RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 111, NO. 7, 10 OCTOBER 2016 1220 *For correspondence. (e-mail: mukundbehera@gmail.com) Plant species richness pattern across India’s longest longitudinal extent Mukunda Dev Behera 1, *, Partha Sarathi Roy 2 and Rajendra Mohan Panda 3 1 Centre for Oceans, Rivers, Atmosphere and Land Sciences, Indian Institute of Technology, Kharagpur 721 302, India 2 Centre for Earth and Space Sciences, University of Hyderabad, Hyderabad 500 046, India 3 School of Water Resources, Indian Institute of Technology, Kharagpur 721 302, India Although the longitudinal pattern of biological diver- sity may not be as striking as the latitudinal pattern, there are climatic gradients associated with habitat, topography and disturbance, which may generate variation in biological diversity along a longitudinal extent. We analysed the pattern of plant diversity using field data gathered from a national level ‘biodi- versity characterization at landscape level’ project, along the longest longitudinal extent of India, varying from desert to wet tropics. Across a 3 latitudinal band (25–27N lat.), the aridity decreased drastically from west to east, and the vegetation varied from tropical thorn, tropical deciduous, tropical semi- evergreen to tropical evergreen forests. In general, the species diversity is maximum in the east, minimum in the centre and intermediate in the west. The propor- tion (with respect to total number of species of each corresponding grid) of woody trees increased by half and shrubs increased by one-third, whereas herbal proportion decreased by half from western to eastern grid. The major predictor of species richness was the number of forest vegetation types, followed by topog- raphy (ruggedness index); together these explained 63% of the variance as revealed from generalized linear modelling analysis. The study concludes that, from west to east, (i) the vegetation type varies from tropical thorn to tropical moist/wet evergreen forest corresponding to aridity; (ii) there is monotonic dras- tic increase of woody tree, moderate increase of shrub, and drastic decrease of herb species, and (iii) plant diversity has a nonlinear distribution pattern attri- buted mainly to variation in the number of forest vegetation types followed by abiotic topography and climate. The study was possible due to the availability of national plant diversity database that followed a uniform field sampling design. Keywords: Aridity index, climate, disturbance, plant life form, topography, vegetation type. PATTERNS of species distribution are a product of many processes, including species history (speciation, migra- tion and extinction), geographic location and environ- mental variables (geology, topography, climate and soil). Earth–sun relationships (axial tilt, orbital cycle, etc.) and abiotic planetary factors (surface physiography, atmos- pheric circulation, etc.) determine spatio-temporal varia- tions in climate. The former always produce a latitudinal gradient in the duration and intensity of incoming solar energy, and thus the first-order pattern of variation in climate. The latter modify this pattern, more prominently over continental scales, where climate varies along an essentially longitudinal gradient 1 . Although latitudinal patterns in species diversity are well known 2 , relatively little is known about the diversity patterns along a longi- tudinal gradient. While longitudinal patterns of biological diversity may not be as striking as the latitudinal pattern (since the latter is associated with large climatic differ- ences from the poles to the equator), there are climatic gradients associated with coastal to interior areas, such as precipitation and temperature, which are expected to give rise to variation in biological diversity along a longitudi- nal gradient 3 . The few studies of longitudinal patterns in diversity can be summarized briefly. In North America Qian 4 found that the alpha diversity of boreal forest changes from west to the east in an asymmetric modal trend. O’Brien 5 argued that the woody plant species richness tends to increase from west to east across southern Africa in a non-latitudinal, essentially longitudinal fashion; con- sistent with the increasing dominance of woody plants as vegetation shifts from desert to evergreen forest. Karren- berg et al. 6 , working within the active zone of the near- natural Tagliamento, NE Italy, found that diversity of woody vegetation was mainly structured by the longitu- dinal gradient, which corresponded to gradients in air temperature and altitude. Spatial patterns in riparian vegetation composition and structure also occur along longitudinal gradients within river catchments 7 . For the semi-arid San Pedro river flood plain, Lite et al. 8 sug- gested that species richness varies along longitudinal gradients with changes in surface and groundwater avail- ability, canopy cover and topographic diversity. India is a vast and topographically varied subcontinent that extends from the tropics to the sub-tropics. Along its northern span, the Thar desert bounds India on the west, the Gangetic plains compose the middle, and the eastern Himalaya and the Brahmaputra valley form the eastern- most boundary (Figure 1). This provides a gradient of arid, semi-arid, tropical dry, tropical moist, subtropical humid and montane climate from west to east. The arid climate of the Thar desert in the west allows the growth of xerophytes and dry succulents. The central Gangetic plains are characterized by semi-arid vegetation in the west, moist deciduous and evergreen species in the north and dry deciduous species in the south. The eastern Himalaya and the Brahmaputra valley support tropical semi-evergreen and evergreen species in the lower eleva- tions, and moist or wet evergreen species of temperate and alpine zones in the elevated highlands 9 . The altitudinal