In September 2019, the French agriculture ministry announced the discovery of the bacterium Xylella fastidiosa, also known as the olive tree disease. It was first detected in Puglia, Italy, in October 2013 when ancient olive trees began to die and over 230,000 hectares of olive groves were cut down. Two trees are said to have been infected in the south of France in the Alpes Maritimes and have since been destroyed to stop the disease from spreading. Over 1m valuable olive trees were killed by the subspecies of the disease that was found this September resulting in the destruction of all trees and plants vulnerable to the bacterium within a three-mile radius.
No cure or prevention for the disease has been found as of yet, which prevents plants and trees’ ability to absorb water. Michel Dessus, the president of the chamber of agriculture in the Alpes-Maritimes, where the two infected trees were discovered, said more tests were needed before swathes of vegetation were destroyed. “Cutting down trees more than a hundred years old needs to be thought about,” he told French television. Palm trees all the way down the Promenade Des Anglais and the Croisette in Cannes have been affected by red beetles in the French riviera too.
The European Plant Protection Organization has declared Xylella fastidiosa a “very serious threat to the European region”, claiming its’ manifestation worsens in dry and hot conditions. As a result, the EU has provided substantial funding to combat the disease, which the European commission describes as “one of the most dangerous plant bacteria” in the world “causing a variety of diseases, with an economic impact for agriculture, public gardens and the environment”.
When an undetected disease was found in bigleaf maple trees in Washington in 2018, studies into the widespread infection uncovered numerous abnormalities as the cause of the decline. Xylella fastidiosa was one of the first theories for the disease, but further examination gave inconclusive results. Another theory linked increased human development with bigleaf maple decline, as well as higher summer temperatures, extreme summer droughts, all of which will only increase in the future. As of the end of 2018, no sign of recovery in dying bigleaf maple was seen with symptoms of the disease including partial or entire crown dieback, discoloration and reduced size of the leaves, crown thinning, and death.
Why should we be concerned with increasing diseases in urban forestry? Trees act as cooling agents able to bring down temperatures by two to three degrees centigrade. When planted near buildings, trees can cut air conditioning use by 30%, and, according to the UN Urban Forestry office, reduce heating energy use by a further 20-50%. One large tree can absorb 150kg of carbon dioxide a year, as well as filter some of the airborne pollutants. As more and more infections wipe out hectares of trees across the world, temperatures rise and the effects of climate change accelerate.
The city of Amsterdam spends $3 million annually to aggressively combat outbreaks of Dutch Elm Disease. The American elm was once America’s most beloved and abundant city tree. As a result of their widespread branches that provided shelter and shade for cities and their ability to grow in urban soil, thousands of Elm Trees were planted across American cities, resulting in mass destruction when the disease hit. Diversity is a main component of a healthy urban forest. Different species are susceptible to various pests and different kinds of damage, so the more diverse kinds of trees that are planted in a community, the less vulnerable the overall forestry will be to invasive species, climate change, or severe weather.
One of the major reasons that Dutch elm disease and emerald ash borer have been so devastating to communities is that many had planted nothing but elms or a very high proportion of ashes. The more closely related tree species are, the more likely they are to be vulnerable to the same pests and damage. The goal is to follow the 5:10:15 rule: In any community, no more than 5 percent of trees should be of the same species; no more than 10 percent should be from the same genus; and no more than 15 percent should be from the same family. When choosing a tree to plant for example, make sure it is suitable for the growing conditions of the site and try something new.
The best way to protect our community’s urban forest is to practice diversity. We must break away from buying the same common trees that everyone knows the name of. We need to select, not only new species, but whole new families of trees, in order to protect our communities from losing all of their shade due to an epidemic of a new pest.
By the end of 2019, the city of Paris is looking to make the city greener by creating ‘urban forests’. The strategy promoted by Mayor Anne Hidalgo will create parks and gardens in an effort to lower the overall temperature by planting 30 hectares and over 20,000 new trees before 2020. This planting scheme is supposed to make 50 per cent of the city’s surfaces vegetated and is part of Paris’ aim to be a carbon neutral city by 2050. In an interview with Paris Match in September 2019 Hidalgo said, “I am convinced that Paris must adapt to changing temperatures, The Intergovernmental Panel on Climate Change forecasts heatwaves at 50 degrees Celsius by 2050. We have an obligation to act today.”
As part of the Biodiversity Information System for Europe, a structure was put in place called the ‘trame verte et bleue’ (Green and Blue Network, GBN) which is “a spatial planning tool covering the entire national territory, with a core objective of stopping the decline of biodiversity by conserving and restoring ecological continuities to ensure provision of ecosystem services”.
In Italy however as a result of the mass dying olive tree epidemic in 2013, the country adopted the National Law on the Development of Green Urban Areas (Law n. 10, 14.1.2013) aimed at promoting green areas for the provision of ecosystem services (air quality, hydrological risks, soil protection and cultural dimensions). As well as this, the Vertical Forest (Bosco Verticale) was launched, a project in the Porta Nuova district of Milan, containing plants roughly equivalent to 2.5 acres of forest (European Commission, 2017). The Metropolitan Area of Milan has created a system of regional parks under Regional Law 86/1983 on protected areas, reaching 39% of the land area in the Province in 2012. The system consists of six regional parks and an extensive agricultural belt called Milan South Agricultural Park (GreenSurge, 2015).
According to a study published by Eyob Tenkir Shikur entitled Challenges and problems of urban forest development in Addis Ababa, Ethiopia; low floristic diversity has resulted in problems with plant selection and high levels of exotic species. The study highlights that the “limited diversity of species may have negative impacts on the survival of dominant species where outbreaks of disease and pest attack are recorded to which these trees are vulnerable”.
An exotic plant is a plant that has been brought into an environment from another climate whereas native plants occur naturally in a region in which they evolved. They are the ecological basis upon which life depends, including birds and people. Without them and the insects that co-evolved with them, local birds cannot survive.
Some of the most commonly used trees and plants in urban forests across the world are Maple, Ash, Pine, Elm, Cedar and Oaks and nearly all of these have been infected by serious diseases over the last ten years. Some of the best trees to plant are various strains of Oak trees, Red Maple, Dogwood, Green ash or Japanese Maple as they develop well and require minimal attention in Urban settings.
Urban and city forests are essential to the environment which make the care for them and infrastructure very important. Having the wrong trees, badly cared for soil, or too much of the same species when added to diseases, fires, floods and storms added to social problems such as air pollution and over development makes for challenging urban expansion in the future and increasing temperatures as a result. In September 2018, British engineers created a seed-planting drone which could help restore the world’s forests called Biocarbon Engineering, a start-up based in Oxford. The drones fire seed missiles across fields, planting hundreds of potential trees in minutes. In 2018, the drones were deployed in a field outside Yangon, Myanmar, in hopes of combating Mangrove deforestation. Today, the seeds have grown into 20-inch tall Mangroves.