Abstract
Urban forests are valuable assets to a community from multiple perspectives. Diversifying urban forests is a common and important management goal to increase resilience in the face of biotic pests and diseases, climate change, and to increase ecosystem services. Multiple design and engineering consultants develop site plans and specify trees for public and private projects, and consultant disciplines differ in their training plant material selection and diversity. Publicly available site plans from Des Moines (79) and Iowa City, Iowa, USA (70) between 2015–2018 that passed planning and zoning review were analyzed to determine the associations between practitioner disciplinary training and the diversity of specified trees at site, city, and landscape scales. We found that consultant discipline was not related to the alpha diversity of a site until site size was greater than 2.42 ha, then civil engineers were associated with lower tree diversity than other consultants. Landscape architects completed a majority of the project plans (43 %), specified the most trees (62 %), and utilized the most diverse plant palette, including 30 species not used by other consultants. However, landscape architects also frequently specified clonally produced cultivars (62 % of all selections) which may limit genetic diversity at a landscape scale, thus potentially undermining diversification efforts by repeating genotypes. Findings from this study suggest that messages pertaining diversification efforts, plant selection, and reduction of genetic diversity resulting from cultivar specification are important for all consultants, but especially for landscape architects who were responsible for specifying a majority of trees and sites. This case study of two Iowa (USA) communities provides evidence of landscape-scale diversity outcomes associated with consultant discipline and the potential for use of public site plan documents to assess such relationships. Further research is needed to determine if these findings are generalizable to other municipalities or states.
Highlights
IntroductionUrban forests offer a multitude of benefits for city dwellers (Ma et al, 2020; Morgenroth et al, 2016; Tyrvainen et al, 2005) and there is increasing interest in diversifying tree canopies to increase ecosystem services, mitigate risks associated with both biotic pests and diseases, and abiotic stresses such as drought and climate change (Conway and Vander Vecht, 2015; Ma et al, 2020; Ordonez and Duinker, 2013; Sjoman et al, 2015)
The analysis revealed that the α-diversity of sites was similar regardless of consultant discipline and city and that α-diversity increased with site size
This case study used publicly available site plans to assess the di versity of trees specified by different consultant disciplines that varied in their professional training about plant materials
Summary
Urban forests offer a multitude of benefits for city dwellers (Ma et al, 2020; Morgenroth et al, 2016; Tyrvainen et al, 2005) and there is increasing interest in diversifying tree canopies to increase ecosystem services, mitigate risks associated with both biotic pests and diseases, and abiotic stresses such as drought and climate change (Conway and Vander Vecht, 2015; Ma et al, 2020; Ordonez and Duinker, 2013; Sjoman et al, 2015). Explicit management of urban forest diversity through taxonomic benchmarks has been widely considered (Ball and Tyo, 2016; Kendal et al, 2014; Raupp et al, 2006), including the 10/20/30 formula that remains influential (Santamour, 1990). If the 10/20/30 guideline were followed, a pest affecting multiple spe cies in a genus would still result in a 20 % loss (Ball and Tyo, 2016). Taxonomic benchmarks must be carefully considered with respect to a multitude of other considerations
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