AOBPreview originally published online on September 30, 2005
Annals of Botany 2005 96(7):1215-1223; doi:10.1093/aob/mci271
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The Relative Importance of Pre- and Post-germination Determinants for Recruitment of an Annual Plant Community on Moving Sandy Land
1 Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, 260 Donggang West Road, Lanzhou 730000, China and 2 NSW Department of Primary Industries, Wagga Wagga Agricultural Institute PMB, Wagga Wagga, NSW 2650, Australia
* For correspondence. E-mail lfengrui{at}vip.163.com
Received: 24 June 2005 Returned for revision: 18 July 2005 Accepted: 8 August 2005 Published electronically: 30 September 2005
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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• Background and Aims The relative importance of pre- and post-germination determinants for recruitment of natural plant communities is rarely explored. An annual plant community on moving sandy land was chosen for a case study. Answers to the following questions were sought: (a) Does recruitment of new individuals within the community of annual plants differ in time and space? (b) Is there spatial concordance between seed deposition, seedling emergence, survival and recruitment? (c) What are the direct and indirect effects of pre- and post-germination determinants on plant recruitment.
• Methods An integrative approach combining investigation of natural recruitment processes with regression, correlation and path analyses was adopted. Data on seed deposition and seedling recruitment were collected by monitoring the number of seeds deposited in the top 5 cm of the soil and the numbers of seedlings emerged and recruited from all annual plants at sites to a range of distances from the existing shrub Artemisia halodendron (Asteraceae) in eight compass directions for two consecutive growing seasons.
• Key Results Community-level recruitment was strongly affected by inter-annual rainfall variation and was highly site- and density-dependent. Low recruitment rate in this system was due to low emergence rate and low post-emergence survival rate. Of the pre- and post-germination determinants studied, it was the number of seedlings which emerged and the post-emergence survival rate that had the greatest direct effects on recruitment, with a combination of both variables explaining the majority of the variance (97 %) in recruitment.
• Conclusions This study suggests that post-germination determinants (emergence and survival) rather than pre-germination determinants (seed deposition) substantially determined the final pattern of recruitment. Although the density of seeds deposited did not have a significant direct effect on recruitment, it contributed to observed variation in recruitment indirectly through density-dependent emergence of seedlings.
Key words: Annual plants, community level, Horqin Sandy Land, path analysis, recruitment dynamics, recruitment success, spatial variation, seed deposition, seedling emergence, seedling survival
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Recruitment in plant populations and communities is considered a multi-phase sequential process, with the final outcome of recruitment depending on pre- and post-germination processes (Garrido et al., 2005
). In general, pre-germination processes include seed production, pre- and post-dispersal seed predation (Vander Wall and Longland, 2004), with the end product being the number of seeds reaching the ground, which can be assessed by the density of seeds deposited in the surface layer of soil. Post-germination processes include seedling emergence, post-emergence seedling survival and final establishment of new individuals (Lortie and Turkington, 2002). Seedling emergence can be considered as a process from seeds deposited in situ to emergent seedlings, with the end product being the number of seedlings. The rate of emergence success can be assessed by the proportion of seeds in the soil seed bank that emerged as seedlings over a given period (i.e. the mean number of seedlings per viable seed). The establishment of seedlings can be considered as a process from emergent seedlings to established plants, with the end product being the number of seedlings recruited. The rate of establishment success can be assessed by the proportion of seedlings that survived by the end of the growing season (i.e. the mean number of recruited plants per seedling emerged). The success of overall recruitment process relies not only on emergence success but also establishment success. The probability of recruitment success can be evaluated using the recruitment rate that is decomposed into the product of the emergence rate and the survival rate (Li et al., 2005b).
In recent years, an integrative approach by considering concomitantly pre- and post-germination processes and their causal relationships has been adopted by several studies to explore the complex mechanisms of the recruitment dynamics in plant populations or communities (Reid, 1989; Jordano and Herrera, 1995; Clark et al., 1998; Dalling et al., 1998; Harms et al., 2000; Rey and Alcantara, 2000; Wenny, 2000; Garrido et al., 2005). These studies have suggested that at population or community level different processes involved in recruitment dynamics may be affected differently by a variety of abiotic and biotic factors, revealing the complex nature of recruitment processes of natural plant communities (Kitajima and Fenner, 2000). For example, some studies have shown that post-germination processes such as seedling emergence and establishment were more vulnerable to the abiotic environment than pre-germination processes (Kitajima and Fenner, 2002; Garrido et al., 2005). Some other studies have highlighted that most recruitment processes such as seedling emergence and survival are density-dependent (Augspurger, 1983; Watkinson, 1987; Goldberg and Barton, 1992; Murray, 1994; Hanley, 1998; Goldberg et al., 2001; Lortie and Turkington, 2002) and microsite-limited (Eriksson and Ehrlén, 1992; Clark et al., 1999; Zobel et al., 2000). To have a better understanding of the complexity of the recruitment dynamics of natural plant communities, there is a need to use an integrative approach combining experimental investigation of natural recruitment processes with regression, correlation and path analyses (Garrido et al., 2005). However, such an integrative research is still limited.
