Recent changes (1930s-1990s) in spatial patterns of interior northwest forests, USA

We characterized recent historical and current vegetation composition and s tructure of a representative sample of subwatersheds on all ownerships with in the interior Columbia River basin and portions of the Klamath and Great Basins. For each selected subwatershed, we constructed historical and curre nt vegetation maps from 1932 to 1966 and 1981 to 1993 aerial photos, respec tively. Using the raw vegetation attributes, we classified and attributed c over types, structural classes, and potential vegetation types to individua l patches within subwatersheds. We characterized change in vegetation spati al patterns using a suite of class and landscape metrics, and a spatial pat tern analysis program. We then translated change in vegetation patterns to change in patterns of vulnerability to wildfires, smoke production, and 21 major forest pathogen and insect disturbances. Results of change analyses w ere reported for province-scale ecological reporting units (ERUs). Here, we highlight significant findings and discuss management implications. Twentieth century management activities significantly altered spatial patte rns of physiognomies, cover types and structural conditions, and vulnerabil ities to fire, insect, and pathogen disturbances. Forest land cover expande d in several ERUs, and woodland area expanded in most. Of all physiognomic conditions, shrubland area declined most due to cropland expansion, convers ion to semi- and non-native herblands, and expansion of forests and woodlan ds. Shifts from early to late seral conifer species were evident in forests of most ERUs; patch sizes of forest cover types are now smaller, and curre nt land cover is more fragmented. Landscape area in old multistory, old sin gle story, and stand initiation forest structures declined with compensatin g increases in area and connectivity of dense, multilayered, intermediate f orest structures. Patches with medium and large trees, regardless of their structural affiliation are currently less abundant on the landscape. Finall y, basin forests are now dominated by shade-tolerant conifers, and exhibit elevated fuel loads and severe fire behavior attributes indicating expanded future roles of certain defoliators, bark beetles, root diseases, and stan d replacement fires. Although well intentioned, 20th-century management pra ctices did not account for landscape-scale patterns of living and dead vege tation that enable forest ecosystems to maintain their structure and organi zation through time, or for the disturbances that create and maintain them. Improved understanding of change in vegetation spatial patterns, causative factors, and links with disturbance processes will assist managers and pol icymakers in making informed decisions about how to address important ecosy stem health issues. (C) 2000 Elsevier Science B.V. All rights reserved.