Spatial heterogeneity of understory vegetation and soil in an Alaskan upland boreal forest fire chronosequence

In this study we characterized spatial heterogeneity of soil carbon and nitrogen pools, soil moisture, and soil pH of the first 15 cm of the soil profile; depth of the organic horizon; forest floor covers; and understory vegetation abundances in three sites (1999, 1987 and 1920 wildfires) of a boreal forest chronosequence of interior Alaska. We also investigated the cross-dependence between understory vegetation distribution and soil characteristics. Our results showed higher microbial respiration rates and microbial biomass in the oldest site and greater net N mineralization rates in the mid-successional site. Although spatial heterogeneity was absent at the scale studied for the majority of soil variables (60%), understory vegetation abundances and forest floor cover, spatial heterogeneity decreased with time after fire for the depth of organic horizon, soil microbial biomass, N mineralization rates and feathermoss cover. Our results also showed that increasing time after fire decreased the number of correlations between understory vegetation and soil characteristics while it increased between forest floor covers and soil characteristics. Overall, our study suggest that fire initially creates a patchy mosaic of forest floor cover, from fire hot spots, where high intensity burning exposes mineral soil, to practically unburned areas with intact mosses and lichens. As time since fire passes, forest floor cover and soil characteristics tend to become more uniform as understory species fill in severely burned areas.