Plant Ecology 146: 137–144, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 137 The effect of environmental factors on the regeneration of Quercus semecarpifolia Sm. in Central Himalaya, Nepal Ole R. Vetaas Botanical Institute, All´ egt. 41, N-5007 Bergen, Norway (e-mail: ole.vetaas@bot.uib.no) Received 12 January 1999; accepted in revised form 14 September 1999 Key words: Canopy disturbance, Himalaya, Lopping, Regeneration niche, Size-class distribution Abstract Poor regeneration of oak forest has been observed in north America and Eurasia. In the Himalaya it has mainly been explained by anthropogenic pressures. This study analyses the regeneration of two Quercus semecarpifolia Sm. forests (2–3000 m a.s.l.), where one forest has almost nil disturbance and the other has different degrees of canopy disturbance. The relationships between biotic and abiotic environmental variables and the number of Quercus semecarpifolia seedlings and saplings (i.e., recruits <10 cm Diameter at Breast Height (DBH)) were analysed by means of Generalized Additive Models (GAM). The two forests were compared with respect to size-class distributions of mature trees (i.e., >10 cm DBH), and the number of recruits. The size-class distributions indicate that regeneration is most reliable in the nearly undisturbed forest. Most recruits were found under high canopy cover and high potential radiation. Canopy disturbance has a negative effect on the number of seedlings. Seedlings seem to prefer a pH of around 6, Loss-on-Ignition between 20 and 30 %, and total nitrogen between 2 and 3 %. In general, there were very few plots with saplings, and there was not a clear relationship between the number of saplings and the soil variables. The size-class diagrams indicate a lack of young trees in the disturbed forest, and it is hypothesized that fire may inhibit recruits reaching the canopy phase. Introduction Quercus is a large genus with many of its five hundred species being canopy dominants in north America and Eurasia (Spurr & Barnes 1973). Poor regeneration of oak woods has been reported in all these regions; in north America, e.g., Carvell & Tyron (1961), Crow (1988), Abrams & Downs (1990), Lorimer et al. (1994), in Europe, e.g., Watt (1919), Mellanby (1968), Shaw (1968), and Andersson (1991), and in Asia, e.g., Saxena et al. (1984), Singh & Singh (1987) and Masaki et al. (1992). Several reasons have been sug- gested to explain poor development of oak seedlings, such as erratic seed production, defoliation, acorn her- bivory, browsing damage to seedlings, decreased or increased fire frequency, and extensive lopping (e.g., Singh & Singh, 1987; Andersson 1991; Lorimer et al. 1994; Thadani & Ashton 1995). Regeneration of canopy dominants is commonly assessed by the distributions of size-classes mea- sured as diameter at breast height (DBH) (Leak 1964; Schmelz & Lindsey 1965; Harcombe & Marks 1978; West et al. 1981). This is based on the survivorship curves and the density diameter relationships devel- oped by applied forest scientists (Pearl & Miner 1935; Leak 1964). It is supported by the tenet that fecun- dity and population growth in plants may be more dependent on size than on age (Harper & White 1974; Buchholz & Pickering 1978). Undisturbed old-growth forests with sustainable regeneration are found to have a reversed J-shaped size-class distribution (West et al. 1981; Parker & Peet 1984; Rao et al. 1990; Bernadzki et al. 1998). A bell-shaped size-class distribution has been attributed to disturbed forest where regeneration is hampered (Parker & Peet 1984; Saxena et al. 1984). The evergreen oak species plays a major role as canopy dominants in the middle hills of Himalaya (2000–3500 m a.s.l.), and are often under strong an- thropogenic pressures, such as lopping for fodder, browsing, and fire. This may reduce their regener-