Modeling urban host tree distributions for invasive forest insects using a two-step approachThis article is part of a larger document. View the larger document here.
Many alien insect species currently impacting forested ecosystems in North America first appeared in urban forests. Unfortunately, despite serving as critical gateways for the human-mediated spread of these and other forest pests, urban forests remain less well documented than their “natural” forest counterparts. While Forest Inventory and Analysis (FIA) data provide good information about the composition of natural forests, only a small percentage of the more than 26,000 communities in the US and Canada have completed any sort of urban forest inventory, and these inventories have commonly been restricted to street trees. We devised a two-step approach that utilizes the available local inventory data to comprehensively model urban host tree distributions at a regional scale. We illustrate the approach for three tree genera – ash (Fraxinus), maple (Acer), and oak (Quercus) – that are associated with high-profile insect pests. Available inventory data include 60 sample-based inventories of entire cities (i-Tree Eco inventories) and 475 street tree inventories. First, based on existing inventories, we use a suite of explanatory spatial variables to model the proportion of the total basal area (as a proxy for forest volume) occupied by each genus. Second, we apply a similar suite of spatial variables to estimate the total basal area of these communities. These estimates will be combined to estimate basal area of each genus in non-inventoried communities and to construct region-wide urban distribution maps for each genus. By merging these maps with similar data on natural forests (e.g., distribution maps developed from FIA plot data), we are able to provide a more complete host setting for spread modeling efforts. Urban FIA projects promise to provide information about the composition of urban forests, but it will be some time before most US urban areas have been inventoried intensely. This modeling approach provides a use for urban FIA data as they become available to better understand urban forests at larger spatial scales.