Urban forestry has lots of useful information to offer, but there is a big lie at the heart of the majority of the work of urban forestry which threatens to discredit the entire discipline if anybody will call their hand on it. It’s the Root Zone Myth.
As with any myth, there is some kernel of embedded truth. In the case of the Root Zone Myth, it’s the fact that, left alone in a field, a tree’s roots will spread about as far as its drip line, which is the outer limit of its leaves. So if the tree’s limbs and leaves were 40 feet in diameter, its root system would be, too. Now, the urban foresters are saying that’s not good enough; a tree’s root system actually extends 2-1/2 times as far as the drip line. So for that tree with a 40-foot canopy, the roots actually extend to a 100-foot diameter.
Left alone in a field, this is generally true for many species of trees. But in town, it’s a myth. Nearly every discussion with an urban forester begins with their declaration that we need to protect the drip line, and that no hard surfaces (paving, sidewalks, etc.) can be permitted within the root zone. They say that if you plant trees near paving, the tree will be “stunted,” or “dwarfed.” Do the trees in the image above look serious stunted to you? But look... paving gets to within 3 or 4 feet of their trunks all around them! How can this be? The urban foresters claim that this is impossible, but I took the picture myself in Charleston, and I can vouch for the fact that it is indeed real.
The Root Zone Myth becomes the Grand Lie of Urban Forestry because the urban foresters really should know better, but they keep repeating the Root Zone Myth while totally ignoring the urban context.
How do they get away with this? They start by telling stories of trees that have had intrusions into their root zones, and parts of the trees have died in response. The stories are true. But what they’re not telling you is that these are old established trees. Of course they’ll be stressed and drop some leaves when a significant portion of their roots are destroyed! But the trees in question are the new ones that they want to prevent you from planting along the street, or in the medians of avenues or boulevards. According to many of them, the tree will only grow as large as the unrestricted area for root growth.
I was in an audience last night where an urban forester and his city and county planner colleagues made all of these allegations, and more. On the one hand, they were alleging that tree roots don’t grow under paving. On the other hand, they were complaining that tree roots that grow under sidewalks buck the sidewalks up over the years. You can’t have it both ways, guys!
Let’s look at it this way. If, as they said, tree roots don’t grow under paving, and the tree canopy will only grow as large as the roots, then canopy streets would be impossible, because if the branches won’t go further than the roots and the roots won’t go under the street, then the branches would never grow over the street. But the best canopy street in Miami Beach is Meridian, which is pictured here, and which was only a few blocks from the room in which we were sitting as they made these allegations!
I don’t know precisely where the roots of the trees in this picture are going, but the branches are clearly going far out over the paving. And this is exactly the sort of street that the urban foresters are saying can’t be done. Worse, if they have their way, they won’t allow you to build it this way. Rather, they’re looking for super-wide swales where the trees are far back from the pavement. Do we really want to outlaw these sorts of streets?
Here’s another question: what does any of this have to do with sustainability? In a hot and humid climate, it’s very important to be able to shade the sidewalk as soon as possible so that it’s more comfortable to walk there. Walkability, as this blog has said many times, is essential to sustainability because a place isn’t green if people have to drive everywhere, no matter how many green points you get for the buildings.
Placing trees far back from the sidewalk in oversized swales may delay shade on the sidewalk for several years, transforming what should have been a pleasant walk into concrete that seems hot enough to cook an egg.
The bottom line is that the Grand Lie of Urban Forestry doesn’t just dismiss canopy streets, which are typically the most beautiful streets in town. It also helps to make the neighborhood far less green by inhibiting walking, which is one of the foundations of sustainable places.
To the urban foresters: With all due respect, you guys really need to dispense with the Grand Lie. It’s destroying your credibility with those of us who choose to observe real life conditions, and to think about what we’ve seen. Because what we’ve seen makes shambles of your Grand Lie. Just walk around town. Just about any neighborhood built before World War II will do, because before then, we knew how to build excellent urban streets, and how to plant the street trees. Observe those trees, and how they live in an urban setting. A city is not an open field. You guys could do a lot of good with all the things you know about trees if you’d just get rid of the Grand Lie and apply all of your knowledge to the urban setting.
