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

Abstract

I examined tree recruitment and mortality over a ten-year period at permanent plots in an urban forest, Forest Park, in Portland, Oregon. The density and diameter at breast height (dbh) for all trees living and dead were measured in 1993 and again in 2003. Data were analyzed using paired Student t-tests. I found significantly fewer live and significantly more dead trees in 2003 than in 1993. The increase in mortality was significant for all species of trees and for all sizes except large diameter trees. Mortality rates ranged from 0% to 67% at my sites. Recruitment was lower at all sites in 2003 with significantly fewer seedlings and saplings. The high mortality and low recruitment resulted in a net loss of trees at all sites. Loss of trees was not offset by increasing tree diameter, which suggests self-thinning is not the cause. No strong correlation with an urban to rural land use gradient was observed. The results may be related to global climate change or pollution. The high mortality of trees of all species in many diameter classes without a concomitant increase in recruitment could lead to dramatic changes in forest structure.

Nativity and seed dispersal mode influence species’ responses to habitat connectivity and urban environments

“…Urbanization also changes the local environmental conditions, including warmer temperatures (the urban heat island effect; Oke, ), increased concentrations of nutrients and pollutants (Pouyat, Szlavecz, Yesilonis, Groffman, & Schwarz, ), and stream incision (deeper stream channels) caused by large run‐off events during storms (Walsh et al, ). These environmental changes affect plant growth (Gregg, Jones, & Dawson, ), recruitment (Trammell & Carreiro, ) and survival (Broshot, ), and thus act as filters limiting the plant species that persist in urban environments (Knapp, Kühn, Schweiger, & Klotz, ; Williams et al, ). The relative importance of dispersal and environmental conditions for structuring urban plant communities has implications for biodiversity conservation in urban landscapes, because there is often a trade‐off between maintaining or enhancing connectivity between protected habitat patches (e.g., by creating corridors; Hobbs, ) versus prioritizing patches for conservation based on their environmental conditions (Fahrig & Merriam, ; Kang, Minor, Park, & Lee, ).…”

Long‐term outcomes of forest restoration in an urban park

“…Creating, restoring, and sustaining forests in urban areas are complicated by habitat fragmentation, invasive species, and degraded soils. For example, forests in the city of Portland, Oregon, have seen a decrease in recruitment and increase in tree mortality between 1993 and 2003 (Broshot ), trends indicating that these forested ecosystems are not capable of sustaining themselves without intervention. When it does occur, natural regeneration in urban areas may or may not lead to a diverse forest dominated by native species (Alvey ).…”