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

The growth of ponderosa pine and associated understory vegetation was evalu ated for a 6 yr period following spring underburning of surface fuels. Unde rburn and control (unburned) plots were paired at 15 replicate sites in pol e-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 mo rtality (6%). A small but significant decline in basal area and volume grow th rates of surviving trees was found in the 6 yr following underburning. T he reduction in tree growth was related to a combination of crown length re duction, O horizon reduction, and site productivity. More productive stands had the highest proportional reduction in growth due to burning. By compar ison, site conditions including stand density, initial basal area, elevatio n, parent material, and soil fertility were not related to the observed gro wth reduction. Understory vegetation showed a mixed response to burning. Sh rub cover, dominated by Purshia tridentata, declined significantly followin g burning and remained well below preburn levels for the length of the stud y, even though one-fourth of all burned Purshia plants successfully resprou ted, Total herbaceous vegetation cover and production were unaffected by bu rning, while species diversity increased slightly, With the exception of th e decline in Purshia cover, the results indicate that low-severity prescrib ed burning has a relatively minor impact on tree-growth and understory resp onse in thinned ponderosa pine stands.