To really get this started in the right direction, you might consider changing the name of your discipline from “Urban Forestry” to something else. Why? Because there aren’t many forests in the city. That may be the core of your problem: you’re confused about your own identity, and you’re acting more like a forester than an urbanist. Forestry happens in the forest. Urbanism happens in the city. Call yourselves Urban Tree Managers or something... I don’t really care, so long as you let us build great streets and sustainable places again.
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Tuesday, August 3, 2010 - 03:39 PM
Thanks for articulating this important point. If you've ever endured a rezoning in your attempt to build a walkable community, you'll appreciate the crucial impact tree setbacks have on the developer's ability to create great outdoor spaces. Tree ordinances are another form of creeping BANANA (Build Absolutely Nothing Anywhere Near Anything) legislation which have the horrible, unintended consequence of encouraging sprawl. Could we have built a single one of America's 20 most beautiful cities if the original developers had been subject to tree setback ordinances?
Tuesday, August 3, 2010 - 03:47 PM
Yep. I've run into the very same roadblock. And yet it's just another case of the specialist who can't see the forest through the trees (pun intended). They envisage a perfect scenario in nature and then attempt to impose it in an urban environment and voila - suburbanism. The same can be said for the macro forest "experts" from American Forests et al who are developing those tree loss maps in every city in the country and arguing that we need to stop development because of the loss of tree. Should the better approach be the planned "reforestation" of the urban areas with urban trees. Beaufort, SC and Charleston, SC are both great examples of a ecological system that has adapted well to human habitation and maintained superb canopies (including when viewed from the air) against the standards of the modern urban forester.
Tuesday, August 3, 2010 - 05:09 PM
It's the sidewalks and sewer pipes that get stunted. Wouldn't it great if we could control trees just put putting sidewalks around them?
Wednesday, August 4, 2010 - 08:36 AM
Tree roots compete for space and research confirms they can grow well outside the canopy dripline. Not all roots are the same. Feeder roots in the top 6-12” of soil take up water and nutrients. Long tensile roots function like ropes that help hold up a tent. Buttress-like roots form a base of support. Roots only grow as deep as the soil profile provides a media that allows oxygen penetration and water availability, usually between 3’-5’.
Your photographs show large trees growing in small openings, often with superficial roots along the surface--not the normal condition. The pavement in all the photos looks relatively new, evidence of tax dollars spent to replace sidewalks that were cracked by tree roots seeking space?
In Gainesville, Florida, we have 50% canopy coverage. We say we are “A City in a Forest” and as a result have comparatively low utility rates, shelter from storm force winds and the many other benefits trees confer. On Gainesville development projects, the surface requirement for trees is 140 sq ft with a minimum width of 9’and soil that can support plant growth to a depth of 3’. A required site meeting prior to landscaping is for inspecting the soil and explaining the importance of planting Florida Grade #1 trees. New subdivisions must have “tree lawns” between curb and sidewalk of 8’ in diameter. I’ve been the City Arborist for 22 years and believe the future urban forest will be abundant, with minimal infrastructure damage from tree roots.
“New urbanist” developers who recognize the importance of trees and actually listen for the reasoning behind the regulations are creating neighborhoods that will have shade. Those who reason that new trees only need a small amount of space because old preserved trees can survive in 5’ diameter openings are building projects that will ultimately disappoint. If the new trees live and grow well, their roots will disrupt pavement (tax dollars) and possibly the new owners of the building. If they don’t live, the community suffers all the losses that go with an inhospitable environment.
Those whose mission is assuring a healthy tree canopy for urban areas have various job titles, but we band together under the concept of urban forestry. I think your understanding is shallow. Planting trees is easy. Having them grow to maturity in an urban setting is very challenging. Urban foresters have developed a variety of solutions to meet these challenges, but it takes real effort to understand them.
Wednesday, August 4, 2010 - 10:16 AM
"Do the trees in the image above look serious stunted to you? But look... paving gets to within 3 or 4 feet of their trunks all around them! How can this be?"
Live oaks are unique in their capacity to occupy all T-zones. The majestic live oaks in and around Charleston College have patches of garden courtyards and other landscape areas proximate to them (and within the root zone). The soils in and around Charleston tend towards a sandy loam, and the brick walkways are often semi-pervious (on a sand setting bed). All of these streetscape features mitigate the suffocation of the root zone under the asphalt street.
