Survivorship and Growth of Seedlings and Saplings in Urban Forests

Research in 1993, 2003, and 2013 showed high rates of tree mortality and low rates of recruitment (new trees) in Portland, Oregon’s Forest Park. To determine if the lack of young trees was an urban phenomenon, we added three control sites in the Mount Hood National Forest in 2013. Last summer, seedlings and saplings were measured at all sites in Forest Park, the Ancient Forest Preserve, and at the control sites. Seedlings are trees less than 2 meters tall, and saplings are trees greater than 2 meters tall but having a dbh of less than 10cm. The 2018 data was compared to that collected in 2013. We measured dbh (diameter at breast height) of saplings and the basal area for seedlings, as well as the tree height and the height of the lowest living branch. In 2018, the control sites had significantly more live trees, more coniferous trees, more shade tolerant trees, and more seedlings and saplings. We also found that the seedlings and saplings in 2018 had a greater diameter at the control sites than the urban sites. We found significantly more dead trees in 2018 than 2013 in Forest Park

Soil Chemistry and Ground Cover in Urban Forests

In 1993, I began doing research at 25 permanent study plots in Forest Park in Portland, Oregon. In 2003 and 2013, I found high mortality of trees in Forest Park including seedlings and saplings (recruitment). To determine if the lack of recruitment was an urban phenomenon, I added three control sites in the Mount Hood National Forest in 2013. Significantly lower numbers of young trees were found in the urban forest than at rural sites. In 2015, I found deeper O horizons, higher C/N ratios, and higher levels of soil respiration at more rural sites. Last summer we collected soil samples at all sites to test for a wider range of elements. We are still processing the soil samples so they can be analyzed and will send them to the Central Analytical Laboratory at OSU later this semester to test for calcium, magnesium, and aluminum, in addition to carbon and nitrogen. To better quantify impacts of urbanization on the forest, we also measured ground cover by herbaceous plants and shrubs, downed woody debris, and canopy cover at all sites. We found significantly deeper O horizons, higher rates of soil respiration, and lower levels of electroconductivity (EC) at rural sites than at urban sites. We believe changes to the soil may be related to the lack of recruitment in urban forests and are looking forward to getting the chemical data back from OSU later this spring to see if there are any differences among locations relative to urbanization. We also will be analyzing the other data we collected. The herbaceous data will be especially interesting. A major die-off of sword ferns has been noticed in the Pacific Northwest, and comparing the data we collected last summer to that previously collected will help determine if the die-off has spread to the Portland area