Fire Frequency and the Spatial Age Mosaic of the Mixed-Wood Boreal Forest in Western Canada

One approach to ecosystem management is to emulate the effects of natural disturbance in producing landscape patterns; this approach requires a spatial analysis of the pattern and an understanding of the processes producing the pattern. Forested landscapes exhibit mosaic patterns of both stand types and ages. This study investigates the spatial mosaic of stand ages produced by high-intensity stand-replacing fires in the mixed-wood boreal forest of western Canada. A high-resolution, accurately dated, time-since-fire map for a large (3461 km2) contiguous area is used to produce the landscape survivorship distribution in which both spatial and temporal changes in fire cycle are statistically tested. Spatial multivariate analysis of the time-since-fire map is also used to investigate the spatial assembly of the age mosaic. Significant changes in fire cycle can be explained by climatic change as well as land use change in the surrounding area. The shift from a short (15 yr) fire cycle to a longer (75 yr) cycle after 1890 in the northern half of the study area coincides with climatic change at the end of the Little Ice Age. In the southern half of the study area, the short fire cycle continues after 1890 due to the spread of human-caused fires from the adjacent area which was settled and cleared for agriculture during the first half of the 20th century. Upon completion of settlement in 1945, the fire cycle becomes significantly longer due to the fragmentation of the once continuous forest that surrounded the study area and from which the majority of large fires propagated in the past. The different fire cycle histories of the two parts of the study area also explain the spatial mosaic pattern of stand ages, sizes, and shapes. The extended period of the short fire cycle through the first half of this century in the southern region results in it being dominated by younger, larger, oblong-shaped polygons with irregular edges: characteristics that describe the shapes of large burns. The northern region has generally older and smaller, more circular, compact polygons that are the remnants of larger much earlier burns that have since been overburned. The polygons in the northern region are more similar in size and shape but less similar in age to adjacent polygons than are those in the southern region. Thus, this study shows how spatial heterogeneity in the landscape mosaic pattern can be characterized and related to the disturbance history of an area. Furthermore, it provides evidence of the impacts on the age mosaic due to forest fragmentation in surrounding areas.