Determining Causes of Pacific Madrone Decline in Urban Landscapes of the Pacific Northwest

In order to determine factors contributing to the decline of the Pacific madrone (Arbutus menziesii), 103 trees were surveyed and rated on a scale of 1–4. The performance rating was then compared with the following plant parameters and site conditions: size, growth habit, bark characteristics, location and size of cankers on the trunk, light exposure, slope, soil characteristics, management practices and competing vegetation. Significant correlation was found between performance and: 1) growth habit—spindly trees did poorly compared to bushy trees; 2) percent cankering on the trunk; 3) persistent bark—trees that experienced peeling bark on their trunks looked poorer than those that retained a thick layer; 4) soil texture—trees growing in silty or clay loam soils performed poorly; and, 5) sprouting at root collar—trees experiencing dieback often produced sprouts near the base of the trunk (results of the completed research, involving 300 trees, are summarized at the end of the paper). The most serious tree decline occurs where urban development exposes trees previously growing in the forest. Trees growing in a forest are tall and spindly compared to trees growing in the open. Thick bark retained under shady conditions is shed when exposed to strong sunlight. Both of these factors may make madrones more susceptible to canker diseases. Knowledge of how a madrone can be expected to perform in a landscape is of value to landscapers desiring to incorporate or plant madrones into a landscape and to landscape managers caring for madrones in existing landscapes. Many persons have noticed that in areas around the Puget Sound the Pacific madrone (Arbutus menziesii) appears to be declining. Many trees can be seen with dead or dying twigs and branches. This has caused concern for the health of the species throughout our area. In fact, at least 3 canker diseases may infect madrone in this area: Arbutus canker [Nattrassia mangiferae (formerly Hendersonula toruloidea; Davison 1972 and Hunt, et al. 1992)], Phytophthora root rot (Phytophthora cactorum) (Stuntz and Seliskar 1943) and madrone canker [Fusicoccum aesculi (Burns and Honkala 1990; cf. Elliott