Plant Ecology 132: 29–38, 1997. 29 c 1997 Kluwer Academic Publishers. Printed in Belgium. The effect of canopy disturbance on species richness in a central Himalayan oak forest Ole R. Vetaas Botanical Institute, All´ egt. 41, N-5007 Bergen, Norway; (Tel: +47 55 583351, Fax: +47 55 589667. E-mail: ole.vetaas@bot.uib.no) Received 5 July 1996; accepted in revised form 12 April 1997 Key words: Biodiversity, Conservation, GLM, Nepal, Vascular plants Abstract Non-epiphytic species richness was studied in different disturbance classes within a Quercus semecarpifolia forest. Nine disturbance classes were defined according to the degree of biomass removal (lopping) and their spatial mixture. Six of these were observed in the study area. The species were divided into three functional groups: climbers, phanerophytes, and field-layer plants. The primary aim was to test if there is an elevated species richness under an intermediate disturbed canopy for (i) all vascular plants, (ii) lianas, (iii) phanerophytes and (iv) field-layer species. The richness of the different plant groups and all species were fitted against the disturbance gradient by means of Generalized Linear Models (GLM). Other environmental variables such as altitude, potential solar radiation, light intensity, canopy cover and soil parameters were also evaluated as predictors. Disturbance classes, canopy cover and light intensity were combined into a new variable, disturbance-complex, using Principal Component Analyses. Phanerophytes did not respond to any variable. Climbers were mostly related to pH and canopy cover, and were the only group related to altitude, nitrogen and loss-on-ignition. Herbaceous plants and total species richness showed a unimodal response to disturbance classes and the complex disturbance gradient, which supports the intermediate disturbance hypothesis. Relative radiation and slope also supported a unimodal response in herbaceous plants, but disturbance had a significant additional contribution to this pattern. The most significant predictor for these two groups was pH. The responses to organic carbon and phosphorus were not significant for any of the subsets. The results indicate that a small-scale lopping regime will enhance species richness of vascular plants; only a few species in the intermediate disturbed forest are weedy ruderals. In such a situation, the conservation policy may accept small-scale human impact as part of the forest landscape. Introduction Species richness is a simple and easily interpretable indicator of biological diversity (Hurlbert 1971; Peet 1974; Whittaker 1977). Theories of biodiversity have been thoroughly reviewed by Huston (1994), and syn- thesized by Palmer (1994). The classical view on richness and disturbance has been that undisturbed sites (e.g. ‘climax-forest’) have maximum species number (Clements 1936, Margalef 1968, Slobodkin & Sanders 1969, Odum 1971, West- hoff 1971, Rao et al. 1990). In recent decades, however, several authors have recognized that disturbance may increase species richness (Connell 1978; Grime 1979; Huston 1979; Naveh & Whittaker 1979; Sousa 1984; Petraits et al. 1989). This prediction is deduced from various simple conceptual models (cf., Peet et al. 1983; Huston 1979, 1994). The spatio-temporal variation and intensity of the disturbance, however, is difficult to incorporate in these models (White 1979; Huston 1979; Petraits et al. 1989; van der Maarel 1993). The effect of disturbance is also relevant for the choice of conservation strategies, particular in densely populated areas. This paper reports on species richness patterns in a temperate Himalayan Quercus forest in the periphery