Impacts of woodchip amendments and soil nutrient availability on understory vegetation establishment following thinning of a ponderosa pine forest

Abstract

Mechanical thinning for fire mitigation has become increasingly widespread in recent years throughout the western United States. A common practice in fire-mitigation procedures is the conversion of slash into chipped mulch (referred to as “woodchips”) that is spread on-site. Here, we investigated: (1) the effect of woodchip amendments on soil nitrogen availability, and (2) the influence of potential interactions between woodchip amendments and soil nitrogen availability on patterns of understory plant establishment in a thinned montane forest in the Front Range of Colorado. We created a gradient of soil nitrogen availability among experimental plots by applying sugar to reduce plant-available nitrogen and applying ammonium nitrate to increase plant-available nitrogen. Soil and resin bag analyses confirmed the effectiveness of the nitrogen manipulations. Nitrogen manipulations did not affect understory species richness or diversity, but increased nitrogen availability promoted a disproportionate increase in the cover of non-native forbs over the three-year post-treatment interval. Woodchips did not alter inorganic nitrogen availability at ambient levels of soil fertility during the first two years of the study, but were associated with increased soil N availability in the third growing season. Woodchip additions were associated with significantly lower species richness, diversity, and overall plant cover than found in unchipped plots. The observed reduction in plant cover affected native and non-native species similarly, although certain species had distinctive responses to woodchip amendments. On the whole, woodchips excluded more species than they promoted, and the physical effects of woodchips appeared to be more important than soil nitrogen status in influencing the understory composition of this area following thinning. At this site, approximately 60% of the plant cover in the three years following thinning operations was comprised of non-native plant species. Thus, the thinning activity, itself, promoted non-native understory plant species cover, but the application of woodchip mulch reduced vegetation cover and native species richness relative to the thinned areas that did not receive woodchips. This effect was largely independent of soil nitrogen availability during the first three years following thinning.