The experiment was conducted on the Horqin sandy land of eastern Inner Mongolia in China, using an annual plant community which occurs here as a case study. Data on seed deposition as pre-germination determinant of recruitment, and seedling emergence and establishment as post-germination factors were collected by investigating the number of seeds deposited in the soil seed bank and the numbers of seedlings emerged and recruited at the sites to a range of distances from the target plant of the existing shrub Artemisia halodendron (Asteraceae) in eight compass directions for two consecutive growing seasons. The experiment was designed to find the answers to the following questions: (a) Does recruitment of new individuals within the community of annual plants differ in time and space? (b) Is there a spatial concordance between seed deposition, seedling emergence, survival and recruitment? (c) What are the direct and indirect effects of pre- and post-germination determinants on plant recruitment?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Study site
The study site is located in the southern part of the Horqin sandy land in eastern Inner Mongolia, China (42°55'N, 120°44'E; altitude approx. 360 m), 520 km north-east of Beijing (Li et al., 2003
). The landscape in this area is characterized by sand dunes alternating with gently undulating lowlands. The soils are sandy, light yellow and loose in structure. The main soil type is classified as an Arenosol in WRB (ISSS, ISRIC and FAO, 1998). The climate is temperate, semi-arid and continental, receiving annual average rainfall of 360 mm, with 75 % of this in the June–September period. Highly variable rainfall in the growing season resulted in unreliable seedling emergence, survival and establishment, as well as seed production in most years (Li et al., 2005b). Climatic conditions in both years of the study (2002 and 2003) were quite dry with only 58 % (207 mm) and 69 % (249 mm) of the long-term average received in 2002 and 2003, respectively. There was little rain or snow during the late autumn–early spring period and this was coupled generally with frequent strong winds. The period from November to May was therefore considered as the major wind-erosion season (Li et al., 2005a), as well as the potential dispersal season for plant seeds (diaspores) (Li et al., 2005b). The dominant wind directions over the wind-erosion season were south, south-west, north and north-west. These winds occurred at high speeds in excess of 4 m s–1 at 2 m height (the threshold wind speed at which sand movement is initiated at the study site; Zhang et al., 2004) on most days of the season.
The study area was originally a grass-dominated steppe community with sparsely distributed woody species (mainly elm Ulmus spp.). When the study commenced, the original vegetation had been substantially degraded, mainly due to the prolonged heavy grazing by livestock (Li et al., 2000). Degraded steppe is generally classified into three main forms: stabilized (light degradation), semi-stabilized (moderate degradation) and moving (severe degradation) sandy lands, which represented a realistic range of historical grazing activities and impacts (Li et al., 2000).
Experimental design
The experiment was carried out on moving sandy land that had been protected from livestock grazing for 2 years since 2000. The moving sandy land had a very low vegetation cover (<5 %), which was dominated by summer annuals and the sparsely distributed sub-shrubs A. halodendron, a native pioneer sand-stabilizing plant (Li et al., 2005c). The most abundant species in the annual plant community was Agriophyllum squarrosum (Chenopodiaceae), accounting for approx. 70 % of the total cover. The remaining species, including Setaria viridis, Eragrostis pilosa, Aristida adscensionis, Chloris virgata, Artemisia scoparia, Bassia dasyphylla, Corispermum macrocarpum, Potulaca oleracea and Salsola collina, represented 30 % of the total cover.
Six adult plants of A. halodendron which had a plant height/canopy diameter of 70/278 cm, 84/310 cm, 85/290 cm, 63/241 cm, 64/269 cm, 67/281 cm, respectively, were chosen as ‘targets plants’. Each of the these six target plants was at least 30 m away from its nearest neighbour, so that the effects of other plants on the respective seed deposition were minimized (Li et al., 2005b).
