It Was a Mast Year….But Why and How?

June 2015

by Linda Martinson 

You may remember walking on the Richland Ridge trails last fall and noticing acorns everywhere, like ball bearings underfoot. And then, noticing this spring a proliferation of small oak seedlings in every open space. It has been ‘a good mast year’ for acorns and therefore for the wildlife at Richland Ridge. 

Mast is the accumulation of nuts on the forest floor, usually acorns in the Southern Appalachians. Approximately every three to five years, an entire population of trees, across a fairly wide geographical area, produces an over-abundance of wild nuts, as many as five to ten times more than during an average year. These bumper crop years are ‘good mast years,’ and there is often a bumper crop of wildlife, such as bear cubs, the following spring because acorns are the most important wildlife food in most deciduous forests. 

  Mast on the forest floor.

Why, and how, do the trees coordinate a synchronous over-abundant nut production in a given year? The variable nut production of individual trees must have evolved as a survival strategy: if the nut production was constant from year to year, the animals that depend on mast would adjust, and almost every nut would be eaten every year. Certainly, the synchronous production of mast among a large number of trees is a superior survival strategy: when there is suddenly an over-abundance of mast produced in the forest one year, the wildlife population cannot eat all of it, leaving plenty to sprout and grow. 

An abundance of seedlings following a mast year.

The big question is how masting trees manage to coordinate the same cycle with other trees over a large area in a given year? And it can be a large contiguous area; synchronous masting has been found among a million trees as far away from each other as 1500 miles. Despite extensive research on synchronous masting, until recently scientists had not developed a complete and definitive answer even though they had identified several factors that affect masting.

Weather, such as seasonal temperature extremes, wind, rain, drought, and other environmental stresses such as fire or pests do affect oak trees, but only in specific areas. Oaks are monoecious, i.e., they bear their male and female flowers on the same tree, so a late spring frost that kills all the flowers on oak trees in a given area certainly precludes a good mast year there. Also, oaks are wind pollinated and there are chemical signals carried from tree to tree by what is called pollen coupling, but pollen is usually effective only within a range of about 200 feet so it cannot account for all of the synchronous tree communication among masting trees. There has to be another component to tree communication beyond weather and other environmental factors and chemical communication through pollen coupling.

An emerging hypothesis explaining tree and other plant communication is from recent research proposing that an extensive underground network exists that connects plants by their roots, serving as a complex interplant communication system, a kind of ‘plant internet.’ The organism that creates this immense biochemical highway is a type of fungus called mycorrhizae. The tiny filaments of the fungi form a symbiotic relation with plants, including trees, by colonizing the roots and sending extremely fine filaments far out in every direction into the soil, like extensions of the plant roots. Mycorrhizae increase the nutrient absorption of the plant 100 to 1,000 times, and these fungi also provide communication networks among the plants allowing them, for example, to develop natural chemical defenses against common pests such as aphids after they receive signals from a plant that has been attacked.

In one thimble full of healthy soil, there are several miles of fungal filaments releasing powerful enzymes that make communication and synchronicity among plants possible. One network of mycorrhizae was found weaving its way through an entire Canadian forest with each tree connected to dozens and hundreds of other trees through enormous networks of mycorrhizae. These fungi networks also connect plants of different species, thereby possibly facilitating interspecies plant communication. 

One characteristic of oak tree germination is its interesting life cycle. Soon after acorns fall, if they aren’t eaten almost immediately, they start to germinate, but they send out roots not shoots. Root growth continues until the ground gets too cold, then it continues in the spring along with the growth of shoots. It is possible that the immense mycorrhizae network connecting the plants in a healthy forest, including oak trees, can determine whether a sufficient number of acorns have developed roots and signal this information to oak trees in a large geographic area. It does sound like science fiction, but it is possible given the size and capabilities of mycorrhizae networks, and it could form a more complete explanation of the widespread synchronicity of masting trees in large forested areas.

Mycorrhizae underground networks have been found to have symbiotic relationships with more than 90 percent of plant species, and what is absolutely necessary for these fungal networks to exist and flourish is that the soil must be undisturbed. Soil erosion, tillage, compaction and other disturbances to intact healthy soil destroys beneficial fungi, and they are slow to recolonize once they are disrupted. If indeed the trees and plants in forests of Richland Ridge are able to communicate with each other through an extensive underground mycorrhizae network, kudos to us for keeping our forests healthy with relatively undisturbed soil. And hooray for good mast years, because as Ralph Waldo Emerson reminded us, “The creation of a thousand forests is in one acorn.”

References:

W.D. Koenig and Johanes Kropp American Scientist. The Mystery of Masting in Trees. July – Aug 2005 Volume 13, Number 4. Page 340

BBC News Summary Science and Environment Fungus Network Plays Role in Plant Communication 10 May 2013