Six-year changes in mortality and crown condition of old-growth pondersoa pines in ecological restoration treatments at the G.A. Pearson Natural Area

Abstract

Ecological restoration treatments using thinning and prescribed burning have been proposed to reverse the decline of old-growth ponderosa pines in the Southwest. However, long-term data on the effectiveness of such treatments are lacking. In 19931994, two ecological restoration treatments and a control were established at the G. A. Pearson Natural Area (GPNA) near Flagstaff, AZ. The thinned treatment removed many postsettlement-aged trees to create tree density and stand structure similar to pre-Euro-American forests. The thinned + burned treatment included prescribed burning of the forest floor in combination with this thinning. The control was a dense stand of pre- and postsettlement trees with no thinning or burning. Crown dieback of presettlement trees decreased by about 3 percent in both thinned treatments over 6 years since initiation of treatments, whereas dieback increased by about 4 percent in the control. Crown dieback was not related to tree age. Change in height of presettlement trees between 1994 and 2000 did not differ among treatments. Of 146 presettlement trees monitored for survival between 1994 and 2000, four died between 1997 and 2000, and all were in thinned treatments. Two of the four trees that died toppled or the stem broke in a severe windstorm in 1997. The other two dead trees died between 1997 and 2000 following the severe regionwide drought of 1996. These two dead trees had large amounts of canopy dieback prior to treatment initiation, suggesting that thinning did not contribute to their mortality. Our results indicate that heavy thinning of postsettlement trees improved the crown condition of presettlement trees at the GPNA over 6 years since treatment, but also may have increased windthrow and wind breakage