To determine the spatial variation in seed deposition around the target plant, densities of annual plant seeds deposited in the top 5 cm of soil were investigated with fixed sampling points at a range of distances from the target plant. Line transects were established along the eight compass directions (at 45° intervals, i.e. north, north-east, east, south-east, south, south-west, west, north-west) centred on each target plant. Along each transect, six sampling points were placed at distances of 0·5, 1, 2, 3, 4·5 and 6 m away from the target plant. At each sampling point, a 20 x 20-cm soil seed sample 5 cm deep was collected in the first week of April before a significant rain event and seed germination. The sampling depth used in this survey was considered to be appropriate as >70 % of seeds were in this depth in a typical Inner Mongolian steppe (Bao et al., 2000; Zhao et al., 2003). On the other hand, it was observed that almost no seedlings emerged when seeds were buried at a depth of >5 cm, but most seed within a depth of 5 cm can emerge in laboratory conditions (L. Y. Zhao, unpubl. res.). The direct germination method was used to assess the readily germinable seed bank (Gross, 1990). Details of the germination experiment can be found in Li et al. (2005b). The survey of the seed bank was repeated for two consecutive growing seasons (2002 and 2003).
To track the consequences of in situ recruitment of an annual plant community, permanent quadrats (1 x 1 m2) were established at a site close (25 cm away) to each of the six sampling points of each direction. In each quadrat, the number of seedlings which had emerged at the end of June and the number of plants recruited at the end of August over the two years (2002 and 2003) were investigated.
To analyse the spatial concordance between seed deposition, seedling emergence, survival and establishment, the recruitment process was divided into two distinct phases: seed deposition to seedling emergence; and seedling emergence to final establishment of new individuals (Lortie and Turkington, 2002). The first phase includes the process of seedling emergence, with the end product being density of seedlings emerged. The emergence success was assessed by the emergence rate, which is calculated as a percentage of the number of seedlings emerged relative to the number of seeds deposited in situ (as estimated by the density of deposited seeds that was determined from soil samples taken close to the permanent quadrats). The second phase includes the process of post-emergence seedling establishment, with the end product being density of plants recruited (final plant density). The establishment success was assessed by the survival rate, which is calculated as a percentage of the number of plants recruited relative to the number of seedlings emerged. Because the success of overall recruitment depends not only on emergence success but also establishment success, the recruitment rate was used to assess the overall recruitment success, which is decomposed into the product of emergence rate and survival rate (Li et al., 2005b).
Data analysis
Data were analysed using a general linear model of repeated-measures. The main factors (year and direction) and their interactions were examined for the responsive variables, including seed deposition (number of seeds deposited), number of seedlings emerged, emergence rate, survival rate, recruitment rate, and final plant density (number of plants recruited). Differences between the eight directions and between the two years were compared using Tukey's post-hoc tests. If a significant interaction between year and direction was detected for a responsive variable, the effect of year within directions and the effect of direction within years were analysed separately. Data for the numbers of seeds deposited, seedlings emerged and plants recruited were log-transformed, and data for rates of emergence, survival and recruitment were arcsine square root transformed prior to analysis to meet the assumptions of ANOVA (analysis of variance: Sokal and Rohlf, 1995).
To quantify the mechanisms of the recruitment dynamics in plant communities and the factors determining their spatial variation, Pearson correlation and simple and multiple regression analyses were used to study the spatial relationships between the variables involved in pre- and post-germination processes. A structural model was constructed based on the results of both correlation and regression analyses. Aside from this, path analysis was also performed to investigate the direct and indirect effects of pre- and post-germination determinants on recruitment (Garrido et al., 2005). The data were analysed for each year pooled over the eight directions and each direction pooled over the two years. All statistical analyses were conducted with MINITAB Statistical Software (MINITAB, 1994).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Effects of year and direction on seed deposition
Seed deposition density varied significantly between the two years (Table 1), with a much higher number of seeds deposited in the surface 5-cm layer in 2003 than in 2002 when the data for the eight directions were analysed together (Fig. 1A). This pattern was also observed in the north, north-west and south-west when the data were analysed separately for each direction (Fig. 2A). Although seed deposition density did not vary significantly between the eight directions when averaged across the two years, there was a significant interaction between year and direction (F7,80 = 6·77, P < 0·0001). In 2002, the number of seeds deposited was highest in the east and lowest in the north-west, whereas the number of seeds deposited was highest in the north-west and lowest in the west in 2003 (Fig. 2A).