Consequently, trenching along the walks and green would likely imperil these trees. I've seen it happen on hundreds of sites; it ain't pretty.
Wednesday, August 4, 2010 - 10:38 AM
yet another example
Steve: I have added a link with another example that points again to the fallacy of the Urban Forester's arguments. Keep chipping away!
Wednesday, August 4, 2010 - 11:01 AM
David Moore, Forestry Manager
This is the first time I have been confronted with this particular issue, but the debate about street trees and their planting space can take almost unlimited forms. One major factor this author did not bring up is tree species, as certain tree species can take to a street tree pit better than others due to their varying tolerances to compaction, flooding, drought, inherent to each species. For instance, if all we ever needed to plant were London planes, then we could probably get away with 3' by 3' holes in the cement and just wait for the tree to push through all its constrictions over time. Meanwhile, a tree that is more sensitive to compaction, like a white oak, cannot grow in a tree pit no matter how large you make it. A group that really has it figured out is Casey Trees from Washington DC.
Feel free to glance through here, you will get the 'scoop' on soil volume and how that relates to tree health, and also learn of innovative engineering methods of expanding the root space below ground while keeping the sidewalk legit on the surface. You can expand root space below by excavating around the pit and replacing the fill with Cornell University Structural Soil (which is basically jagged pebbles that allow for roots to grow through them), or by connecting street tree pits underground so the trees all share the same soil volume. Combine this with permeable bricks or pavement over the 'underground' root space and it blends perfectly into the landscape and you have a great design solution!
Sidenote: Trees and people can coexist beautifully in the urban environment, but most of our misconceptions of urban trees usually start with an assumption based on our experiences with other types of city infrastructure that are much less dynamic than a large woody long-lived compartmentalizing perennial :)
-David Moore, Forestry Manger
New York Restoration Project email@example.com
Tree_Space_Design_highres.pdf 4.6 MB
Wednesday, August 4, 2010 - 03:57 PM
I'm all for new urbanism. I love those tree-lined streets, too. I understand what new urbanists are trying to do with street trees. But I have a feeling that those nice tree-lined streets in the pictures were once even nicer. The trees probably once had a lot more room. A lot of streets have been widened or the sidewalks have been moved or added. The streets may have even been gravel roads at one point. New developments use modern paving and probably have lower standards for the amount of space trees need. Maybe some research needs to be done on what these tree-lined streets were originally like. No one wants dead street trees.
Saturday, August 7, 2010 - 02:16 AM
I think this is a very worth while topic and thank Steve for bringing it to us. Although I can see many sides to this conversation the simple fact is that not all municipalities view this equally and policy varies from place to place. Last year I was working as a landscape designer in Orlando, FL where new trees require spacing on the upper end of the size requirements. I now work in construction management in New York and witness saw cutting of 5'x7'6" rectangles out of existing sidewalks with a min. 4' of excavation and spec soil back-fill to make way for new street trees. After a couple successful growing seasons, these new tree pits are given a granite paver treatment along the perimeter further reducing the surface area by roughly 8" all around. I honestly am not sure why these pavers are added but I have noted that in a majority of the cases, older trees with this treatment demonstrate that the roots will buckle the pavers and general stop there, thus minimizing damage to the actual paved sidewalk.
I guess only time will tell, but I truly believe that these trees (for the most part) will survive. Of course there are MANY other factors besides tree pit size that will determine the success rate in these intensely urban environments. From soil compaction and canine contamination (which is why I'm a fan of tree grates and pit guards) to vandalism and plain arborcide, you would be surprised at how many people tell me "I DO NOT WANT THIS TREE!" (which could be another very useful discussion).
In the end the NYC Urban Foresters have either done their homework and "get it" or are taking a huge gamble in order to implement one of Mayor Bloomberg's PlaNYC initiatives (a million trees don't come cheap: www.milliontreesnyc.org). Either way, I am glad to be a part of the greening of one of the great global cities and look forward to seeing the outcome.
