Life of a Maple Part 5: Maple Incline and Decline

Healthy sugar maple forest.

It is very likely that prior to European settlement that deciduous forests of eastern North America were actually more disturbed than they are today.  This presettlement disturbance however was much different from the disturbance we see in our forests today.  Today, we see grazing, logging, invasive species, and widespread agriculture as the major forms of disturbance.  During presettlement days fire was the most common type of disturbance of the forest.  Just like the modern disturbances, it is very likely that fire was also human caused in a vast majority of cases.  Given the wet nature of eastern deciduous forests, it is extremely unlikely that fire could have been anything other than human caused.  While today's disturbances are typically an after though to land use, Native Americans purposely used fire to manipulate the landscape, increasing its productivity, and health. 

Fire however, strongly works against the maple tree.  Remembering back to previous installments of this series on maple trees, you might remember that maples prefer very stable, undisturbed habitats.  Anytime fire comes around one of these habitats where maples have become established, the maples are killed off.  As a result, prior to European settlement of the eastern deciduous forest oaks, a fire adapted species were far more abundant, and maples, a fire intolerant species were far less abundant.  By some estimates in some locations there may be up to three times more maples today than there was in the 1800's.  The increase of maples over the last century is a result of fire suppression by European settlers.  It was only on the best soils, in the most ideal habitats where fire didn't touch that maples were found in the 1800's and prior. 

Sugar Maples in fall.

As fire was suppressed and forests began to stabilize, maples began to expand there range.  Maples increased, invaded, and replaced forests that historically had been filled with oaks.  Oak forests typically are far drier and have poorer soil than ideal maple forests.  Oak forests were naturally more prone to fire and therefore easily survived.  But without fire maples moved in. 

Maples moving into areas of less than ideal soil wasn't the best thing for the forest.  Being maples are extremely picky about their environment, living in these less than ideal soils made them especially sensitive to drought.  Oaks are adapted to drought but maples are not.  Maples ideally overcome drought simply by living in the best soils in the forest.  But in less than ideal soils, the maples were damaged during drought.  The damage did not end with drought though.  Drought damage made the tree more susceptible to other problems such as fungal infections and insect damage which often end up killing the tree.  So the incline of maples was a direct result of fire suppression allowing maples to move into marginal habitats.  Maple decline is a result of maples living in these marginal habitats.

Forest where many of the maple trees are dying due to "maple decline".

Life of a Maple Part 4: Maple Syrup

If the Sugar Maple tree is famous for anything, it is famous for maple syrup.  Early each spring as the snow melts, maple syrup farms spring to life from the Midwest U.S., Northeast U.S., and Southeast Canada.  I personally have payed a few visits to these farms and they are always quite an interesting experience.  The weather is typically beautiful with temperatures between 30 and 50 degrees.  This is at least beautiful compared to the previous winter months.  Snow is typically on the ground but melting, which is producing the next most memorable thing about most maple syrup farms: mud.  The farms I've visited are always unbelievably muddy.  They are so muddy in-fact that horses are often used to gather sap.  Horses are used rather than tractors or other vehicles simply because they don't get stuck in the mud! 

The story of how maple syrup is made begins with the previous late summer.  Late in the summer, the maple tree stops growing and instead stores energy in the form of starch.  This starch is stored in the trees sapwood through the winter.  Come spring when sapwood temperatures reach about 40 degrees the starch is converted to sugar by an enzyme and moves out of the wood and into the tree sap.  Rising temperatures, particularly in the morning as the sun comes up, cause the sap to rise through vessels.  The sap rises towards the trees twigs and branches where the sugar will help the tree to begin flowering and budding. 

A large maple tree with two taps and buckets for collecting sap.

As the sap rises, if a tap is in place, some of the sap will drip out of the tree.  This sap generally contains about 2-3 percent sugar and is collected in buckets hanging from the tap.  One Sugar Maple tap can produce 5 to 15 gallons of sap.  Once the sap is gathered from multiple trees it is boiled down to evaporate off the water and concentrate the sugars to form maple syrup.  Typically 40 gallons of sap will produce about 1 gallon of maple syrup.  While sugar maples are the most common tree for producing syrup, red maples, black maples, silver maples, and even boxelder trees (also in the maple family) all can produce syrup. 

Maple syrup was first discovered by and utilized by Native Americans.  Europeans quickly picked-up on the practice and refined it to the practice we see today. 

Earthworm Invasion

Northern Maple forest without earthworms.

As odd as it might sound, earthworms are not native to the northern United States and Canada.  Why? Well, as glaciers receded from the northern portion of North America 11,000 years ago, they left behind a bitterly cold and extremely muddy waste land.  These glaciers reached from the north to their southern the extent of present day norther Iowa, Illinois, and Indiana.  Along this southern extent of the glacial line and northward there have been no earthworms until recently.  Worms simply couldn't survive the frigidly cold temperatures and frozen tundra when glaciers were present in these areas.  Further south however, where glaciers never reached, earthworms have been around for a long time. 