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Effects of year and direction on post-germination recruitment processes
There were significant differences between the two years in all recruitment performance measures except for the survival rate (Table 1). Across all eight directions, the number of seedlings emerged, emergence rate, recruitment rate and final plant density were significantly higher on average in 2003 than in 2002 (Fig. 1B–D and F). In addition, there were significant interactions between year and direction in all recruitment performance measures such as the number of seedlings emerged, rates of emergence, survival and recruitment and final plant density (Table 1), indicating that the effects of year varied between directions (Fig. 2).
There were also striking differences in the recruitment performance between directions in terms of the seedling density, rates of emergence, survival and recruitment rate and final plant density, but direction effects varied between years (Table 1). Across the two years, cumulative seedling emergence over the period from late April to the end of June was highest in the east and lowest in the south-east, with a difference of 89 %. However, cumulative seedling emergence over the same period was highest in the west and lowest in the north-east in 2002 (1·3-fold difference) and highest in the north-east and lowest in the south-east in 2003 (1·3-fold difference) (Fig. 2B). Across the two years, the emergence rate was highest in the west and lowest in the south-west (1·5-fold difference). However, the emergence rate was highest in the north-west and lowest in the north-east in 2002 (3·1-fold difference) and highest in the north-east and lowest in the south-west in 2003 (3·7-fold difference) (Fig. 2D). The survival rate was highest in both the south-east and south-west and lowest in both the south and north-west when averaged across two years (96 % difference), whereas this variable was similar in the eight directions in 2002 and significantly higher in the south-west than in other directions except for the south-east in 2003 (Fig. 2E). Across two years, final plant density was significantly higher in the south-west than in other directions except for the west, east and south-east, whereas this variable was highest in the north-west and lowest in the north-east in 2002 (91 % difference) and highest in the south-west and lowest in the north-west in 2003 (3·5-fold difference) (Fig. 2C). The recruitment rate was highest in the west and lowest in the north-east when averaged over the two years (1·4-fold difference), but it was highest in the north-west and lowest in the north-east in 2002 (2·8-fold difference) and highest in the west and lowest in the north-west in 2003 (8·9-fold difference) (Fig. 2F).
Spatial concordance between seed deposition and plant recruitment
The structural model derived from the pooled data across two years (Fig. 3C) showed that the number of plants recruited was not associated with the number of seeds deposited in situ but positively correlated with the number of seedlings emerged. Similar patterns were observed in the two years, despite the between-year variability in the relationship between seedling density and final plant density between years (Fig. 3A and B). There was a positive relationship between seedling density and final plant density in 2002, but no significant relationship was found between them in 2003.
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There were significant negative relationships between the number of seeds deposited and the emergence rate and between the number of seedlings emerged and the survival rate for the two years and for the two years combined, indicating the negative density dependence of seedling emergence and survival (Fig. 3). Path analysis showed differential direct and indirect effects of pre- and post-germination determinants on recruitment (Table 2). Seedling density and survival rate had far greater direct effects on recruitment (path coefficients: 0·741 and 0·868) than did seed deposition density and emergence rate (path coefficients: 0·207 and 0·132). Path analysis also showed that seed deposition density affected recruitment indirectly through its positive effect on seedling density and negative effect on survival rate. Seedling density had an effect on recruitment indirectly through its negative effect on survival rate. Emergence rate had an effect on recruitment indirectly through its positive effect on seedling density and negative effect on survival rate (Table 2). Although patterns were similar in the two years, there were some differences in certain aspects of the recruitment dynamics between years (Table 2). Seedling density and survival rate alone explained only 47 % and 19 % of the variance in recruitment, but a multiple regression using both seedling density and survival rate as the independent variables explained 97 % of the variance in recruitment when log-transformed data for the two years were analysed together (Table 3).
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There was a tendency that the direction with the lowest seed deposition density had the highest rate of recruitment success as well as largest number of plants recruited. In contrast, the direction with the highest seed deposition density yielded the lowest rate of recruitment success and lowest number of plants recruited in both years (Fig. 2).
