Pine-oak forest dynamics five years after ecological restoration treatments, Arizona, USA

Five years after ecological restoration treatments in a ponderosa pine-Gambel oak forest, we re-measured permanent plots to assess changes in forest structure and understory vegetation. The treatments were (1) thinning to emulate pre-fire-exclusion conditions + prescribed burning (FULL restoration), (2) minimal thinning around old trees + burning (MIN), (3) burning alone (BURN), and (4) CONTROL. We expected tree growth and understory abundance to be greatest in the least dense (FULL) treatment. Probably due to drought as well as treatment effects, basal area, tree density, and canopy cover declined 3-20% over the 2000-2004 time period. Smaller trees and those with greater crown scorch were most likely to die. Tree growth differed significantly by species and treatment; ponderosa pine grew faster than oak and the FULL treatment had the highest pine basal area increment and quadratic mean diameter. Understory plant cover and richness differed only slightly by treatment, generally varying more with pre-existing conditions and climate. Exotic species were present but exotic cover and richness were less than that reported after comparable treatments or wildfires in the region. Compared to historical reference conditions at the time of the last surface fire, 1887, the FULL was less dense and all treatments were relatively low in basal area, due to 20th century harvesting of most of the large pines. At current growth rates and without additional mortality, the FULL treatment may be similar to historical forest structures in ∼20 years. Tree densities in other treatments are expected to remain above historical levels. The dynamics of stands following alternative restoration treatments are of high interest for management because large areas have been proposed for treatment but there is limited data on effects.