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

Abstract

Global change has a large and growing influence on forests, particularly in urban and urbanizing areas. Compared to rural forests, urban forests may experience warmer temperatures, higher CO 2 levels, and greater nitrogen deposition, with exacerbated differences at urban forest edges. Thus, comparing urban to rural forests may help predict future effects of global change on forests. We focused on the conifer western red-cedar (Thuja plicata) to test three hypotheses: at urban forest edges, relative to rural forests and urban forest centers, trees experience 1) higher temperatures and nitrogen levels, 2) lower seedling recruitment, and 3) greater growth. We additionally tested anecdotal reports that 4) tree seedling recruitment in urban and rural forests is much lower than in "pristine" old-growth forests. To test these hypotheses, we quantified air temperature, soil nitrate, adult T. plicata growth and seedling recruitment in five urban and three rural parks at both forest edges and centers. We also quantified T. plicata recruitment at five old-growth "pristine" sites. Temperatures were highest at urban forest edges, and soil nitrate was highest in urban forests. In urban relative to rural forests, we observed greater T. plicata growth, but no difference in seedling densities. However, seedling densities were lower in urban and rural forests than in old-growth forests. In all, our results suggest urban influences enhance adult T. plicata growth, but not seedling recruitment. Recruitment in urban and rural forests was reduced compared to old-growth forests, implying that fragmentation and logging reduce T. plicata seedling recruitment.

Prevalent vegetation growth enhancement in urban environment

“…Another study along a rural-urban gradient in Baltimore showed that ecosystem productivity in secondary succession fields increased 60% and 115% for the suburban and urban sites, respectively, relative to the rural site (5). Growth enhancement in urban settings has also been reported in the conifer western red cedar (Thuja plicata) in Seattle (21) and in pine and oak forests in the Florida Panhandle (22). These isolated field studies corroborated our findings from remote sensing that enhancement of the EVI is overwhelmingly positive across cities.…”

Plants in the city: understanding recruitment dynamics in urban landscapes

“…For example, in urban forested natural areas in Seattle, Washington, logging and the selective removal of old‐growth trees, which contribute a disproportionate amount of total seed within populations, have been hypothesized to drive local source limitation and limit natural regeneration of conifer species (O'Brien et al . ; Ettinger et al . ).…”

“…Edge effects, human activity, and soil contamination are just a few of the many urban drivers that may limit plant growth and survival (eg O'Brien et al . ; Pregitzer et al . ).…”

“…Source limitation may also shift as a result of direct human activity, which may selectively introduce or remove individuals in a landscape. For example, in urban forested natural areas in Seattle, Washington, logging and the selective removal of old-growth trees, which contribute a disproportionate amount of total seed within populations, have been hypothesized to drive local source limitation and limit natural regeneration of conifer species (O'Brien et al 2012;Ettinger et al 2017). Despite the inherent effects of urban drivers on adult plant abundance and distribution, there are few studies that have quantified the effects on relative seed availability in the landscape.…”

“…Edge effects, human activity, and soil contamination are just a few of the many urban drivers that may limit plant growth and survival (eg O'Brien et al 2012;Pregitzer et al 2016). Other factors in urban sites, such as elevated tropospheric ozone, temperature, and nitrogen deposition, have been shown to promote the establishment and growth of urban plants (eg Gregg et al 2003;O'Brien et al 2012;Zhao et al 2016). In addition to being species-specific, these impacts may be region-specific; in grasslands, for instance, high levels of nitrogen facilitate the growth of only a few species, especially more common ones, which alters not only interspecific competition dynamics but also community composition (Zeeman and Morgan 2018).…”