Conifer– Ceanothus interactions influence tree growth before and after shrub removal in a forest plantation in the western Cascade Mountains, USA

Abstract

Ceanothus velutinous is often considered to be an aggressive competitor with young conifers in the western United States. Using a conifer (noble fir, Pacific silver fir, Douglas-fir, and western hemlock) plantation in western WA where Ceanothus had become established, we assessed how conifer–shrub crown interactions affected individual tree growth before and after cutting Ceanothus from around individual trees. For all conifer species, trees surrounded by but with leaders above Ceanothus were 18–70% taller than open-grown trees, and trees with leaders below Ceanothus were 22–50% shorter than open-grown trees; suggesting that Ceanothus had both positive and negative effects on tree growth. Few seedlings (8%) of Douglas-fir were found under Ceanothus. However, because of shade-intolerance, Douglas-fir growing under Ceanothus exhibited the greatest reduction in height (53%) compared to open-grown trees. The most shade-tolerant species, Pacific silver fir, had the least reduction in height (22%) under Ceanothus, yet because of its slow growth, most of its seedlings (62%) were found growing under Ceanothus. Twelve years after cutting Ceanothus, growth rates increased for all four tree species independent of the type of previous interaction with Ceanothus; however, mean tree heights and diameters for all four conifers remained substantially lower for trees that had been overtopped by Ceanothus compared to trees with leaders not overtopped at the time of release. Thus, the benefits of growing with or being released from encroachment by Ceanothus will vary with conifer species and the degree of conifer–shrub interaction and may persist for many years.