Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region

CATENA-01889; No of Pages 9 Catena xxx (2012) xxx–xxx

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Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region Dongli Wang a, Juying Jiao a, b,⁎, Dong Lei a, c, Ning Wang b, Huadong Du b, Yanfeng Jia b, d a

College of Resources and Environment, Northwest A&F University, Yangling 712100, Shaanxi, PR China Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources, Yangling 712100, Shaanxi, PR China Henan Building Materials Research & Design Institute Co., Ltd., Zhengzhou 450000, Henan, PR China d College of Water Conservancy, Shenyang Agricultural University, Shenyang 110866, Liaoning, PR China b c

a r t i c l e

i n f o

Article history: Received 14 August 2012 Received in revised form 3 November 2012 Accepted 5 November 2012 Available online xxxx Keywords: Erosion-resistance Seed size Seed displacement Seed redistribution Eroded slope Rainfall simulation

a b s t r a c t Seed was regarded as a limited factor of vegetation regeneration, and seed removal by water erosion may affect plant distribution on eroded slopes. This study aimed at understanding how seed morphology affects seed removal, consequently, interacts with plant distribution. Rainfall simulation experiments were conducted over 1 m2 plots with slope of 20° at intensity of 120 mm/h for 30 min, accompanying with vegetation survey of 70 sample plots from 16 eroded slopes. The seed loss ratio (SLR) of 60 species ranged 0–100%, while the seed displacement ratio (SDR) was 3.3–100% and the seed displacement distance (SDD) was 3.2–157.5 cm. Seed mass played a key role in the seed removal process, while seed shape, seed appendages, and seed ability to segregate mucilage could modify seed removal sometimes. Seed morphology resisting water erosion like big mass, extreme elongated shape, appendages, and mucilage segregation was useful for species to develop on eroded slopes. However, there was no uniform relationship between species distribution and seed removal by water erosion. Some species with seeds resisting water erosion prefer gentle slope to eroded slope, while some species with high seed removal can develop on eroded slope. Some species that distribute on eroded slope maybe mainly determined by plant strategies or soil surface characteristics. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Soil erosion is a natural geomorphic process on slopes in semiarid region, which disturbs the vegetation development and drives ecological degradation (Jiao et al., 2009; Poesen, 1987). Soil erosion influences vegetation generation and recovery not only by reducing soil water holding capacity and nutrients stored in the soil, but also by removing the seeds, seedlings, fragments of plants or even entire plants (Cerdā and Garcia-Fayos, 2002; Guerrero-Campo and Montserrat-Martí, 2000). On the other hand, vegetation was identified as the essential and available approach to control soil erosion and to recover degraded ecosystem (Xu et al., 2006). Seeds are the base of vegetation development, regeneration and recovery after disturbance (Harper, 1977). However, the failure of plant revegetation because of seed limitation is ubiquity in degraded ecosystems (Calviño-Cancela, 2007; Eriksson and Ehrlén, 1992; Herrera and Laterra, 2009). In eroded environments, seed limitation is caused not only by seed production, seed predation, but also by seed removal through water erosion. ⁎ Corresponding author at: No. 26, Xinong Road, Institute of Soil and Water Conservation, Yangling, Shaanxi 712100, PR China. Tel.: +86 13474375827; fax: +86 29 87012210. E-mail address: [email protected] (J. Jiao).

Soil erosion affecting seed removal or even vegetation had gained attention until 1994 (Chambers and MacMahon, 1994; García-Fayos et al., 1995). Previous researches have provided insight into the effect of seed morphology on seed removal. Under simulating rainfall, seed size was found the main factor affecting seed removal and the shape became important when the seed mass reach a threshold, as well as the seed removal could be modified by the presence of seed appendages or by the ability of seeds to segregate mucilage when wetted (García-Fayos and Cerdà, 1997). Some species were mainly affected by seed weight, as well as some by seed shape, or seed surface structure, or seed appendage (Jiao et al., 2011). A field experiment found that seed shape seems to be a key factor in post-dispersal movement, the flatter the seeds were, the more seed remained on the slope (Isselin-Nondedeu et al., 2006). Seed is regarded as the most important limiting factor in plant colonization (Calviño-Cancela, 2007; Eriksson and Ehrlén, 1992; Foster and Tilman, 2003), which determines the plant distribution and the vegetation composition. In eroded environment, seed scarcity caused by seed removal through water erosion maybe the most important factor to disturb species colonization on eroded slopes (Guārdia et al., 2000). In semiarid Meaiterranean, it was concluded that seed removal susceptibility related to soil erosion had the potential to affect plant communities, because the seed susceptibility to removal is lower for plants living on steep slopes than that of species living in

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Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

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communities of flat sites (García-Fayos et al., 2010). It was also deduced that seed translocation by water erosion contributed to the distribution of vegetation in the Chinese hill-gully Loess Plateau (Jiao et al., 2011). Thus, seed removal by water erosion leads to a secondary seed dispersal and seed redistribution, consequently, may affect vegetation composition and species distribution (García-Fayos et al., 1995; Han et al., 2011; Jiao et al., 2011). The Chinese Loess Plateau is located in a semiarid region, which is subjected to severe soil erosion and degradation. There were many eroded slopes that formed in the hill-gully Loess Plateau of China, with clear vertical zonations from the watershed down the slope (Zheng and Kang, 1998). Soil erosion process on different erosion zonation varied with erosion intensities and soil erosion patterns changing (Xiao et al., 2002). The geomorphic processes affect plant cover, structure and composition (Thornes, 1985). It could be assumed that species distribution on eroded slopes maybe shaped by seed removal responding to soil erosion process in the region. The related researches in the region showed that most seeds in the rainfall simulating experiment were displaced, and the degrees of seed loss and displacement varied among species (Han et al., 2011); seed germination and seedling survival seem to be more important than seed loss in determining establishment (Jiao et al., 2011). However, these results were obtained with seeds from only 16 species. The relationship between seed morphology and seed removal, and the effect of seed removal through soil erosion on species distribution should be further studied at an abundant species level. Thus, we operated a simulation rainfall experiment with seeds from 60 species and investigated plant distribution on 16 eroded slopes in the hill-gully Loess Plateau region, in order to determine the effects of seed morphology characteristics on seed removal and plant distribution. The objectives of this study were to determine whether the removal of seeds by water erosion could explain the plant distribution on the eroded slopes in the Chinese hill-gully Loess Plateau region.