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DISCUSSION |
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Spatio-temporal variability in seed deposition and plant recruitment
At the community level the deposition of seeds for annual plants was highly year- and direction-specific (Table 1). The observed variation in seed deposition between years was probably due to the inter-annual variation in seed production, seed dissemination and germination, although some other factors such as pre- and post-dispersal seed predation may still have significant effects (Vander Wall and Longland, 2004
; Garrido et al., 2005). The observed between-direction variation in seed deposition could largely be attributed to the spatial variation in the frequency and speed of the prevailing winds over the dispersal season (Li et al., 2005b). The role of the wind in shaping the spatial pattern of seed distribution has been reported widely (McEvoy and Cox, 1987; Bond, 1988; Greene and Johnson, 1993; Tackenberg et al., 2003; Bullock and Moy, 2004).
Recruitment of new individuals to the community of annual plants was found to vary markedly with years, indicating that the recruitment process was strongly influenced by annual climatic conditions, consistent with several other studies suggesting a strong influence of inter-annual variation in rainfall on patterning of recruitment in plant populations for annual species in the arid and semi-arid grasslands (O'Connor and Roux, 1995; Sternberg et al., 2000; Zhang et al., 2004). In the present study, the higher numbers of seedlings emerged and recruited in 2003 could largely be associated with the high rainfall in 2003 (249 mm) compared with 207 mm in 2002. It is likely that this between-year difference in rainfall may translate into a different available soil moisture environment, leading to significant differences in emergence success and establishment success between years, as supported by the significantly higher rates of emergence and recruitment in 2003 than 2002 (Fig. 1). It is important to note that seedling emergence rate for the early season (late April to the end of June) was significantly higher in 2003 than 2002, but seedling survival rate for the subsequent season was higher in 2002 than 2003 (Fig. 1D and E), although this effect was not statistically significant (F1,80 = 2·05, P = 0·1566). This pattern arose probably because of inter-seasonal rainfall variation between years. It was observed that the soil moisture condition over the May–June period was more favourable for seedling emergence in 2003 than in 2002, while the soil moisture condition for the subsequent season was less favourable for growth and survival of juvenile seedlings in 2003 than in 2002.
There was the remarkable variation in the recruitment performance across different directions; while the among-direction variation was also found to vary significantly between years (Fig. 2), indicating year-specific effects of direction on recruitment of annual plants. This result generally agreed with the observations from other studies, suggesting that recruitment in plant populations and communities is microsite dependent (Eriksson and Ehrlén, 1992; Zobel et al., 2000; Garrido et al., 2005). Although the present study cannot explain the mechanisms of this direction-specific recruitment, one of the possible explanations could be due to heterogeneity of soil resources (water and nutrients in particular) at the sites across the different directions (Wijesinghe et al., 2005), leading to the differences in recruitment between directions. Another possible explanation for the observed direction-specific recruitment could be due to differences in competition for key resources such as water across the different direction sites, probably because of the differences in the densities of deposited seeds and emerged seedlings between directions. Such a situation is most likely to occur in a semi-arid sandy land system where the availability of soil water to plants is often limited (Li et al., 2005c). It is possible that the directions with higher seed deposition densities may have stronger competition for water during emergence of seedlings compared with those with lower seed deposition densities, suggesting greater density dependence for emergence of seedlings in the high seed density direction than in the low seed density direction. Similarly, it is expected that there was stronger density dependence for seedling survival in the high seedling density direction than in the low seedling density direction. An important consequence of this density dependence could be: (a) the direction with the highest seed density may have the lowest emergence rate, whereas the direction with the lowest seed density may have the highest emergence rate; (b) the direction with the highest seedling density may have the lowest survival rate, whereas the direction with the lowest seedling density may have the highest survival rate, as has been shown in the present study (Fig. 2).
Overall, the rate of community-level recruitment for annual plants in this moving sand land system was quite low. Only 8 % of seeds that were deposited in the top 5 cm of soil germinated, or emerged in the field and survived for long enough to be included in the seedling census at the end of the growing season. Low recruitment rate was primarily attributable to low emergence rate, as the proportion of seeds in the soil seed bank that emerged as seedlings was, on average, 18 % over the period from late April to the end of June (a major emergence period). Furthermore, low recruitment rate was also associated with low post-emergence survival rate. Approximately 46 % of the seedlings that emerged over the period from late April to the end of June had died by the end of the growing season. In the present study, the primary cause leading to mortality in juvenile seedlings was generally due to desiccation during the summer drought. Precipitation in the study area was highly variable over the growing season; hence it was not surprising that short periods of drought occurred frequently (Li et al., 2005b). A study by Li et al. (2000) at the same site showed that the rain-wetted surface layer would soon dry out rapidly when exposed to sunshine and high winds because of the low vegetation cover on moving sandy lands. It was observed that in the bare sandy land the top 7–9 cm of the soil layer wetted during a rainfall event dried out rapidly when exposed directly to the sun in the summer months (Li et al., 2005b).
