General best management practices for creating resilient forests

By Christine Buhl, PhD, Oregon Department of Forestry

Hot droughts have led to an increase in damaged and dead trees across the Pacific Northwest. However, many resources are available to help landowners create forests that are more resilient and better prepared for a changing climate. This post highlights current strategies from universities and natural resource agencies to improve forest resilience.

Drought impacts are particularly visible in trees, which as perennial plants, are exposed to multiple stressors year after year. Tree mortality often seems sudden, evidenced by prematurely dropping leaves, needles turning red or tops dying back. Such cases are frequently attributed to a single acute stressor, like an insect outbreak. However, indicators of chronic stress, such as thinning crowns, stress crops of cones or reduced annual growth rings, were observable all along. But forest health experts are frequently called in only when trees are already dead or dying, with few, if any, treatments available. Instead, we must plan ahead with preventative strategies to improve tree health and resilience, helping trees avoid or recover from damage.

A common concept in forest health is the cumulative impact of multiple stressors over time, which results in final tree mortality. In short, tree mortality often results not from a single factor but from a combination of stressors. These include predisposing factors beginning from planting a tree from a seed lot that is not adapted to conditions of a site, inciting factors such as drought and perhaps additional contributing factors, such as the presence of heightened beetle activity or root disease. In addition, the changing climate exposes urban trees and rural forested stands to conditions that are too extreme or that they may not have had time to adapt to. Conditions such as increasing temperature and infrequent or inconsistent precipitation are further intensified by their timing, duration, frequency and rate of change. Droughts are a common precursor that determines if a tree has enough resources to defend itself or rebound from damage caused by insect outbreaks and intense wildfires. There is direct damage to tissues such as roots and vascular tissues when a tree is moisture-stressed, which reduces their ability to obtain and transport moisture throughout the tree. There is also indirect damage when moisture stress reduces defenses that prevent, mitigate or allow trees to bounce back from damage caused by insects, diseases, fire and other stressors.

Many of these issues in our forests, like unnaturally intense wildfires, climate change-induced drought and insect pest outbreaks, can be prevented or mitigated through credible and effective strategies. These strategies include planting the right tree in the right place, reducing competition for water, promoting stand diversity and encouraging natural ecological processes that keep pests at endemic levels.

Many resources from universities and natural resource agencies are available to guide landowners in decisions that increase tree resilience to future conditions. Although predictive models in these tools are not certain, they serve as guides indicating the range of possibilities. These tools are continually refined, and new ones are always under development.

Here are some general best management practices for creating resilient forests:

1. Be aware of tree species growth requirements and major pests: USDA PLANTS Database
2. Plant trees within their range rather than along the edge for best performance. Current tree species ranges: USFS Individual Tree Species Parameter Maps
3. Plan beyond current species ranges by accounting for predicted climatic conditions
   • Predicted species ranges under low to high intensity predictive climate models: Species Potential Habitat Tool
   • Predicted within-species seedlot (i.e., seed originating in and adapted to conditions present at the time within a specific zone) transfer to sites with matching conditions based on low to high intensity predictive climate models: Seedlot Selection Tool
4. Account for site features that influence microclimate such as soil type, elevation, aspect, etc.
5. Establish young trees well to ensure proper development of tissues such as roots: Guide to Reforestation in Oregon 

Reference:

Manion, P. D. (1991). Tree disease concepts (2nd edn). Prentice-Hall.