But roots are not the only issue for city trees. Biodiversity, including a variety of species and age, increases survival rates and resistance to natural processes, Mr. Ward says. A notorious example is the Dutch elm disease, which decades ago wiped out US populations of the Dutch elm tree. Ward still fields calls from city planners desiring to plant elm trees on their Elm streets.
"They want to know, 'Should we plant it all in elms?'" he says. "I tell them, 'No.' "
Ward praises Boston officials for undertaking a "very ambitious" 100,000 tree project, but cautions that a city tree's life – especially that of an urban sidewalk tree – can be stunted by factors from drought to human interaction.
"These trees take a coalition of people to basically babysit, water, and tend to," says Mr. Ward. "In the end, aftercare makes all the difference."
Tips for urban tree survival
1. Consult with a certified arborist before planting. He or she will advise about the best tree species for the soil, space, and overall environment of where you want to plant.
2. Improve the landscape before you plant. Soil particles need to have optimum moisture and air movement, but soil compaction often causes urban trees to have poor aggregation. This can be helped by adding organic matter such as compost or peat moss.
3. Prepare the tree appropriately before it's planted. Almost all trees used to be planted using the "balled and burlap" method, where the rootball is wrapped in soil and burlap. Now, trees can also be successfully planted using a "bare root" method. Consult with your arborist for the best planting method for your tree and environment.
4. After planting, mulching is crucial for survival. Water and cover the upturned soil area with wood chips about 4 inches deep and at least 6 inches away from the trunk.
5. Make sure the mulching protects the tree from lawn mowers and trimmers.