Chapter 4: Tree Species Recommendations

Data and Methods

The i-Tree Species tool ranks tree species best on potential benefits derived from tree species at maturity. Users input their geographic location of interest and rank various tree benefits on a scale of 0 (not desired) to 10 (highly desired). A database of about 1,600 tree species is then reduced based on plant hardiness of the selected area (i.e., tree species not hardy to the selected location are eliminated from potential selection). The remaining species are then sorted based the user-selected weighting of the following desired benefits:

  • Air pollution removal
  • Air temperature reduction
  • Ultraviolet radiation reduction
  • Carbon storage
  • Pollen allergenicity
  • Building energy conservation
  • Wind reduction
  • Streamflow reduction (storm water management)

Methods for estimating the benefits and species hardiness zones are detailed in Nowak (2008). Species selections are also noted as to whether they are invasive to the area (based on state invasive species lists), sensitive to various air pollutants or at risk to various insects or diseases. This information is provided to aid local users in selecting species that are not only functional, but that can survive given various risks (pests, pollution) and will not cause environmental issues due to invasive tendencies. The ultimate species selections should be made by local experts and the species list is provided as a guide to help with these local species selections.

The Popple River watershed has a USDA hardiness zone of 4. For this assessment, streamflow benefit weighting was set to 10 and other benefits set to 0. Based on this weighting, the top 10% of the species that are hardy to zone 4 were selected.

Results

The top 10% of the final recommended species list for the watershed are given in Table 4.1. Hardiness ranges are uncertain for 18% of the species. Only 1 of the species is considered invasive: White poplar.

Table 4.1. Recommended list of species for the watershed. Note that only the top 10% of matches from the list are displayed.
Sensitivitya
Scientific Name Common Name O3 NO2 SO2 Pest Riskb
Ulmus americana American elm I/S ALB, DED, WM
Tilia americana American basswood I I GM, WM
Ulmus glabra Wych elm ALB, DED
Betula alleghaniensis Yellow birch I S ALB, LAT, WM
Magnolia acuminata Cucumber tree
Tilia platyphyllos Bigleaf linden I GM
Larix kaempferi Japanese larch I S
Populus deltoides Eastern cottonwood I ALB, WM
Larix laricina Tamarack GM
Larix decidua European larch I/S S GM, PSB
Picea abies Norway spruce MPB, PSB, SPB, SBe, WSB
Tsuga canadensis Eastern hemlock I HWA, SPB
Acer platanoides Norway maple S I ALB, WM
Aesculus hippocastanum Horsechestnut ALB
Tilia cordata Littleleaf linden GM
Acer rubrum Red maple I I ALB, WM
Acer x freemanii Freeman maple ALB
Celtis occidentalis Northern hackberry
Aesculus flava Yellow buckeye S ALB
Betula papyrifera Paper birch S ALB, GM, LAT, WM
Tsuga caroliniana Carolina hemlock HWA, SPB
Populus x canadensis Carolina poplar GM
Larix lyallii Subalpine larch GM
Larix siberica Siberian larch
Juglans nigra Black walnut TCD
Aesculus glabra Ohio buckeye I ALB
Populus x canescens Gray poplar GM
Populus fremontii Fremont cottonwood WM
Fraxinus pennsylvanica Green ash S S ALB, EAB, WM
Pseudotsuga menziesii Douglas fir I/S DFB, FE, PSB, WSB, DFBS
Pseudotsuga macrocarpa Bigcone douglas fir S DFBS
Populus alba* White poplar GM, WM
Fraxinus americana White ash S EAB, WM
Larix occidentalis Western larch S GM, WSB
Populus angustifolia Narrowleaf cottonwood I GM, WM
Populus x brayshawii Hybrid balsam poplar S GM
Populus x heimburgeri Heiburger’s poplar GM
Populus heterophylla Swamp cottonwood GM, WM
Populus x hinckleyana Hinckley poplar GM

a Sensitivity: “S” indicates sensitive to pollutant; “I” indicates intermediate rating between sensitive and tolerant to pollutant; and “S/I” indicates a mix of sensitive and intermediate ratings in the literature. O3 = ozone; NO2 = nitrogen dioxide; SO2 = sulfur dioxide.
b Pest abbreviations: ALB - Asian longhorned beetle; DED - Dutch elm disease; WM - winter moth; GM - gypsy moth; LAT - large aspen tortrix; PSB - pine shoot beetle; MPB - mountain pine beetle; SPB - Southern pine beetle; SBe - spruce beetle; WSB - Western spruce budworm; HWA - hemlock woolly adelgid; TCD - thousand canker disease; EAB - emerald ash borer; DFB - Douglas-fir beetle; FE - fir engraver; DFBS - Douglas-fir black stain root disease
* Invasive species

Discussion

There are numerous species with a potential to intercept rainfall and transpire water to help reduce storm water runoff. While all healthy trees will provide these functions, larger trees with more leaf area tend to provide more of these services than smaller tree species. This species list is only intended as a guide for species selection related to storm water reduction. Trees provide numerous other benefits and i-Tree Species could be used to refine this selection if other benefits are desired or considered more important than storm water runoff.

As the recommended list is only based on hardiness zone and estimated benefits, local expert knowledge on which tree species are locally available and suitable is essential to refine this list. Some species on this list will not perform well locally due to various site conditions (microclimate, soils, available planting space for roots and crown, etc). Other species may be exotic or invasive species, or at risk to local insects and diseases, all of which could lower the ecosystem benefits of those species. There is currently no way in i-Tree Species to isolate or exclude species classified as non-native, invasive, or susceptible to a particular pest or disease. This list is a starting point for managers and should be used in conjunction with a more rigorous species selection process.

Land managers making species selections should consider choices that will enhance the local diversity of urban forests. Diverse forest ecosystems have been found to be more resilient when exposed to pests (Guo et al., 2019) and disease (Haas et al., 2011). Increasing the resilience of forest ecosystems therefore increases the likelihood trees will survive such exposures and continue to provide ecosystem services.

Species selections should ensure that the tree can survive under the local conditions and remain healthy throughout its potential life span. Species recommendations are based on the size of tree at maturity and do not consider species life span. Life span is a critical element to consider in species selection also as long-lived species have the potential to provide greater benefits through time. Life span estimates are currently being added to i-Tree Species. When combined with local expertise and priorities specific to a project or location, this recommended species list can hopefully guide planners and project managers to plant the right tree in the right place for the right reasons.