Influence of residual forest patches on post-fire bird diversity patterns in jack pine-dominated ecosystems of northern Lower Michigan

Abstract

Stand-replacing fires are part of the natural disturbance regime that maintain jack pine (Pinus banksiana Lamb.) forests of northern Lower Michigan. Currently, the occurrence and extent of wildfire is much reduced relative to the past and young jack pine plantations specifically managed for Kirtland’s warbler (Setophaga kirtlandii Baird) are a prominent feature on the landscape. Stringers, or patches of residual pre-fire forest, are a unique feature left behind after wildfires which are not often accounted for in plantation management. In this study, we examined the ecological value of stringers from an avian biodiversity perspective by comparing bird assemblages found within stringers to those in the surrounding forest (non-stringers). Our objective was to answer two research questions: (1) do stringers have unique bird communities relative to the surrounding vegetation (forest or plantation)?; and (2) how much of the variation in bird communities can be explained by differences in vegetation structure and composition between stringers and non-stringers? We conducted breeding and non-breeding season point counts and used abundance data to compare bird species found within stringers and non-stringers at seven sites grouped by time since disturbance. Species richness was significantly higher (P=0.01) in the stringers when the non-stringers were 30–40years old, with 32 species discovered in the stringers and 29 species in the non-stringers. During the breeding season, bird assemblages differed between stringers and non-stringers when the non-stringers were <12years old (multi-response permutation procedures; T=−10.11, A=0.07, P=<0.00), but no differences were observed when non-stringers were 30–40years old or >40years old. Non-breeding bird communities differed between stringers and non-stringers only when the non-stringer was a recently planted (<5years old) plantation (T=−2.15, A=0.09, P=0.02). Differences in bird assemblages appear to be driven by the vegetation structure of stringers and non-stringers where fires were recent, but increasing similarity in vegetation structure occurs with time since fire; over time the importance of stringers for avian biodiversity is reduced. Our results suggest that stringers are important for bird communities, especially in recently disturbed areas, and that these biological legacies should be considered where jack pine management attempts to emulate natural patterns and processes.