers of seeds and seedlings than either fine or intermediate particle sizes (ANOVA, P = 0.005; Fisher's protected LSD. P [is less than] 0.05).
As in the uniform particle sizes, P. viscosum had similar numbers of seeds retraining in the different particle-size mixtures and very low emergence (Fig. 4).
Relationships between seed entrapment and seedling emergence and survival
In the uniform particle sizes, both seedling emergence and seedling survival were significantly related to seed entrapment and retention (number of seeds and seedlings) within the potential zone of emergence (0-1 cm) for most species (Table 3). The relationships between emergence and entrapment wet-e highest for A. lanata, a species in which entrapment and emergence occurred only in small particle sizes, L. argenteus, a species with highly similar patterns of entrapment and emergence, and D. cespitosa, a species with both high entrapment and emergence. Species with low emergence, A. glauca and P. viscosum, exhibited no relationships between entrapment anti emergence. Although entrapment and survival were significantly related for several of the species, the strength of the relationships as indicated by the [R.sup.2] values was low (Table 3). High numbers of seeds were trapped in larger particle-size soils, but few seedlings of any species survived in particle sizes larger than 1-2 mm. When only those particle sizes in which most emergence or survival occurred ([is less than] 0.5 to 2-4 mm) were included in the regressions, the [R.sup.2] values were much higher for the relationship between entrapment and emergence (0.33 to 0.95) and between entrapment and survival (0.09 to 0.69) for species in which a significant relationship was found. The magnitude of the [R.sup.2] values comparing seed entrapment and seedling survival corresponded to seed mass. Values of [R.sup.2] for species with the highest to lowest mass were, respectively: L. argenteus, [R.sup.2] = 0.69; A. scribneri, [R.sup.2] = 0.56; P. diversifolia, [R.sup.2] = 0.32; D. cespitosa, [R.sup.2] = 0.45; S. acaulis, [R.sup.2] = 0.20; and A. lanata, [R.sup.2] = 0.09. Thus, a higher percentage of the total seeds trapped survived as seedlings after the first growing season for species with higher seed mass.
TABLE 3. Results of linear regressions evaluating the relationship between seedling emergence or survival and seed entrapment (seeds and seedlings) for uniform particle sizes [R.sup.2] is the coefficient of determination and P is model significance; n = 36).
Seedling Seedling emergence survival Species [R.sup.2] P [R.sup.2] P Silene acaulis .15 .018 .10 .066 Potentilla diversifolia .12 .040 .06 .160 Lupinus argenteus .44 .001 .10 .055 Agropyron scribneri .13 .033 .13 .033 Deschampsia cespitosa .31 .001 .23 .001 Antennaria lanata .88 .001 .11 .049 Agoseris glauca 0 .902 0 .878 Polemonium viscosum 0 .853 0 .853
Seed entrapment and retention in the different particle sizes depended on seed size, the presence or absence of specialized appendages or seed coats, and the nature of the appendage or seed coat. The same general patterns observed for seed entrapment and retention in the uniform-particle-size classes were apparent in the mixed-particle-size classes. Regardless of appendage Or seed coat type, large-seeded species (L. argenteus, A. glauca, and A. scribneri) had higher overall entrapment in larger particle sizes, while smaller seeded species (S. acaulis and A. lanata) had high entrapment in small and intermediate particle sizes. Species with intermediate-sized seeds (P. diversifolia and D. cespitosa) had the highest entrapment in intermediate particle sizes. This general pattern is consistent with past results (Chambers et al. 1991) and indicates that physical relationships between seeds and surface soils greatly influence seed entrapment and retention within the zone of potential emergence.
Differences in numbers of seeds trapped and seed distributions among the soil particle sizes depended on the nature of the appendage or seed coat. Species with specialized appendages or seed coats can exhibit unique patterns of seed entrapment that depend on the characteristics of the appendage, the seed's interaction with the soil surface, and the environmental conditions (Pears 1979, 1981, Peart and Clifford 1987, Stamp 1989a, b). Deschampsia cespitosa exhibited higher entrapment than any other species. This species has antrorse hairs at the caryopsis' base that can anchor seeds into soil (see Peart 1979). Precipitation can increase downward movement of seeds (van Tooren 1988), and a heavy snow soon after seeding probably increased entrapment in small particle sizes, while the antrorse hairs promoted retention. Despite low overall entrapment and retention, A. lanata also exhibited high entrapment and retention in the smallest particle sizes. This species has small seeds with light pappuses that collapse completely on moist soil but resume their original shape upon drying (see Sheldon 1974). If the seed is not incorporated into soil, the pappus can expand on drying and be moved by the next wind gust. The moist conditions soon after sowing probably facilitated entrapment of the few seeds retained in the smaller particle sizes.
The large-seeded species, A. scribneri and A. glauca, had similar entrapment patterns despite different appendage types. Agropyron scribneri has a hygroscopic awn that straightens under humid conditions and, if the seed has something to push against, the awn can propel the seed into the soil. When dry, the awn is curled and under windy conditions this can cause a circular seed movement (J. C. Chambers, personal observation). Agoseris glauca has a rigid pappus that partially collapses on moist soil. The dry pappus results in sporadic, if not circular, movement on the soil surface. Despite larger surface areas and lower mass, both A. scribneri and A. glauca exhibited higher entrapment over the range of particle sizes than the large-seeded species without an appendage. L. argenteus. The appendages of these species appear to interact with the soil surface to facilitate higher burial over the range of particle sizes.
