Signs of Spring 9: Natural History of the Sugar Maple

Sugar maple. Photo by D. Ramsey. Wikimedia Commons

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(The observations of the sugar maple on the Penn State New Kensington Nature Trail were made in 2006. The trail, sadly, is no longer accessible for hiking.)

It’s early March, and I am walking down the Pine Trail section of the Penn State New Kensington Nature Trail. A fresh two inches of snow fell last night and covered the patchy, half-melted trail surface with a smooth, unbroken layer of white. Walking is especially treacherous today because you have no idea if there are old leaves or a sheet of ice beneath each footfall. I’ve decided that it’s too slippery to hike down into the ravine, so I cut perpendicularly off of the trail to follow the edge of the ridge top. I have to push through thickets of raspberry and multiflora rose that fill the tight spaces in between the trees. Thorns dig into my denim coat and pants and snap away as I walk on.

After about 60 yards of bushwhacking I come to the Spicebush Trail and turn right to follow it along a seldom walked dead-end path that runs toward the back meadow of the campus.  Ten yards down this trail is the tree I am looking for: a very venerable looking sugar maple. Its bark is gray and very knobby and fractured. There is a hollowed out space in its trunk that runs all the way through the tree. I bend down and can look through it to the forest beyond. The tree stands on the top edge of a long slope that runs down through a young beech forest to the stream below. The tree is a foot and a half in trunk diameter and fifty feet tall.  Its lower branches spread out laterally to command a large volume of the predominantly yellow poplar canopy. A chaotic network of upper branches inter-twine to form its dense upper crown.

I dig down through the snow with my boot and uncover some of last season’s leaves. They are wet and brown and shaped in the familiar five-pointed leaves of the Canadian flag (and a Molson’s beer label, and a Toronto Maple Leaf’s  hockey sweater). I put my hand onto the trunk almost expecting to feel a pulse from the sap which I assume is rising inside the bark. For breakfast I had some pancakes that were covered with some concentrated sap that had been gathered from some Michigan sugar maples. In nearby woods, other sugar maple trees are already being tapped to gather the dilute sap which will be cooked down into syrup and sugar. This tree, though, will get to keep all of its sugar.

Sugar maple. Photo by B. Marlin. Wikimedia Commons

The sugar maple is a significant tree of our northeastern forest. Further north, it may form dense pure stands, but here in Western Pennsylvania, it is usually found mixed in with other types of trees. It is a very versatile tree and, economically, quite valuable. The wood of this “hard” maple is used extensively in furniture manufacturing and for floors, and the maple sugar industry generates hundreds of millions of dollars in sales each year.

The sugar maple grows best on deep, well drained soils and, unlike the red maple, is relatively uncommon on any site that is excessively dry or excessively wet. It is a tree of cool, moist climates and habitats, and its distribution throughout the northern sections of the eastern United States is being reduced possibly by the impacts of global warming. Some forestry experts contend that the continued warming of our climate will so reduce the viability of sugar maples that the maple sugar industry of the United States will cease to exist within the next hundred years.

Photo by NPS. Public Domain

Maple sap begins to flow when day temperatures rise above freezing and night temperatures fall below freezing.  Data collected from sugar maple harvesters show both a warming trend and a shortening of the sap flow period. Sap flow begins now a week earlier and ends ten days earlier than it did just forty years ago. The net loss of those three days represents a ten percent decline in the thirty day sap flow season, and this shrinkage is expected to continue (S. Samuels, New York Times, April 28, 2007).

The sugar maple tree in front of me, based on its diameter and height, is between eighty and ninety years old. It is in the early prime of its life which could, if all goes well, last three hundred to four hundred years (although that hole through its trunk could shorten its potential life span!). It could continue to grow in height for another 60 years and keep growing in diameter for many decades after that. Sugar maples, not unlike people, have smooth and regular looking “skin” (their bark) when they are young but get increasingly “interesting” (wrinkled, lumpy, gnarled) as they age. This tree is just beginning to undergo the transition into a more “mature” bark appearance.

So why is this tree here?

We know that all sugar maple trees come from sugar maple trees, so, eighty or ninety years ago there had to be a parental sugar maple tree in this area. It may have been right here on the edge of this long, ravine slope, or it may have been a hundred yards or more away. The winged samaras of this species can travel moderate distances on the wind. The parental tree had to be at least twenty-two years old (the age of first flowering) although it was probably much older than that.

The human history of this Nature Trail site describes the clear cutting of the pre-settlement forest in the early 1800’s. The original forest was predominately white oak, American chestnut, and yellow poplar with a rich mix of other tree species that included a small percentage of sugar maple. This initial removal of the forest cleared the land for a short period of agricultural use. When the land was abandoned a few decades later, a secondary forest of poplars, white ash, oaks, and an increased percentage of both red and sugar maples arose. In the early decades of the twentieth century this secondary forest was cut again. A few older white oaks were left to act as “seed” trees, but, apparently, no sugar maples were left within the area of the Nature Trail itself. The tertiary forest that then arose (and which we still see today) was dominated by yellow poplar and white ash, with scattered white and red oaks, red maples, and a few sugar maples.

