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Plant Ecol (2007) 191:67–75 DOI 10.1007/s11258-006-9214-4 ORIGINAL PAPER Reproductive allocation, seed dispersal and germination of Myricaria laxiflora, an endangered species in the Three Gorges Reservoir area Fang-Qing Chen Æ Zong-Qiang Xie Received: 7 February 2006 / Accepted: 11 September 2006 / Published online: 10 November 2006 Springer Science+Business Media B.V. 2006 Abstract Myricaria laxiflora is an endangered plant that grows in the flood zone along the Yangtze River in the Three Gorges area from 70 m to 155 m above sea level. To understand the spatial distribution patterns of the species and to provide information for developing conservation strategies, we used field surveys to study its seed reproduction and dispersion, and used growth chambers to study seed germination. Results showed that M. laxiflora produced many flowering branches, inflorescences and seeds. Seeds were very small and output was high although biomass allocation to reproduction was low (~4%). Reproductive allocation was strongly correlated with the biomass of stems and leaves. Seeds were dispersed either by the wind or the river current. Wind-dispersed seeds usually settled within 25 m from parent plants leading to a clumped distribution of individuals in populations. Water-dispersed seeds often landed and established on strands of firth where the fine F.-Q. Chen Z.-Q. Xie (&) The Laboratory of Quantitative Vegetation Ecology, Institute of Botany, Chinese Academy of Science, Beijing 100093, P.R. China e-mail: [email protected] F.-Q. Chen Graduate School of Chinese Academy of Science, Beijing 100049, P.R. China e-mail: [email protected] sediment and gentle sloping were available. Seedlings that emerged from water-dispersed seeds were distributed along the water flood line.The life-span of M. laxiflora seeds was about 7 days. Seeds could germinate within 24 h when they absorbed adequate amounts of water. Soil water content was a key factor limiting the establishment ability of M. laxiflora. Experiments showed that the minimum soil water content for germination to occur was 10% on sand or 17% on sandy soil substrates, and the optimal conditions were on saturated soils. The water content of sandy soils on the riverbank was lower than 10% in autumn, the dry season, and seeds were able to germinate only on sandy beaches that were intermittently inundated by the fluctuating river current. These characteristics of seed dispersal and germination limit the ability for M. laxiflora to expand its distribution. These results provide information essential for the conservation and reintroduction of this endangered species. Keywords Soil water content Biomass Riverbank Distribution Three Gorges area Introduction A large of proportion of plant species are rare and have limited distributions (Manfred et al. 2004; Mills and Schwartz 2005). For conservation 123 68 purposes, it is important to determine what traits limit a species from expanding its geographic range, prevent it from increasing in abundance, or allow it to persist even at low abundances. The geographical distribution of plant species is related to the development, dispersal ability, and germination requirements of seeds (Ford et al. 1983). Many plant species are rare and endangered just because the processes of development, dispersal, and germination of diaspore throughout are hampered or limited (Manfred et al. 2004). Understanding these processes not only will contribute to understanding the underlying mechanisms that promote a species rarity but also provide important information for species conservation. Resource acquisition and resource allocation to reproduction determine seed output. In perennial plants, the reproductive strategy typically shifts towards an increase in the proportion of resources allocated with plant age (Zhang and Jiang 2002; Pickering and Arthur 2003; Zhao et al. 2004). Seed shape and volume, an important factor for adaptation of species to their environment, determines the dispersal distance and spatial area of seed coverage (Kelly and Purvis 1993; Jakobsson and Eriksson 2000). Seed-dispersal patterns not only determine the potential area for plant recruitment to occur but also serve as a template for subsequent processes, such as competition and genetic exchange (Tilman and Kareiva 1997; Lookingbill and Zavala 2000; Joseph et al. 2005). Effective seed dispersal is influenced by spread time, spread range, recruitment ability of seeds, spatial distribution of establishment sites, and landscape structure (Horn et al. 2001; Oliver et al. 2003; Soons et al. 2004). The dispersal of seeds and other propagulum is often a complex, multi-step process. Seed-dispersal patterns can be studied by following the fate of marked seeds from their sources, using genetic markers to establish the source of dispersed seeds (Ouborg et al. 1999; He et al. 2004), or by documenting variation in seed deposition or density with distance from their source (Willson 1993). Seed-dispersal patterns also can be inferred from the distributions of