Douglas-firs with dying branches and tops, and occasionally entire trees were a common site in the Willamette Valley this spring. Branch dieback in Douglas-fir is not a new phenomena, but it is very extensive this year and alarming to landowners. Questions have been raised as to the causes of dieback and whether it is likely to spread. Examination of the dying branches in the early spring has occasionally shown signs of infection by canker fungi or attacks by insects, but in many cases dying branches have no signs of these opportunistic organisms. It is important to note that while dieback symptoms expressed as red foliage become visible in late winter or early spring, the actual damaging event may have occurred months earlier or may even have resulted from an accumulation of damage over several years. The 2001 branch dieback in Douglas-fir lacks a consistent association with insect attack or disease and this strongly suggests abiotic (non-biological) causes for the damage.

Although there can be several causes of branch dieback in Douglas-fir, most of the current damage appears related to water stress. Trees with dieback often share several characteristics that make them more susceptible to water stress. One is that many damaged trees are under 30 years of age and growing in shallow, rocky, or droughty (heavy clay) soils. These sites may not have supported conifer forests in the past and are probably unsuitable for Douglas-fir over the long term. Trees with dieback symptoms are also found along roads, the edges of stands, or growing in the open. Douglas-fir in these exposed locations is subject to higher temperatures and levels of evapotranspiration (water loss). On these exposed sites severe water stress occurs in late summer and may kill the tops or branches of trees. But why does the damage show up in the spring? The expression of damage symptoms is delayed because wet cool conditions during the fall and winter months prevents the foliage from drying and turning red. When temperatures rise in the spring the drying process accelerates and the damage suddenly becomes visible. Other abiotic causes of branch dieback in Douglas-fir include low temperature injuries and root dieback from excessive moisture. A detailed discussion of the branch dieback phenomena and what can be done to reduce tree stress can be found at www.odf.state.or.us/ fa/FH/fhn/deadbr99.pdf.

Since additional trees will be damaged by drought in the summer of 2001, more branch dieback and tree mortality will become visible in the spring of 2002. As the drought develops, insect and disease problems may increase where trees have been weakened by water stress. Areas such as abandoned Christmas tree plantations and overstocked stands of Douglas-fir may suffer tree mortality. These biotic factors could become more important if drought conditions persist into 2002.

Douglas-fir has been widely planted in the Willamette Valley and in some cases has become established on very marginal sites. The visible damage to Douglas-fir during this drought event points out areas where this tree is not particularly well adapted or proper stand management has been lacking. On droughty sites, Willamette Valley ponderosa pine or Oregon white oak maybe a better choice for reforestation and long term management.

Respectively forest entomologist and pathologist with the Oregon Department of Forestry in Salem.