Walking Palms

I don't know about you but Socratea exorrhiza has to be the strangest species of palm that I have ever laid eyes on. Native to Central and South America, its peculiarly stilted appearance has earned it the common name of "the walking palm." Whereas most palms invest in heavy trunks, the walking palm sends out a lanky set of stilt-roots upon which the palm grows like some otherworldly tripod.

There has been a lot of debate over the last few decades as to the purpose of these stilt-roots. In 1961 it was suggested that they were an adaptation for living in swampy areas. To date, no evidence of this has been found. Others have suggested that these roots are relatively cheap to produce compared to a solid trunk, thus allowing more investment in growing taller in a shorter amount of time while still maintaining structural integrity. This sounds attractive and is probably part of the puzzle. However, I feel that a particular study published in 1980 offers the best explanation.

Tropical forests are full of decomposition. The omnipresent threat of rot means there is a constant rain of limbs and snags from the canopy above. Trees regularly topple as well. For most plants, getting flattened by such debris is usually fatal. This is not necessarily so for the walking palm.

It has been observed that walking palms flattened by a fallen limb or tree can actually "walk" themselves out from underneath. Since most of the trunk is capable of producing stilt-roots, it doesn't take long for a new anchor to become established. Once this occurs, the palm is free to continue its journey into the canopy.

Getting squashed isn't the only worry either. Light is a premium in the deep shade of a rainforest understory. It is also short lived. A hole in the canopy that provided ample light one week may quickly close in the next, removing the life-giving rays of the sun. If a plant were able to "move around" it could potentially relocate to a sunnier spot. In a sense, this is what the walking palm does.

Walking palms are positively phototropic, meaning they lean towards a light source. Leaning can put stress on a trunk to the point that the tree topples over. The walking palm gets around this by sending down those stilt-roots, which provide support as it chases light through the canopy. In a sense, this palm "walks" itself around the forest in search of the best light. Whereas most trees are stuck where they germinate, the walking palm has, in a sense, freed itself from such restrictions. As such, older trees are often found far from their original germination point.

Once in a favorable location, the walking palm will right itself and continue upwards. At this point, the old trunk and roots are superfluous and will often rot away. For a young tree, this process can happen in as fast as two or three years. This is an incredible feat considering the time scale most trees operate on. Personally I would love to be able to observe a forest with walking palms over a few decades. Seeing how their positions change with time would be fascinating. At the very least, their bark is often covered in epiphytes, which offers a lot to comb over on our timescale.

Photo Credits: Hans Hillewaert (Wikimedia Commons) and John H. Bodley and Foley C. Benson (infographic)

Further Reading:

http://www.revistas.ucr.ac.cr/index.php/rbt/article/view/5955

http://www.jstor.org/stable/2387775?seq=1#page_scan_tab_contents