Short-Term Belowground Responses to Thinning and Burning Treatments in Southwestern Ponderosa Pine Forests of the USA

Steven Overby,Stephen Hart

doi:10.3390/f7020045

Abstract

Microbial-mediated decomposition and nutrient mineralization are major drivers of forest productivity. As landscape-scale fuel reduction treatments are being implemented throughout the fire-prone western United States of America, it is important to evaluate operationally how these wildfire mitigation treatments alter belowground processes. We quantified these important belowground components before and after management-applied fuel treatments of thinning alone, thinning combined with prescribed fire, and prescribed fire in ponderosa pine (Pinus ponderosa) stands at the Southwest Plateau, Fire and Fire Surrogate site, Arizona. Fuel treatments did not alter pH, total carbon and nitrogen (N) concentrations, or base cations of the forest floor (O horizon) or mineral soil (0–5 cm) during this 2-year study. In situ rates of net N mineralization and nitrification in the surface mineral soil (0–15 cm) increased 6 months after thinning with prescribed fire treatments; thinning only resulted in net N immobilization. The rates returned to pre-treatment levels after one year. Based on phospholipid fatty acid composition, microbial communities in treated areas were similar to untreated areas (control) in the surface organic horizon and mineral soil (0–5 cm) after treatments. Soil potential enzyme activities were not significantly altered by any of the three fuel treatments. Our results suggest that a variety of one-time alternative fuel treatments can reduce fire hazard without degrading soil fertility.

Highlights

When Euro-Americans settled in the southwestern United States of America (USA), the ponderosa pine
For the variables we analyzed pre‐treatment, there were no significant differences within blocks among the assigned treatment units
There was a significant reduction in forest floor mass for firetime interaction significantly different (Table S2)

Summary

Introduction

When Euro-Americans settled in the southwestern United States of America (USA), the ponderosa pine Lawson) forests they encountered were more open landscapes than today, with clusters of pine trees interspersed in meadows dominated by grasses [1,2]. Frequent fires (every 2–20 year), herbaceous competition, and periodic drought maintained the pre-Euro-American forest structure with densities of 30–140 trees ha1 [3,4]. Following Euro-American settlement, fire exclusion, livestock grazing, and removal of large pre-settlement trees increased stand densities dramatically [5,6,7]. Removal of the dominant trees, coupled with a large seedling recruitment of ponderosa pine in 1919 [8,9], increased stand densities to the current 727 trees ha on average in Arizona [5], with some stands exceeding 2000 trees ha1 [6,7].

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