Sinker Root System in Trees with Emphasis on Soil Profile

Abstract

Anchorage, water and nutrient uptake and transport are well-known functions of tree roots. However, recent studies ascribe more complex physiological and ecological role to tree roots. This is more particularly so in many tree species where roots have a characteristic dimorphic spread having (1) the surface roots that have a subterranean horizontal spread a few metres around the trunk and (2) sinker roots that go vertically downwards to 10 m and beyond. Increasing evidence is accumulating that the surface and sinker roots form a very dynamic water facilitating system in the soil. This is discussed under three main heads. Firstly, the sinker roots have access to groundwater moisture and even the capillary fringe of the deep water tables making the transpiration and the vital shoot processes sustainable, even when the upper soil profiles are dry. Such roots also cause biodrainage of the water table preventing it from rising to surface layers and making the soil waterlogged. Secondly, the sinker and surface roots form an integrated conduit in the soil that causes upward hydraulic redistribution of the deep soil water to soil surface. Interestingly, this water may also be used by shallow-rooted herbaceous vegetation for its sustainability during episodes of drought. Thirdly, a downward hydraulic redistribution from the surface roots in moist topsoil to the deep soil through the sinker roots may recharge the deep dry soil profiles for future use. The sinker root system, therefore, enables hydraulic redistribution sustaining dry season transpiration and photosynthetic rates of the parent tree and surrounding shallow-rooted vegetation, prolonging the life span of fine roots and maintaining root–soil contact in dry soils and storing rainwater down into deeper soil layers for dry season utilization.