Our tree may have begun its life as a stump sprout or it may have begun as a new seed that was blown in from a nearby parental tree. The parental tree would have flowered in early April a couple of weeks before it leafed out. The parental tree would have had both male and female flowers and the pollen produced would have been spread by the wind. At one time, it was thought that bees directly pollinated sugar maple leaves, but bees are no longer considered to be involved in sugar maple reproduction.

Photo by K. McFarland, Flickr

The seed is encased in a relatively large, double winged samara that develops through the summer and reaches maturity in the late summer or early fall. Two weeks or so before leaf fall the samaras drop from the tree and spread through the forest system also on the wind. The seed overwinters on the forest floor (often covered by fresh fallen sugar maple leaves)  and germinates often before the snow cover has melted. Each germinating seed produces a very strong root that pushes down through even thick layers of leaf litter into the underlying layer of moist, mineral soil. The germination rate is very high (up to 96% of the fallen seed germinate!), but at least half of the seedlings will not survive their first year of life. Still, sugar maple seedling densities in the understory can be very high (over 150,000 per acre!).

The seedlings need to stay moist and are quite vulnerable to even moderate drying. Once the seedlings reach heights of two to four feet they have sufficient root masses to sustain themselves even under the drying impacts of rising levels of sunlight. The extra sunlight then drives more rapid photosynthesis in the sugar maple leaves and stimulates vertical growth of the seedling. Growth of sugar maple seedlings, saplings, and “pole” trees is slower than most other hardwoods (about one foot per year for the first thirty or forty years), but the trees high tolerance of shade enables its slow, steady survival in the shaded understory of many types of forest.

Photo by D. Sillman

White-tailed deer browse heavily on sugar maple seedlings, so our tree required some aspects of chance and “luck” to avoid being eaten during its seedling and early sapling stages. The deer population in Western Pennsylvania became explosively large during the early years of this trees life. A browsed sugar maple, though, is able to re-sprout, so, possibly, our tree was partially consumed by a deer but recovered. The impact of deer browsing may favor the survival of American beech trees (another slow growing, shade tolerant hardwood species, but one that is not palatable to white tailed deer) over sugar maple.

Eventually, a fortuitous opening in the over-story above our surviving sugar maple possibly through wind throw or wind breakage of one of the dominant yellow poplars or white ashes (a commonly observed phenomenon in this forest), enabled our tree to push up into the canopy and attain its mature aspect and configuration.

Sugar maples are defoliated by gypsy moths and forest tent caterpillars but are, overall, fairly resistant to insect damage. They are quite sensitive to air pollution and road salt, and they may, as mentioned earlier, be under some stress due to global warming. Many animals eat sugar maple seeds including red, gray, and flying squirrels and a variety of small rodents. Porcupines feed on the bark of sugar maples.

Photo by J. St Johns, Flickr

The sugar maple is the poster tree for autumn. Its bright red, yellow and orange leaf colors gloriously mark the end of the growth season and announce the onset of winter. When the sugar maples, like all deciduous trees,  get ready to shed their leaves in the fall, they undergo several well defined stages of change.

First, in response to the duration of the dark period of the day reaching a critical length, the leaves begin to generate large numbers of cells right at the junction of the leaf’s stem and its branch. These cells greatly increase in number but not, at first, in their individual sizes. This layer of cells (the “abscission layer”) slowly starts to interfere with the flows of sugars out of the leaf and nutrients into the leaf.

The lack of nutrients entering the leaf stops the synthesis of the new chlorophyll molecules that are needed to replace the ones that wear out in the ongoing process of photosynthesis. Chlorophylls are, of course, the pigments that give plants their characteristic green colors. Initial cessation of chlorophyll production makes the leaves appear a bit paler and less intensely green than they were during the height of summer.

Continued breakdown of the chlorophylls then starts to unmask the other pigments (the “accessory” pigments of photosynthesis: the carotenoids and xanthophylls) that had been present in the leaves all summer long). As these pigments are “revealed” the leaves then “turn” orange (from the carotenoids) or yellow (from the xanthophylls) before they finally fall.

The chlorophylls trapped in the leaves continue to power photosynthesis and fixation of carbon dioxide into sugar molecules. Sugars, then accumulate in the blocked leaves. This accumulation of the sugars also has an effect on eventual leaf color because the sugars stimulate the synthesis of anthocyanin pigments in the leaf. These pigments generate purple or bright red colors in the leaf and were thought (by the late W. D. Hamilton, the famous “Bill Hamilton” of biology!) to possibly protect the leaf (and particularly next year’s delicate leaf buds) from insect damage.

So, possibly the beauty of the fall color change is an aesthetic reflection of an important mechanism of self-protection by the tree!

I am back on the Nature Trail looking at the sugar maple on the meadow branch of the Spicebush Trail. There are small, red flower buds starting to swell on its branches. They will open in a few weeks and start the year’s reproductive cycle. A few weeks later, the tree will begin to leaf out. Around me, in the snow covered leaf litter, seeds from last year’s cycle are about to germinate and send their root radicals down through the litter layer into the underlying mineral soil. Surviving seedlings from previous years stand almost invisibly as thin woody stalks barely breaking through the surface of the snow. Scattered arrays of saplings are partially hidden in the surrounding thickets of raspberry and spicebush. The maple forest is slowly growing around this parental tree.