Spatial and temporal patterns of forest disturbance and regrowth within the area of the Northwest Forest Plan

Understanding fine-grain patterns of forest disturbance and regrowth at the landscape scale is critical for effective management, particularly in forests in western Washington, Oregon, and California, U.S., where the policy known as the Northwest Forest Plan (NWFP) was imposed in 1994 over > 8 million ha of forest in an effort to balance environmental and economic tensions. We developed approaches to create disturbance and regrowth maps for forests within the area of the NWFP from the results of LandTrendr, a temporal segmentation algorithm described previously only at the pixel scale. Maps were developed from 674 Landsat Thematic Mapper and Enhanced Thematic Mapper + images distributed across 22 separate scene areas, and were assessed for validity at 2360 points using TimeSync, a time-series validation and interpretation tool. Unlike maps derived using other techniques, maps derived from the segmentation approach were unique in providing simultaneous detection of abrupt disturbance, chronic disturbance, and ongoing vegetative growth with consistency across large areas and across time. Maps were then used to address six core monitoring questions focusing on the distribution of disturbance across time, ownership categories, and ecoregions. Forest was disturbed at rates that varied by ownership category and state, ranging from 9% to > 39% of forest area over the period 1985 to 2008, with highest cumulative disturbed area on private and native lands in Washington and Oregon and lowest disturbed area on federal protected lands in Washington. Effects of court injunctions and subsequent implementation of the NWFP lowered forest disturbance rates, particularly in Oregon, and also caused decreases in the relative magnitude of disturbance on those lands relative to private lands. State-managed forests showed forest disturbance rates that varied considerably among the three states, with the highest rates in Washington state and lowest in California. Affected by large, stochastic fire events, protected lands in both Oregon and California showed disturbance rates similar to those found on actively managed federal lands. Protected lands also experienced high rates of chronic disturbance, often associated with insect-related mortality. As expected, moisture-limited ecoregions recovered vegetation more slowly than those where moisture was not limiting. Vegetative regrowth rates showed substantial variation among ownership categories, suggesting that differential forest policies may affect vegetative recovery rate. Taken together, these results emphasize that forest management policies do have manifestations at the landscape scale, but that detection of these manifestations is best achieved with mapping approaches that can detect both abrupt and longer-duration processes within the Landsat archive.

► Methods to map forest disturbance and regrowth from annual Landsat time-series data.
► Disturbance and regrowth by ownership and ecoregion in the Pacific Northwest, U.S..
► State and federal forest management changes are captured in time-series data.
► Large stochastic events dominate forest disturbance on protected lands.
► Regrowth rates vary considerably by ecoregion.