Evaluating the role of cutting treatments, fire and soil seed banks in an experimental framework in ponderosa pine forests of the Black Hills, South Dakota

Abstract

Pinus ponderosa Laws. (ponderosa pine) forests have changed considerably during the past century, partly because recurrent fires have been absent for a century or more. A number of studies have explored the influence of timber harvest or burning on understory production in ponderosa pine forests, but study designs incorporating cutting and prescribed burning in an experimental framework are needed to identify mechanisms responsible for the observed changes. In this study, we first characterized the disturbance history and the soil seed bank of a ponderosa pine stand in the northern Black Hills. We then experimentally addressed the effects of prescribed burning and overstory reduction on understory vegetation. Before Anglo settlement of the area, the mean fire interval was 14 years and no fires were recorded after 1879. Cessation of fires, prolific regeneration of ponderosa pine, and subsequent logging in 1903 has led to a very dense, even-aged ponderosa pine stand with very little understory vegetation and very few viable seeds in the soil seed bank. Only 57 individual plants, or 186 seeds/m 2, emerged from 1080 soil samples. Response of understory vegetation during the first growing season after application of treatments was sparse, with no significant treatment effect. There were, however, significant treatment effects during the second growing season. Total understory biomass ranged from 5.8 kg/ha on untreated plots to 1724 kg/ha on clearcut, unburned plots. Herbaceous dicots comprised over 90% of total understory biomass. Both understory species richness and evenness responded to treatments, but understory woody plant density did not respond to either treatment. Paucity of viable seeds in the soil seed bank does not appear to constrain recruitment of understory vegetation in dense ponderosa pine forests of South Dakota.