Spatial heterogeneity of the soil seed bank in the tropical semi ...

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80 SOIL SEED BANK AT WAGOMUWA NATIONAL PARKespécies de capim, sementes de algumas espécies florestais pioneiras e espécies infestantesagrícolas. Estes bancos seminais no solo são muito heterogéneos mostrando frequentesagrupamentos de sementes. Esta heterogeneidade afecta fortemente a composição da vegetaçãolocal, a estação de frutificação e os padrões de dispersão de sementes. As sementes de espéciesflorestais lenhosas dificilmente se encontravam na pastagem implicando que a invasão dasflorestas nas pastagens não ocorre espontaneamente. Já as sementes das espécies da pastagemeram frequentemente dispersadas na interior da floresta principalmente pelos herbívoros. Se afloresta, ou as suas margens, forem disturbadas, estas sementes germinarão e estabelecer-seão,e de que resultará uma redução do coberto arbóreo e, subsequentemente, a expansão dapastagem.Key words: Proximity to seed source, seed dispersal, spatial heterogenity, tropical semideciduousforest.IntroductionSpatial heterogeneity of species and seeddensities of soil seed banks is well known for someecosystems like tropical humid forests (Whitmore1983) and temperate grasslands (Rice 1989; Rusch1992; Young et al. 1981). Grasslands often containlarge number of seeds and their seed banks arepersistent. For example, Young et al. (1981)reported 27,400 seeds m -2 from Stipa grassland inCalifornia. Tropical rain forests contain largenumber of seeds in the soil of many pioneer species(Whitmore 1983). In contrast, the density of seedsin tropical dry forests is found to be low. Forinstance, Lieberman (1979) found a maximum of160 seeds m -2 in a dry forest in Ghana while Hall& Swaine (1980) reported 100 - 700 seeds m -2 in adry forest in the same country. In a tropical semideciduousforest at Sigiriya Sanctuary, Sri Lanka,soil seed density of 166 ± 127 seeds m -2 wasreported (Perera 2005 in press).Soil seed banks in tropical humid forests arefound to be highly heterogenous (Garwood 1989;Uhl et al. 1981). Some factors which results in highspatial heterogeneity of soil seed banks are theseed dispersal patterns of the different species(Hall & Swaine 1980; Nepstad et al. 1996; Swaine& Hall 1983; Uhl et al. 1981), edaphic factors (e.g.drainage) (Garwood 1989), fire and seed predation(Perera, unpublished data). Such constrains affectseed germination while the time of disturbancerelative to flowering may limit the number of seedsand species that invade a site (Epp 1987).According to Geritz et al. (1984) and Hartshorn(1980), proximity to a seed source is important indetermining the successful colonization andregeneration of species in a disturbed site. Thisseems to be more relevant to the colonization ofsmall canopy gaps in forests.Tropical semi-deciduous forests in Sri Lankaare highly disturbed and large canopy gaps occuras a result of shifting cultivation (Perera 2001).Regeneration of tree species do not occurfrequently in these large gaps (Holmes 1954;Perera 2001; Rosayro 1961) and, therefore, it isnecessary to assist natural regeneration (Perera etal. 1995; Perera 2001).Soil seed banks play a major role in naturalregeneration after disturbances such as fire,logging and overgrazing in humid environments(Grime 1981; Roberts 1981; Swaine & Hall 1983),and determining the composition and density ofseed banks is considered as an essential step inartificial restoration of degraded vegetation (vander Valk 1989). Therefore, it is necessary toexamine whether the disturbed tropical semideciduousforests of Sri Lanka could berehabilitated by using their soil seed banks.The objectives of this study were to examinethe spatial distribution of seeds in the soil seedbank of a tropical semi-deciduous forest of SriLanka and factors responsible for the spatialheterogeneity. It also assesses the potential role ofthe soil seed bank in forest regeneration andevaluates how far the proximity to forests affectsin the regeneration of forests in grasslands, whichare situated adjacent to forests.
PERERA 81Materials and methodsStudy siteThe research was conducted at theWasgomuwa National Park in the dry zone of SriLanka (7 o 34' - 7 o 57' North and 80 o 51' - 81 o 05'East) (Fig. 1), which extended over 37,063 ha area(Green 1990). The forest and adjacent grassland inthe southern half of the park were selected for thestudy. The soils in the area are reddish brown withsome alluvial deposits, and the pH varies between6 - 8 (Jayasingham 1991). Generally, the meanannual rainfall in the dry zone of Sri Lanka isabout 1000-1900 mm year -1 (Rosayro 1950). Therain is mainly brought by the north-east monsoonbetween October and February. The south-westmonsoon brings less rain between March and May.Dry periods occur in between monsoonal periodsthough intermonsoonal rains also occur. Manmade fires periodically burn the grass vegetation.The vegetation of the area is mainlycharacteristic of a tropical semi-deciduous forestsof Sri Lanka (Jayasingham 1991). Characteristicspecies include Berrya cordifolia (Willd.) Burret,Chloroxylon swietenia DC, Drypetes sepiaria(Wight and Am.) Pax. and Hoffm. and Diospyrosspp. Over the forested area, 155 species belongingto 47 families have been identified including 107trees, 34 shrubs, 13 lianas and 2 climbers(Jayasingham 1991). The Euphorbiaceae,Leguminosae and Rutaceae are the dominantfamilies (Jayasingham 1991). Some parts wereheavily disturbed by a cyclone in 1978 and theseareas have more light demanding forest pioneerspecies.Jayasingham (1991) identified twograssland types in the area and named them asImperata grasslands and Ischemum/Eragrostisgrasslands. The former occurs widely over thearea and is dominated by Imperata cylindricawhile the latter occurs towards the easternboundary and is dominated by Eragrostis sp.and Ischemum sp.Soil samplingSampling was done at the early period of themain rainy season (November 1995). Fourtransects were established at right angles to theforest-grassland boundary in randomly selectedlocations across slight altitudinal gradientdistances of 0, 5, 25, 125, 625 and 3125 metrewere marked along these transects from theforest-grassland boundary into the forest andthe grassland. In most of the places, the lengthof the grassland was less than 3 km from theboundary. In such transects, the soil sampleswere collected from the furthest available point(fp) in the grassland, which varied between 2000to 3125 m.At each distance along the transect, 20 mlines perpendicular to the transect were marked.Ten soil samples (surface area = 15 x 15 cm;depth = 4 cm) were collected at 2 m intervalsalong these lines. Soil samples were brought tothe shade house thatched with transparentplastic corrugated sheets, and the 10 soilsamples collected at each distance were mixedtogether and spread over sterilized sand beds(surface area of the sand bed was one squaremeter and depth of the sand layer was fivecentimeters). The beds were well watered usinga hand held fine sprayer and germinants wereobserved and recorded for 110 days. Seventydays after the original spreading the soil surfacewas lightly scarified in order to expose buriedseeds if any. Six sand beds were left withoutany soil to detect any contamination in theshade house. Dicot seedlings which could not beidentified when very young were potted andidentified after 7-8 months.Fig. 1. Location of the study site and transects.
Page 1: PERERA 79Tropical Ecology 46(1): 79
Page 5 and 6: PERERA 83due to abundance of Agerat
Page 7 and 8: PERERA 85by Hurlburt’s rarefactio
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Page 11: PERERA 89van der Valk, A.G. & R.L.

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