The changing functions of fungi in forest succession.

Abstract

The ecological roles of fungi in forest settings are incredibly diverse, but two of the most crucial services provided by members of this kingdom include fixation of nitrogen into forms useable by plants and decomposition of organic matter to allow the recycling of certain nutrients into the soil. As forests recover from disturbances such as burning, different nutrients become limiting to plant growth at different stages of succession—for example, nitrogen tends to be a limiting factor in younger forests. Our group hypothesized that nitrogen-fixing species of fungi (i.e. mycorrhizae) should be more abundant than decomposing (i.e. saprophytic) species in younger forests due to the competitive advantage in resource availability given to plants who had formed mutualistic relationships with mycorrhizae; in other words, we predicted that the primary function of fungi in a forest ecosystem would shift from mycorrhizal to decomposing as the forest increased in age. To test this, we performed a transect of three differently aged forests, represented by “burned plots” in various stages of secondary succession, and categorized each species of fungi found as either mycorrhizal or saprophytic. Additionally, we took soil samples from all forests to establish the relative ratio of carbon to nitrogen in each. Though no significant differences in soil nutrient percentages were found between the different plots, we found a significantly higher proportion of nitrogen-fixing:decomposing fungi in the youngest plot sampled. This result indicates that the primary function of fungi in a successive forest may indeed change from mycorrhizal to saprophytic (or, at least, less mycorrhizal) as the forest reaches different stages of recovery and that this change is strongly reflected in the composition of the fungal community, though several factors limited our ability to wholly assess the dominant functional group of fungi present in any particular forest. The capability to understand the changing dynamics of fungal function and distribution as compared to the specific needs of a particular ecosystem could be valuable for researchers attempting to describe the vast majority of fungal species that remain unknown yet still play unique, critical ecological roles in their environment.