2.2. Rainfall simulation experiments Rainfall simulation experiments were operated in the Rainfall Simulation Hall of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau. The lateral sprinkling automatic rainfall simulation system (Zheng and Zhao, 2004) and the experimental soil bins of 2 m × 0.5 m × 0.5 m were adopted. The experimental soil was the loess from An'sai County located in the typical hill-gully Loess Plateau. And more details about the rainfall simulator, soil bins, and soil characteristics were described by Han et al. (2011). Seeds were placed on 70 cm–150 cm (from the top down bin) of soil surface in lines which were located at 10 cm interval (Fig. 1). There were 2–3 species in a line according to seed size, and 10 seeds of each species were placed evenly in a line. The larger seeds were placed in the down line so as not to affect the runoff path. Three replications were performed for each rainfall simulation experiment at a slope gradient of 20° under the rainfall intensity of 120 mm/h with duration of 30 min, basing on the rainfall simulation experiments by Han et al. (2011). Before each rainfall simulation experiment, the rainfall intensity was calibrated for 6 min to meet the demand of rainfall homogeneity (above 80%). During the rainfall experiment, runoff samples were collected at 3 min interval. After each rainfall simulation experiment, the number of seed losses were counted to calculate the total seed loss ratio (SLR=the lost seeds/the total experimental seeds× 100%), and the lost seeds include the seeds in the runoff samples, in the sediment samples, and on the outlet of V-shaped collecting device. The number of seeds displaced from its original position was also counted to calculate the seed displacement ratio (SDR=the displaced seeds/the total experimental seeds×100%). The distance of each displaced seed from its original position was measured to obtain the seed displacement distance (SDD), and the seed displacement distance of seeds lost out of soil bin was calculated as the distance from its original position to the end of outlet of soil bin. Seed removal included both seed loss and seed displacement.

2. Materials and methods 2.3. Vegetation survey 2.1. Seed collection and seed morphology observation Vegetation survey was conducted at Zhifanggou watershed, Songjiagou watershed, and Laocaowan watershed in the hill-gully Loess Plateau during August and September in 2010. To determine plant distribution on eroded slopes in the study site, 326 quadrats of

10 cm 25 cm

15

20

0

cm

0

cm

70

cm

cm

50

Seeds from 60 species (Appendix A) were used in the experiments. The 60 plant species were widely distributed in the hill-gully Loess Plateau region. For each species, seeds were collected from at least 15 individuals during their mature periods, air-dried and stored in paper bags under laboratory conditions for rainfall simulating experiments. The seed mass (M) was measured by sensitive balance (over 1/10,000 level). In order to measure the mean seed mass accurately, we divided seeds into three categories according to seed mass. For big seeds (>100 mg), 10 groups of five seeds were weighed. For small seeds (10–100 mg), 10 groups of 10 seeds were weighed. For extremely small seeds (b 10 mg), 10 groups of 100 seeds were weighed. Seeds of Gramineae were weighed with awns and seeds of Asteraceae were weighed with pappus and hairs. The length (longest axis, L), width (intermediate axis, W) and height (shortest axis, H) were measured by slide caliper with five replicates to determine seed size. Seed volume (V= L × W × H), surface area (S= L × W), mass area ratio (S/M) and density (D= M/V) were calculated based on the mass, length, width and height of the seeds. Seed shape was characterized by the Flatness Index (FI = (L+ W)/2 H) (Poesen, 1987) and Eccentricity Index (EI = L/W) (Cerdā and Garcia-Fayos, 2002). The Flatness Index ranged from a value of 1 for spherical seeds to greater values for flat or spindle shapes. The Eccentricity Index ranged from 1 for spheres, circles and ellipsoids to values greater than 2 for spindle seed shapes (Cerdā and Garcia-Fayos, 2002). We also observed the presence of seed appendages (awns, hairs, wings, etc.) and the ability of seeds to segregate mucilage in contact with water (seeds were checked to have the ability to segregate mucilage after steeped in water for 24 h). The seed morphology of the 60 species was shown in Appendix A.

Fig. 1. Seed location in lines on the soil surface.

Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

D. Wang et al. / Catena xxx (2012) xxx–xxx

70 sample plots from 16 eroded slopes were investigated. The sample plots were selected by line transection method along eroded slope, and the sample plot had a homogeneous natural population or community. For each sample plot, 3–6 quadrats of 2 m × 2 m were selected, and the altitude, aspect and slope angle were also recorded. For each quadrat, the density, coverage and frequency of each species were observed. To avoid the influence of vegetation succession on the plant distribution, we sampled only the sites with no signs of cultivation. The erosion condition of sample plots varied widely, with the vegetation coverage of sample plots ranged from 4.8% to 50.7% and the slope gradients ranged from 3° to 70°. The locations of study site and sample plots were shown in Fig. 2.

