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INSTAAR, University of Colorado andThe Regents of the University of Colorado, a body corporate, contracting on behalf of the University of Colorado at Boulder for the benefit of INSTAAR are collaborating with JSTOR to digitize, preserve and extend access to Arctic, Antarctic, and Alpine Research.Environmental factors may affect the frequency of diseases, e.g., by affecting the growth of a host plant, by affecting the developmental stage of individual plants and plant populations (Jennersten et al., 1983; Carlsson et al.In this study we focus on Vaccinium vitis-idaea L., a dominant field layer species in heath forests of Fennoscandia, and its fungal pathogen Exobasidium splendidum Nannf. The distribution of E. splendidum is restricted to high altitudes or high latitudes (Sundstr6m, 1964; Nannfeldt, 1981). In northern Finland, E. splendidum is frequently observed in V. vitis-idaea stands with natural and anthropogenic disturbances (Pehkonen, pers. obs.). The frequencies of diseases caused by pathogens have often been found to increase as a consequence of abiotic and biotic disturbances, which cause changes in environmental factors and vegetation (Harper, 1977;Dinoor and Eshed, 1984; Shearer and Dillon, 1996). We can therefore ask whether the predicted climate warming and increasing disturbances in northern areas will have a major effect on the outcome of the disease caused by E. splendidum in V. vitis-idaea stands. Due to the high ecological and economic importance of V. vitis-idaea in these areas it is significant to clarify the effects of pathogens on its population dynamics, and consequently, on plant communities. In general, this study will give information about a poorly studied field in 428 / ARCTIC, ANTARCTIC, AND ALPINE RESEARCH Abstract We examined the host-pathogen system of an evergreen dwarf shrub Vaccinium vitis-idaea and its fungal pathogen Exobasidium splendidum in two habitats, a cleared forest (disturbed habitat), and an untouched forest (undisturbed habitat). Our aims were to find out how environmental factors, plant community characters, and the characters of V. vitis-idaea vary among the two habitats and whether these characters correlate with the frequency of E. splendidum. Our results suggest that environmental factors in the disturbed habitat are more favorable for the disease through increased growth of the host V. vitis-idaea and possibly by increased growth and reproduction of the pathogen. The high emergence rate of new V. vitis-idaea ramets in the disturbed habitat resulted in a younger ramet age structure as compared with the undisturbed habitat. Consequently, the ratio of ? 2002 Regents of the University of Colorado
