Evaluating ponderosa pine regeneration rates following ecological restoration treatments in northern Arizona, USA

Ecosystem restoration is emerging as a dominant ponderosa pine (Pinus ponderosa var. scopulorum) forest management objective in the semi-arid southwestern USA. Restoration consists predominantly of mechanical thinning in overly dense stands and reintroduction of ground fire, and therefore can meet a variety of community and agency objectives associated with forest ecosystem health, wood production, fire protection, recreation and scenic beauty. Ecosystem restoration and long-term multi-aged stand management creates the growing space, microclimate and seedbed conditions necessary for natural regeneration via density reduction and regular understory burning. Several long-term research studies have demonstrated regeneration > 100 seedlings/ha within a few years of overstory treatment when a relatively dense overstory canopy is left as a seed source; based on the literature, the reduced density of overstory seed-producing trees following restoration treatment still supplies sufficient viable seed over time for regeneration. Our remeasurement data of treated sites showed only 18-41 seedlings/ha in the absence of fire due perhaps to the reduced density of seed-producing overstory trees or lack of adequate (fire prepared) seedbed. Repeated prescribed burning of surface fuels can enhance the seedbed, but limits seedling survival typically during the first two decades. Survivors develop into fire resistant saplings at 10 cm stump diameter and 3 m height. Seedling density in a restoration study plots that has received two burning treatments, in 1992 and 1998, was only 12 seedlings/ha. Seedling density in the 30 growing stock level treatments at Taylor Woods averaged only 7 seedlings/ha following one burning cycle. However, even this relatively low natural regeneration rate is sufficient to supply the 3.6 trees/ha/decade needed to sustain tree densities like those present before Euro-American settlement.