Saturday, August 7, 2010 - 12:20 PM
Steve, you are correct in that many trees planted a long time ago are big and healthy. You probably agree that most of those trees are growing in residential areas with more open space and not in highly urbanized areas. Engineering standards were likely different at those locations when those trees were planted meaning that soils were less compacted and better penetrable by roots than they are these days. This means that roots of those older trees are probably growing a little farther than just in the tree pit.
There are big differences between soils in regards to compactability, nutrient and water availability etc, sands are better penetrable (even compacted) than clays, for example. Plus, different tree species tolerate different conditions. It is therefore a bad idea to generalize. As you said, walk around with open eyes and look at trees that are growing in highly urbanized areas. If it were as you say all those trees should be growing just as fast and healthy as trees in the woods. Look at the pavement around those trees as well. Are these trees really healthy and big without growing out of their tree pit? How many start cracking pavement? Is that what we want? Trees probably wouldn't do that if the conditions withing the tree pit were adequate to support growth. Plus, trees are oftentimes removed if they crack gray infrastructure. That's not sustainable either because we'd have to start from scratch.
There is a big difference between trees that are planted in small cutouts and large trees whose roots are cut to build a street or building. Cutting roots is always a bad idea, even though it's oftentimes unavoidable.
As David said, there are many designs available to increase available/penetrable soil volume to improve tree growth thus you can plant trees closer to a sidewalk and provide shading somewhat soon. The better the conditions the faster the tree will grow so if you decide to plant a tree in a small cutout with poor soil quality you will have to wait a bit until you get some benefits. I'm sure you have seen the many large parking lots with small and unhealthy trees growing in small cutouts. They are trying to grow trees in unsuitable conditions. Those trees will probably never be big enough shade your car.
Tuesday, August 10, 2010 - 10:08 AM
Alert reader Eddie Suarez sent this link to a photo of a tree that refused to take no for an answer.
Thursday, August 12, 2010 - 12:45 AM
Wow, this is great!! Exactly what I was hoping for... lots of thoughtful commentary and resources. From my shallow understanding, here are a few comments:
A. Interrupted sidewalks aren't the end of the world. Miami Beach handles them very well by grinding down the high lip... and Miami Beach isn't the world beacon of municipal efficiency. Rather, they're right down there with typical American cities, working to figure things out as they go along. So there may be better solutions... but this one isn't bad.
B. My original post makes no mention of species. That's what I was hoping those with a deeper understanding of trees might contribute, and they did... thanks! Special thanks to David Moore for the resources he posted. Much appreciated!
C. Most of the images I posted were from Charleston, south of Market Street, where the buildings almost certainly are older than the trees. Or if not, they were the same age. Because these buildings were built in a city, it's extremely unlikely that the trees originally grew along a dirt road. So what you're seeing isn't likely so far from their centuries-earlier condition. If you have photos from the mid-19th century, please post them and prove me wrong! Let's all learn something in this discussion.
D. As for ideal conditions, this is the big point of the post: growing trees in an urban setting isn't ideal, but this isn't a toggle switch... it isn't ideal-or-nothing. Rather, urban trees can grow to very serviceable size in less-than-ideal settings. And that's OK. I've been called a tree-hugger many times, but in reality, I've never observed that a tree has the same capacity for love and affection as a puppy or a person... so I don't have reservations about planting trees in places where they might be less "happy" than in a forest.
Monday, August 16, 2010 - 04:46 PM
To me, it's less about trying to cram trees everywhere in order to make the places we want. Its more about making the places we want and making them suitable for trees. We do not spend the time or money to build proper growing environments for urban trees. Giving the trees "space" is not always the only answer, and that seems to be your argument. We can design fantastic soil volumes under the pavement that will allow many trees to thrive and give us those "great places" we all so badly desire. If we simply add more trees without fixing the problem below the paving, we will only be disappointed by dead or pathetic trees that provide little or no value to the place.
Another very important part of this is tree species. We really need to find out what the best trees for urban plantings are for our location….and that may not line up with a designer’s (or forester’s) favorite species list. Locally, our city forester has outlawed Planetree/Sycamore as a street tree. Really? It is a bit messy and not without problems, but it is one of our best street tree species. Great trees in an open lawn are not always the best species for the street, but few designers or foresters seem to understand this.
