Trees make good urban citizens

The Boston Globe recently checked in on the city of Boston’s promise to plant 100,000 trees by 2020 and found execution woefully lacking. Mayor Thomas Menino made the promise in 2008, responding to concern over climate change. Urban trees across the nation are aging and dying. Planting trees helps absorb carbon and reduce energy use. Last year Boston notched a net gain of only 4,000 trees.

What would Central Park be without its trees? New York is way ahead of Boston in planting new and replacement trees.

    My city's director of urban forestry, Marc Welch, reports he gets more requests to remove street trees than to plant them. Some residents regard trees as pests dropping leaves on their yards. Others want them out because of roots growing into pipes or making driveways and sidewalk surfaces uneven. Some of our street trees need to be removed because they’ve died, falling victim to drought, vehicle strikes, and nonnative insect pests such as winter moth and gypsy moth.

    Marc is pro-tree, though. He and the Newton Tree Conservancy are bucking the trend, planting new street trees since. In my post of May 14, I described the planting process.

Planted last month

    Trees do much more than offer shade in summer. One of my favorite garden authors, Toby Hemenway, describes the many ecological services provided by trees: 

• Creating insect habitat and hunting grounds for birds. One example: as the air warms in the morning, the leafy canopy stays cool longer than the air just above it. Insects swirl around in the layer of warm air, allowing birds to find food.

Cedar waxwing feasting on fruit of an amelanchier tree

• Conserving water. Soil stays moist under the shade of the tree’s leaves, watering plants and contributing to stream flow.

• Filtering groundwater. As leaves transpire, releasing water through pores in the leaves called stomata, the tree cleans out impurities from the water it draws up from the ground. 

• Making rain. Up to half of rain over tree-covered land comes from water transpired from leaves. In addition, pollen and dust that mixes with air as it flows through the leaves form nuclei for raindrops, seeding the clouds.

Trees make rain for the plants and creatures around them

• Harvesting moisture. Fog condenses on cool leaves, gathering water even without rain.

• Sequestering carbon. During photosynthesis, leaves take in carbon dioxide from the air and release oxygen.

• Stirring breezes in summer and blocking wind in winter. In warm weather, convection starts air moving when the tree absorbs sunlight and mixes warm air with cooler air near the ground. A tree can also act as a windbreak, reducing heat loss from buildings in winter.

Conifers make good winter windbreaks

• Preventing erosion. Leaves catch falling rain and funnel it toward the tree’s trunk, dispelling energy so that soil is not displaced. Leaf litter and roots keep soil in place.

• Making their own fertilizer. Pollen, dust, bird and insect droppings, bacteria and fungi collect on leaves and fall to the ground with rain, carrying plant nutrients and also organisms that will help break down organic matter in soil into forms roots can use.

Trees build soil

Our predecessors knew that trees enhance city life. By continuing to plant trees along our streets, we can maintain that environmental benefit.