THE URBAN CHALLENGE

Multifunctional urban corridors address the needs of pedestrian and vehicular traffic alike, and trees have an important role in providing shade, generating clean air, lowering air and pavement temperature, and creating usable and comfortable open space. The most significant problem for tree growth is soil compaction under impermeable pavement engineered to meet load-bearing requirements.

Other problems include lack of available soil nutrients, lack of water, root damage during sidewalk construction and road repair, and lack of proper tree maintenance. When roots encounter dense soil, they change direction, stop growing, or adapt by remaining abnormally close to the surface where they find more oxygen to sustain microbial and root metabolism. This superficial rooting makes the urban trees more vulnerable to drought and can cause pavement cracking. It is unthinkable we will be resilient in our communities if we do not aggressively invest in new ways to plant and manage our urban forest as part of our infrastructure costs.

GREEN BELOW GROUND

This project aims to resolve the underground conflicts between root growth and the infrastructural needs to support pavement, power utilities, drainage, stormwater, and waste; achieving the green infrastructure we are seeking and need above and below ground. More specifically, the urban ground is no longer conceived as the horizon that separates the underground infrastructure from the visible space of the city.

This shallow datum, which has been in the past the surface of much paving pattern design efforts in urban design, is now expanded conceptually to address the two opposite but interconnected realms of green infrastructure: the “under” and “above” ground, with their processes, composition, and interdependent potentiality.

Our argument is that today, with decreasing municipal budgets allocated to tree maintenance, it is more cost-efficient to respect the natural branching pattern of a tree species and to invest more in the infrastructural requirements of the root system at the moment of planting. We need to achieve the spatial effect desired for our “green roof” less by professional pruning, which in Puerto Rico is not available, and more by manipulating from the start the urban soil and stormwater infiltration conditions that we have at our disposal.

THE SAMAN BOULEVARD

The Saman Boulevard shows that it is possible to engineer very large trees into sidewalks to create a dignified urban space where the city does not exist yet. It is a manifesto against utility companies and municipal ordinances which, in order to reduce maintenance pruning, mandate the use of small-scale trees or trees too widely spaced in our cities.

Trees are most effective, healthy, and hurricane-resistant if grown closely together with branches and roots intermingling with each other to create a continuous network, as they do in their natural habitat. The Saman Boulevard is not a return to the past. Instead, it advocates that it is still possible to plan our districts by introducing large, uniform tree plantings as a unifying element around which urban space and transportation may be organized. This can be achieved if trees are engineered in the project as an integral part of the road and above-ground and underground infrastructure, just like sidewalks and sewer lines, and not as decoration, like benches and trash receptacles.

THE UNNATURAL FOREST

Samanea saman is a fast growing, drought tolerant, long-lived tree that has been used in many tropical countries for urban avenue plantations. This species can be found naturalized in disturbed areas and dry savanna conditions in the center and south of Puerto Rico, and the island of Vieques. It can develop an 80-foot wide umbrella-shaped crown when the root system has plenty of space to expand—and it will under the cantilevered planks of our design. Since canopies will span over many driving lanes, what woks functionally as a four-lane highway will appear formally at the scale and appereance of an urban boulevard.

The use of one singular botanical species in the boulevard provides a visual syncopation that improves our perception of space and movement but makes us also want to slow down. It achieves a homogeneous foliage texture plus continuous filtered light pattern that are both lost when mixing trees of different sizes and species. This is important in this area of Science City where the urban fabric is still a future projection and space is boundless. A random arrangement of different kinds of trees to mimic a natural forest would fail because it would lack the complex dynamics of organisms and organization that gives the natural forest an its beauty and meaning. Paradoxically, an orderly pattern of closely spaced trees with a continuous, green tunnel effect is the best adaptive geometry to convey the impression of being in a forest even if we are in the middle of an urban or suburban area. The uninterrupted quality of light and shade is important to achieve this aesthetics.

WOODLAND EDGES: TRANSITIONAL LANDSCAPES

The landscape and hardscape treatment of Laboratory Street is different on the two opposite sides, acknowledging the different site conditions and projected land use. Much like an urban “ecotone," this street gathers around a transitional or hybrid zone where two different planned activities and programs meet and interface. Unlike the unified, symmetrical bold space of the Saman Boulevard, Laboratory Street is an interstitial space with two very different edge conditions that take predominance over the center.

