- Variation in modern pollen from tropical evergreen forests and the monsoon seasonality gradient - 551 Journal of Vegetation Science 14: 551-562, 2003 © IAVS; Opulus Press Uppsala. Abstract. This study analyses the pollen signature of tropical lowland forests (< 1000 m a.s.l.) in the Asian monsoon cli- mate. Its aim is to investigate how well the pollen data can reproduce the vegetation patterns in tropical India, and how the variations in the pollen composition are related to the gradient of decreasing plant moisture availability (measured by the ratio of actual over equilibrium evapotranspiration) that is associated with the strong seasonality of precipitation that characterizes the monsoon climate regime. We used canonical correspondence analysis (CCA) to relate the variations in the pollen composition of 71 surface soil samples from evergreen and semi-evergreen forests dis- tributed along the western coast of south India (8° 48' N-15° 08' N), with the climate characteristics of the sampling sites. We show that variations in plant moisture availability strongly determine variations in the pollen composition; for example evergreen and semi-evergreen forests can be distinguished on the basis of their pollen assemblages. Variations in the mean temperature of the coldest month associated with elevation also determine distinct pollen assemblages; for example ever- green forests above 800 m a.s.l. present different pollen signa- tures than those below this altitude/temperature limit. Varia- tions in the relative abundance of some pollen taxa are strongly related to plant moisture availability and taxa indicators of climate can be identified. Hence, modern pollen assemblages from tropical forests in south India carry considerable infor- mation about vegetation patterns and climate. Paleoclimatic changes, notably in the monsoon season, could be quantified. Keywords: CCA; Dry season; Indicator taxa; Western Ghats. Abbreviations: IVI = Importance value index; PANN = Mean annual rainfall; MTCO = Mean temperature of coldest month. Introduction Pollen data from dated sedimentary sequences have been widely used to reconstruct vegetation patterns during past periods (e.g. Prentice & Webb 1998) and to infer the changes in climate that have led to the observed changes in vegetation distribution (Howe & Webb 1983; Bonnefille et al. 1990; Cheddadi et al. 1997; Bonnefille & Chalié 2000). Modern pollen surface samples from known environments provide an important way to es- tablish whether specific types of vegetation have a suf- ficiently distinctive pollen spectrum to allow their iden- tification in the fossil record despite inter-species differ- ences in pollen production or post-depositional modifi- cation. These investigations can also help to isolate the bioclimate parameter(s) determining the limits of veg- etation types (Bush 2000). These relationships then can be used to improve inferences about past climates using pollen-based reconstructions of vegetation patterns. Recent investigations of modern pollen surface sam- ples from the Indian subcontinent have shown that tem- perature controls vegetation types along altitudinal gra- dients (Bonnefille et al. 1999). Precipitation-related dif- ferences between evergreen and deciduous forests also can be distinguished on the basis of their pollen spectra (Anupama et al. 2000; Barboni & Bonnefille 2001). However, seasonality of precipitation, and specifically the length of the dry season, may be a more important determinant of the distribution of evergreen forest types in southern India than precipitation (Pascal 1988). In this paper we present analyses of modern pollen surface samples from the evergreen forests in this region. Our aims are to establish whether different floristic types of evergreen vegetation can be distinguished by their pol- len assemblages and to investigate the bioclimatic con- trols on evergreen vegetation types. Study area This study concerns different forest types located in the western Ghats, a narrow mountainous chain along the west coast of south India. We focused on the area between 8° and 15° N, well studied by botanists and ecologists who provided maps of vegetation and bioclimate. We restricted our study to areas below 1000 m a.s.l. Variation in modern pollen from tropical evergreen forests and the monsoon seasonality gradient in SW India Barboni, D. 1, 2* ; Bonnefille, R. 2 ; Prasad, S. 1 & Ramesh, B.R. 1 1 French Institute of Pondicherry, 11 Saint Louis Street, B.P.33, Pondicherry, 605001 India; 2 C.E.R.E.G.E., Europôle Méditerranéen de l’Arbois, B.P. 80, F-13545 Aix-en-Provence Cedex 4, France; * Corresponding author; E-mail barboni@cerege.fr