Within recent decades however, earthworms began showing-up in these formerly glaciated soils of the north.  This might not seem that weird until you realize that earth worms travel an average of 5 or 6 yards a year.  Over 11,000 years that equals only about 40 miles, which is a ridiculously slow rate that wouldn't have even allowed them to travel across an entire state.  Even if you double or triple that distance it doesn't even come close to the distance the worms would need to travel to show up in these northern forests.  So how did they move thousands of miles in just a few decades?  The only explanation is that humans carried them.  Fishermen and gardeners are especially notorious for carrying earthworms long distances.  As a result, worms were accidentally introduced to new locations hundreds of miles away from the nearest native worms. 

Northern Maple forest with earthworms.

Most people might think this is a good thing.  Worms are very good for garden soil after all.  The reality is though, worms are not very good for northern forest soils.  Worms are extremely efficient at what they do, which is break down organic materials such as dead leaves.  They do this extremely rapidly, moving nutrients from dead organic materials into the soil quickly.  As a result, plants cannot absorb the nutrients as fast as need and much is lost when water washes it out of the soil.  The burrowing action of worms also functions to compact forest soils, making it more difficult for plants to survive.  While some plants are well adapted to earthworms crawling around through their roots, other plants are extremely sensitive.  Sugar Maples, one of the dominant plants in these northern forests, is extremely sensitive to earthworms. Establishment of maple seedlings where earthworms are present becomes very difficult.  Northern forests with earthworms have far fewer plants than forests without earthworms.  Simply by changing soil and forest floor structure, the earthworm has a huge effect on the overall habitat. 

Fortunately, earthworms have not taken over every single forest in these northern areas.  Also fortunate is the fact that worms only travel about 6 yards a year.  This means, if people quit transporting worms to new areas in the north, populations of worms aren't going to expand much. 

Great Lakes Worm Watch

Life of a Maple Part 3: The Maple Tree and Sunlight

When it comes to soil, Sugar Maples are pretty picky.  When it comes to sunlight however, maples aren't picky at all.  Other trees, such as oaks, prefer to have as much sunlight as possible through out their entire lifespan.  Maples however can do quite well with very low levels of light early on in life.  This is a very fortunate adaptation being the most ideal soils for maples are typically going to be located in the shade of large trees.  Lots of, but not complete, shade aids the germination and early sprouts of maples.  However, maple seedlings will often have stunted growth in very low light situations.  Small seedlings and saplings are capable of surviving many years in the shade of larger trees.  Other sun loving trees such as oaks simply would die due to lack of sunlight.  These small maple trees simply wait until the larger tree dies and is removed by ice storms, wind, or disease.  The wait for an older maple to die can be a long one though being they are capable of living 500 years. 

Once these over-story trees are out of the way, smaller trees that had waited patiently in the shade for years suddenly make a bolt for the sky until becoming a dominant tree in the forest canopy.  This cycle can then repeat itself many times over with younger maples replacing older maples.  This self sustaining process of the Sugar Maple forest will continue unless significant disturbance such as fire or major drought take place.  If disturbance does happen, plants that require more light, such as grasslands or oak forests, will replace the maple trees.  Given time though, and lack of disturbance, after a hundred or more years the maples will replace sun loving trees such as oaks and will again dominate the forest.  This process of one plant community replacing another plant community is called succession.  Maple forests typically are the last stage in succession, which is called the climax plant community. 

Slow growth, long life, and tolerance for shade are what make the maple a climax forest species.  Faster growing trees with shorter lives typically require lots of light and occupy areas after a major disturbance such as fire.  The slow growing maple tolerates the shade and out live these faster short lived species.   Shade tolerance is one of the most important adaptations maples have to being a late successional climax tree.  There are a number of more minor adaptations that aid in the overall shade tolerance of maple.  First off, maples form large thin leaves that gather light very well.  Leaves lack pubescence, or hairiness, which would block light.  These leaves also grow to orient themselves in a manner that helps them gather the most sun light.  Pigments inside of the leaves also are especially adapted to gathering far red light which is abundant in shady environments.  Lastly, maples produce a huge number of leaves in their canopies in order to catch as much light as possible.  Such a great density of leaves are produced by Sugar Maples that the top 10 percent of leaves gather 60 percent of the total sunlight. 

Life of a Maple Part 2: Soil and Roots

A moist maple forest with rich soil.

A Sugar Maple seed doesn't get to choose where and what type of soil it gets to land on.  Typically, where the seed lands is a result of wind direction and strength at the time it falls.  Where ever the seed germinates and begins to grow is where it will spend the rest of its life.  The unfortunate majority will die long before reaching a foot in height.  Often, predators such as deer and squirrels, find the young seedling far to appetizing to pass it by.  Many seedlings will also unfortunately find themselves in soil that is less than ideal.  As far as deciduous forest trees go, the Sugar Maple is quite picky, much like Goldilocks.  The soil can't be too wet or the roots will suffocate as they drown in the water soaked soil.   Neither can the soil be too dry or the roots will dehydrate.  Nor can the soil have too much clay or too much sand.  They soil has to be just right. Even when the soil has just the right texture (meaning the right amounts of clay and sand) and the right amount of water, the soil might not be good enough.  The soil also has to have high levels of nutrients.  Soils with low nitrogen or calcium may prevent healthy growth and longevity of maples.  Even then, maples seem to prefer very deep soils deposited by glaciers over any other type of soil.  The maple is very picky...

