Research: Silva Cell Suspended Pavement System Outperforms Structural Soil in Tree Growth

A recent research study conducted by Bartlett Tree Research Laboratories tackled the question of the best methodology to grow trees in cities, with the ultimate goal of reaping benefits that mitigate the impact of climate change. The study (Comparison of Tree Responses to Different Soil Treatments Under Concrete Pavement) was produced by Dr. E Thomas Smiley, Dr. Kelby Fite, and James Urban (FASLA).

The results of a two-phase, decade-plus research study examining the performance of suspended pavement green infrastructure systems generally — and the Silva Cell specifically — are definitive: Silva Cells outperformed other soil media in just about every important tree-growing metric.

The implications of the study are far-reaching, particularly considering the climate change consequences experienced around the world, from urban heat island effect to significant storm events. Municipalities, perhaps more than ever before, are seeking to establish resilient long-term strategies.

One of the primary solutions for city planners is to include trees in their streetscapes: the benefits of urban tree canopies in mitigating heat island effect, managing stormwater, and improving air quality — as well as mental and physical health — are well established. That said, paving and city infrastructure often reduce the lifespan and growth of trees. So, what is the best method for growing healthy trees in an urban environment?

Perhaps the most important factor in the health of an urban tree is the soil conditions. The first phase of the Bartlett study examined the performance of five different soil treatments: gravel-based structural soil, expanded slate structural soil, expanded slate alone, compacted control, and supported pavement.

This ten-year study, begun in 2004, produced clear results: trees grown over suspended pavement (like a Silva Cell system) were “significantly larger in all growth metrics than other treatments.” This includes tree trunk diameter and height as well as visual ratings of foliar color.

The second phase of the research was established in 2014, a four-year study exploring six planting variations: sand-based structural soil, compacted control, open control, gravel-based structural soil, the Stratacell, and DeepRoot’s Silva Cell system.

Once again, the Silva Cell outperformed the competition.

As with the first phase of the study, the Silva Cell system led all planting variations in trunk diameter and height, as seen in the tables below.

Also of note, trees grown using Silva Cells saw a significantly higher average number of large roots (those over 1.2cm in diameter) than any other media.

Likewise, when it came to maximum root spread and the mean weight of the tree parts, the Silva Cell was once again the top performer.

The results are clear — and the study’s own conclusions reaffirm the benefits of a suspended pavement Silva Cell system. The first study proved that “suspended pavement treatment trees grew larger and generally appeared healthier than the other treatments.” The second phase of the research showed that “with the count of larger roots, the greatest number were found in the Silva Cell system treatment. With root depth, the Silva Cell and Stratacell treatments had significantly more roots deeper in the soil. Overall root weight was significantly greater with the Silva Cell, Stratacell, and open control treatments.”

When it comes to providing a healthy soil environment for urban trees, the DeepRoot Silva Cell is clearly the leading choice for tree-growth performance.

See the study itself here, beginning on page 303.

Below is a visual progression of tree growth throughout the years (trees planted in suspended pavement are found at the bottom, growing larger and faster than their counterparts).

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