On one side, where a dense and tall office-building envelope will be developed, a string of various tree species will articulate both sidewalk and linear parking spaces with their different height and form. On the other, where a future park will be built, the need for not blocking views into the park open space calls for visual integration and a planting design that is transparent and quite porous. To value or discredit this streetscape from a purely spatial structure or visual perspective would be erroneous. It is a landscape of utility, transiency, and mobility, articulating differential experiences at its center and on the two sides. This, along with the physical difference in sun exposure, moisture levels, and wind patterns created by the built side and the open park side, requires a relatively sharp transition in vegetation because of its “urban habitat” change.

THE PALM WOODLAND EDGE

On the park side of Laboratory Street, three different varieties of palms are arranged densely in a linear space that has a multilayered cross section in its interior: Roystonea borinquena, Acrocomia media and Carpentaria acuminata. The palms alternate dynamically defining a bike path from the sidewalk and the park on the other side. They seem to sprout from a playful arrangement of trapezoidal flush planter openings, which provide interest with their jagged edges and entice the biking experience.

Above ground, a layering effect of pinnate leaves of different textures will be experienced. We specified different sizes at planting to obtain an incremental difference of height at three intermediate levels. We clustered the Royals above the Carpentaria in the shared space, and the Acrocomia at a mid-level between those two. This layering effect may disappear somewhat at maturity, as the Carpentaria will catch up with the Royals, but the Acrocomia will lag maintaining an intermediate height. At that point, it will be primarily the trunk’s differential features that will provide visual contrast among the three.

THE TREE WOODLAND EDGE

The name given to this landscape typology is only figurative. It does not imply that we are emulating or approximating a woodland edge plant community by irregularly planting trees and understory vegetation with the intent of making it look more natural. However, in Laboratory Street, we can achieve a partial “reforestation” of the sidewalk with a diversity of species that can adapt to confinement, drought, and poor soil conditions if we adopt the correct soil and drainage infrastructure that improves those limiting conditions.

The trees will offer noise and car pollution buffer, and physical protection to people walking at the edge of the street in a relatively narrow sidewalk with few islands sided by an intermittent parallel car parking lane. They will humanize the scale of the tall office buildings nearby. The planting selection focuses on textural compatibility or contrast, and green color graduation, not color of flowers and not even branch structure or individual tree form. We are purposefully “crowding” smaller trees with larger ones, as large size trees are often not available in Puerto Rico. Selection includes desirable native species that have proven to adapt well in tough urban conditions, though the urban forest in Puerto Rico is primarily composed of exotic and naturalized species.

In the Comprehensive  Cancer Center  site  nearby,  we used both of them in an alternating linear pattern over a background field of Bermuda grass, with the shorter species in the foreground or higher ground and the taller species in the depressions. The grass ribbons act as a series of dams or weirs perpendicular to the stormwater flow across the sloping terrain, intercepting with their stems and powerful roots a large quantity of runoff thereby eliminating the need for an oversized stormwater drainage infrastructure.

PERVIOUS OVER STRUCTURAL SOIL

The difference between Laboratory Street and Saman Boulevard above ground is mirrored by the different approach for underground stormwater infrastructure design. Rather than suspending the concrete sidewalk to protect the structure and porosity of the soil from compaction, in this case we “suspend” the soil among the load-bearing infrastructure of angular crushed stone without overfilling the voids, which would compromise aeration and bearing capacity.

CU-Structural Soil is the material specified under pavement for this green stormwater application. Pervious concrete is a concrete paving made primarily of small aggregate and cement mix, with no sand. The goal of CU-Structural Soil is to ensure that the crushed stone elements touch one another creating a rigid skeleton that can be compacted to 95% peak density to meet engineering requirements for load-bearing specifications, while the soil between stones meets tree requirements for root growth. In this medium, the different palm roots will interlace quickly, holding up steadily the palm trunks during heavy storms.

VEGETATED INFILTRATION SUMP

This stormwater design approach carries often the name of bio-retention cell, or rain garden. It is a planted infiltration trench or depression filled with a permeable soil mix and drainage layers that intercept and slow the erosive path of surface runoff water created by surrounding paved areas, thereby recharging the aquifer.

While most rain garden projects focus on the functional and ecological aspect of the sump, creating informal depressions with all sorts of drought-tolerant plants that also tolerate inundation, in Science City the formal aspect of what is seen above the trench is an important component: a drifting repetitive pattern, recalling a parterre, is scattered in specific areas and becomes almost the signature for the Saman Boulevard entrance points.