seedlings (Schupp and Fuentes 1995). It is difficult to study seed dispersal 123 Plant Ecol (2007) 191:67–75 in wetlands using seed traps as flooding washes seeds out of the traps. Seedling and establishment will reflect the pattern of seed dispersal given that germination, establishment and survival are not limiting. The distribution of seedlings around their parent plants can be used to estimate seed dispersal patterns. This approach has been applied effectively in studies on seed dispersal of tree species (Houle 1982; Clark 1998). Favorable conditions for seed germination and seedling establishment differ among species, which can result in the zonation of species along environmental gradients (Boeken et al. 2004; Kunstler et al. 2004). Parolin et al. (2004) found that the combination of different adaptive characteristics of seed germination and seedling development resulted in a variety of growth strategies among trees, which could explain species distribution patterns and zonation along the flooding gradient within Amazonian floodplain systems. Studies on seed traits and seed germination can provide important information for the conservation of endemic species (Manfred et al. 2004). Myricaria laxiflora is restricted to the riverbanks of the Yangtze River in the Three Gorges area. Scattered populations have a narrow distribution (north latitude 2941¢43¢¢–3103¢57¢¢, east longitude 10658¢38¢¢–11055¢55¢¢; altitude 70–155 m) (Wang et al. 2003; Chen et al. 2005a, b). The highest water level of the Three Gorges Reservoir will reach 175 m with a new flood zone between 145 m and 175 m when the dam is completed in 2009. All the habitats of M. laxiflora will be plunged under water owing to the construction of the dam (Changjiang Water Resource Commission 1997; Wu et al. 1998; Chen et al. 2005b). Species reintroductions at the higher water mark are most likely required in order to preserve this species. The present study aims to uncover the factors that restrict the distribution of M. laxiflora and provide important information for the establishment and management in this species if it is to be reintroduced. The specific objectives are to (1) quantify plant reproductive allocation, (2) determine spatial patterns of seeddispersal, and (3) determine the factors that influence seed germination. Plant Ecol (2007) 191:67–75 Materials and methods Study species Myricaria laxiflora, a shrub species of Tamaricaceae, is 1–1.5 m in height. Its old branches are red-brown or purple-brown in color. Branches of current year are green or red-brown. Every year, after 3–6 months dormancy induced by flooding, it begins to sprout leaves and grows in September. The species usually sets flowers and fruits from October to April in the following year, depending on the time of its appearance above water. Racemes are usually terminal and 6–12 cm in length. Capsule are narrowly conical and 6–8 mm in length. Seeds are 1–1.5 mm in length. Seeds are mainly dispersed by wind, but they also can be dispersed by river water flow. Following germination will occur on sand beach in the following weeks after seeds’ disperse under favorable environmental conditions. Reproductive allocation We sampled 20 individuals of each size group in a field population distributed in Zigui County in April 2003. Plants were dug up and separated into roots, stems, leaves, and inflorescence. Plant age was determined based on its tree-ring from the stem basis. All plant material was dried to a constant mass in an oven at 60C and weighed. The proportion of the total biomass accounted for by inflorescences was used as an estimate of reproductive allocation. Seed dispersal Seedling dispersal of wind-dispersed seeds was investigated on two common riverbank landform types, convex and concave banks. Seedling abundance was determined around five parent plants on each landform type. The number of seedlings in the distribution area was counted in concentric circles around each parent plant with radial intervals of 1 m. A secondary dispersal pattern was studied by investigating the distribution of seedlings that were water dispersed and established on river bends where no parent plants were present. We investigated six contrasting sites with 69 to survey the relationship between seedling establishment and environment. The appropriate habitat for water-dispersal seed germination and seedling establishment was indicated in a sketch map to give a concept model. Seed germination Inflorescences were collected from riverbank, Zigui county, Hubei province in October. All fruits were stripped off the inflorescences manually, and then wrapped in cheesecloth. The wrapped fruits were put in a plastic bag and kept in an icebox at 5C. Mature seeds were selected for germination experiments. As seeds germinate and seedling established on riverbanks during several days following seed dispersal, the critical environmental factors limiting seed germination and seedling establishment were soil water content and soil type. A factorial experiment was designed to test the effects of soil type and soil water