By Tommy McCarthy and Kevin Andros, Environmental Educators

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September is a fantastic month to learn more about trees. They still have their leaves in order to help with practicing identification, and there is so much about them to observe as we head into autumn. We can make a point of observing what color changes different species exhibit, the timing of those changes, and what else aside from leaves they might be dropping. As we move ever closer towards another autumn equinox, I’ve found myself thinking about what might be going with trees that we can’t easily observe.

 

One of the things that I anticipate noticing in the fall aside from the beautiful foliage is the crop of acorns produced by oak trees. Year after year, there are differences in the abundance of these charming cap-wearing nuts that drop from the canopy. Years of extreme abundance are known as “mast years”, and these years of increased production can happen with a number of tree fruits we are familiar with; including “helicopter” seeds from maples, beechnuts in their spiky casings, and hickory nuts in their thick shells. Last fall, I took particular note of maples. There were days that it almost seemed to be raining these seeds as gusts of wind sent them playfully spiraling to the ground. 

 

We might just think that it was a good year or a bad year for certain species when we notice these patterns, but fascinatingly, there is an effective strategy behind them. The abundant years produce more fruit than can be eaten by the wildlife in the forest, and the years of scarcity end up acting as a population control for the would-be pilferers of seeds. This is how trees increase their success rate when it comes to reproducing. I mentioned that I noticed maples having a mast year last fall, and sure enough as spring rolled around, the seedlings were prolific (as were the chipmunks!).  But knowing that mast years are a strategy to ensure reproductive success causes another question to arise - how do trees coordinate it? Conditions like frost, fires, and droughts can certainly play a role, but research reveals that regional conditions like those can’t possibly tell the whole story. Millions of trees as far as 1500 miles apart have been found to synchronize their increased production. How might they all “know” that the time has come to spend that additional energy and create a mast year? 

 

One theory for how many trees enter mast years at the same time comes from mycorrhizae. This hard-to-pronounce word refers to many species of fungi that have symbiotic, mutually beneficial, relationships with trees. This aspect of nature is under-appreciated and rarely talked about outside of arborists and mycologists even though it is an amazingly beneficial feature of ecosystems. 

 

Before delving into mycorrhizae it is important to mention that when I say fungi I am not talking about mushrooms. Mushrooms are the fruiting body of the fungi just like how the apple is the fruiting body of an apple tree. In this example the “apple tree” of a fungi is the underground root-like structure called mycelium. Mycelium is present year round, under every step you take in a forest, and mushrooms are only formed when conditions are just right. 

 

Fungi that are mycorrhizal have the survival strategy of working with the roots of plants to survive. In most cases the fungi receives carbohydrates made from photosynthesis from the plant. In exchange the fungi can bring water and nutrients to the plant. It is like the fungi are a more efficient root system that can reach further. In the natural world this relationship is incredibly important as about 90% of terrestrial plants are symbiotic with mycorrhizal fungi. For humans these relationships are pivotal to the success of many agricultural plants. Major crops like corn, wheat, rice, and cotton rely on mycorrhizal fungi to create healthier soil, improve access to nutrients, improve the growth rate, and increase their yield. 

Wait a minute weren’t we talking about trees and mast years? Sorry I got caught up in how cool mycorrhizal fungi are and thought it would help out to see the big picture behind mycorrhizae. Mast years may be the result of fungi forming a network between trees underground. Trees are able to indirectly communicate with each other by releasing chemical signals through the mycorrhizal network (also known as the "wood wide web"). When trees begin to communicate about nutrient and water availability their seed production begins to sync up. This synchronization is a key piece that makes up the mast year puzzle.