Tree-growth and understory responses to low-severity prescribed burning in thinned Pinus ponderosa forests of central Oregon

Abstract

The growth of ponderosa pine and associated understory vegetation was evaluated for a 6 yr period following spring underburning of surface fuels. Underburn and control (unburned) plots were paired at 15 replicate sites in pole-sized ponderosa pine forests of central Oregon. The burns were generally low in severity, as noted by low O horizon mass reduction (24%) and tree mortality (6%). A small but significant decline in basal area and volume growth rates of surviving trees was found in the 6 yr following underburning. The reduction in tree growth was related to a combination of crown length reduction, O horizon reduction, and site productivity. More productive stands had the highest proportional reduction in growth due to burning. By comparison, site conditions including stand density, initial basal area, elevation, parent material, and soil fertility were not related to the observed growth reduction. Understory vegetation showed a mixed response to burning. Shrub cover, dominated by Purshia tridentata, declined significantly following burning and remained well below preburn levels for the length of the study, even though one-fourth of all burned Purshia plants successfully resprouted. Total herbaceous vegetation cover and production were unaffected by burning, while species diversity increased slightly. With the exception of the decline in Purshia cover, the results indicate that low-severity prescribed burning has a relatively minor impact on tree-growth and understory response in thinned ponderosa pine stands.