It’s the 40th anniversary of Earth Day! Let’s celebrate with a round-up of a posts that shed some light into why cities matter, from the perspective of mayors, urban planners, businesspeople and community activists. Have something to share? Link to the best Earth Day stories from across the Web in the comments of this post.

VIVA MEXICO

Mayor Marcelo Ebrard writes for the Huffington Post on how Mexico City reduced its carbon footprint through a  comprehensive sustainability initiative, including major investments in public transportation. “Our efforts to control atmospheric pollutants include replacing aging taxis, microbus and government fleets with lower emission vehicles, introducing a bike-sharing program, and building a world-class bus rapid transit system,” he says. Learn about how Mayor Ebrard is commemorating Earth Day with the city’s new EcoBici bikesharing program.

GUERILLA GARDENS

The Root magazine reports on how urban farmers from African-American communities in cities across America are fighting obesity and “food deserts” with small-scale, communal agriculture projects.

MOBILE SOLUTIONS

Denver, Colo. celebrates Earth Day by launching the United States’ biggest bike-sharing program with B-Cycle, which aims to have 1,000 bikes by 2011.

THIS LAND IS YOUR LAND

Patrick Phillips, CEO of the Urban Land Institute, explains why better integration of land use planning and transportation planning matters. “Earth Day 2010 marks a pivot point for land use and community building,” he says. “Looking forward to Earth Day 2050, it’s important to consider how the impact of urban design and development meets residents’ expectations for livability, amenities, flexibility and choice.”

PlaNYC GROWS UP

Earth Day 2010 marks the third-year anniversary of New York City’s PlaNYC2030, described by Mayor Michael Bloomberg as a “long-term sustainability” plan. Read these recommendations on how the plan should start focusing more on neighborhoods, community boards, and civic and environmental groups, as provided by Tom Angotti, director of the Hunter College Center for Community Planning & Development.

AMONG THE TREES

The City of San Francisco throws a launch party for the Urban Forest Map, an interactive user-generated inventory of San Francisco’s urban forest.

EARTH DAY? HOW ABOUT EARTH MONTH?

General Motors Corp. demonstrates its commitment to the environment by launching Earth Month, with each day of the next 45 days highlighting one of its “green” initiatives.