content on seed germination by using three soil types, eight soil water content treatments, and four replicates. The three soil types were sand substrate, sandy soil, and a clay soil. All soil materials were taken from the study area. Soil water content treatments (w/w) included 5, 9, 13, 17, 21, 25, 29, and 31%. The experimental unit consisted of 50 mature seeds placed on a plate filled with 50 g of a soil substrate treated with one of the eight soil water content treatments. Seed germination trials were performed in growth chamber in October, under 12 : 12 h light to dark cycle. Temperature was kept constant at 18C, the average temperature in field in October. Water content was adjusted every day, and germinated seeds were counted and removed on a daily basis. The experiment lasted for 7 days. One hundred seeds were placed on a sandy substrate with saturated soil water content and germinated in a culture box at 18C to study the germination dynamics of seeds over time. Germinated seeds were removed every 2 h over a 48 h period. Some collected inflorescences were kept under natural conditions (Temperature 13–23C, Humidity 75%). One hundred seeds released from fruits were germinated on a sandy substrate with saturated soil water content at 18C every day for 123 70 Plant Ecol (2007) 191:67–75 a week to observe changes in seed viability over time. 25 20 The correlation between biomass and plant age was examined by Pearson’s correlation coefficient. Simple Linear Regression Analysis with the dependent variable ‘inflorescence biomass’ and independent variables ‘stem biomass’ and ‘leaf biomass’, respectively, and Stepwise Linear Regression analysis with dependent variable ‘inflorescence biomass’ and independent variables ‘stem biomass’ and ‘leaf biomass’ were conducted. Multiple analysis of variance (MANOVA) with the dependent variable ‘seed germination’ and fixed factors ‘soil type’ and ‘soil water content’ was used first to investigate the effects of soil type, soil water content, and their interaction on seed germination. Then the effects of variables were analyzed using one-way analysis of variance (ANOVA) after In-transformation of data and means were compared with LSD test. All analyses were conducted using SPSS software (11.0). Seedling (%) Data analysis 15 10 5 0 5 10 15 20 Dispersal distance (m) Fig. 1 Dispersal distance of seedlings from parent plants. The seedlings emerged from wind dispersed seeds on convex riverbanks. All the 1-year-old seedlings in the distribution areas were counted. About 90% seedlings were distributed within a distance of 10 m from the parent plant Inflorescence biomass was significantly related to the biomass of the stem and leaves but not the root. The following forms are their correlation equations. The relationship between inflorescence biomass and leaf biomass: Results Y ¼ 0:131 þ 1:622 Xleaf Reproductive allocation The relationship between inflorescence biomass and stem biomass: Myricaria laxiflora began to flower and to fruit 1–2 months after the floodwaters receded, and many plants in the field were in flower and fruiting from October of the current year to April in the next year after the flood season (June– October). Inflorescence biomass usually accounted for about 4% of the total plant biomass (Fig. 1), however, seed production was very high and seeds were small (0.15 g per 1,000 seeds). A mature plant typically developed 5–60 flowering branches, and the length of a flowering branch ranged from 10 cm to 100 cm. At times, a flowering branch developed a raceme, which would have 20–170 fruits per raceme. Each fruit produced about 100 seeds. A flowering branch sometimes developed only a few branches but it also developed racemes so that the seed output of individual plants was consistently high (from 5 · 103 to 10 · 106). 123 Y ¼ 0:569 þ 0:0642 Xstem r ¼ 0:919; P\0:001 r ¼ 0:655; P\0:05 The relationship between inflorescence biomass and leaf biomass/stem biomass: Y ¼0:0612 þ 0:0059 Xstem þ 1:549 Xleaf r ¼0:920; P\0:001 The biomass of inflorescence, stems, leaves, and roots were positively related to plant age (Table 1). We could infer indirectly that seed output was also positively related to plant age. Plants usually began to fruit in 2-year-old plants. However, reproductive allocation did not show any positive relationships with age. Reproductive allocation reached the highest levels in 3- to 4year-old plants and then declined in 5- to 6- yearold plants. Plant Ecol (2007) 191:67–75 71 Table 1 Biomass and biomass allocation of different aged Myricaria laxiflora plants Age (year) Total mass (g) Mean ± SD Root mass (g) Mean ± SD Stem mass (g) Mean ± SD Leaf mass (g) Mean ± SD Inflorescence mass (g) Mean ± SD Reproductive allocation (%) Mean ± SD 1–2 3–4 5–6 n r 23.99 ± 10.19 56.29 ± 33.12 82.65 ± 34.67 20 0.80ns 7.23 ± 3.75 21.75 ± 21.69 38.40 ± 14.27 20 0.82* 14.59 ± 6.21 30.29 ± 11.94 39.38 ± 18.60 20 0.76* 0.85 ± 0.07 1.66 ± 1.24 1.68 ± 1.01 20 0.34ns 1.31 ± 0.55 2.59 ± 2.16 3.19 ± 1.59 20 0.45ns 5.47 ± 0.90 5.63 ± 5.10 3.77 ± 0.87 20 0.44ns Mass means the dry mass; n is the