Do trees reduce water runoff during flood events? An assessment of soil hydrological properties in three different land-use types of varying tree density

Abstract

The effect of trees on soil hydraulic and physical properties has been of interest for several centuries. While a variety of relevant studies have been undertaken in Europe, Northern America, Asia and Africa, very few studies have been conducted in New Zealand. This study aims to add some understanding and awareness to the overall pool of knowledge regarding New Zealand’s ecosystems, soils and the link to its vegetation. Three different land use types were tested; a grassland paddock (P), a 20-year-old restoration forest (RF) and a 400-year-old native Kahikatea dominant forest (KF). The main assessments included infiltration rate measurements (IR) and soil moisture content (SM). Hereby a single-ring infiltrometer and a Time Domain Reflectometry (TDR) device were used. The average infiltration rate values tended to KF > RF > P, with the Kahikatea forest soil scoring 18 times higher (200.58mm/hr) than the pasture (10.97mm/hr) and 9 times higher than the restoration forest (21.92mm/hr). The average soil moisture content resulted in P = RF > KF with an overall difference of only 10.6% between the paddock/restoration forest and the Kahikatea forest (74.2%). The experiment proved that trees promote infiltration rates, water storage capacity and reduce surface runoff, hence decreasing the risk of perpetuated flood events. Trees are therefore a suitable tool for practicing natural flood management, to slow the spread and lower the level of a possible flood.