Mapping the Distribution and Conservation Status of Mature and Old Growth Forests Across Coastal Alaska and British Columbia

Abstract Old growth forests in coastal Alaska and British Columbia represent some of the most expansive temperate rainforests on Earth, supporting high biodiversity, substantial carbon stocks, and the cultural heritage of Indigenous communities. Despite their importance, spatially consistent information on the distribution of old growth and mature forests in this region remains limited. We addressed this gap by integrating data across two national forest inventory programs with remotely sensed predictors. Forest inventory plots were classified by forest type and successional status (old growth, mature, or other) and linked to Landsat imagery, disturbance history, climate variables, and topographic metrics. Using this data, forest type and successional status were spatially predicted at 30-m resolution using Gradient Nearest Neighbor imputation. Across the ~ 30 million ha study region, mountain and western hemlock were the most widespread forest types. We identified 1.2 million ha of old growth forest and 2.3 million ha of mature forests, representing ~ 12% of the total land area. Alaska yellow cedar and white spruce exhibited the highest proportions of mature and old growth forests (43% and 46%, respectively), while widespread species such as Sitka spruce had relatively low percentages (4.1%). More than 40% of mature and old growth forests occurred within Inventoried Roadless Areas (IRAs) and Old Growth Management Areas (OGMAs), which lack permanent legislative protection. OGMAs contained the highest aboveground C densities, while IRAs had the greatest total aboveground carbon. These findings highlight the vulnerability of mature and old growth forests to policy or management changes that could influence their long-term persistence at a regional scale.