BLACK WALNUT: SEED DISPERSAL—SPHEROIDAL “FRUITS”—SLOPE AND GRAVITY

Black Walnuts seed dispersal. Hard, impact and decay resistant spheroidal fruits (drupes) like those of the Black Walnut (Juglans niger) can roll long distances from its parent tree and this process may be an important factor in its seed dispersal.

An ancient and much-admired Black Walnut (Juglans niger) tree grows not far from my home on Long Island’s north shore.  At this time of the year (late October of 2017)  the near baseball-size green spherical seeds (known as a “drupe” in botany) hang heavily from the tree like big green Christmas ornaments  After every windstorm many fall to the ground and roll away from the drip line of the tree, many often accumulating in in long “windrows” in depressions downslope from the tree.    In areas with steep slopes where this species grows,  I have observed the green dupes roll tens of meters from the base of the tree. In one instance accumulating in an intermittent stream bed, then to be presumably washed down stream during a heavy rainfall.
Simple rolling downhill, under the influence of gravity seems a natural means of seed dispersal for this species, but one which seems little discussed by ecologists and botanists who study this species.

The green husk of the fruit is very tough and thick, as anyone knows who has attempted to get at the very nutritious and tasty nut-meats inside the shell.  The drupe is hard as a baseball and resists deformation when it hits the ground even from great heights.  The green thick husk is impregnated with a strong herby smelling substance ( I find it pleasant) that appears to slow the decay process.  However, when decay does take place, the partly decayed darkened husks will stain your hands a near-]permanent brown color.  I have heard from old timers that this (decayed husk) substance  was used as a staining agent and mordant (a substance which will combine with a dye and fix it permanently in a fabric —or on your skin) by our local colonists.  Local woodlore claims  that black walnut husks were used to make a brown fabric dye, a permanent ink, and even to dye the graying hair of early colonists.  As a consequence of the sturdy physical character and decay-resistant  chemical substances in the husk, the drupes tend to resist deformation, biological decay and physical abrasion and remain in their original sphere-like shape over a long time, enabling it to continue to roll over relatively long periods of time, encouraging its movement  from its parent tree.

An old timer I knew with a great store of wood lore and a powerful desire and taste for walnut meats used an interesting technique to remove the tough husks.  He secured a two-foot square piece of 3/4 thick exterior plywood in which he drilled a walnut-sized hole in the center.  He placed the plywood square on top of a sturdy metal pail (or an empty ten gallon  plastic paint pail) and setting the green-husked drupe on the slightly smaller drilled hole, he struck the green orb with a heavy mallet.  The walnut passed through the hole into the pail  while the green husks remained on the outside.  He wore heavy gloves to protect his hands.

When I admired this technique he remarked that it was a great improvement over his original system which was simply stomping the partly decayed drupes on the asphalt surface of his driveway.

I often wondered how these huge seeds were dispersed. The most common explanation is that they are commonly eaten by our local Gray Squirrels (Sciurus carolinensis)  These arboreal rodents ( as well as Sciurus niger the Fox Squirrel) are known to practice a hoarding system in which they bury the drupes.  The fruits are collected and carried away from the tree by the squirrels and buried in scattered locations to protect their nutritious hoard from the depredations of other squirrels.  Some of the buried fruits of course are lost or never recovered and thus sprout into new trees at some distance from the parent tree.

I am certain that this is a major means of seed dispersal for this species.  I have seen grays eating the dehusked walnuts and often find the fragments of opened nuts at squirrel feeding posts on the tops of large boulders or tree stumps.  I also observed others carrying the big heavy drupes in their jaws.

But here I suggest that another method of seed dispersal for the Black Walnut is simple rolling.  The spherical drupe of  the black walnut can roll long distances away from its origin simply by the near universal fact of topographic slope and gravity.  This “seed” is almost perfectly adapted to moving over the earth’s surface on even gentle slopes, and to remain intact as a sphere and continue that motion over relatively long periods of time during the fall season.  I suggest that even in the absence of rodents the tree would continue to effectively disperse its seeds based on the spherical nature of its seed.