Brahma Toleti

Soil characteristics were studied in Allan Gardens, an historic urban park located in downtown Toronto, using in situ measurement data and geostatistical analysis. The applicability of various kriging interpolation models was investigated with the goal of mapping prediction surfaces for soil characteristics including bulk density, compaction, and moisture availability. Soil samples were collected at 40 locations for analysis of bulk density. Soil compaction data were measured at 76 and 200 mm depths for 117 locations in late spring 2007. Soil characteristics at these depths are important to the urban forest because most trees and shrubs have extensive feeder root networks in the top 300 mm of soil. To capture soil response to a precipitation event during one of Toronto's driest summers on record (2007), soil moisture data were collected at the same depths and locations used in the soil compaction investigation. Pre-rainfall soil moisture data were collected on August 20 with a second set of data collected on August 2rh, following a precipitation event on August 25th of 5 mm. Mapping of these data using kriging enables the creation of interpolated prediction surfaces, with the advantage that the results can be statistically validated in the form of standard error surfaces. Depending on model inputs, the prediction surfaces (interpolated maps) generated using ordinary kriging differed markedly. An optimal model for each set of data was determined by comparing cross validation statistics. The prediction and standard error surfaces of soil bulk density, compaction and moisture showed significant spatial variability across the 4.5 ha urban park. Results of this investigation reveal that soil characteristics within Allan Gardens Park are not spatially consistent concerning their ability to support tree root growth and access to soil water. Indicator kriging reveals that the spatial range of sub-optimal conditions expands appreciably during prolonged periods of rainfall absence. Soil remediation is recommended in several areas of the park to protect and enhance conditions for a vigourous and resilient urban forest in Toronto's downtown.