Seedling growth and survival of western red cedar (Thuja plicata), six years later

Background/Question/Methods

Previous research revealed low rates of recruitment and high rates of mortality for tree seedlings in urban forests, especially in more urban areas. Possible causes for the high mortality included grazing by deer and elk or rodents. The study was designed to examine the effect of mammalian predation on western red cedar seedling growth and mortality in an urban forest. In 2005, western red cedar seedlings were planted in an urban forested area, Forest Park in Portland, Oregon. Nine study sites, three in each section (urban, middle and rural) of the park were located along an urban-rural land use gradient. At each site, 27 seedlings were planted. Each tree was randomly assigned to one of three groups: deer exclusion, rodent exclusion or control. Each seedling was measured prior to installation of exclusion devices in March 2005. Tree height, width, basal diameter, percent of branches grazed and mortality rate were measured and have been measured every June since. 

Results/Conclusions

The measurements made in summer 2011 revealed that tree seedlings grew significantly better (increased height, width and basal diameter) in the middle and rural sections than in the urban section of the park. Trees in deer exclusion devices grew significantly more than those in rodent exclusion devices or controls. Trees in the rural section had significantly higher levels of grazing by deer and elk than those in the urban section, although trees in deer exclusion devices throughout the park had significantly less grazing. Seedling mortality at the sites ranged from 3.7% to 70.4%. Mortality was not significantly related to predation. Our conclusions were that although deer and elk grazing did result in less growth in tree height, it did not affect tree width, increase in basal diameter, or mortality. We are following the seedlings in an attempt to ascertain the causes of the observed differential mortality.