Barcelona’s Smart City Expo looks at ways for cities to transition to a more circular economy.

Vertical vegetables and trees with their own e-mail address might sound like inventions of Dr. Seuss, but they’re actually part of the World Economic Forum’s top 10 list of urban innovations for 2015.

James Pennington, a specialist at the Forum’s circular economy initiative, addressed a packed room at the Smart City Expo in Barcelona this morning, offering these two examples as some of the best solutions being implemented around the world. Moving from a linear to a circular economy, where products and materials are used to their maximum utility, will be key to a sustainable future for cities.

Trees with their own e-mail is an approach taken by Melbourne, Australia, often referred to as “the garden city” for the leafy canopy created by its abundance of trees. But by 2009, as a result of an extended drought and severe water restrictions more than 40 percent of those 70,000 trees were stressed or dying. The implications were more than visual. Tree canopies are one of the most cost-effective ways for cities to reduce the urban heat-island effect and counter climate change.

Melbourne mapped all its trees in 2013, giving each one a unique e-mail address so citizens could easily report damage or problems. The result has been a level of civic engagement beyond anything anticipated: Along with reporting, citizens have been sending love letters to their favorite trees. Talking to trees may be as old as time, but now it can be documented — more than 3,000 e-mails over the past two years.

The benefit for the city has been a population that values and cares for the urban forest around it. The city’s website Urban Forest Visual enables residents to zoom in and identify exact trees and see their life expectancy and species.

Melbourne’s program will go beyond replacing dead and dying trees and will extend the canopy cover from 22 percent of the city to 40 percent by 2040. They will also substantially increase tree diversity to reduce the forest’s vulnerability. Meanwhile, the “treemail” idea is still generating dialogue — with the trees.

The second example James Pennington gave was vertical farming, or the more catchy “vertical vegetables,” where people grow food in the urban environment. The impetus for bringing the farm to the city is twofold. By 2050, according to the UN, 2 billion more people will be living in cities and will need to be fed. At the same time, a study by the Ellen McArthur Foundation found that 45 percent of vegetables go to waste before they reach the table because of inefficient supply chains.

Vertical farming shortens that chain and offers other efficiencies by using hydroponic systems, which need 10 times less water than conventional agriculture.

One company, Freight Farms, is upcycling shipping containers than can be located in unused urban spaces and stacked to multiply their capacity. No natural light is needed since the plants inside grow under energy-efficient LEDs.

What Freight Farms is doing, says the WEForum report, is mimicking London’s original design centuries back which included a band of farmland around the city to provide food security for its residents. Like the best circular economy, the idea is coming full circle.