Given the high level of rainfall in rainforest habitats, there is always a risk of soil erosion, and landslips that can lead to trees falling over. In addition, constant high rainfall can wash away the ‘tree food’; the decomposing leaf litter and the soil nutrients.
Buttress and stilt roots
To increase stability, maximise the speed and potential of nutrient (food) harvesting, and allow the trees to have their roots near the soil surface, many rainforest trees have trunks that flange out and widen significantly at the base before joining directly onto surface feeding roots. Some of these stem base ‘buttresses’ can stretch many meters from the trunk and stand several meters high. The buttress roots create a large surface area and allow the tree to spread at the base, which increases stability, and allows the development of surface feeding roots that slow the movement of water and increase the potential to access nutrients.
Other tropical root types include ‘stilt' or 'prop’ roots, that look like a witches broom, such as those on some Mangroves (for example, Rhizophora spp.).
Remarkably, there is one tree that grows in the opposite direction to all other trees, growing down rather than growing up! The ‘Strangler Fig’ begins its life as a seedling in the canopy of the forest. Small seeds are dispersed by birds and bats and sometimes lodge in an epiphyte or on a branch high in the canopy. Aerial roots then grow and slowly descend toward the ground, sometimes 40 metres or more below.
Only in the moist shaded rainforest can aerial roots survive without drying out.
After many years the roots take hold in the soil and then the Fig begins to strangle its host. Slowly, over decades and centuries, it grows to overtop and shade the original tree, and finally encases it in its own woody tomb. When the host tree finally dies and rots away the Strangler Fig is left in its place, with only the hollow inside the Fig showing where the original tree once lived.