By Glenn Ahrens, OSU Forestry and Natural Resources Extension Agent – Clackamas, Marion, & Hood River Counties
Dying Douglas-fir trees have been a common sight in western Oregon in recent years due to drought-related problems, exacerbated by diseases and insects that attack stressed trees. History shows that periodic drought is to be expected in western Oregon and Douglas-fir forests are adapted to summer heat and drought to some extent. But extreme drought causes tree morality and raises concerns. In the news about climate change, there is a lot of talk about higher levels of heat and drought being the “new normal”. Is the recent drought “normal” or “abnormal” for western Oregon? Either way, how can we manage our forests to improve their resilience – their ability to withstand climatic extremes?
Historical climate data shows that the summer of 2015 was extremely warm and dry, but not the worst on record in western Oregon (climate data from Estacada Station 2E http://www.ocs.orst.edu/oregon-climate-data ). The Estacada station was chosen as an example because it is in the heart of the west Cascades and it has consistent records back to 1909. Summer 2015 ranks 2nd warmest (average July daily maximum temperature) and 6th driest (rainfall June-July-August) over the last 105 years. 2015 is one of only three years ranked in the top 10 for both heat and drought at the same time.
Regardless of any trends in climate change, it is clear that periodic extremes in heat and drought can be expected. Forest management practices to maintain forest resilience – the ability to recover from stressful or damaging events – can play an important role in sustaining forest health in the face of climatic extremes. While foresters have been most concerned with this on the drier east side of Oregon, attention to forest resilience and “stress relief” for trees during drought cycles is important on the west side as well.
Thinning and vegetation management in forests west of the Cascades crest are often employed to improve growth rate and quality of residual trees. These treatments can also be used to increase forest resilience by providing relief from competition stress, by managing fuel loads and reducing hazards and risks of severe fires.
The recent extreme conditions in western Oregon emphasize the need to assess forest resiliency and consider some management activities if warranted. Techniques that can be used to increase resiliency and reduce risks include:
- Monitoring your woodlands to assess risks due to wildfire, drought, insects, and disease.
- Thinning to maintain crown vigor and increase growing space for all residual leave trees. Forests become increasingly dense as the individual trees grow, increasing competition and intensifying stress. Competition stress increases trees’ vulnerability to climatic extremes.
- Periodic control of grass and brush in the understory of forests to reduce soil moisture depletion and fuel hazards.
- Timely removal of trees downed by significant storms to manage threat of beetle outbreaks within stands.
- Favoring more drought tolerant species in thinning or planting
- Fuel break strategies utilizing natural fire breaks such as creeks and roads, designated fire trails kept clear, and shaded fuel breaks – where canopy density and shade is maintained, but understory fuel is eliminated.
- Retention of some large wood on the ground can retain site moisture, especially in areas shaded by forest canopy or topography.
As with any management activity, there can be tradeoffs involved with treatments aimed at increasing forest resilience. While a thinning generally improves tree vigor and therefore some long-term resiliency, thinning also produces some stress, creates slash debris or other changes that may increase some vulnerabilities in the short term. Newly thinned stands are often somewhat more vulnerable to wind or ice damage. Thinning opens the understory to more light, which may improve understory growth and wildlife habitat, but also increases heat and drying and the amount of grass, brush, and slash fuels. A manager may need to consider vegetation control and cleanup to reduce fuel hazards after thinning.
Managing for resilient forests draws upon the basic foundation of successful woodland management: getting to know your soils, climate, and topography, and how each tree species is suited to your site. Applying this knowledge with an eye toward periodic climatic extremes and possible warming trends involves active management treatments such as thinning and weeding to reduce moisture stress, and fuels reduction to reduce fire hazards.