individuals; r is the Pearson’s correlation coefficients between plant age with biomass (total, root, stem, leaf inflorescence) and reproductive allocation. *The correlation is significant at P < 0.05 level; nsThe correlation is not significant at P < 0.05 level Seed dispersal Seeds were dispersed primarily by wind. Field investigations showed that wind-dispersal resulted in the distribution of seedlings in fanshaped zone. Landform played an important role in determining the seed dispersal pattern. In the riverside convex riverbanks seedling distribution zone, about 90% of the seedlings were distributed within a distance of 10 m of the parent plant (Fig. 1). To the contrary, in concave or sunken riverbanks seedling distribution zone, about 90% of the seedlings were distributed within a distance of 5 m of the parent plant. Both of the seedling distribution patterns only indicated the seed dispersal patterns around parent plant. Some seeds probably traveled too far to be followed their fate. The seed hairs on the surface of M. laxiflora seeds enhanced the ability for seeds to adhere to the soil surface and germinate. The seed hairs also helped seeds to float on the water surface and spread to new habitats over long distances when washed off the riverbank. The landscape of the riverbank influenced the establishment and resulting spatial patterns of water-dispersed seeds. Water-dispersed seeds were deposited on banks of river bends where the fine sediment and gentle sloping were available and germinated along the water flood line (Fig. 2). Seed germination MANOVA tests showed that both the effects of soil type, soil water content, and their interaction had a significant effect on seed germination Fig. 2 The distribution of seedlings germinated from water-dispersed seeds. Seedlings typically established on river bends where the fine sediment and gentle sloping were available (Table 2). Seed germination increased with an increase in soil water content and reached maximum germination rates when soils were saturated (Fig. 3). The water holding capacity of the soil and soil pore space differed among soil types, and, consequently, seed germination differed significantly among soil types. Seed germination was highest on the sand substrate and lowest on the sandy soil substrate. The minimum and optimal soil water content for seed germination also differed among soil types. Seeds began to germinate on the sand substrate at 10% soil water 123 72 Plant Ecol (2007) 191:67–75 Table 2 The effect of soil type and soil water content on germination of Myricaria laxiflora seeds Treatments Type III of squares df Mean square F Mean ± SD P Soil water content Soil type Soil water content*soil type Error Total 20277.78 8055.25 5886.31 1238.67 76356.00 7 2 14 48 72 2896.83 4027.63 420.45 25.81 112.26 156.08 16.29 23.83 ± 23.35 23.83 ± 23.35 <0.001 <0.001 <0.001 The result of MANOVA with fixed factors soil water content and soil type 60 Sandy soil Sand Caly soil 60 50 40 30 20 10 0 5 10 15 20 25 30 35 Soil water content (%) Fig. 4 Effect of soil type on seed germination (means ± SE). Seeds germinated in all three kinds of soils, and sand was the most fitted one. However, the clay soil was suitable for seed germination when soil water content ranged from 25% to 30% Discussion The seed reproduction of M. laxiflora is very high and its seeds are very small and light. High seed a a 40 b 30 20 100 c 10 0 b c c 5 10 15 20 25 30 35 Soil water content (%) Fig. 3 The effect of soil water content on the seed germination of Myricaria laxiflora seeds. Values are means of all pots exposed to each moisture level (means ± SE). One-way ANOVA with dependent variable germination percentage and fixed factor revealed that the germination percentage under treatment with different letters is different significantly at P < 0.05 level. The germination percentage increased with an increase in the soil water content but declined when soil water content was over 30% 123 Seed germination (%) Seed germination (%) a 50 70 Seed germination (%) content, and the highest germination rates occurred at 23% soil water, which was the saturated soil water content where a water film appeared on the surface of the sand substrate. The water film allowed the seed hair to cling to the substrate surface. The minimum and optimal soil water content for seed germination on the sandy soil were 17% and 26%, respectively, and 13% and 29%, respectively, for the clay soil (Fig. 4). Seed germination decreased when the water layer over the soil surface was too deep. Seeds of M. laxiflora on saturated soils began to germinate within 8 h at 18C, and germination increased sharply between 11 h and 15 h. All seeds germinated within a 24-h period (Fig. 5). Seed vitality and germination declined after 24 h since the time of released from the fruit, and seeds almost lost their ability to germinate after a week (Fig. 6). 