Baltimore City’s tree cover increased from 27 percent to 28 percent between 2007 and 2015, based on data analyzed by the U.S. Forest Service and in collaboration with City of Baltimore and the University of Vermont. Tree canopy coverage for cities in the United States ranges from less than 20 percent to upwards of 40, with several cities, including Detroit and coastal Los Angeles, experiencing a loss during the same time period.
Baltimore’s 1 percent net gain equates to an additional 200 acres of tree coverage for the city, and thereby provides residents with a continued increase in benefits such as a reduction in utility bills and storm water runoff. This is great news for Baltimore. Despite challenges in the years to come from exotic insects and increasingly violent storms, the U.S .Forest Service and the city’s Tree Baltimore Program will continue efforts to reach a goal of 40 percent cover.
Tree canopy cover measurements refer to the extent of the outer layers of leaves, or canopy, of a tree or group of trees. Percentage tree canopy cover measures the portion of an area that has tree coverage when viewed or measured from above. Amount of tree canopy coverage is typically a reflection of a variety of factors — including intentional planning and investment. Studies throughout the United States have repeatedly shown that most communities are losing tree canopy due to a wide range of threats, including insects, disease, natural disasters and development.
The tree canopy change detection in Baltimore was carried out using high-resolution aerial imagery and 3D airborne laser-guided data detection, known as LiDAR. Change was mapped at the individual tree canopy level for every single tree in the city, overcoming the limitations of other studies that used coarser resolution data or sample-based approaches.
Why should we care about the extent of tree cover? As it turns out, tree coverage has immense benefits not only to Baltimore City residents, but also to surrounding communities. U.S. Forest Service and related research has demonstrated that there can be a beneficial correlation between trees and public safety (more trees, less violent crime and crimes against property), regardless of socioeconomic factors. Urban forests have positive impacts on human health (reduced asthma and other respiratory problems) and economic development (increased property values and business performance), while exposure to nature can relieve ADHD symptoms and improve academic performance. Further, trees improve water quality and reduce flooding by acting as purifiers and sponges during and after rainfall, filtering fertilizers, pesticides and pollution that washes off roads and other impervious surfaces. Increasing porous cover by investing in trees and green space has water quality benefits for the Chesapeake Bay and downstream communities. This is especially essential in cities like Baltimore, where sidewalk storm water drains connect directly to the bay.
Baltimore’s 1 percent tree canopy cover increase is encouraging but leaves room for improvement. The actual story of tree canopy change in Baltimore is more nuanced than simply stating that tree canopy increased. Over the 2007-2015 time period, areas throughout Baltimore experienced both substantial gains and losses. Locally, these changes can lead to positive effects (e.g., reduced peak summer temperatures) and negative ones (e.g., loss of wildlife habitat).
Tree canopy loss, whether due to human activities, such as construction, or natural events, such as a severe storm or the currently unfolding outbreak of Emerald Ash Borer, which is expected to decimate the region’s ash tree population, can be instantaneous and dramatic. Tree canopy increases resulting from new plantings, natural regeneration, and growth, are slow processes that take time and commitment.
Increasing or even maintaining tree cover in a city is not an incidental event. It typically requires collaboration and investment by multiple government and non-profit entities, as well as the efforts of private citizens who choose to plant trees on their property. In Baltimore City, TreeBaltimore operates as the umbrella organization that supports and coordinates all tree planting and greening efforts within the city — including city agencies such as Baltimore’s Departments of Recreation and Parks, Public Works, Transportation and Planning; and non-profits such as Blue Water Baltimore, the Baltimore Tree Trust, Baltimore City Forestry Board, Baltimore Green Space, and Parks and People Foundation. Trees planted today will help buffer against loss and deliver benefits for generations.
Promoting Baltimore's tree canopy in the future will continue to require not only additional tree plantings but an investment in preserving our existing trees.
Erik Dihle (firstname.lastname@example.org) is the city arborist for Baltimore City Recreation and Parks’ urban forestry division. Morgan Grove (email@example.com) is a scientist and team leader for the U.S. Forest Service’s Northern Research Station.