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Let Trees Connect

“A forest is an interdependent community. Resources are shared, and life in isolation is a death sentence.”

—Becky Chambers, 2019

I love to see how ecologists’ contemporary understanding of forests as living systems has caught the imagination of popular writers today. This quote from Becky Chambers is extracted from a beautiful memorial description (that should get her an honorary Landscape Architect title at least). I’m sharing this not to blatantly promote her work—although everything she writes is profound and delightful—but to highlight that the connectivity of trees as living systems, between each other, their mycelial networks, and their plant communities, is no longer a discussion reserved for academics and scientists. It has entered the popular collective memory, being highlighted everywhere I look, from the lyrical expansive writing of Richard Powers in The Overstory: A Novel, or the poetic allusions in Paul Harding’s Tinkers, to best-selling science writing including The Hidden Life of Trees by Peter Wohlleben, and The Language of Trees: A Rewilding of Literature and Landscape by Holten and Gay.

I realized just how ubiquitous this common understanding has become when I saw mycorrhizal inoculants on the shelf next to herbicides at Home Depot this weekend. 

As landscape architects, urban planners, arborists, and urban forestry professionals, we in the ‘green industry’ have embraced these stories and have started to radically change how we think about planting design and maintenance. The celebration of interconnectivity has focused on new works, restoration planting, new contiguous stands of trees in parks, and tree protection best practices—developing new tools, methods, and standards for understanding how we can protect critical root zones during site improvements. 

Our team at Sasaki has been diving into this work, implementing tree protection and invasive removal best practices at Bonnet Springs Park, giving access to visitors to both the canopy and the understory of a fantastic stand of heritage oaks and wetlands. 

As an industry, we are now versed in presenting planting design as plant communities, researching and collaborating with ecologists to understand commensal and mutualistic relationships between species to build more resilient garden and habitat spaces. These practices are not unique to our work at Sasaki, but now are (or should be) part of professional expectations in the field of landscape architecture.

Good planting design and preservation now require not only understanding the critical root zone needs of trees we want to preserve, but also respecting the developed relationships between those heritage trees and their supporting planting communities, potential pressures from invasive species and pests, and the potential for allelopathic relationships between species.

In the urban forestry world, we knew we had a problem in the 1990s when the average street tree lifespan in the United States was calculated at 10 years. Since then, researchers and soil scientists have developed sophisticated specialty soil systems, with species-specific soil volume requirements, to construct enabling soils in highly urbanized areas. Landscape architects are implementing gravel- and sand-based structural soils and elevated pavement systems, to give roots a chance to breathe in urban conditions. These measures are now all part of contemporary best practices in urban forestry planting design and part of the core educational resources provided by the International Society of Arboriculture.

The challenge in urban forestry, and with street trees in particular, is that people get the most benefit when we are as close as possible to the trunks of the trees. This results in many isolated trees in the urban environment. We design complex and expensive systems to fit trees into a highly artificial environment and are asked to guarantee their success. However much we may aesthetically want rows of uniform trees to make those iconic, cathedral-like, tree-covered streets, the isolated planting approach makes the trees vulnerable. They are vulnerable to disease and pests when planted in monoculture. They are vulnerable to heat stress when surrounded by paving. Isolated street trees are often planted with inadequate soil volumes due to compaction, or inadequate water resources due to the low available water holding capacity of structural soils, and they are vulnerable to girdling roots and root rot in compacted soil conditions. 

Trees are not columns of shade structures—they are living, dynamic organisms, adapted to their unique habitats of messy, living systems, and they do better together, in stands. In Florida, ecologists have learned that planting groups of at least five canopy trees together at a time can double the stand’s resistance to hurricanes. In Germany, we have learned that trees sharing root zones will warn each other of pest attacks, triggering the release of chemical defenses and tannins in nearby individuals not directly impacted. 

We must ask, where can we plant stands of trees in our cities that work together as forests, allowing trees to grow, thrive, die, and bring those more resilient natural cycles into our cities?

When we design stand-alone tree pits, we are isolating the tree, and the success or death of that tree is completely dependent on the constructed system we provide. So while we always want to be able to have bosques of trees in urban plazas and hardscapes for cooling, shade, and biophilia, we should take care to plan for connectivity to overcome those risks of isolation. We must ask, how do we connect our horticultural soils, even under pavements and structures? Where can we feasibly open up our sidewalks, and allow for bigger planting beds of supportive understory species? Where can we plant stands of trees in our cities that work together as forests, allowing trees to grow, thrive, die, and bring those more resilient natural cycles into our cities?

This next generation of urban forests must be more than 5×5 tree pits and struggling slow-growing trees. They need to be complex, biodiverse, natural systems, integrated into our urban communities. This can still be accomplished in a wide range of aesthetic manners to accommodate history, culture, and preference—from shared rooting zones under cobbled plazas to discrete Miyawaki-style microforests. Today’s writers are capturing the public’s imagination about the complexity and beauty of trees. As designers, urban foresters, and regulators, we can reveal those stories, and celebrate trees as members of our shared urban ecology. 

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