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greater than 10% were selected when the CCA ordination diagram was produced, the species with weight of the IV less than 10% always distribute rarely and can't represent the characteristics of species distribution. Differences of the seed morphology variables on seed removal indexes were tested using one-way ANOVA analysis. Seed morphology variables in correlation with seed removal indexes were analyzed using Pearson correlation coefficient. The difference of seed removal indexes in different plant types were tested with a Monte Carlo permutation test for significance (Pb 0.05). 3. Results

2.4. Data analysis

3.1. The species composition of experimental seeds and vegetation on eroded slopes

To further understand the seed removal susceptibility, seed removal was categorized by the Hierarchical Cluster Analysis, basing on the seed loss ratio, the seed displacement ratio, and the seed displacement distance. Canonical correspondence analysis (CCA) was selected to determine the distribution of species, based on the max length of gradient which was 3.417 in Detrended Correspondence Analysis (DCA) and unimodal model was adaptive. The aspect and slope gradient were used as environmental factors to analyze the plant distribution patterns, while species importance values (IV=Relative density+Relative coverage+Relative frequency) were used as species data. The species with weight of the IV

The species of 60 experimental seeds belonged to 28 families (Appendix A). The species belonging to Leguminosae (9 species), Gramineae (7 species), Asteraceae (7 species), Rosaceae (5 species) and Ranunculaceae (4 species) occupied 53.4% of the 60 experimental species. Meanwhile, 122 species belonging to 38 families were surveyed on the eroded slopes, with the species of Asteraceae, Gramineae, Leguminosae, and Rosaceae accounting for 54.8% (Table 1). Among the 60 experimental species, perennial herbs (48.3%), shrubs (18.3%), and annual/biennial herbs (20%) were the major life forms, while these three life forms of standing vegetation on the eroded slopes totaled 85% (Table 1). Thus, the 60 experimental species can represent the

Fig. 2. The locations of study site and the sample plots.

Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

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D. Wang et al. / Catena xxx (2012) xxx–xxx

composition structure of natural vegetation composition on the eroded slopes in the hill-gully Loess Plateau region.

3.2. Seed morphological characteristics The seed morphological characteristics of 60 experimental species were extremely abundant (Appendix A). There was a wide variation in seed size and shape among species, with a range of seed mass from 0.16 mg to 357.43 mg, seed length from 0.8 mm to 19.9 mm, seed width from 0.6 mm to 11.0 mm, seed height from 0.3 mm to 10.6 mm, seed surface area from 0.98 mm2 to 126.97 mm2, special surface from 0.36 mm2/mg to 19.95 mm2/mg, seed volume from 0.48 mm3 to 1433.90 mm3, seed density from 0.02 mg/mm3 to 0.73 mg/mm3, the FI from 1.07 to 12.93 and the EI from 0.67 to 19.57. The seeds from 24 species had appendages, including awns, hairs, wings, pappus and styles. There were only two species, Dracocephalum moldavica and Linum usitatissimum, of which seeds had the ability to segregate mucilage in contact with water.

3.3. Seed removal Seed removal characteristics varied obviously among species (Appendix A). The seed loss ratio (SLR) of 60 species ranged from 0% to 100%, with seeds of 4 species (Astragalus discolor, Potentilla Chinensis, Salsola ruthenica, and Rehmannia glutinosa) were lost entirely and 5 species (Stipa grandis, Ziziphus jujuba, Cirsium leo, Prinsepia utilis, and Rosa xanthina) had no seed lost. Seeds of all species were displaced, while the seed displacement ratio (SDR) was 3.3–100% and the seed displacement distance (SDD) was 3.2–157.5 cm. The SDR of 53 species was over 80%, while that of 3 species was lower than 10% and of 4 species was 10–80%. The SDD was mainly distributed over 100 cm, occupying 55% of 60 species, while the SDD was shorter than 50 cm for 17 species (28.3%) and was 50–100 cm for 10 species (16.7%). Based on the data of SLR, SDR, and SDD, seed removal characteristics of 60 species were classified into 5 categories (Table 2) by using the Hierarchical Cluster Analysis. Seeds of 19 species in category 1 were the easiest to remove, with an average SLR, SDR, and SDD of 89.7%, 99.1% and 131.9 cm, respectively. Seeds of 15 species in category 2 were easier to remove, with an average SLR, SDR, and SDD of 51.3%, 95.3% and 114.6 cm, respectively. The main difference of these two categories was SLR. Seeds of 20 species in category 3 were harder to remove, with low SLR (11.3% in average) and short SDD (51.1 cm in average). Seeds of 3 species in category 4 and 3 species in category 5 were scarcely removed or even had none to remove. Table 1 The species composition of vegetation on eroded slopes. Family

Num. of species

Percentage (%)

Life form

Num. of species

Percentage (%)

Asteraceae Gramineae Leguminosae Rosaceae Ranunculaceae

24 17 15 11 5

19.6 13.9 12.3 9.0 4.1

76 19 19 4 2

62.3 15.6 15.6 3.3 1.6

Liliaceae Lamiaceae Umbelliferae Violaceae Others⁎

4 4 3 3 36

3.3 3.3 2.5 2.5 29.5

Perennial herb Shrub Annual herb Semi-shrub Annual/biennial herb Tree

2

1.6

⁎ There were two species in each family, such as Euphorbiaceae, Chenopodiaceae, Asclepiadaceae, Rubiaceae, Scrophulriaceae, Ulmaceae, and Polygalaceae. There was only one species in each family, such as Valerianaceae, Primulaceae, Alliaceae, Elaeagnaceae, Campanulaceae, Selaginellaceae, Gentianaceae, Oleaceae, Equisetaceae, Vitaceae, Solanaceae, Loganiaceae, Thymelaeceae, Cyperaceae, Sapindaceae, convolvulaceae, Rhamnaceae, Linaceae, Iridaceae, Boraginaceae, and Bignoniaceae.

