The influence of urbanization on forest stand dynamics in Northwestern Oregon

Abstract

Urbanization has been shown to affect forest stand characteristics in nearby natural areas. The purpose of this study was to examine forest structure in a naturally forested area, Forest Park in Portland, OR and adjacent forested land. Tree community structure was examined at 25 sites distributed along an urban-rural land use gradient. All trees in three quadrats per site were identified to species and the diameter at breast height was measured. ANOVA was used to examine differences in species richness and diversity, and tree density and importance value among four categorical areas. Tree species richness and diversity, and the density, diameter and importance values of shade-tolerant (later successional) species of trees decreased with urbanization. Sites nearer the city of Portland had significantly fewer shade tolerant saplings and young trees and were dominated by earlier successional species of trees as compared with sites at the far end of the study area. The forest structure in the city section of the park was very similar to that in the significantly younger middle section. The lack of young shade tolerant saplings and young trees appears to be the result of urbanization, although the mechanisms for such a loss are unknown at this time. Such a lack of recruitment may interfere with normal successional processes at more urban areas of the park.

Conifer growth and reproduction in urban forest fragments: Predictors of future responses to global change?

“…For example, introduced species, which we observed at some urban and rural sites, are known to negatively impact native species through competition (Mack et al 2000) or herbivory (Nystrand and Granström 2000;Pigot and Leather 2008;Strauss et al 2009). Regardless of cause, the lack of regeneration is consistent with other observations of early life-history stage trees in urban areas (Broshot 2007;Lehvävirta and Rita 2002;Zipperer 2002). With this absence of regeneration, a currently abundant and characteristic species of urban and rural forests, T. plicata, may become rare in future forests.…”

“…4). Other studies have addressed recruitment in regional forests: van Mantgem et al (2009) did not find any decrease in recruitment over the past decades in the west in general, while Broshot (2007) documented reduced regeneration of T. plicata and other conifers in an urban core compared to less urban areas in our region. These studies both differ from our study in that they measured recruitment to the sapling life stage rather than to the seedling stage.…”

Mortality and recruitment in an urban forest (Forest Park in Portland, Oregon) between 1993 and 2003

“…Many urban forest ecosystems exhibit reduced species diversity (Airola and Buchholz 1984, Sharpe et al 1986, Pysèk and Pysèk 1990, Tait et al 2005, Broshot 2007). Tree stand characteristics affected by urbanization include changes in density, basal area and/or importance value of trees in urban forests (Airola and Buchholz 1984, Rudnicky and McDonnell 1989, Broshot 2007. Tree growth rates have been shown both to increase (Iakovoglou et al 2001) and decrease (Quigley 2002;Carreiro and Tripler 2005).…”

“…Several studies show urbanization may be interfering with normal succession processes in forests (Kowarik 1990, Matlack 1993, Leak and Smith 1996, Porter et al 2001, Quigley 2002, Komuro and Koike 2005, Broshot 2007). Tree mortality and recruitment are important parameters in determining how succession will proceed, but limited studies have been done in urban areas.…”

“…Most of the urban studies have assessed street trees (Gilbertson andBradshaw 1985, Nowak et al 2004), which do not necessarily respond similar to trees in natural forested urban areas. Several workers have reported reduced recruitment in urban forests (Greller 1975, Airola and Buchholz 1984, Fahey 1998, Lehvävirta and Hannu 2002, Zipperer 2002, Bertin et al 2006, Broshot 2007, and some have reported increased tree mortality in urban areas (Davis 2004, Nowak et al 2004, Carreiro and Tripler 2005, Broshot 2007). Nowak et al (2004) reported that tree mortality in urban, forested areas was moderate as compared to that in more urban areas, but made no comparison with more rural forests.…”