Urban Park Naturalization:
Building Resilience & Sustainability for Toronto's Green Space
1 April 2010
A report prepared for Toronto Parks &Trees Foundation
Andrew Millward, Ph.D.
Assistant Professor of Geography
Principal Investigator, Urban Forest Research &
Ecological Disturbance (UFRED) Group
1. Executive Summary
This report has been prepared for the Toronto Parks &Trees Foundation for the purpose of sharing with it, and the City of Toronto, results of a 2008/09 soil study conducted in Toronto’s Kew Gardens Park.
1.1 Purpose of the Project
This project investigated the potential for naturalization to improve degraded soil conditions within an urban park containing remnant oak forest. It was hypothesized that soil degradation, caused by decades of trampling and general use, had diminished conditions to a point that may be compromising the health and vitality of existing trees as well as contributing to the inhibition of natural regeneration of certain desired tree species. Of specific concern was the observation that red oak (Quercus rubra) and white oak (Quercus alba) were no longer naturally regenerating.
We designed the project to compare and contrast the growing medium between naturalized and non-naturalized portions of Kew Gardens. Few scientific studies have investigated urban park naturalization as a management initiative designed to improve growing conditions for trees. None, to our knowledge, have produced maps of soil properties for such an investigation.
1.2 Research Team and Funding
Ryerson’s Urban Forest Research & Ecological Disturbance (UFRED) Group, led by Dr. Andrew Millward and assisted by Kamal Paudel and Anna Banaszewska (Masters Students) and by Suzanne Briggs (Undergraduate Research Assistant) conducted the research reported on herein during the summer and fall of 2008, and during the summer of 2009. Funding to support this effort was provided by Ryerson University and the Toronto Parks & Trees Foundation.
The following soil physical properties were measured at 104 sample sites in Kew Gardens Park: texture, bulk density, compaction, and surface water infiltration rate. These data were then mapped using a statistical point interpolation method, ordinary kriging, to produce prediction surfaces for each measured soil property. Soil conditions found within three naturalization enclosures (in place for seven years) were then compared with those found in adjacent managed parkland using a combination of the aforementioned prediction surfaces and difference in means tests.
The soil in Kew Gardens Park falls into three texture classes that include, in order of dominance, loamy sand, sand and sandy loam.
A statistically significant difference in bulk density values was found at a depth of 10 cm when soil in the enclosures was compared to soil measured in adjacent, but non-naturalized oak forest: F(1,59) = 43.34, p < 0.000. Soil measured within the naturalization enclosures had lower bulk density. There was no statistically significant difference in bulk density when comparing soils for these same locations at the 30 cm depth: F(1,59) = 0.88, p = 0.352. Overall, bulk density showed spatial variability across the park; lower values, more amenable to plant root growth, were consistently found within the naturalization enclosures when compared to the park as a whole. Results show that root restrictive bulk density values (> 1.8 g/cm3) were spatially correlated with locations of high pedestrian traffic, and with corridors used by park maintenance vehicles. While restricting root growth and respiration, bulk density values in this range also impair soil water infiltration. There were no bulk density measurements above the root restrictive threshold found within the naturalization enclosures at either 10 or 30 cm depths.
Soil compaction, at all depths, was statistically significantly lower in the naturalization enclosures compared with soil measured in adjacent non-naturalized oak forest. Difference in means test results for each depth are as follows: 10 cm, F(1,59) = 32.5, p < 0.000; 30 cm, F(1,59) = 12.32, p = 0.001; 45 cm, F(1,59) = 12.01, p = 0.001. The highest soil compaction values measured in the park were made at a 30 cm depth and were also strongly spatially correlated with locations of high park use. We found that soil compaction measurements taken in non-naturalized oak forest came very close to the threshold value of 2000 kPa, beyond which tree root growth is restricted.
There was a statistically significant difference in soil water infiltration rates when soil within the naturalization enclosures was compared with soil in the adjacent non-naturalized oak forest: F(1,59) = 81.51, p = 0.000. The infiltration rate, on average, was eight times quicker for soil measured inside the naturalization enclosures.
1.5 Conclusions and Recommendations
Patterns of vehicle usage, bicycle and foot traffic in Kew Gardens Park have collectively, and cumulatively, resulted in regions of soil quality deterioration characterized by high compaction and reduced water infiltration. Locations of park naturalization were highly spatially correlated with physical soil conditions more favorable for plant growth. Naturalization, as a park management strategy, has the ability to improve soil physical properties, thereby enhancing conditions for tree and shrub growth and, in some cases, restoration of native species. The results of this study can serve as an important reference for park managers when evaluating possible techniques for maintaining and enhancing urban soils. However, such a technique, implemented as a result of anthropogenic-caused degradation, will have varied degrees of success based on longevity, geographic extent, and enforcement.
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