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 d isturbance in producing landscape patterns; this approach requires a spatia l analysis of the pattern and an understanding of the processes producing t he pattern. Forested landscapes exhibit mosaic patterns of both stand types and ages. This study investigates the spatial mosaic of stand ages produce d by high-intensity stand-replacing fires in the mixed-wood boreal forest o f western Canada. A high-resolution, accurately dated, time-since-fire map for a large (3461 km(2)) contiguous area is used to produce the landscape s urvivorship 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 chan ge as well as land use change in the surrounding area. The shift from a sho re (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 Lit tle Ice Age. In the southern half of the study area, the short fire cycle c ontinues after 1890 due to the spread of human-caused fires from the adjace nt 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 continuo us forest that surrounded the study area and from which the majority of lar ge 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 stan d ages, sizes, and shapes. The extended period of the short fire cycle thro ugh the first half of this century in the southern region results in it bei ng dominated by younger, larger, oblong-shaped polygons with irregular edge s: characteristics that describe the shapes of large burns. The northern re gion has generally older and smaller, more circular, compact polygons that are the remnants of larger much earlier burns that have since been overburn ed. 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 mos aic due to forest fragmentation in surrounding areas.