3.4. Plant distribution The distribution of 53 species was classified into five types by CCA ordination (Fig. 3), and the species–environment correlations were 0.854 and 0.771 in axes 1 and axes 2, respectively (p = 0.002). Five species of type I were always found on the extremely steep (>45°) and south-facing slopes, with a very dry and eroded environment. The 10 species of type II were often distributed on the steeper (26°~ 45°) south-facing eroded slopes. The 18 species of type III were distributed on the gentle slopes (b25°), the top of knolls or the deposited zone along the gully thalweg, where soil erosion is weak and soil water is relatively available. The 10 species in type IV were always found on the steeper north-facing slopes (26°~ 45°), where soil water is more available. The 10 species of type V were always found on the very steep and north-facing slopes (>45°). 3.5. Effect of seed morphology on seed removal There were negative correlations between seed removal indexes and seed mass, length, width, height, surface, volume, FI, and EI, respectively. However, there were positive correlations between seed removal indexes and seed density and specific surface, respectively. The correlations between the SDR and seed mass, width, height, surface, volume reached a 0.01 significance level, respectively; while the SLR and the SDD correlation to seed length was closer than the other variables. However, there were no significant relationship between seed removal indexes and seed density, special surface, FI and EI, respectively, except the correlation between the SDD and EI which reached a 0.05 significance level (Table 3). So, the seed removal by water erosion related to seed size is stronger than the seed shape, while some species with special shape modified seed removal. However, the seed removal could not be exactly explained by seed mass or shape. Other features, such as mucilage secretion and the presence of appendages like hairs or wings affect seed removal as well. For example, the seeds of Dracocephalum moldavica and Linum usitatissimum performed resistance to be lost (SLR were 6.67% and 10%) and displaced (SDD were 7.00 cm and 49.1 cm) for the ability to segregate mucilage in contact with water. The seeds with appendages (Stipa bungeana with awn, Taraxacum mongolicum with hair, Clematis fruticosa with style, Patrinia heterophylla with wing, and Bidens tripartite with Setaias, et.al.) also had low seed removal performance values. 3.6. Effect of seed removal on plant distribution No significant differences in seed removal indexes (SLR, SDR and SDD) of the species were detected in relation to the plant distribution types (p = 0⋅630; p = 0⋅754; and p = 0.769, respectively). The average SLR (42.5%) and SDR (88.6%) of species distributed on gentle slope or flat site were lowest in five plant distribution types, while the average SLR and SDR of species distributed on steep slopes (both north-facing and south-facing) were higher than that on extreme steep slope (both north-facing and south-facing) (Table 4). Besides, the SLR of the species exhibits considerable variation (ranged from 0% to 100%) in each plant distribution types, with the exception of the species distributed on south-facing extreme steep slope (variation range was 86.7%) and north-facing steep slope (variation range was 80%). Moreover, the variation range of the SDR of the species distributed on steep north-facing slope was smaller (13.3%), compared with the ones on the other slopes (40–63.3%). The average SDD of the species of five plant distribution types were also very high (>80 cm), with the seeds of the species distributed on steep north-facing slope and extreme steep north-facing slope displaced longer than the ones on gentle slope, steep south-facing slope and extreme steep south-facing slope (Table 4). Besides, the SDD of the species distributed on gentle slopes (6.7–160 cm), extremely steep south-facing slopes (6.7–122.5 cm), steep south-facing slopes (6.7–144.3 cm) and

Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

D. Wang et al. / Catena xxx (2012) xxx–xxx

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Table 2 The removal characteristics among different seed removal categories. Cluster category

1 2 3 4 5

Num. of species

19 15 20 3 3

Percent (%)

SLR (%)

31.7 25 33.3 5 5

SDR (%)

SDD (cm)

Min

Max

Ave

Min

Max

Ave

Min

Max

Ave

66.7 30 3.3 0 0

100 66.7 26.7 6.7 0

89.7 51.3 11.3 3.3 0

93 88.3 70 36.7 3.3

100 100 100 60 10

99.1 95.3 96.17 52.2 6.67

106.7 100 18.8 3.2 9.3

157.5 134.4 86.5 24 28

131.9 114.6 51.1 11.4 9.9

extremely steep south-facing slopes (3.2–160 cm) varied greatly, while there was smaller variance for the SDD of the species distributed on steep north-facing slopes (46.0–141.5 cm).

rainfall and vegetation survey, plant distribution and seed removal characteristics can be categorized as follows: 4.1. Species distributed on gentle slope

4. Discussion

In our study, the gentle slopes were on the top of knolls or the deposited zone along the gully thalweg, where soil erosion is weak and soil water is relatively available. The 18 main species distributed on these habitats for their favorable conditions with the seed removal varied widely (SLR was 0–100%, SDR was 60–100%, SDD was 6.7– 160 cm, respectively). Among the 18 species, five species (Viola philippica, Dracocephalum moldavica, Artemisia scoparia, Artemisia hedinii, and Viola japonica) performed low seed removal for their ability of secreting mucilage and five species (Ixeridium chinense, Taraxacum mongolicum, Geranium wilfordii, Leymus scalinus, and Cirsium setosum) for their appendages, and one species (Glycyrrhiza uralensis) for its bigger mass and flatter shape. Although the seeds of these species have the mechanisms to resist removal by water erosion, these species prefer gentle slope to eroded slope. However, the other seven species, whose seeds have obviously no erosion-resistance, perform moderate or high seed removal. Thus, we conclude that species distribution on gentle slope is not related to seed removal entirely.

1.0

Seed removal is closely related to seed morphology in our study. Seed mass played a key role in the seed removal process, while seed shape, seed appendages, and seed ability to segregate mucilage when wetted can modify seed removal sometimes, which was in accordance with previous findings (Cerdā and Garcia-Fayos, 2002). Besides, we can conclude that seeds having the following morphological characteristics could resist water erosion and were useful for species to distribute on eroded slopes: seed with big mass, seed with an extremely elongated shape, seed with appendages of awns, hairs, pappus, and seed with the ability to secrete mucilage. However, our study showed that there was no uniform relationship between species distribution and seed removal through water erosion, with the seed removal varying widely in each plant distribution types, especially some species with high seed removal can develop on eroded slopes (> 25°). Basing on the results of simulating Aspect