The reason the maple is so very picky is because of its roots.  Just like branches of deciduous trees shed their leaves annually, larger roots also shed tiny roots annually and with dry weather.  Maples produce an abundance of these fine roots at very shallow depths, right where the nutrients are highest.  It has been estimated that 60 percent of annual productivity of maples is actually contained within these roots.  This is quite amazing when you consider the great density of leaves maple trees produce annually.  The fact that so much of the tree is in-fact these very sensitive tiny roots makes the whole tree very sensitive to whatever happens on or in the most shallow layers of soil.  Trampling by foot traffic, vehicles, or cattle can damage these roots as well as cause the soil to dry out, killing the roots and potentially killing the whole tree.  If fire burns across the ground, the surface soil will be significantly dried out also potentially killing the roots.  The heat of the fire can also kill the roots very easily.  Pollution, such as acid rain, can change the chemistry of the soil, also killing fine roots and damaging the overall tree.

Fortunately, the maple tree does have some adaptations that help make it at least a little less sensitive to changes in the surface soil.  For one, the overall root system of maples is capable of hydraulically redistributing moisture from deep within the soil to more shallow soils.  The thick shade of maples also helps to prevent evaporation of moisture from the soil.  Also, the fact that maples transpire, or "exhale", large amounts of water vapor while photosynthesizing helps cool the environment and increase humidity.  Fallen leaves are very absorbent and are a very effective mulch that help hold moisture in the soil.  All of this helps moisture to be retained within the soil where it can be utilized by the tree and prevents moisture from evaporating into the environment.  All around, the maple works to keep its environment as moist as possible.

Sugar Maple tree in fall.

Life of a Maple Tree: Part 1 Seed to Sprout

I'll be starting a new series on the blog about the life cycle of the maple tree, specifically the sugar maple.  This is sort of a follow-up to the series on oaks and hickory trees.  Maples are sort of a logical follow-up to the oak-hickory forest being they are later successional species to the oaks and hickories.

Sugar Maple leave

The life of a maple tree begins with the charismatic "whirlybird" seed which fall like helicopters from the mother maple.  Often masses of these seeds will blow off of mature maples and twirl to the ground on windy fall days.  Technically, these "whirlybird" seeds are called samara, which are simple seeds with a flattened papery wing-like portion.  The whirlybird nature of these seeds helps the wind to carry them a long to new locations, often hundreds of yards away.  Then hopefully, the seed will be able to sprout and develop into a new tree.

Once on the ground, the maple seed prefers moist and undisturbed locations, such as in a maple forest or an oak-hickory forest that has not been disturbed by fire.  This is because the maple seed is not well protected.  While the wing portion of the samara is good for transporting the seed with the wind, it doesn't do much else.  The seed requires a moist area, and is easily killed by damage from trampling animals, dehydration, or heat from fire.  Once on the ground though the seed becomes actively searched out for by numerous small animals such as rabbits, squirrels, and mice.  Predation really isn't too much of a problem though, the maple tree typically produces so many seeds that it overwhelms predators.  Predators have plenty to go around and there are still plenty of seeds left over to germinate and sprout. 

Sugar Maple samara seeds.

If not found by seed predators on the ground, the seed than requires the cold of winter in order for it to germinate.  Without cold, the seed will not germinate.  Many species of trees, such as oaks, have a difficult time establishing themselves in soil covered with a thick layer of leaves.  Oaks therefore require the ground to be disturbed by fire so their acorns can sprout and grow.  The maple however, does not have this problem and prefers undisturbed forest ground cover, often thickly covered with dead leaves.  Once germinated, the root easily penetrates through thick moist layers of leaves from the previous year. 

Another oddity of the maple is that it prefers shade.  The maple does not like competition with other small plants such as grasses and shrubs.  It does to quite well though when growing under the canopy of mature trees that shade-out other plants.  In-fact, maple seeds germinate and grow best where there is 50 percent or more shade.  In these areas tiny maple seeds can sprout by the thousands, often leading to a carpet of young maple trees.  The problem though is, once germinated there is so little light in these areas the trees will not grow very large and growth will be stunted.  Again though, the maple is adapted to this situation, being able to survive, but not grow, in minimal light retirements for many years.  The tiny stunted tree simply waits until older larger trees casting shade on the forest floor die.  Once these larger trees die, the tiny maple tree grows rapidly in the new sunlight. 

During the potentially long period of time that a maple seedling remains a small stunted tree it is important that the forest remains undisturbed.  Fire and drought both will easily kill these seedlings.  Predators, such as deer, also heavily browse on "carpets" of small maple seedlings.  Usually though, plenty of seedlings survive predation with drought and fire being the big killers.