Of Bedrock and Oaks

100_1311Not too far off of the southern most extent of our land, on the property of my neighbor, stands a most impressive tree.  This giant red oak tree grows on a high south facing ledge.  The tree is somewhere between five and six feet in diameter, and more than 100 feet wide from the branch tip on one side to the branch tip on the other side.  The tree is about 100 feet tall.  It has branches that are three feet in diameter. By evidence of the acres and acres of oak trees that surround this tree, it has been a major force on the landscape.  Nearby there are many 3-4 foot diameter trees.  Within four to five hundred feet there are numerous 2-3 foot diameter red oak trees, and for a distance of more than a quarter mile red oaks from saplings to two feet in diameter dominate the landscape.


What is most impressive is not the huge size of the tree, but rather the location of this tree on the landscape..  This red oak is growing in an area that only contains a thin layer of soil about 6-12 inches thick.  Underneath the soil there is hundreds of feet of bedrock, schist bedrock, to be more specific.  This tree, which must way scores of tons, appears to be teetering on top of the bedrock outcrop. I have spent many a moment, standing in the shadow of this monument, trying to answer these questions.


To a fancier of trees like me this is no small accomplishment.  First, there is the issue of ballast.  How does such a gigantic tree grow with no soil into which it can anchor its roots?  Typically we expect a tree to have nearly the same size of root mass as branch mass.  With so little soil where does this root mass grow?  Second, where does this tree, which must use hundreds of gallons of water a day during the growing season, get it’s water?  Usually the fine root hairs on the root structure absorb groundwater from the soil.  Finally, what about nutrients?  Large trees require significant quantities of nitrogen, phosphorous, and potassium.   In most cases, trees get most of these nutrients from the soil.  Again, with so little soil how does this tree get the vital nutirients required for growth and survival?


The ballast question is most interesting.  First, it is necessary to recognize that oak trees grow slowly, and when they are growing on bedrock they grow very slowly.  To digress for a moment, this 5-6 foot diameter red oak could be 400 years old.  To put that in perspective, the acorn that produced this mighty oak was possibly germinating somewhere around the year 1600.  That is 20 years prior to the pilgrims landing at Plymouth Rock, 150 years prior to the French and Indian War, 176 years prior to signing of the Declaration of Independence, 260 years before the Civil War, and 369 years prior to the first human walking on the moon.  Over this long period of time the roots to this red oak tree have been growing into the plates of the schist bedrock.  How is this possible?  All of the plates in the bedrock in our area are tilted upward at an angle of roughly 45 degrees.  This is the result massive force caused by the colliding of continents millions of years ago.  Te tilted plates allows water to enter the the space between the plates.  It freezes in the winter and the frozen ice expands the space between the plates.  These spaces between the plated bedrock provide a place for the roots to grow.  Over years, and years, and years, the roots wind there way through various passage ways in the bedrock.  As the roots grow in size they fill the bedrock pores, producing huge amounts of friction and force, anchoring the tree to the bedrock mass as if it were cemented in place. 


This tree is also growing down wind of a steep northern slope.  It is protected from the winds, particularly during large ice storms that otherwise could damage the tree. Century after century this tree has weathered the worse that out climate could offer, and that is no small accomplishment.


The answer to the water question is also related to the tilted bedrock plates..  A twelve inch sugar maple can use 500 gallons of water a day during the peak of the growing season, I shudder to think what the water requirements are for a 5-6 foot diameter red oak. Nevertheless, this tree having survived hundreds of years, is having no problem filling its water requirements.  This well plated schist bedrock extends over the entire dome of the mountain that lies to the north and upgradient of the tree.  Precipitation migrates though the soil and quickly into the space between the bedrock plates. This type of bedrock is well known for water storage.  In fact, I have an artesian well placed in this same bedrock that for the last twenty years, has poured out over 18,000 gallons of water a day out the top of the well.  This large oak has relied on this same type of plentiful water source for 400 years.


Nutrients?  Same answer.  There are acres and acres of soil that sits on the bedrock dome.  The nutrients in the soil leach into the fractures in the bedrock.  These nutrients get carried by rain water into the stored water in the bedrock.  The roots of  the oak intercept this water and extract the nutrients to feed the large oak towering on the bedrock above. 


Another significant piece to the puzzle that explains this tree’s remarkable survival is its aspect.  This stately tree sits on a wide ledge and faces south below the top of the dome to the north.  The sun warms the huge mass of bedrock, creating a microclimate of slightly increased temperature.  The tree will leaf out earlier in the spring, and stay active longer into the autumn, giving it a longer growing season.  A longer growing season allows more time for the leaves to photosynthesize, producing much needed sugars that will be stored in the roots during the winter.  This warm microclimate also keeps the tree warmer in the winter which is less stressful on the tree. 


Despite the facts that solve the riddle of how this tree could so handily survive what appears to be a hostile environment, I am still in awe every time I find myself in the presence of this great oak.  It has lived a long and useful life and has many, many years to live on.  Standing next to his tree I often find myself feeling very small, even weak. . I am inspired by its strength, beauty, and longevity.   The tree seems to have a determination that I have a hard time defining. I find myself wondering what it would say if it could only speak to me. And then it occurs to me that maybe, just maybe, it has. 


Originally written in November of 2005




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