Woodland Establishment Reduces Nutrient Losses to Waterbodies in Urban Catchments: A Review of the Evidence

Abstract

AbstractSystematic review of peer‐reviewed literature was undertaken to establish benefits of urban forests on reducing nutrient concentrations in adjacent or downstream waterbodies. Following screening, a small number of articles (40) were found relevant, representing studies quantifying non‐point source nutrient losses from urban and peri‐urban environments. Evidence was split between plot‐ and catchment‐scale. Plot‐scale studies often included evaluations of engineered nature‐based solutions. At catchment‐scale, studies of streamwater quality typically investigated influence of contributory catchment nutrient sources. Wide ranges of beneficial reductions were apparent, and at both scales not all studies identified significant benefits. Summarizing against this backdrop, at plot (micro‐) scale woodland reduces mean concentrations in runoff, soil or groundwater by an average of 44.2% for total nitrogen (TN) and 47.0% for total phosphorus (TP). At catchment (meso‐) scale, evidence suggests a 20% areal addition of forest at the expense of mixed urban fabric can reduce mean concentrations by 15.7% and 12.6% for TN and TP respectively. Additionally, some articles reveal potential drawbacks reducing benefits provided specifically by street trees and riparian woodland. Leaf litter falling on impervious surfaces can heighten risk of TP leaching to streams, but has little impact on TN. Riparian woodland was found to have complex water quality impacts. Canopy cover suppresses stream channel biological nitrogen uptake, which based on all evidence appears considerable. However, unshaded headwaters can foster accelerated primary productivity with undesirable downstream consequences. Overall, gathering further evidence is encouraged, given current uncertainties, especially to address differences between impervious, permeable and riparian urban woodland settings.