Modeling Urban Host Tree Distributions for Invasive Forest Pests Using a Multi-Step Approach

Many invasive pest species currently impacting forested ecosystems in North America first appeared in urban forests. Despite serving as critical gateways for the spread of forest pests, urban forests remain less well documented than their “natural” forest counterparts. Only a small percentage of communities in the US and Canada have completed any sort of urban forest inventory, and these inventories usually have been limited to street trees. We devised a multi-step approach that utilizes the available local inventory data to 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 included 60 sample-based inventories of entire cities (i-Tree Eco inventories) and 500 street tree inventories. First, we used co-kriging to estimate the whole-city tree compositions based on street tree inventories. Next, we used boosted decision trees to model the proportion of the total basal area (as a proxy for forest volume) occupied by each genus in non-inventoried communities as a function of a suite of environmental and demographic variables. We then modeled total urban forest basal area on canopy cover of these communities using a generalized additive model. We combined these estimates to construct region-wide urban distribution maps for each genus. By merging these maps with similar data on natural forests, we are able to provide a more complete host setting for spread modeling efforts