The Impacts of 23 Years of Low-Intensity Prescribed Fire On Litter and Soil Properties in a Midwestern Oak Forest.

For the last 23 years, low-intensity prescribed fire has been used with the intent of increasing shade-intolerant trees species and herbaceous plant diversity in the East Woods of The Morton Arboretum, Lisle, Illinois, USA. The impacts of these fires on the belowground ecosystem have yet to be measured. Soil (0 to 10 cm) and litter samples were collected on 40 plots in burned and un-burned control areas over a two-year period. Invertebrates, microbial activity and biomass, available nutrients, and organic matter pools were measured and compared with vegetation composition and structure from these same plots. We found few differences comparing the effects of periodic (once every three to four years) and annual prescribed fire. Exceptions included greater canopy openness, higher ratio of spring to summer herbaceous plants, and a decrease in soil invertebrate richness with the greater fire return interval. Compared to un-burned controls, burn plots had greater canopy openness, greater herbaceous richness, and a lower spring/summer herbaceous ratio. Indices of litter and soil invertebrate diversity were not significantly impacted by prescribed fire. Burned plots had higher soil pH, electrical conductivity, Ca, K, Na, total N, particulate organic matter (POM), and total organic C. Principal components were positively correlated with soil biochemical properties (C, N, Ca, K, POM, invertebrate indices) and confirmed differences between burned and un-burned plots observed with the ANOVA analyses. A stepwise least squares model with aspect, soil pH, potential N mineralization, and Bray P predicted herbaceous richness on these plots. Our results confirm others showing prescribed fire to increase soil organic matter decomposition, nutrient mineralization and nutrient availability. Forest structural changes with fire appear correlated with soil properties. The observed increases in available soil nutrients with fire likely increase growth of trees and herbaceous plants growing in more open canopies as a result of these prescribed fires. Our data show low intensity prescribed fires do not decrease C storage or negatively impact invertebrate diversity.