Sda Bi

I Lse

Cf

Gs Gu Eh Gw Ah

III

Sv Dm Sco Isi

Psep Agi

Asca Cch Sbu

Cse Ic Asco Vj Csq Pb Tm

Ha

Od

Lst

Pte Ld

II

Pta Slope

Asa Mr

Vp

Sbi Sj

Psp

Sg

Mo Iso

Sa

IV

Vd

Ll

Sce Va Rk

By

V

Sch Kc Ar

Psc

Ch

-1.0

Lcu

-1.0

1.5

Fig. 3. CCA ordination diagram of species in relation to the environmental gradient. (TypeI: Bi—Bothriochloa ischcemum, Sda—Sophora davidii, Cf—Clematis fruticosa, Agi- Artemisia giraldii, Pse—Periploca sepium. Type II: Sbu—Stipa bungeana, Cch—Cleistogenes chinensis, Asca—Astragalus scaberrimus, Lst—Linum stelleroides, Ld—Lespedeza davurica, Ha— Heteropappus altaicus, Pta—Potentilla tanacetifolia, Pte—Polygala tenuifolia, Od—Oxytropis discolor, Asa—Artemisia sacrorum. Type III: Gs—Gueldenstaedtia stenophylla, Cse—Cirsium setosum, Gu—Glycyrrhiza uralensis, Eh—Euphorbia humifusa, Pb—Potentilla bifurca, Ic—Ixeridium chinense, Asco—Artemisia scoparia, Vj—Viola japonica, Gw—Geranium wilfordii, Ah— Artemisia hedinii, Sv—Setaria viridis, Csq—Cleistogenes squarrosa, Isi—Incarvillea sinensis, Vp—Viola philippica, Tm—Taraxacum mongolicum, Sco—Salsola collina, Dm—Dracocephalum moldavica, Lse—Leymus scalinus. Type IV: Iso—Ixeris sonchifolia, Va—Vicia amoena, Rk—Roegneria kamoji, Sa—Scorzonera austriaca, Ch—Cleistogenes hancei, Lcu—Lespedeza cuneata, Mo—Melilotus officinalis, Psp—Poa sphondylodes, Sj—Saussurea japonica, Sce—Serratula centauroides, Type V: By—Bupleurum yinchowense, Mr- Melica radula, Sg—Stipa grandis, Vd— Viola dissecta, Psc—Patrinia scabiosaefolia, Sbi—Swertia bimaculata, Kc—Koeleria cristata, Ar—Allium ramosum, Sch—Siphonostegia chinensis, Ll—Leontopodium leontopodioides,).

Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

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Table 3 Seed morphology variables in correlation with seed removal indexes.

SLR SDR SDD

Mass

Length

Width

Height

Surface

Volume

Density

SS

FI

EI

−0.306* −0.836** −0.349**

−0.500** −0.464** −0.523**

−0.369** −0.732** −0.320*

−0.371** −0.788** −0.385**

−0.410** −0.811** −0.423**

−0.311* −0.799** −0.351**

0.145 0.026 0.096

0.203 0.252 0.181

−0.121 −0.144 −0.117

−0.233 −0.030 −0.315*

Significance level notations: *P = 0.05; **P = 0.01.

4.2. Species with low seed removal distributed on eroded slope The species of Artemisia Giralaii, Artemisia sacrorum, C. fruticosa, S. bungeana, Roegneria kamoji, S. grandis, Linum stelleroides, Ixeris sonchifolia, and P. heterophylla can colonize on eroded slopes, which maybe mainly because of their seed morphologies of erosionresistance. The seeds of A. Giralaii, L. stelleroides and A. sacrorum can resist removal by secreting mucilage when wetted, while seeds of S. bungeana, R. kamoji and S. grandis can retain on the eroded slopes by increasing the friction through the awns, seeds of C. fruticosa through styles and seeds of I. sonchifolia through hairs as well. Seed of P. heterophylla is structured with a plane surface of wing and a globe on it, whose special structure was helpful to balance the jointing edge when splashed by raindrops and made seed unremovable despite its small mass. For these species, their distribution on eroded slope is related to its low seed removal because of their special seed morphologies. 4.3. Species with moderate seed removal distributed on eroded slope The Sophora davidii can distributed on the extreme eroded slopes, although the seeds of S. davidii were removed by water erosion moderately (SLR= 56.7%). The S. davidii with hard seeds can reduce the risk of drought and erosion for its low and slow seed germination strategies (Zhang et al., 2010), and the Vicia amoena has the same strategy to adapt to the eroded environment. Besides, the seedlings of S. davidii have a strong resistance to be destroyed and removed by raindrop, runoff or sediment (experimented in 2010, unpublished), and have a strong capacity to survive on extreme eroded slope. 4.4. Species with high seed removal distributed on eroded slope In our simulating rainfall experiments, the seeds of Bothriochloa ischaemun, Heteropappus altaicus, Astragalus scaberrimus, Oxytropis discolor, Potentilla tanacetifolia, Lespedeza davurica, Cleistogenes chinensis, Poa sphondylodes, Serratula centauroides, Saussurea japonica, Cleistogenes hancei, Melilotus officinalis, Lespedeza cuneata, Siphonostegia Chinense, Swertia bimaculata, Koeleria cristata and Periploca sepium were easily lost (the SLR ranging from 83.3% to 100%), while the species can inhabit on the eroded slopes. The B. ischaemun, C. chinensis, P. sphondylodes and C. hancei are the main perennial herbs on eroded slopes or even severe eroded slopes, especially the B. ischaemun has a large coverage and a high occurrence. The seeds of B. ischaemun with large production can inform soil seed bank in the area (Wang et al., 2011b), and germinate quickly to seize

