Identifying the spatial scales of forest structural change in two boreal regions

Abstract

Global environmental change alters forest dynamics, but the effects vary regionally and the changes often occur at various spatial and temporal scales. Hence, and due to the slow ecosystem responses to environmental changes, long-term, multi-scale studies are needed to understand how forests respond these changes. We studied scale-dependent changes in the structure of unmanaged boreal forests. We asked (1) at what spatial scales do structural changes occur, and (2) whether these scales differ between different regions and landscapes.

We studied 2 km × 2 km forest landscapes at two different boreal regions: Northeastern Finland (three landscapes) and eastern Quebec, Canada (two landscapes). For each landscape, we produced two raster maps depicting forest structural change between two points in time. We analyzed these maps with multiresolution Bayesian scale space approach, where (1) the spatial scales at which the most salient structural changes occur were identified, (2) the scales were used to decompose the maps depicting forest structural change, and (3) the scales of changes were quantified from the extracted components. To produce the maps depicting forest structural change, we visually estimated canopy cover in contiguous 0.1-ha cells from a recent and two historical stereopairs of aerial photographs, and formed time series with total lengths varying between 38 and 52 years, depending on the landscape. Using field measurements and tree ring data, we reconstructed canopy cover in the year each photograph was taken for a random sample of 66 of the 0.1-ha cells, and used the reconstructions to calibrate the interpretations, and to quantify the interpretation error. We compiled the calibrated interpretations to raster maps, and produced maps depicting canopy cover change by subtracting canopy cover maps of consecutive time points.

At both studied time intervals, forest structural changes occurred at multiple spatial scales in landscapes with different disturbance histories and tree species compositions. The spatial scales of changes ranged between 0.1 ha and > 100 ha. In each landscape, the scales at which the structural changes occurred were similar between the both studied time intervals. Between landscapes, the scales of changes were similar at small scales, but differed at large scales. These preliminary findings indicated that similar small-scale structural changes were typical for both studied boreal regions. Structural changes occurred also at large spatial scales, but these scales of changes differed between landscapes.