The 30-year outcome of assisted regeneration treatments in a burned and salvaged Interior Alaska boreal forest

Abstract

This study contributes to the understanding of the persistence of silvicultural treatments into the stem exclusion stage of forest development in an experiment originally designed to test the effectiveness of various white spruce (Picea glauca Moench [Voss]) regeneration practices. Many studies in the North American boreal forest address the effect of silvicultural treatments on a single tree species, specifically white spruce in the great majority of cases. The experiment measured in this study provided an excellent opportunity to compare treatment effects on white spruce density and growth. The Rosie Creek Fire Tree Regeneration Installation experiment represents an operational-scale, spatially-explicit, replicated design on a single site disturbed consecutively by high-severity wildfire and clearcut salvage harvest. Three hierarchical factors, each with multiple levels, were examined: landform type, ground scarification methods, and white spruce regeneration methods. All three of the experimental factors exercised continuing influence on the patterns of white spruce regeneration and growth. The treatment effects did not attenuate over time for white spruce, and we found statistically significant effects that the original researchers could only describe as tendencies. However, relatively few studies address treatment impacts on non-target species or determine how the silvicultural treatments affect a site's overall woody biomass production. Experimental silvicultural practices targeted in this study to improve white spruce survival had profound effects on other dominant upland tree species such as quaking aspen (Populus tremuloides Michx.) and Alaska birch (Betula neoalaskana Sarg.). Interior Alaska timber species demonstrate different regeneration strategies to post-disturbance environmental conditions, especially residual organic soil layer thickness and spatial configuration of surviving potential seed sources. Effective silvicultural practices must consider each species' unique reproductive biology, and clonal sprouting as a source of aspen persistence was a particularly important example in our study. Site differences, such as we found between the slope and ridge landforms, are a key consideration for implementing effective silvicultural practices. Significant interactions between the regeneration treatments and landform types proved to be critical to meet specific reforestation objectives, particularly the different herbaceous vegetation cover types, presence/absence of aspen clonal rootstocks, and spatial configurations regarding seed sources. Managing mixed species stands, which are common in the lightly managed portions of the boreal forest, requires not only the consideration of the future crop tree, but the interacting effects of silvicultural practices on all tree species.