Abstract:

Degradation of the natural environment in karst area, especially in southern China, is a serious issue to be considered. It is difficult to restore vegetation through planting, as is done in other degraded ecosystems, since many karst areas are full of sheer craggy cliffs and outcrops of rock, with shallow and patchy soil. Encouraging regeneration of vegetation becomes crucial. Seeds buried in soil are one of the important sources for plant regeneration. Exploration of the soil seed bank will provide the basis for evaluating the restoration potential of degraded land, and also serve for restoration purposes. A comparison study was carried out to examine the soil seed banks of different karst vegetation types representing different degradation/restoration stages at three typical karst locations in southern Yunnan Province (23°16′-23°59′N, 104°24′-104°58′E). These vegetation types were: 1) secondary forest; 2) scrub; 3) grassland. Ten soil samples of 10 cm×10 cm×10 cm, further divided into 0-2 cm, 2-5 cm, and 5-10 cm depth fractions were taken from two 50 m transect lines with a 10 m interval for each vegetation type at each location. Soil samples were washed through a coarse sieve (4 mm) and a fine sieve (0.21 mm) and then put into a local greenhouse for germination and taxon identification.It was found that there were abundant seeds stored in karst soil of secondary forest, scrub and grassland at each location. Seed densities varied between 4 090-14 930 seeds·m－2, and the number of species identified varied between 23-35 among all of the soil seed banks. Seed density and number of species identified were higher under secondary forest, and little difference was found between that of scrub and of grassland. Within the top 0-10 cm of soil, seeds per volume decreased gradually from 0-2 cm depth layer to 2-5 cm and then 5-10 cm layer. The rate of this decrease was relatively lower under secondary forest. Herb was the richest whether counted in number of species or seed density, shrub came next, and tree and liana seeds were scarce in all soil seed banks across different vegetation types and locations. Tree and shrub species and their seed density were higher under secondary forest, but lower in grassland. A few species dominated the seed bank with higher seed density and accounted for a high proportion of the seed bank. The percentage of the three richest species exceeded 40% in all of the soil seed banks. High similarity was found between one and another soil seed bank. The similarity index between any two seed banks varied between 0.6-0.7 within the same location, and 0.4-0.6 across locations. 9.3% of the total species could be found in all of soil seed banks studied. The dominant species was common. The top three species in terms of seed number were three of Ageratum conyzoides, Conyza stricta, Eupatorium adenophorum, Emilia sonchifolia, Elsholtzia fruticosa, and Spiraea martinii var. pubescens in all studied soil seed banks. Washing soil samples through sieves encouraged the germination of seeds, especially the small herbaceous seeds, and made the karst seed bank density higher compared with other studies. Soil storage condition, mainly related to the vegetation coverage strongly influenced the depth distribution of seeds. Seed physiology, sampling time, and the fragmentation of karst vegetation were the main reasons behind the higher proportion of herb species and seeds, as well as the higher similarity between seed banks.