80 60 40 20 0 8 10 12 14 16 18 20 22 24 26 Time (hr) Fig. 5 Germination of Myricaria laxiflora seeds over time on soil with saturated water content. Seeds began to germinate in 8 h and nearly all of them germinated within 24 h Plant Ecol (2007) 191:67–75 73 Seed Germination (%) 100 80 60 40 20 0 1 2 3 4 5 6 7 Time (Day) Fig. 6 The change in Myricaria laxiflora seed vitality over time following release from the parent plants output is an ecological adaptation by species that experience low seedling recruitment (Willson 1993; Parolin 2001). M. laxiflora experiences many environmental stresses during seed dispersal and germination, such as drought and flooding, and only a few of seeds survive and germinate. High seed production results in a greater chance of having a seedling successfully establish at a new site. Seed dispersal is the critical stage of a plant’s life history that allows it to move. Dispersal patterns not only dictate the potential range of new seedlings but also provide a means to access new suitable environments that ultimately determine the spatial pattern and distribution of new recruits (Harper 1977; Guariguata and Pinardc 1998; Kunstler et al. 2004). Howe and Smallwood (1982) suggested that seed dispersal has three potential benefits: escape from competition, colonization of new suitable habitat, and high probability of seedling establishment. The ideal dispersal system maximizes all three benefits, but few, if any, single means of dispersal seem capable of doing so. Some forms of seed dispersal comprise two phases, with distinctly different mechanisms (Wall and Longland 2004). Because seeds of most wetland species can float, secondary dispersal of seeds by water currents is common in wetland species (Cook 1987; Middleton 1999). Seeds of M. laxiflora are dispersed by two mechanisms, i.e., wind and water. The dispersal distance of seeds by wind is related to landform and wind strength. Most of the wind-dispersed seeds scatter within 20 m of the parent plant in a fan-shaped zone, so that natural populations of M. laxiflora seedlings have clustered distribution patterns. Secondary dispersal by the river current allows for long distance dispersal of seeds from upstream toward downstream. As a result, M. laxiflora tends to have scattered and isolated populations along the Yangtze River in the Three Gorges Reservoir (Wang et al. 2003). Secondary dispersal promotes the exchange of genes among the populations at a certain extent (Li et al. 2003). Fiedler and Ahouse (1992) classified rare species into one of several groups based on their geographic distribution and population size: (1) narrow distribution but large population sizes; (2) narrow distribution and small population sizes; and (3) wide geographic distribution but small population sizes. Conservation efforts have focused on rare species with narrow distributions, discrete occurrences, or strict habitat preferences. All rare species have specific key environmental factors that limit their distributions. For M. laxiflora, the soil water content of the site where seeds dispersed was a key factor limiting their germination and thus distribution. Seeds of M. laxiflora mature in fall, which is the dry season in the Three Gorges area, but the soil water content of most soils is lower than the minimum required for germination to occur during this time. Because the life-span of seeds is very short under natural conditions, seeds will lose their ability to germinate if suitable soil conditions are not met during this time. The restricted distribution of this species in the flood zone along the river is therefore most likely due to limitations imposed by the low soil water content of upland soils during seed dispersal and germination. All the natural habitats of M. laxiflora will be submerged, when the Three Gorges dam is completed and water levels rise to 175 m above the original river levels. The seasonal hydrological environment also will change from summer flooding with winter drought to summer drought with winter flooding (Chen et al. 2005b). The life cycle of M. laxiflora is not adapted to these reversed seasonal hydrological conditions and will not survive making in situ conservation impossible (Chen et al. 2005a). Ex-situ reconstruction of 123 74 the populations is the only viable approach to preserve the species. The flood zones above 175 m in branches of the Yangtze River, which have similar physical and ecological features of the original habitats of M. laxiflora and are not affected directly by the dam, might be suitable sites for population reconstruction. Liu et al. (2006) reported that the gene diversity in natural populations of M. laxiflora was moderate, while genetic differentiation among natural populations was significant. Natural populations should be reintroduced as many as possible in order to conserve genetic diversity of the species. Acknowledgements We would like to thank Dr. Ke-Ping Ma, Dr. Ruth E. Sherman and Dr. Zheng-Qing Li for their helpful comments on this manuscript. The Three Gorge Project Construction Committee of the State Council provided fund for this research. Two anonymous referees gave us very interesting comments to the final improvement of this work. 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