NOAA/WDS Paleoclimatology - Fule - G.A. Pearson Natural Area - PIPO - ITRDB AZ617

We report on survival and growth of ponderosa pines (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) two decades after forest restoration treatments in the G. A. Pearson Natural Area, northern Arizona. Despite protection from harvest that conserved old trees, a dense forest susceptible to uncharacteristically severe disturbance had developed during more than a century of exclusion of the previous frequent surface-fire regime that ceased upon Euro-American settlement circa 1876. Trees were thinned in 1993 to emulate pre-fire-exclusion forest conditions, accumulated forest floor was removed, and surface fire was re-introduced at 4-year intervals (full restoration). There was also a partial restoration treatment consisting of thinning alone. Compared to untreated controls, mortality of old trees (mean age 243 yr, max 462 yr) differed by < 1 tree ha-1 and old tree survival was statistically indistinguishable between treatments (90.5% control, 92.3% full, 82.6% partial). Post-treatment growth as measured by basal area increment of both old (pre-1876) and young (post-1876) pines was significantly higher in both treatments than counterpart control trees for more than two decades following thinning. Drought meeting the definition of megadrought affected the region almost all the time since the onset of the experiment, including three severe dry years. Growth of all trees declined in the three driest years but old and young treated trees had significantly less decline. Association of tree growth with temperature (negative correlation) and precipitation (positive correlation) was much weaker in treated trees, indicating that they may experience less growth decline from warmer, drier conditions predicted in future decades. Overall, tree responses after the first two decades following treatment suggest that forest restoration treatments have led to substantial, sustained improvement in the growth of old and young ponderosa pines without affecting old tree survival, thereby improving resilience to warming climate.