This paper develops field data analysis and theoretical hypotheses concerning the role of climatic factors and fungal interactions in the control of fungal populations. We studied three ‘resource unit restricted fungi' colonizing Pinus sylvestris needles along a climatic gradient. We investigated both competition and succession. A general linear model was used for statistical analysis. Despite interference competition at the scale of the individual needle, the first two colonizers, Lophodermium pinastri and Cyclaneusma minus, coexisted and the frequencies of both increased with altitude. This increase seemed to be related to the greater humidity at altitude. The initial stages of development of the final colonizer, Verticicladium trifidum, were strongly dependent on the first two colonizers on the individual needles. The observed decrease in frequency of V. trifidum with increasing altitude may be due to interactions with the early colonizers, with no direct effect of altitude on V. trifidum. Fungal colonization at low and high altitudes can be interpreted by accounting for both climatic effects and fungal interactions expressed as interference and succession.