Restoring dry and moist forests of the inland northwestern U.S.

Abstract

The complex topography of the inland northwestern U.S. (58.4 million ha) interacts with continental and maritime air masses to create a highly variable climate, which results in a variety of forest settings. Historically (1850 to 1900), approximately 20% of the area was covered by dry forests (Pinus ponderosa, Pseudotsuga menziesii), and an estimated 18% was covered by moist forests (Pinus monticola, Tsuga heterophylla). Frequent surface fires burned over 75% of the area of dry forests; today, these fires burn approximately 45% of the area. In the dry forests, successful fire exclusion and harvesting allowed dense stands of Abies grandis, Pseudotsuga menziesii, and small Pinus ponderosa to develop. Historically, forest canopies (Pinus ponderosa, Larix occidentalis) and their nutrients were located well above the soil surface; fine roots and microbial activity were located deep in mineral soils, thus protecting them from wildfire. In contrast, the Abies- and Pseudotsuga-dominated forests of today contain nutrient-rich crowns that extend to the forest floor. Nutrients and microbial activity are located near the soil surface, increasing their susceptibility to loss from wildfire.