the occasion to establish. The H. altaicus, L. davurica, and Potentilla tanacetifolia also have a high occurrence, and owing to the fact that these species also can produce numerous seeds (surveyed in 2011, unpublished), correspondingly, have a certain scale of soil seed bank (Jiao et al., 2011; Wang et al., 2011a). The species of small seeds can increase the chance to inhabit by producing large numbers, informing a large soil seed bank or being easily dispersed by wind (Leishman et al., 2000; Moles et al., 2004; Thompson et al., 1993; Venable and Brown, 1988), as well small seed can germinate quickly to increase survival (Wang et al., 2007; Zhang et al., 2010). All of these strategies aid the species of S. chinense, S. bimaculata, Melica scabrosa and Bupleurum scorzonerifolium to colonize an appropriate site on severe eroded slope. Besides, the P. Chinensis and S. bimaculata have large seedling density and the B. ischaemun has a strong vegetative propagation capacity (Wang et al., 2010), which strengthen the plant to inhabit on the slopes with severe erosion. Thus, species distribution on eroded slope was determined by other plant strategies (including seed production, seed dispersal, soil seed bank, seed germination, seedling establishment, and species reproductive allocation, etc) rather than by seed removal. In addition, seeds are not easily eroded in the fields, which is first evidenced on badland landscapes in southeast Spain (seed loss rate is 0.48%–27.35%) (Cerdā and Garcia-Fayos, 1997). On one hand, the awns, hairs, pappus and other structures on the seed surfaces are useful for seed to resist against erosion. On the other hand, the slope characteristics determine the seed redistribution and plant distribution by changing overland flow hydrological parameters (Cerdā, 1997; Cerdā and Garcia-Fayos, 1997). The patchy distribution of the vegetation related to the plant distribution in the slopes for the accumulation of water, sediments, litter, nutrients and seeds on the tussock (Cerdā, 1997). In the Chinese Loess Plateau, patched and banded vegetation, a population of B. ischaemun, or A. giralaii, or Artemisia gmelinii, or S. bungeana, is developed on eroded slope and is accompanied with other species, which is proved by the fact that some species with high seed removal like Polygala tenuifolia can often be found in the clumps of some dominant species (observed in the field, 2010). Depressions and mounds created by animal may trap seed as well, for example, hoof prints strongly reduced the traveled distance of post-dispersed seeds (Isselin-Nondedeu et al., 2006). Main companion species of A. scaberrimus (SDD was 144.3 cm) and O. discolor (SDD was 137.5 cm) were mainly distributed on south-facing steep slope, which maybe because of seed trapping. Therefore, plant distribution can't be explained only by seed removal through water erosion on bare slope and plant life history strategies, but is determined by the eroded slope conditions. 5. Conclusion

Table 4 The seed removal indexes in different plant distribution types. Plant Num. SLR (%) distribution of Min Max types species I II III IV V

5 10 18 10 10

0 86.7 0 100 0 100 16.7 96.7 0 100

SDR (%)

SDD (cm)

Ave

Min

Max Ave

Min

46.9 61.8 42.5 68.0 58.7

60 60 60 86.7 36.7

100 100 100 100 100

Max

Ave

91.3 6.7 122.5 81.6 93.3 6.7 144.3 98.7 88.6 6.7 160 83.3 97.7 46.0 141.5 111.1 91.0 3.2 160 112

In conclusion, seed size plays an important role in determining seed removal, while the seed shape, seed appendages, and seed ability to segregate mucilage when wetted can modify seed removal sometimes. Besides, seed morphology of erosion-resistance like big mass, extreme elongated shape, appendages like awn, hair, pappus, and mucilage segregation are useful for species to develop on eroded slopes. However, there was no uniform relationship between species distribution through seed removal by water erosion, while plant strategies played key role in species distribution on eroded slopes as

Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

D. Wang et al. / Catena xxx (2012) xxx–xxx

well. Some species can colonize on eroded slopes with large production, wide seed dispersal, seedlings survival, and strong vegetative propagation. In addition, plant distribution is also determined by the patched distribution of vegetation, the depressions and mounds, and the hoof prints on eroded slopes, which influence seed redistribution, then plant distribution as well.

7

Acknowledgments We thank the Key NSFC project (41030532), the Knowledge Innovation Program of the Chinese Academy of Sciences (KZCX2-EW-406) for funding this research, and acknowledge the assistance of the Rainfall Simulation Hall of the State Key Laboratory of Soil Erosion and Dryland

Appendix A. Seed morphological characteristics and seed removal characteristics of 60 experimental species. Appendage

SLR (%)

SDR (%)

SDD (cm)

