Oscillants are a phenomenon involving pools of groundwater at the base of larger trees that rapidly and cyclically fill up and then empty again due to wind induced rocking movements of the tree and its root ball.

Causes

The cause is strong winds rocking large trees back and forth, often imperceptibly. This rocking motion of the whole root ball acts like a giant syringe, sucking liquified water up on the 'blow' and pushing it back on the 'lull'.

Such movements eventually create large cavities beneath the tree concerned as the water is effectively liquified soil that may overflow and then drain away, leaving a void beneath the tree. The tree's grip on the soil is weakend and this may eventually lead to the tree falling, however trees are both dynamic and responsive, often growing new or strengthening existing roots to help compensate for this loss of adhesion to the substrate.

Cyclic Oscillants

Uses

Trees with anchor-like tap roots, such as oaks, are less prone to developing oscillants, whilst trees such as lime, willow, and sycamore with wide-spreading root systems and mainly horizontal-growing surface roots are prone to developing them on wet soils. Oscillants can become the temporary homes or shelters of small mammals for breeding and rearing young or simply escaping predators.

Observation

Oscillants are best located after a period of heavy rainfall and for safety reasons, during moderate storms.