Growth, mortality, and predation of western red cedar (Thuja plicata) seedlings in an urban forest

Previous research has shown low survival rates among tree seedlings in urban forests. Our research was designed to ascertain whether mammalian predation on young conifers has a role in low seedling recruitment. Nine study sites were located in Forest Park in Portland Oregon, three in each section (urban, middle and rural) along an urban-rural land use gradient. At each site, 27 young western red cedar (Thuja plicata) seedlings were planted. Each tree was randomly assigned to one of three groups: deer exclusion, rodent exclusion or control. Each was measured prior to installation of exclusion devices (March 2005). Tree height, width, basal diameter, percent of branches grazed and mortality rate were measured annually. Summer 2006 data showed that trees exhibited significantly greater growth at sites further from the city. The percent of grazing was significantly greater for trees in the middle section. Grazing pressure correlated negatively with the number of buildings and distance from the forest edge. Data collected in June 2007 will be presented. Preliminary results show increased mortality and grazing. Environmental parameters including light and soil conditions also will be reported.