A

14.11

3.2± 0.2

4.7 ±0.2

1.9±0.07

15.10

28.72

0.49

1.07

2.08

0.67

None

30.0

93.3

100.1

SS

2.13

3.2± 0.2

1.8 ±0.05

1.2±0.04

5.73

6.63

0.32

2.69

2.17

1.83

None

86.7

93.3

129.0

PH

7.47

3.1± 0.09

3.6 ±0.09

2.1±0.1

11.13

23.00

0.33

1.49

1.62

0.85

None

16.7

70.0

60.3

PH

1.66

1.5± 0.05

2.2 ±0.02

0.7±0.03

3.42

2.33

0.72

2.06

2.77

0.68

None

100.0

100.0

144.3

23.77

3.1± 0.04

4.0 ±0.09

3 ±0.05

12.23

36.11

0.66

0.52

1.20

0.77

None

56.7

96.7

106.0

PH

1.45

1.5± 0.06

1.9 ±0.08

0.8±0.05

2.92

2.38

0.61

2.01

2.11

0.81

None

86.7

100.0

151.7

PH

1.34

1.5± 0.03

1.7 ±0.06

0.8±0.03

2.63

2.03

0.66

1.96

2.10

0.87

None

96.7

100.0

137.5

12.22 2.28

2.6± 0.08 3.0± 0.31

3.2 ±0.2 1.8 ±0.2

2.5±0.06 1.5±0.1

8.50 5.40

21.54 8.08

0.57 0.28

0.70 2.37

1.16 1.60

0.82 1.62

None None

53.3 96.7

86.7 100.0

113.1 141.5

PH

0.43

2.0± 0.09

0.7 ±0.03

0.5±0.04

1.48

0.69

0.62

3.42

2.90

2.67

Awn

83.3

100.0

122.5

PH

1.68

5.3± 0.2

0.9 ±0.05

0.7±0.02

4.79

3.18

0.53

2.85

4.65

5.78

Awn

3.3

93.3

18.8

PH PH

8.08 3.28

15.8± 0.5 10.1± 0.3

1.1 ±0.03 1.8 ±0.08

1 ±0.05 0.9±0.06

16.87 18.16

16.16 16.45

0.50 0.20

2.09 5.53

8.82 6.55

14.85 5.58

Awn Awn

0.0 16.7

36.7 100.0

3.2 46.0

AH PH

0.66 0.32

1.9± 0.04 5.0± 0.3

1.1 ±0.04 0.9 ±0.1

0.8±0.02 0.8±0.1

1.99 4.42

1.56 3.65

0.42 0.09

3.02 13.80

1.88 3.56

1.76 5.67

None Awn

90.0 86.7

100.0 100.0

157.5 117.8

PH

0.16

3.1± 0.2

0.6 ±0.05

0.3±0.02

1.81

0.48

0.33

11.33

6.99

5.39

Hair

90.0

100.0

119.3

PH

0.39

2.3± 0.2

1.4 ±0.06

0.6±0.07

3.24

1.95

0.20

8.35

3.08

1.60

Pappus

83.3

100.0

120.9

PH

6.09

6.1± 0.1

2.1 ±0.05

1.3±0.05

12.87

16.92

0.36

2.12

3.12

2.87

Hair

33.3

100.0

118.6

PH

1.61

2.8± 0.04

1.2 ±0.03

0.8±0.03

3.49

2.75

0.59

2.16

2.56

2.25

Pappus

93.3

100.0

120.8

AH

5.22

1 ±0.06

0.8±0.04

20.20

16.32

0.32

3.87

12.93

19.57

Setaias

3.3

100.0

37.6

PH PH

13.78 0.79

5.4± 0.1 3.9± 0.09

2.5 ±0.16 1.3 ±0.05

2.2±0.1 0.9±0.04

13.54 5.24

29.14 4.67

0.47 0.17

0.98 6.63

1.83 2.95

2.13 2.95

Pappus Hair

0.0 13.3

100.0 100.0

22.6 48.3

PH

0.34

2.6± 0.7

0.8 ±0.2

0.4±0.2

2.05

0.91

0.38

6.02

3.80

3.22

Hair

16.7

96.7

69.7

A

10.50

5.4± 0.4

3.2 ±0.08

1.7±0.08

17.33

29.75

0.35

1.65

2.51

1.67

None

23.3

100.0

85.4

S

335.13

12.2± 0.6

9.9 ±0.2

9.5±0.2

120.28

1143.60

0.29

0.36

1.16

1.24

None

0.0

3.3

28.0

SS

49.11

8.5± 0.5

7.9 ±0.3

7.6±0.2

67.42

509.58

0.02

6.87

1.09

1.07

None

3.3

60.0

24.0

PH

0.23

0.8± 0.1

1.2 ±0.02

0.7±0.04

0.98

0.66

0.35

4.20

1.49

0.69

None

100.0

100.0

146.0

151.36

8.5± 0.3

7.4 ±0.3

4.7±0.2

62.70

294.55

0.51

0.41

1.69

1.15

None

0.0

10.0

9.3

Rosaceae

Ranunculaceae

Asclepiadaceae

Oleaceae

S

19.9± 0.4

S (mm2)

EI

Robinia psendoacacia Lespedeza davurica Glycyrrhiza uralensis Astragalus scaberrimus Sophora davidii Astragalus adsurgens Oxytropis discolor Vicia sepium Melilotus suaveolens Bothriochloa ischaemun Stipa bungeana Stipa grandis Roegneria kamoji Setaria viridis Cleistogenes chinensis Phragmites communis Heteropappus altaicus Circium japonicum Saussurea japonica Bidens tripartita Cirsium leo Taraxacum mongolicum Ixeris Chinensis Pyrus betulaefolia Rosa xanthina Cotoneaster multiflorus Potentilla Chinensis Prinsepia utilis Pulsatilla chinensis Clematis fruticosa Clematis aethusifolia Thalictrum aquilegifolium Cynanchum thesioides Periploca sepium Syringa oblata Forsythia suspensa

Asteraceae

H (mm)

FI

Leguminosae

Gramineae

W (mm)

SS (mm2/mg)

Life form

PH ABH

L (mm)

D (mg/mm3)

Species

S

M (mg)

V (mm3)

Family

PH

2.09

4.4± 0.07

1.1 ±0.04

0.9±0.01

4.80

4.52

0.46

2.30

2.90

3.94

Style

13.3

100.0

43.5

SL

3.28

5.8± 0.3

2.9 ±0.21

1.0±0.05

16.43

16.79

0.20

5.00

4.21

2.02

Style

6.7

100.0

53.1

PH

2.67

3.8± 0.1

2.6 ±0.06

1.0±0.08

9.71

9.87

0.27

3.63

3.13

1.49

Style

6.7

100.0

42.4

PH

1.38

5.7± 0.3

1.6 ±0.03

1.2±0.05

9.42

11.38

0.12

6.81

3.05

3.47

None

60.0

100.0

133.7

PH

7.29

7.9± 0.3

4.8 ±0.2

1.0±0.03

38.39

37.55

0.19

5.27

6.53

1.64

Hair

26.7

100.0

54.8

S

5.51

8.2± 0.07

1.8 ±0.05

0.9±0.01

14.84

12.64

0.44

2.70

5.85

4.48

Hair

86.7

100.0

119.8

S

4.53

9.3± 0.5

2.4 ±0.1

1.0±0.06

22.85

22.43

0.20

5.04

6.00

3.81

Wing

26.7

100.0

77.3

S

3.97

6.8± 0.2

2.3 ±0.09

1.0±0.03

15.97

16.42

0.24

4.02

4.47

2.94

None

43.3

100.0

126.9

(continued on next page)

Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003

8

D. Wang et al. / Catena xxx (2012) xxx–xxx (continued)A (continued) Appendix Family

Species

Life form

Liliaceae

Asparagus cochinchinensis Allium chrysanthum Corispermum hyssopifolium Salsola ruthenica Dracocephalum moldavica* Leonurus artemisia Patrinia heterophylla Crotalaria sessiliflora Hippophae rhamnoides Ostryopsis davidiana Ailanthus altissima Erodium stephanianum Rhus typhina Radix Rudix Lonicera ferdinandii Raphanus raphanistrum Ziziphus jujuba Berberis virgetorum Rehmannia glutinosa Linum usitatissimum* Polygala tenuifolia Lappula myosotis Incarvilea sinensis Platycladus melitoloides

PH

12.03

3.0± 0.04

2.7 ±0.05

2.1±0.08

7.99

16.57

0.73

0.66

1.36

1.10

None

PH

2.98

3.1± 0.05

2.2 ±0.04

1.1±0.03

6.92

7.67

0.39

2.32

2.41

1.43

AH

2.01

3.2± 0.03

1.7 ±0.04

0.7±0.01

5.56

3.78

0.53

2.77

3.63

AH

1.33

2.6± 0.04

1.6 ±0.05

1.2±0.07

4.33

5.06

0.26

3.24

AH

1.20

2.6± 0.05

1.5 ±0.04

1.0±0.03

3.93

3.88

0.31

ABH

1.08

2.4± 0.04

1.4 ±0.03

0.9±0.05

3.44

3.10

PH

0.81

2.2± 0.03

1.2 ±0.02

1.1±0.04

2.59

AH

2.53

5.3± 0.3

4.5 ±0.4

0.6±0.04

S

6.25

2.8± 0.1

2.1 ±0.03

S

11.51

6.5± 0.3

A

10.70

ABH

Chenopodiaceae

Lamiaceae

Valerianaceae Papilionaceae Elaeagnaceae Betulaceae Simarubaceae Geraniaceae Anacardiaceae Rubiaceae Caprifoliaceae Umbelliferae Rhamnaceae Rberidaceae Scrophulariae Linaceae Polygalaceae Boraginaceae Bignoniaceae Cupressaceae

M (mg)

L (mm)

W (mm)

H (mm)

S (mm2)

SDR (%)

SDD (cm)

50.0

100.0

102.2

None

50.0

100.0

134.4

1.84

None

50.0

83.3

126.7

1.83

1.62

perianth

100.0

100.0

156.0

3.28

2.08

1.71

None

6.7

60.0

7.0

0.35

3.18

2.13

1.72

None

90.0

100.0

112.5

2.88

0.28

3.20

1.53

1.94

Wing

6.7

100.0

67.4

23.76

2.88

0.18

9.39

8.46

1.18

None

66.7

100.0

148.5

1.8±0.03

5.98

10.68

0.59

0.96

1.38

1.31

None

3.3

93.3

36.6

4.4 ±0.07

2.6±0.06

28.25

73.90

0.16

2.46

2.07

1.49

None

13.3

93.3

86.5

5.2± 0.1

3.6 ±0.1

1.5±0.04

18.52

27.07

0.40

1.73

2.99

1.44

None

76.7

96.7

138.0

9.03

8.3± 0.2

1.5 ±0.03

1.5±0.02

12.67

18.48

0.49

1.40

3.36

5.37

Awn

3.3

100.0

19.4

A PH S

8.18 9.67 3.85

3.1± 0.02 3.1± 0.08 3.9± 0.1

2.4 ±0.1 3 ±0.09 2.9 ±0.2

1.7±0.04 2.1±0.06 1.1±0.07

7.44 9.29 11.15

12.66 19.23 12.18

0.65 0.50 0.32

0.91 0.96 2.90

1.61 1.47 3.09

1.26 1.03 1.36

None None None

63.3 13.3 3.3

100.0 90.0 100.0

107.8 71.9 31.9

BH

1.18

3.3± 0.1

2.6 ±0.04

0.8±0.04

8.48

6.97

0.17

7.19

3.57

1.30

None

53.3

96.7

108.9

S

357.43

11.5± 0.2

11 ±0.3

126.97

1343.90

0.27

0.36

1.07

1.05

None

0.0

6.7

22.5

S

7.86

4.5± 0.04

2 ±0.04

1.5±0.02

9.27

13.86

0.57

1.18

2.20

2.22

None

86.7

93.3

116.7

PH

0.15

1.3± 0.03

0.9 ±0.03

0.8±0.03

1.21

0.99

0.16

7.87

1.37

1.48

None

100.0

100.0

106.0

AH

0.85

2.7± 0.1

1.4 ±0.08

0.7±0.06

3.94

2.62

0.32

4.64

3.13

1.88

None

10.0

86.7

49.1

PH

2.72

2.9± 0

2.1 ±0.05

1.4±0.06

6.08

8.66

0.31

2.24

1.76

1.41

None

50.0

100.0

117.0

ABH

2.76

3.9± 0.2

2.3 ±0.1

1.4±0.2

9.12

12.47

0.22

3.31

2.28

1.67

Setaias

66.7

83.3

102.3

PH

0.58

4.2± 0.1

2.7 ±0.1

0.5±0.04

11.48

5.72

0.10

19.95

6.97

1.56

Wing

56.7

100.0

120.5

13.41

5.3± 0.4

3.2 ±0.1

2.5±0.07

16.64

40.85

0.33

1.24

1.72

1.65

None

53.3

90.0

100.0

A

10.6 ±0.2

V (mm3)

D (mg/mm3)

SS (mm2/mg)

FI

EI

Appendage

SLR (%)

Note: *—Seed can segregate mucilage in contact with water.

Farming on the Loess Plateau and Ansai Ecological Experimental Station of Soil and Water Conservation, CAS.

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Please cite this article as: Wang, D., et al., Effects of seed morphology on seed removal and plant distribution in the Chinese hill-gully Loess Plateau region, Catena (2012), http://dx.doi.org/10.1016/j.catena.2012.11.003