City Trees

September/ October 2011

City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!

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Page 35 of 39

Porous Pavements: Can They Improve Street Tree Growth? by Dr. Justin Morgenroth, Lecturer, New Zealand School of Forestry average lifespan for open-grown or forest trees far exceeds that for street trees. Growing bigger, healthier, long-living street trees has long been a goal for urban foresters—but with new techniques and products con- tinuously rolling out, it pays to be discerning. G One means of achieving bigger, healthier street trees is to surround them with porous, rather than impervi- ous pavements ... or so we've heard. Porous pave- ments aren't new. In fact, they've been incorporated in cities for decades, but to a limited degree due to concerns about clogging, cost, and ravelling in cold climates. Despite these issues, their alleged benefits ensure their continued installation. The popularity of porous pavement increased in the 1980s in the United States due to legislation asso- ciated with the Clean Water Act. The regulations pertained to stormwater management and required decreasing the amount of surface runoff, as well as treating water at the source. Since both of these requirements are met by porous pavements, there has been greatly increased installation of porous surfac- ing for compliance purposes. Though porous pavements have primarily been used to manage stormwater, they have the potential to improve tree growth in our heavily paved cities. They allow for increased infiltration of precipitation into soil and for air to be exchanged between the soil and atmosphere. Since trees require adequate water and sufficient soil oxygen for normal function, it makes sense that porous pavements should improve tree growth. But despite these obvious benefits, porous pavements have never really lived up to their prom- ise. Some research that I conducted recently at the University of Canterbury in Christchurch, New Zealand may help explain why. The research, conducted over two growing seasons, showed that the use of porous concrete pavements (11% void ratio) could improve tree growth. The aver- age height and diameter growth of fifty Oriental plane 36 enerally we think that pavements and street trees don't mix, and maybe they don't; after all, the This is the experimental site in Christchurch, New Zealand as of March 2009. Tree growth had been influenced over two growing seasons by modifying soil compaction rates and installing porous or impervious pavement. trees (Platanus orientalis) increased by 30%. Trees surrounded by porous, rather than impervious, pave- ment also had the largest root systems. But this is only part of the story. One goal of the study was to contrast different pave- ment profile designs. After all, not all pavements are made equal. Consider a sidewalk versus a highway; the pavement designs will be vastly different. One element of pavement profile design we could easily manipulate experimentally was subgrade compaction. Some trees were planted in an uncompacted fine sandy loam with overlying pavement, while others were planted in compacted soil with overlying pave- ment. The roots of trees planted in compacted soil (mean soil strength = 2411 kPa) experienced soil strength roughly three times greater than those in uncompacted soil (mean soil strength = 841 kPa). The resulting pavements were designed to two very different standards. The decision to include a compacted subgrade as a covariate proved crucial. Without it, we'd still believe that porous pavements are a silver bullet, able to improve tree growth in all conditions. But this is sim- City Trees

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