Mollisols provide clues about types of vegetation that historically existed on national forests
Soil surveys on the Umatilla National Forest. Photo credit: U.S. Forest Service
When looking at a vegetation management project, it is easy to focus solely on what the trees need and how restoration can reduce the amount of overcrowded forest stands on the landscape. But forest ecosystems are far more complex, serving as home to many different plant and animal species. Overcrowded forest conditions alter the quality and sustainability of these habitats, but landscape-scale restoration presents an opportunity to be thoughtful about where we create or enhance habitats to provide future forests that are more resilient to the effects of a changing climate.
Soils often can give us clues as to the types of vegetation that historically or naturally should occur on a site. Mollisols, for example, are the soils of grassland ecosystems. Mollisols are characterized by a thick, dark topsoil, resulting from the long-term build-up of organic materials derived from plant roots.
In the Blue Mountains, mollisols exist in many settings including ridges, shoulder slopes, mid-slopes or flat lands that are currently forested. The presence of mollisols suggests these landforms historically were dominated by grasslands, with sparse and widely spaced individual trees or localized patches of trees. In the absence of fire disturbance, trees have grown into the previously open grasslands. Today, many of these areas are completely covered by a forest of trees.
So what is the problem? In some areas, mollisols can occur in soils that are shallow to very shallow in depth or have some other root limiting factor. In the Blue Mountains, many of these areas are currently overwhelmed by overstocked forest conditions. While shallow mollisols may have the nutrient capacity to support timber stands, the shallow soil depths tend to be droughty and typically cannot sustain a forest full of trees. Historically, a dense forest would have been uncommon and/or short-lived because fire or insects would have killed the trees. An unintended consequence of excluding fire from the landscape is that we now have unstable, unsustainable forested conditions growing over shallow mollisols when there should be dry open grasslands.
When considering the extended drying trends predicted within climate change models, the ability of shallow soils to support trees will likely be further degraded or constrained. We cannot control climate trends and drought, and we cannot know how climate change will affect a specific place, but we do know that some tree species are more resistant to drought and fire than others. We also can use landscape features, like mollisols, to help design appropriate treatments that are more likely to be sustainable over the long term. By managing density and composition, forest managers support the growth of desired vegetation that can thrive under changing climate conditions. These treatments will reduce or mitigate drought-related stress and improve forest resilience. While reducing the number of trees might not necessarily affect overall water consumption, active management can reduce competition for nutrients and create healthier forests and grasslands that are better able to resist drought stress, insect attack, and uncharacteristic wildfire.
The Forest Resiliency Project provides an opportunity for land managers to take a landscape-approach to creating a more resilient ecosystem. The Forest Resiliency Project is being designed to set up our forests to be resilient against natural disturbances in the face of a changing climate. By reducing stand density across large landscapes (thinning the number of trees on an acre), adjusting species composition (altering the species of trees and other vegetation growing on a site), and creating mosaic forest patterns, we can reduce the risks from uncharacteristic wildfires, and allow fire to play its natural role on the landscape.