Planting Trials in Northern Arizona Ponderosa Pine Forests

Abstract

Mechanical tree thinning and prescribed fire have been widely proposed for restoring ponderosa pine (Pinus ponderosa) forests in the southwestern United States. However, these restoration activities often result in intensely disturbed sites, such as landings for equipment and temporary roads, which managers may need to revegetate. Managers may also wish to augment the forest understory through revegetation after thinning and burning treatments. We present five planting trials conducted as part of restoration projects in northern Arizona ponderosa pine forests in order to assess survival of 11 species and the influence of various microsites created by restoration activities. In one trial, five-year survival of four transplanted graminoid species ranged from 1% to 23% among species and from 0% (unthinned forest) to 22% (unburned slash) among microsites. Also illustrating microsite effects, over three times more Fendler’s meadow-rue (Thalictrum fendleri) survived when outplanted below Gambel oak (Quercus gambelii) than in openings created by restoration pine thinning. Only 4% of purple locoweed (Oxytropis lambertii) survived in an unsuccessful attempt at revegetating a decommissioned road on dry cinder soils. Conversely, two years after outplanting on a different road on moister soil, mountain muhly (Muhlenbergia montana) survival was 72% (89% of which were fruiting), and 40% of plants of the biennial ragleaf bahia (Bahia dissecta) were surrounded by new seedlings. Results illustrate that successes occurred without supplemental watering even in dry years; outcomes hinged on species or planting microsite; and heavy mortality occurred in some contexts. Owing to financial costs and logistical constraints, revegetation may be most suited for localized disturbances caused by restoration activities or for creating revegetated islands for understory augmentation.