On a side note, I wanted to know what the best street trees species for my city was so I asked 17 local arborist, foresters, nurseryman and educators to numerically rate each tree (378 species/cultivars). I they averaged out their ratings and developed this list. Go to www.gouldevans.com, and click on “Planning/Landscape Architecture” to see the list.
Monday, August 16, 2010 - 07:01 PM
Thanks, Robert! You're clearly correct that the planting details have to be right for the tree to flourish in an urban environment. It's also a fact that many places are downright unwalkable in the heat of summer with no shade. Try walking around downtown Dallas in July, for example. So in places where it regularly gets 90° or warmer, I'd suggest that shade is pretty much a prerequisite of walkability. So lets get the details and the species right so that we can provide the walkability that is an essential building-block of sustainability.
Friday, August 20, 2010 - 12:56 PM
There is no doubt that trees are key to walkability. I have also been arguing that point for a long time. I would also agree that a stunted ugly tree is usually better than no tree at all. We need to get the spacial form of the street and building setback correct, and add the trees to the mix to make it feel “right”. We just usually spend no time understanding trees. I have seen great places built, then they place the wrong tree in a pathetic hole. No one cared enough to find out how to do it right and what tree species would give them the best long term success.
One reason I think 99.9% of the population does not understand why street trees are important is because our cars are too perfect of an isolation chamber. I live in first-tier older suburb and drive most places. My car is an old roadster. No top, no AC and primitive by today’s standards. What is does, is allow me to experience some of what a pedestrian experiences. Things like heat, shade, odors/fragrances and many other things. It allows me to experience the spacial definition of the street…or lack of it. It’s not quite like walking, but it helps me understand what works and what doesn’t much better than a traditional automobile.
Trees are the one element that really improves the experience of my drive. If I am in a newer expansive suburb where the trees are too far to experience, I really hate the drive. If I am on a road with a superb tree canopy over the travel lane, I really enjoy the drive. If you want to show a client what is good or bad about r.o.w. design, put them in a primitive roadster (any convertible might do) and drive them around. Walking is better, but time might not allow for it.
Sunday, August 22, 2010 - 03:37 PM
"One reason I think 99.9% of the population does not understand why street trees are important is because our cars are too perfect of an isolation chamber"
Precisely, Robert! Our lack of tolerance of anything other than perfect conditions precludes a sustainable range of conditions. This really is central to sustainability. On the other hand, we'll never bridge the gap between our perfect isolation chambers and the really brutal conditions on treeless Dallas streets in summertime, for example, or dark-roofed houses with 8' ceilings. So what really must happen is that we've gotta make each environment, whether indoor or outdoor, as comfortable as possible using natural measures. Only if that happens do we have a real chance of people becoming conditioned enough to the local environment to leave their isolation chambers and the near-perfect conditions inside them.
Friday, November 5, 2010 - 07:59 AM
Danny Burbage, Urban Forester, Charleston
Rather than calling urban foresters liars and suggesting that we change the name of our profession, I wish you had taken the time to contact one of us who is nurturing large trees in confined urban spaces. We are working very hard to educate others in the field on how to strike the proper balance between much needed large shade trees and the tight confines of our cities. I would have been happy to meet with you while you were in Charleston. You do yourself and others a disservice by painting with such a broad and uninformed stroke.
Danny Burbage, Urban Forester, City of Charleston, SC
Friday, November 5, 2010 - 08:53 AM
There are countlless urban foresters across the country that are quiet aware of the root zone reality and the differences between particular species growth patterns in open fields vs. city streets. The variables are all over the map which makes growing large trees in urban areas such a complex challenge. Your suggestion to rename the urban forestry profession is off base and ignores the science and research that has gone into this profession for the past 50 years or so. It is "forestry in the city " in that we try and grow and design forest plants into our urban areas. Your comment ..."Nearly every discussion with an urban forester begins with their declaration that we need to protect the drip line, and that no hard surfaces (paving, sidewalks, etc.) can be permitted within the root zone. "... seems a bit off base. I've known and deliberated with many UF's and I don't think that this is where the majority are coming from. Sure it would be nice to protect the root zone, but arborsits and UF's know this is the ideal and reality is something much different.
Tom Knowles, Certified Arborsit
Board Memebr - TreesSC