312 CONCLUSIONS In general, the greatest amount of tree growth and healthier trees were seen in non-compacted, low-density soil media treatments, either under supported pave- ment or with an open soil surface. There were no growth or health differences based on the system that supported the pavement. Lesser growth was seen in trees growing in com- pacted soil media. When the soil media was com- pacted, regardless of the substrate, there was less growth and a generally less healthy appearance abo- veground. This is consistent with other studies that have shown the negative impacts of soil compaction (Kristoffersen 1999; Smiley et al. 2006; Rahman 2013; Fite et al. 2014; Urban and Smiley 2014). In Study 1, the supported pavement treatment trees grew trees larger and generally appeared healthier than the other treatments. Trees planted in the com- pacted soil treatment did surprisingly well consider- ing the soil density. When the cherry trees were removed from this treatment, it was seen that the roots did not penetrate the compacted soil but rather grew upward and outward from the edge of the root ball, likely finding lower-density soil just below the pavement and at the edge of the plots. Trees in the ESSS plot grew less well than trees in the GBSS plot. This may be due to the smaller size of the stone, which could result in less space between stones and a higher level of soil compaction (Gra- bosky and Bassuk 1995; Grabosky and Bassuk 1996). Trees planted in the 100% expanded slate (ES) media had a high mortality rate, resulting in the plot being removed from the study in 2007. This mortality was most likely based on the amount of water retained by the porous stone (personal observation). That treatment would not be considered acceptable in most urban situations unless water was carefully managed. Concrete cracking was significantly greater in the GBSS treatment. Smiley (2008) showed that a GBSS treatment is more likely to result in concrete sidewalk cracks based on the trees production of fewer, but larger diameter roots. In Study 2, the tree growth differences based on soil media treatments were more obvious than in Study 1. This may be attributed to the tree species selection. Liriodendron are fast growing in the Char- lotte, NC region, are known to have a root system that does best in low-density, sandy soils, and are inhib- ited by higher densities (Francis 1979). ©2019 International Society of Arboriculture Smiley et al.: Tree Responses to Soil Treatments Under Pavement Treatment-based growth and health differences in general separated into two groups. The open control, Stratacell™, and Silva Cell™ treatment trees grew significantly larger than the compacted soil, GBSS, and SBSS treatments. When there were differences in foliar color, it followed the same pattern with the open control, Stratacell™, and Silva Cells™ being a healthier green color. Root growth differences were seen in the mean maximum root depth, counts of roots greater than 1.2 cm diameter, and in the overall weight of the root sys- tem. With the count of larger roots, the greatest num- ber were found in the Silva Cell™ treatment, followed by the open control and Stratacell™. With root depth, the Silva Cell™ and Stratacell™ treatments had sig- nificantly more roots deeper in the soil. Overall root weight was significantly greater with the Silva Cell™, Stratacell™, and open control treatments. Root differences are potentially based on soil den- sity and oxygen availability in the soil profile. Soil moisture does not appear to be a factor, since the highest and lowest soil moisture level were found in the compacted soil and SBSS treatments, respec- tively, both of which had lower levels of root devel- opment. The Silva Cell™ and Stratacell™ treatments had moderate moisture contents, and the open control had higher moisture level near the surface. This high moisture at the surface can be attributed to the non- coved surface that the other treatments did not have. The soil beneath the pavement subsided more when the soil was not heavily compacted. The com- pacted control, SBSS, and GBSS treatments had sig- nificantly less subsidence than the other treatments. The Stratacell™ treatment experienced much more subsidence than all other treatments. This was attributed to the redistribution of soil with the infiltra- tion of rainwater. It was noticed that with the first three large rain events after tree planting, the trees in this treatment were moved deeper into the soil as the soil beneath the root ball was washed into other por- tions of the Stratacell™ structure. Those affected trees were replanted at the original depth after the addition of more soil. In one replicate a shed snake skin was found partially under the pavement, which points to potential problems with having a large gap. SUMMARY Soil treatments that provided a low-density growing media resulted in the largest and healthiest trees. There
November 2019
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