The Effect Of Season Of Prescribed Fire On Richness And Abundance Of Breeding Bird Species And Vegetation Structure In Minnesota Lowland Brush Ecosystems

Abstract

Lowland brush ecosystems provide critical habitat for diverse breeding bird communities in the western Great Lakes region of the United States. These ecosystems are disturbance-dependent and historically experienced fires occurring throughout plant growing and dormant seasons. Disturbance in brushland landscapes influences woody vegetation structure, which can increase in extent, density, and height without frequent or effective disturbance events. Woody vegetation structure has been shown to be important for breeding bird communities in upland and forest systems, but this has not been studied in lowland brush ecosystems. Additionally, there are few studies on how bird communities and lowland brush vegetation structure respond to fire. Currently, lowland brush ecosystems in the Upper Midwest are predominately burned during spring months when plants are dormant. This may not mimic historical fire effects because before fire-suppression and control, fires occurred throughout snow free seasons, including summer and fall. In forests and grasslands, plant and bird species have been shown to respond distinctly to different seasons of fire, with the response of bird species being related to changes in vegetation structure and its role in nesting and foraging. No studies on season of fire have been done in lowland brush ecosystems. We assessed baseline models relating bird species richness, bird total abundance, and abundance of frequently detected bird species to woody vegetation structure in lowland brush ecosystems. Frequently detected bird species included golden-winged warbler (Vermivora chrysoptera), sedge wren (Cistothorus platensis), and veery (Catharus fuscescens) which are Minnesota Species in Greatest Conservation Need. We then used a Before-After-Control-Impact experimental design to evaluate the magnitude in change in response to spring, summer, and fall fire treatments of the same bird and vegetation variables. This allowed us to determine vegetation characteristics that are important to breeding birds and how the responses of birds and plants relate to season of fire. Stem height and stem height diversity, which was a measure of vertical structural diversity, were related to the most frequently detected bird species and bird species richness. Although these vegetation variables did not respond significantly to spring, summer, and fall fire treatments compared to controls, they exhibited decreasing trends after fires. Additionally, changes in stem height were nearly statistically significant. Veery and yellow warbler (Setophaga petechia) decreased in abundance after summer (veery) and spring and fall (yellow warbler) season treatments and were related to stem height in baseline explanatory models. We posit that these species decreased in abundance due to decreases in stem height. In contrast, chestnut-sided warblers (Setophaga pensylvanica) increased in abundance after spring and fall fires. This species was also related to stem height and the number of woody plant species. Therefore, chestnut-sided warblers may have been responded to additional changes in vegetation from fire and notably, chestnut-sided and yellow warblers exhibited opposite responses to the same fire seasons even though these species exhibit similar life history traits. Bird total abundance increased after summer and fall fires, the 2 seasons when prescribed fires are not typically conducted in the Upper Midwest. In our explanatory baseline models, the null model best explained bird total abundance and so although we were unable to relate this response to vegetation measurements, we suggest this response be considered in future management. Based on our results and the mix of responses to spring, summer, and fall fire seasons, conducting prescribed fires during different seasons may support different breeding bird species. The overall breeding bird community may also benefit, especially if prescribed fire is implemented during the summer when plants are growing. Adding summer burns to disturbance management-regimes that are often restricted to the spring in lowland brush ecosystems may also provide managers with larger burn-windows.