Urban tree growth modeling

There is little information available that documents and models the growth of trees under heterogeneous urban conditions, despite the fact that understanding tree growth is fundamental to quantifying the ecosystem services trees provide and to choosing the best species for a given location. This paper provides an overview of the state of the science. Two general approaches to modeling tree growth are described. The first, empirical models, use field measurements and statistical methods to predict tree growth. I-Tree Eco and i-Tree Streets are described and their results compared; the Lindenmayer-Systems approach, which combines tree growth equations and computer animation, is also presented here. The second approach includes process-based and hybrid models, which translate nutrient and carbon uptake into tree growth. These models are highly complex, which makes them computer processor-intensive and difficult to quantify in terms of their uncertainty. Finally, the USDA Forest Service Reference City master database is introduced. The database includes 17,000 trees representing 171 species in 16 climate regions. From this data, 2,600   regionally specific growth equations predicting a number of tree dimensions were developed. One important outcome of these equations is the determination that species growth can differ significantly owing to variations in climate, tree care, and soil.