The relative importance of pre- and post-germination determinants to plant recruitment
Data from the present study showed that there was no significant relationship between the number of seeds deposited in situ and the number of seedlings emerged and recruited, but the seedling density was positively correlated with the number of seedlings recruited (Fig. 3). This indicates that post-germination determinants (emergence and survival) had stronger effects on recruitment than pre-germination determinants (number of seeds deposited), which is consistent with the observation of Garrido et al. (2005) on pre- and post-germination determinants of spatial variation in recruitment in the perennial herb Helleborus foetidus (Ranunculaceae) in south-eastern Spain. They reported that the pattern of H. foetidus recruitment was little affected by the number of seeds deposited in situ, but was more strongly influenced by the number of seedlings emerged.
Path analysis was considered to be a powerful analytical tool for revealing the complexity of the recruitment dynamics of plant populations (Garrido et al., 2005). The use of path analysis in this study revealed that both seedling density and post-emergence survival rate had the greatest direct effects on the community-level recruitment and therefore were the most important determinants of recruitment success. This conclusion was also supported by the multiple regression analysis indicating that a combination of seedling density and survival rate explained 97 % of the variance in recruitment (Table 3). Path analysis also showed that, although seed deposition density did not have a significant direct effect on recruitment, it affected recruitment indirectly through its positive effect on seedling density and negative effect on emergence (Table 2). The emergence rate generally had a weaker direct effect on recruitment, but it significantly affected recruitment indirectly through its positive effect on seedling density and negative effect on survival (Table 2). In the present study, the effects of seedling density on recruitment were primarily attributed to its negative effects on survival rate, suggesting the existence of density-dependent regulation on seedling survival, which has been reported in many studies (e.g. Janzen, 1970; Augspurger, 1983; Watkinson, 1987; Hanley, 1998; Harms et al., 2000; Kitajima and Fenner, 2000). The final consequence of this density-dependent survival was that the sites best for emergence would be the worst for survival, as indicated by the present experimental data on spatial variation in seedling density and survival rate in both years (Fig. 2). There was a clear tendency that the direction with the highest seedling density had the lowest survival rate, whereas the direction with the lowest seedling density had the highest survival rate (Fig. 2B and E), similar to the observation by Garrido et al. (2005).
One of the most important findings from the present study is that the effects of pre- and post-germination determinants on recruitment varied markedly between years, suggesting that the relative importance of pre- and post-germination determinants for recruitment patterning was year-specific. Furthermore, the effects of pre- and post-germination determinants on recruitment were also found to vary with directions (data not shown), again suggesting that the relative importance of pre- and post-germination determinants on recruitment was direction-specific. These results may reflect different mechanisms of plant recruitment dynamics in each year and each direction. Further research is needed for a better insight into the mechanisms and patterns of recruitment dynamics in plant communities, by considering a range of abiotic and biotic factors that could potentially affect the different processes involved in recruitment dynamics.
In conclusion, this study has revealed important aspects of the recruitment dynamics of an annual plant community and the factors determining its spatial variation in a moving sandy land system. The recruitment process was strongly affected by inter-annual rainfall variability and was highly direction- and density-dependent. Low recruitment rate within the community of annual plants was due to a very low emergence rate and a low post-emergence survival rate, indicating that the unfavourable physical condition could be the major limitation to germination, seedling survival and establishment for annual plants in this system. It is suggested that the post-germination determinants rather than the pre-germination determinants substantially determined the final outcome of recruitment. The number of seedlings emerged and seedling survival rate were identified to be the two most important determinants of spatial variation in recruitment due to the fact that these two variables had the strongest direct effects on recruitment, and a combination of both variables explained most of the variance in recruitment. Although density of seeds deposited did not have a significant direct effect on recruitment, it contributed to observed variation in recruitment indirectly through density-dependent seedling emergence.
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ACKNOWLEDGEMENTS |
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The authors wish to thank all technicians involved in the project for their field assistance. We also thank the Naiman Station of Desertification Research, the Chinese Academy of Sciences (CAS) for providing working facilities. This study was supported by the China National Key Projects for Basic Scientific Research (TG2000048705), and the CAS International Partnership Creative Group ‘The Cold and Arid Regions Territorial Surface Process Studies’.
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LITERATURE CITED |
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