Naturalization as a strategy for improving soil physical characteristics in a forested urban park

Ensuring good soil quality is essential to promoting plant growth in urban parks and building ecological resilience into a cityscape. Periodically used to restore a degraded urban ecosystem, parkland naturalization is a management approach designed to facilitate the return of an area to a natural state by largely discontinuing maintenance activities and restricting public access. This study investigates the potential for parkland naturalization to improve soil quality in a forested portion of Kew Gardens Park, Toronto, Canada by comparing soil physical properties in three six-year-old naturalization enclosures with those found in adjacent managed parkland. Soil texture, compaction, bulk density, and surface water infiltration rate were measured at 104 sample sites, while ordinary kriging was used to interpolate two-dimensional prediction surfaces representative of soil properties. Sand and loamy sand were the dominant soil texture classes found across the study site. Highly compacted soil (>2,000 kPa) and soil with a bulk density >1.8 Mg/m³ (values sufficiently elevated to restrict tree root growth and respiration and impair soil water infiltration) were spatially correlated with high pedestrian traffic areas and corridors used by festival and park maintenance vehicles. In contrast, compaction and bulk density measurements in the naturalized areas were at or below thresholds known to impair root function (`(X)] {{\overline{X}}} at 10 and 30 cm depth: 849 and 1,311 kPa, 1.15 and 1.51 Mg/m<sup>3</sup>, respectively). Similarly, water infiltration rates were rapid ( `(X)] = 470mm/hr ) \left( {{{\overline{X}}} = {47}0{\hbox{mm}}/{\hbox{hr}}} \right) within the naturalization enclosures but retarded to the point of surface pooling in parkland subjected to regular public use. In the absence of quantitative baseline data, our use of spatial analysis demonstrates that parkland naturalization is a good management practice for restoration of soil physical characteristics. While our results show improvements to soil properties in a relatively short period of time, variability in the soil response rate to parkland naturalization will be dependent upon disturbance history as well as on soil and climate type.