The mucilaginous seed coats of P. viscosum resulted in a nearly uniform seed distribution among the different particle sizes. In a previous study, this species exhibited higher entrapment in intermediate than in small or large particle sizes (Chambers et al. 1991). Continual wetting and drying in these particle sizes may result in lower adhesion than in the smallest particle-size soils, and horizontal seed loss.
Depending upon seed morphology, exposure to the environment may result in progressive seed loss over time. Chambers et al. (1991) found similar total numbers of small-, intermediate-, and large-seeded species trapped in the 0-1 cm interval over a similar range of soil particle sizes. In the current study, both large- and small-seeded species were trapped in lower numbers in the 0-1 cm depth than species with intermediate-sited seeds. Although small seed size alone can promote seed entrapment and facilitate burial (Harper et al. 1970. Thompson et al. 1993), moderate seed size may result in the highest retention within the potential zone of emergence. Constant exposure to the environment apparently results in higher horizontal or vertical movement of small and large seeds over this range of particle sizes. Seed predation has not been quantified for alpine species, but seed-eating beetles and birds occur on the site (Chambers 1995) and may have further reduced seed densities of the large-seeded species.
Seed germination (emergence) and especially seedling survival depended on the ability of different-particle-size classes to meet the biological requirements of the species. Although some seedling emergence occurred in particle sizes of [is less than] 0.5 4-8 mm, almost no seedlings survived in particle sizes [is greater than] 1-2 mm. The larger particle sizes did not provide the necessary root/soil contact and/or have the nutrient- or water-holding capacities to meet the biological requirements for growth and survival. Although overall emergence did not differ among the mixed-particle-size classes, seedling survival was higher in soils with predominantly small particle sizes. These data, along with those for the uniform-particle-size soils, indicate that particle-size thresholds exist above which there are predictable declines in seedling survival, and that these thresholds are applicable over the range of seed morphologies studied.
Larger seed size (mass) often conveys an advantage for initial seedling growth and survival in stressful environments (Harper et al. 1970, Venable and Brown 1988). Survival of larger seeded species was not higher in larger particle sizes, but for those particle sizes in which significant emergence occurred ([is less than] 0.5 to 2-4 mm), seed entrapment and retention and seedling survival were more highly related for larger seeded species than for smaller seeded species. Although larger seeds were trapped in lower numbers in small particle sizes than small seeds. the number of seeds that survived the first growing season as seedlings was higher for large-than small-seeded species in these same particle sizes.
For many of the species, the highest entrapment and retention occurred in particle sizes in which they had little or no emergence. Seedling emergence was significantly related to seed entrapment in the uniform-particle-size soils only for species with reasonably high emergence, and the [R.sup.2] values were often low. Survival over the entire range of particle sizes was even more poorly related to entrapment. The differences in seed and seedling distributions observed for the various life stages can be attributed to differences in the environmental factors that influence them. Entrapment and retention depend primarily on interactions between seed morphological attributes and soil surface characteristics (Chambers et al. 1991, Johnson and Fryer 1992). In this study, small seed size, antrorse hairs, hygroscopic awns, pappuses, and mucilaginous seed coats appeared to result in higher entrapment over the range of particle sizes. Effectiveness of the different seed morphologies was mediated by continued exposure to the environment. Both precipitation and soil freezing and thawing can affect the horizontal or vertical movement of seeds in soils directly (Sheldon 1974. Watkinson 1978, van Tooren 1988) or indirectly by changing the structure of surface soils (Sheldon 1974, Young et al. 1990) and altering their susceptibility to later movement by wind or water. In contrast to seed entrapment, seedling emergence and survival are determined by relationships between species' physiological requirements and surface- or near-surface temperature, light, nutrient, and water regimes (Chambers 1995). Thus, seedling establishment depends not only on soil physical properties, but on soil chemical properties and nutrients. A relatively small range of particle sizes exists in which the necessary conditions for establishment are met.
The differences observed between seed entrapment and seedling survival have important implications for seedling establishment patterns in exposed soils. Soil surface characteristics have been shown to influence seed entrapment, and thus, seedling establishment patterns in a variety of laboratory and field situations (Harper et al. 1965, Sheldon 1974, Reichman 1981. Eckert 1986, Peart and Clifford 1987). This study indicates that landscapes with varying surface roughnesses due to differences in soil particle sizes should result in somewhat predictable seed distributions based upon relationships between seed morphology and soil particle size. However, surface roughness characteristics that promote seed entrapment, such as larger particle sizes, permit seedling establishment only if small particle sizes exist in high enough percentages to provide the proper environmental conditions. Thus, understanding species' establishment patterns in these types of environments requires knowledge of how relationships between seed morphology and soil characteristics affect entrapment patterns and of soil-particle-size thresholds for establishment.
Technical assistance was provided by Craig Biggart, John Binder, and Nolan Precce. The manuscript benefitted from the comments of Carol Augspurger, Ken Thompson. Gene Schupp, Steve Vander Wall, Jim Haefner, Jim Young, and Bob Blank. The use of trade or firm names in this paper is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service.
(1) Manuscript received 5 December 1994: revised 28 February 1995: accepted 4 March 1995.
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JEANNE C. CHAMBERS Intermountain Research Station, U.S. Department of Agriculture Forest Service, Reno, Nevada 89512 USA
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|Author:||Chambers, Jeanne C.|
|Date:||Oct 1, 1995|
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