- Soil nutrient fluxes and vegetation changes on molehills - 23 Journal of Vegetation Science 11: 23-30, 2000 © IAVS; Opulus Press Uppsala. Printed in Sweden Abstract. The hypothesis that mole burrowing activity alters soil nutrient fluxes and that, as a response to the new condi- tions, a specialized guild of species develops on the molehills, was tested in an area located in the southwestern Spanish Pyrenees, on a spectrum of montane grassland communities that varies from xeric to temporally waterlogged. Evidence for an association between disturbance and nu- trient availability was reported for nitrogen. Mole-disturbed soils had elevated amounts of inorganic nitrogen compared to soils in surrounding pastures. At the first stages of mound revegetation, changes in nitrate flushes and in species com- petitive relationships following disturbance appeared to facili- tate the establishment of ruderal and non-mycorrhizal species. The diversity of the whole grassland was enhanced by the existence of these sets of species, abundant on mounds and rarer in the pasture. However, the difference was mainly quantitative, as exclusive colonizers of molehills were not found. Keywords: Disturbance; Grassland; Growth form; N-miner- alization; Revegetation. Nomenclature: Aseginolaza et al. (1996). Introduction The structure and dynamics of grassland communi- ties result from a variety of causes, including responses to environmental gradients, disturbances and competi- tive interactions (Hobbs & Mooney 1991). Distur- bances caused by small mammals are known to play an important role in the dynamics of herbaceous commu- nities (Collins & Barber 1985; Hobbs & Hobbs 1987; van der Maarel 1993; Fields et al. 1999). Competition among species that colonize recently created mounds is expected to be low and species revegetating the dis- turbed ground are probably responding more to the abiotic conditions of the new environment than to po- tential competitive constraints (Rabinowitz & Rapp 1985). The opening of free space, contrasting tempera- tures, drier microclimate and increased light reaching the soil are among the environmental conditions chang- ing when a portion of grassland is disrupted by small burrowing mammals (Grubb et al. 1969; McConnaughay & Bazzaz 1990). Physical properties of soils and soil nutrient fluxes are also likely to be altered by burrowing activity (Hobbs & Mooney 1985; Reichman & Jarvis 1989). In spite of its importance, studies on nutrient cycling following disturbances smaller than fire and forest clear-cutting are scarce (Vitousek 1985) and published results show little agreement. Regarding the influence of burrowing animals on N-nutrient fluxes, some studies find an in- crease in the amount of nitrogen associated with the mounds (Laycock & Richardson 1975; Kalisz & Davis 1992) whereas others conclude that nitrogen decreases compared to the surrounding grassland (Inouye et al. 1987; Koide et al. 1987). These differences are due to the nitrogen-form analysed, the type of soil or the habits of the different burrowing mammals. Altered soil properties are likely to affect vegeta- tion developing on mounds. Consequently, floristic composition both on and off mounds may be different and changes in species richness and plant diversity may occur. Changes in community composition may lead to changes in the life-form spectrum of the pas- ture. Growth-form correlates with other physiological and morphological traits of the plants and has been used as a tool to characterize community responses to different disturbances (Gómez-García et al. 1995; McIntyre et al. 1995, 1999) and to define guilds in plants with specific life strategies (e.g. Grime et al. 1987). Up to now, no single growth form has been found to be the most successful in colonizing small- mammal mounds. Many studies have found a large number of annuals on disturbed soils (Foster & Stubbendieck 1980; Tilman 1983) and in some cases this abundance seems to be directly related to gap size (Korn 1991). Annuals act as opportunists, having a short life span and high fecundity that enables them to rapidly colonize open spaces created by disturbances. When grown from seed, annuals have a higher photo- synthetic capacity and a higher relative growth rate than perennials, although they are more vulnerable to biotic stresses such as pathogens and herbivores (Bazzaz & Morse 1991). Soil nutrient fluxes and vegetation changes on molehills Canals, Rosa-Maria 1* & Sebastià, Maria-Teresa 2 1 Departamento de Producción Agraria, Universidad Pública de Navarra, Campus Arrosadia s/n. 31006 Pamplona, Spain; 2 Tecnologic Forestal de Catalunya, Pujada del Seminari s/n, 25280 Solsona, Lleida, Spain; * Fax +34948169732; E-mail rmcanals@unavarra.es