The natural disturbance regime on the Tongass is wind. Most of the forest is subject to periodic frequent small wind events that break or blow down single trees or small groups of trees. These small wind events have created a patchy forest with great biodiversity. There are some places on the Tongass that are subject to less frequent, larger wind events that can blow down hundreds of acres at a time. These large wind event forests are somewhat less patchy and diverse. The old growth (OG) forest and associated wildlife that exists here has developed over thousands of years to adapt to these particular wind disturbance patterns. Historically, most of the OG timber harvest on the Tongass National Forest (TNF) has been by the clear cut method. Clear cut harvesting was and is the silvicultural prescription of choice because it is economical, efficient, and usually produces fast growing, even-aged stands. Depending on the scale, clear cutting can closely mimic the disturbance of a large wind event. There are some negative impacts of clear cut harvesting, most notably:
- The loss of biodiversity, as the more diverse OG stand is converted to a more homogenous even aged stand;
- The eventual loss of wildlife habitat as the young stands enter stem exclusion as the crowns close and limit light to the forest floor;
- The loss of some forest functions, such as loss of large dead wood (both standing and down);
- The altered snow melt and rain fall through patterns.
These negative impacts are more pronounced on those more diverse portions of the forest that are a result of small wind disturbances.
There are alternative, partial harvest prescriptions that better replicate the natural disturbance of small wind events than do clear cuts. Selective harvest, shelterwood harvest (when the OG shelter trees are left within the stand), and clear cuts with reserves have been around for many years and are used on the TNF.
The Nature Conservancy, with input from industry, federal and state resource managers, and conservation representatives, developed a selective helicopter harvest prescription for OG stands that provides guidelines for the harvest of valuable timber and the protection of forest function and diversity.
Variable retention harvest prescriptions are replacing clear cut prescriptions in other parts of the country. These partial harvest prescriptions leave strategic old growth scattered throughout the harvest area to continue forest function, provide diversity, and “life boat” vulnerable species. The residual OG can either be dispersed as single trees or aggregated as groups of trees. Wind concerns would dictate the use of aggregate retention on the Tongass. Some blow down due to wind is to be expected with any retention as it is a natural occurrence; the key is to avoid excessive blow down.
Progressive partial harvest prescriptions include the protection and management of large down dead wood, large standing dead trees (snags), and cull (rotten) near dead green snags. These trees have little if any commercial value and dozens of species depend on dead and rotten wood for at least a portion of their lifecycle. Retention patches can be anchored around large dead wood and other legacy trees.