Influence of soil properties on street tree attributes in Singapore

<p>Soil quality is thought to be a primary driver of street tree performance and thus a major concern for urban forest growth, health and longevity. This research was conducted to evaluate the influence of soil physical, chemical and biological properties on Singapore's street trees. In total, 338 plots, 1014 trees and 32 species across Singapore's five regions were sampled. Singapore's street trees are skewed towards smaller diameters (<50 cm) and largely represented (61 %) by five species, four of which are non-native. Most soil properties in Singapore's streetscapes are likely not limiting for trees: verge (5 to 7 m), bulk density (1.05 to 1.45 Mg m^-3), P (1.52 to 2.87 mg kg^-1), organic C (7.8 to 11.4 %), Ca (924 to 1772 mg kg^-1), Mg (313 to 631 mg kg^-1), Na (130 to 208 mg kg^-1), Cr (143 to 212 mg kg^-1), Pb (55 to 74 mg kg^-1), Ni (15 to 30 mg kg^-1) and microbial biomass C (265 to 457 mg kg^-1). Soil pH (6.04 to 7.63) is not as acidic as the humid-tropical soil commonly found in Singapore. Soil K (23 to 130 mg kg^-1) was found to be relatively low and potentially limiting. Individual soil properties and multi-factor models were poor predictors of urban tree attributes across Singapore, but models improved when examined at regional scales. Relatively high soil quality, uniformity of streetscape soils and fast growth rates of these trees are proposed as explanations for why soil properties appear to poorly predict street tree attributes in Singapore.</p>