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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Report Examines the Pros and Cons of Vertical Greenery

By Kevin Wilcox

New research examines the benefits and challenges of incorporating extensive greenery into tall buildings by focusing on the early months of Bosco Verticale.

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Research based on an examination of the 900 trees that cover the Bosco Verticale skyscrapers in Milan revealed that some species adapt better to the challenging environment than others, and trees on the upper floors are more stressed, requiring 20 percent more water. Wikimedia Commons/Luca Nebuloni

June 30, 2015—Architects and urban planners, searching for new ways to bring greenery and trees into densely developed urban centers, have increasingly turned their attention to the walls, balconies, and roofs of tall buildings. The concept offers many advantages, but the question remains: can trees thrive in these artificial conditions?

The Council on Tall Buildings and Urban Habitat (CTBUH) recently released the results of research that indicate, at least in the first year, trees can be healthy and grow naturally when planted and maintained in and on high-rise structures. The report, " Vertical Greenery: Evaluating the High-Rise Vegetation of the Bosco Verticale, Milan" was conducted by Elena Giacomello, an adjunct professor at the Università Iuav di Venezia, a design-focused university in Italy, and Massimo Valagussa, a director of the Minoprio Foundation in Milan, Italy.

The Italian bosco verticale translates as vertical wood or vertical forest, and that name is apropos for the two striking residential towers comprising the Bosco Verticale skyscrapers in the Porta Nuova district of Milan. Approximately 900 trees are divided between the 76 m tall tower and the 110 m tall tower, making them look like manicured shrubbery that tower above the city. The greenery on the two towers covers a staggering 10,142 sq m of the exterior, roughly 40 percent of the surface.

The research was sponsored by a $20,000 grant provided by the global consulting firm Arup and awarded through the CTBUH's Research Seed Funding Review Committee. The goal of the seed grants is to provide initial support for research projects that can then continue on with alternative sources of funding. 

"We had seen this project from afar for quite some time," says Antony Wood, the executive director of the CTBUH. He says the committee was eager to further understanding within the building industry of the benefits and challenges of incorporating significant greenery into tall buildings.

The benefits are already well understood, and include sequestering carbon, producing oxygen, reducing the heat island effect, providing sound deadening, improving the building's energy efficiency, and adding protection to the envelope. "The science is absolutely proven," Wood says.

However, the jury is still out on how those benefits compare with the challenges of placing trees at great heights. The authors provide extensive details of how the tower was designed and engineered to accommodate the extensive use of trees, how the trees are sited along the tower, and the triple-redundant safety system that the engineers developed to secure the trees in place.

"When you look at the building you just think, 'Wow, it is a vertical forest.' It's unprecedented," Wood says. "When you see what they had to do, for example, to absolutely avoid the chance of one of these trees becoming uprooted in a storm and damaging the building or falling below and killing someone, it's remarkable."

The team conducted the research between June and October of 2013 and April and June of 2014. During the first period, the towers were still under construction, but trees had been installed in the lower levels. During the second research period, all of the trees were in place.

The researchers measured leaf chlorophyll and nutrient content to assess the health of the trees, which represent 23 species and were planted as relatively mature examples. The team analyzed chlorophyll florescence and the content of heavy metals in the leaves to determine the level of stress in the trees. The research indicates that some species seem to adapt better to the challenging environment, and that trees on the upper floors are more stressed, requiring 20 percent more water.

"These trees are being taken out of their natural habitat," Wood explains. "Those higher up—that are subject to high wind speeds and different environmental pressures—are under greater stress than those lower down. What we don't know is what are the longer-term ramifications on the trees themselves."

Because the building wasn't yet fully occupied, the researchers investigated energy use on the basis of predictive models. They developed a computer model that projects that the extensive greenery will reduce energy demand by 7.5 percent.

Wood, who is an associate professor in the College of Architecture at the Illinois Institute of Technology, says that ultimately the question of including such extensive greenery in tall buildings has greater ramifications than simply whether or not the costs and benefits for that specific building pencil out.

"Over the last 10 years I've been working with my students to really find what I call a 'new vernacular' for tall buildings—buildings that are not just steel and glass, that guzzle energy, that are the monotonous palette of architecture that is homogenizing cities around the world," Wood says. "The incorporation of vertical greenery creates a new aesthetic language for our buildings, relevant to the challenges of our age. This is a language that changes with the seasons. Plantings [are] dynamic, organic material that can bloom, flower, fall, die back, and grow. Just think of the dynamism that gives the facade.

"If we were to magnify this—if every building had vegetation—the impact of reducing the urban heat island effect by 3° or 4° [F] is absolutely massive, not just on the building, but on the people and other buildings throughout the city," he says.

Wood says that because the research was conducted so early in the life of Bosco Verticale, the CTBUH hopes researchers will revisit the topic in five years to further answer the questions about how trees fare in an urban environment at the full height of the buildings.

 

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