Individual cases of tree survival following grade increases in which soil aeration systems were incorporated, are often held up as testament to the effectiveness of these systems. The number of these successes paints a convincing picture. However, soil aeration systems have not yet been subjected to thorough scientific scrutiny. Other urban tree care measures, such as cavity filling and wound painting, were also founded on "conventional wisdom" and widely accepted by a public that felt that doing "something" was an improvement over no treatment at all. Just as scientific study proved those techniques ineffective, research may show aeration systems to be equally questionable.
A two-year, controlled field study of aeration systems was conducted on 11-year old white pines (Pinus strobus) growing in a plantation near Clemson, South Carolina. Trees without fill were compared to those whose root zone grade had been raised 6 to 8 inches by 6 tons of fill. Clayey subsoil fill, having no components of an aeration system, was compared to crushed rock fill, with and without aeration piping. All fill treatments contained the load-bearing geotextile base recommended by Harris (1) and a top surface cover of water-repellent geotextile and tar paper, intended to simulate the water-shedding properties of asphalt paving.
Anaerobic conditions under the fill treatments were less severe than anticipated. Soil fill reduced O2 and increased CO2 content slightly, but not nearly enough to produce concentrations unfavorable to tree growth and development. Soil aeration under rock fill, with and without piping, was equivalent to that of trees with no fill or covering.
Table: Average percent soil air composition by volume within the root zones of Pinus strobus treated with crushed rock or soil fill.
|Percent by volume|
|Soil Air Constiuent
*denotes significant difference from the control.
(image: White pines of study two years after treatment/installation.) Physiological processes of the trees were largely unaffected by the fill and cover treatments. Treated and untreated trees were equally active photosynthetically and produced similar amounts of shoot growth. Measurements of predawn water potential of the foliage showed that treated trees received an adequate supply of water in spite of the water-shedding tar paper which covered their root zones. Predawn potential, an indicator of root zone water availability, ranged from -3 to -9 Bars throughout the growing season, but was consistent among treated and untreated trees on individual days of measurement. Water was not a limiting growth factor, as some of the highest photosynthetic rates appeared on days when predawn potential was lowest.
Why did these fill treatments not produce the severe conditions observed by Yelenosky (4)? As a simulation of construction site conditions, this study differed from most actual situations in several important ways. Study trees were treated individually with islands of fill and "paving" surrounding them. Hence, the study trees may have gained some access to air and water at the outer edges of the treatment areas, while a tree located within a vast parking lot would have little advantage from edge effects.
Another confounding factor observed on construction sites but not addressed in this study is the effect of soil compaction on root zone aeration. Fill materials were placed on load-bearing geotextile fabric with a front-end loader which stayed outside of the treatment areas. Consequently, significant compaction did not occur. Soil bulk densities beneath the fill were low--averaging 1.08 g cm-3 with pore space comprising 59% of soil volume. Although similar precautionary measures can be taken in actual practice, soils are often so compacted prior to fill application by other construction activities that soil aeration is already limited. This was probably the case in Yelenosky's study where measurements were taken in soils under fill layers that had been rolled and packed down with grading equipment in preparation for asphalt paving.
Confounding factors and the limited scope of this study make it difficult to give conclusive statements regarding the usefulness of soil aeration systems. Further studies must first be designed and conducted which address the following factors: varying species tolerance to poor soil aeration, effects from greater depths and different types of fill, and soil compaction's role in aeration problems. The results thus far seem to indicate, however, that fill may not be as large of a culprit in creating poor soil conditions and tree decline as the other damaging activities that normally accompany it. Perhaps more important than the actual soil aeration system in insuring a tree's survival following grade changes is the increased level of care and protection from damage afforded to that valued tree on the construction site.