Native vs. nonnative trees: comparative impacts on landscape biodiversity

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1 Native vs. nonnative trees: comparative impacts on landscape biodiversity Examples of mandates Seattle, WA new developments Neighborhoods around Philadelphia, PA natives only Riverside, IL planting on parkways restricted UK and Oregon tree removal Mandate rationale Loss of native species with development Highway revegetation efforts Ecological restoration guidelines Sense of place Perception #1: Loss of biodiversity due to nonnative species Invasive plants have no natural enemies to controls their spread. Once they take over an area, complex native plant communities, with hundreds of different plant species supporting wildlife, are reduced to a monoculture. Perception #2: Only native species can fill ecological roles Perception #3: Natives are superior to introductions adaptation to local environment reduced maintenance integration with other species sense of place Benefits of using native plant species are unquestionable Less water No fertilizer Fewer pesticides Attract wildlife Create oxygen Prevent floods Purify water Prevent weeds Aesthetics Save money Biodiversity Research question Do native and nonnative trees differ in how they affect community diversity?

2 Study parameters Peer-reviewed scientific articles Urban settings o Private residences & residential streets o Botanical gardens & public parks o Remnant natural areas o Golf courses o Exempt: ecological restoration sites o Exempt: geographically isolated areas Organisms other than trees considered in biodiversity analysis Study sites North America Canada (3 provinces), United States (11 states) Central America Mexico, Puerto Rico South America Argentina, Brazil, Peru, Uruguay Europe Bulgaria, Czech Republic, Denmark, France, Germany, Slovakia, Spain, Switzerland, Turkey, UK Asia China, India, Japan, Nepal, Philippines,Taiwan Middle East - Israel Africa South Africa Oceania - Australia, New Zealand Plant biodiversity Older trees important for epiphytic species o Bark is thick and coarse o Some epiphytes are host specific Compared to forest interiors, edge habitats o richer in total species o richer in forest specialists than forest interiors Edges act as refuges for rare and endangered species Home gardens and other cultivated areas serve as conservation refuges for endangered and vulnerable plant species. Bird biodiversity Urbanization favors omnivorous, carnivorous, granivorous, and cavity nesting species Gardens provide fruit and nectar Forests provide mostly seed Urban areas have wide range of resources Negatives: lawns, lack of water, lack of protection for ground nesters or foragers Native fruit-eating birds can increase invasive species over native relatives

3 Insect biodiversity Many native pollinators have become dependent on introduced plants as their native hosts disappeared. Community, botanical, and residential gardens support a wide diversity of insects (especially pollinators). Pest insects Resistance of introduced trees to pests often the same as or greater than natives Damage higher in natural forests than urban or ornamental settings Generalists can feed on plants in several families nativeness doesn t matter Animal biodiversity Introduced tree species, including invasives, can meet and diversify habitat needs. Richness and diversity of animal species enhanced by: o Habitat structure (canopy cover, vertical diversity, tree and shrub density and diversity) BIMR o Larger and/or connected sites BIM o Older, larger trees BIMR o Hollow trees IMR o Woody debris - R o Moderately disturbed sites (suburbs, public greenspaces) B o Profusely flowering species with seasonal diversity I o Permanent water source I o Herbaceous/grass cover MR o Native vegetation specialists only Natives better than introduced? they need less water require fewer chemical pesticides require little or no fertilizer attract beneficial wildlife are aesthetically pleasing add biological and genetic diversity save money create oxygen we breathe help in preventing floods purify water prevent weeds species from establishing The rationale, revisited: Definitions of native and alien are value judgments. Not all introduced species are invasive. Urban areas are not natural.

4 Native species often not adapted. Introduced species provide ecological benefits. Vegetation diversity, structure and function more important to biodiveristy than nativeness. Mandating native trees in urban areas is not a science-based policy. Utilize introduced species to enhance ecosystem biodiversity Selection strategies: tree surveys Age distribution Percent canopy cover Size distribution Species diversity Selection strategies: comparing urban to native sites Air pollution Drought Heat Light Salt Soils Pests and disease Selection strategies: diversifying palette Identify needed roles in plant community food value nesting value structural diversity species diversity - natives & nonnatives Selection strategies: predicting invasiveness Decision tree (Reichard and Hamilton, 1997): non-vegetatively reproducing trees hybrids slow-growing species from noninvasive taxonomic groups Management strategies: Plant alternative resources before removing invasive species utilized by native birds Remove invasive species favored for nesting during the winter Maintain a vertically-diverse assemblage of trees, shrubs, and groundcovers to enhance vegetation structure. Protect middle-aged trees to ensure they become old trees Plant trees in diverse groups rather than as isolates or monocultures

5 Enhance vegetation at landscape edges ( buffer strips ) and leave alone Maintain deadwood and other nesting structures in place when possible Reduce managed, open lawns when possible Protect quality of surface waters Reduce soil compaction with organic mulch Adopt less intrusive practices Special situations: golf courses Out-of-bounds areas similar to forests outside courses Large percentage of edge Increase deciduous and coniferous tree cover Vegetate around water Reduce amount of highly managed turf Special situations: residential Educate property owners as to how well-managed gardens and landscapes contribute to urban biodiversity Beyond arboriculture Science-informed decision-making must occur in planning and design of urban green spaces. Cannot overemphasize the importance of edge habitat in enhancing biodiversity across the board Urban areas are not natural ecosystems natives-only policies reduce planting palettes and ultimately biodiversity. Non-native trees species have demonstrated value in: o storing carbon o supporting biodiversity, and o enhancing urban landscapes Community, botanical, and residential gardens as well as public greenspaces with noninvasive, introduced species support biodiverse communities. Neighborhood well-being positively associated with species richness, abundance of birds, and vegetation cover For more information: Dr. Linda Chalker-Scott, WSU Associate Professor and Extension Horticulturist lindacs@wsu.edu URL: Blog: http// Books: http// Facebook page: Facebook group: Washington State University gardening page: