We examined factors that influence the expansion of alder forests in a wetland isolated by dikes and drainage ditches in northern Japan. Using aerial photographs, airborne laser scanner data, and existing elevation data, we described the spatial distribution and heights of alder forest in 2002 and the spatial distribution in 1977. We also measured water level and water quality in 2002. The mean water level was slightly lower than the ground surface, and water level fluctuation was minimal. The study site was mesotrophic, but salt concentration was high. The alder forest expanded around the area adjacent to the alder forests present in 1977, and the canopy cover ratio increased over a wide area. A generalized linear model suggested that water level and quality and the previous distribution of reproductively mature trees controlled the expansion pattern of the alder forest. A decrease in water level fluctuation promoted initial establishment, while an increase in fluctuation promoted height growth. A decrease in salt concentration or acidity promoted both initial establishment and growth. Flood protection by dikes may change spatial distribution of alder trees by narrowing water level fluctuation, restricting fruit dispersal, and reducing tree mortality.