Sugar maple (Acer saccharum)  in the Hubbard Brook forest:

        Sugar maple is one of the dominant canopy species of the northern hardwood forest found at all but the highest elevations in the Hubbard Brook Experimental Forest.  It is one of the most shade tolerant hardwood species in the forest, and therefore, has increased its presence in the canopy as the forest has matured since logging in the late 19th and early 20th centuries.  Mature sugar maple is currently suffering a decline in the northeastern U.S. for reasons that are not yet entirely clear.  A few trees in the Hubbard Brook forest appear to be suffering but most seem to be in good health.  Sugar maple regeneration, however, is minimal or absent in many areas, perhaps due to competition from the very dense beech regeneration in most areas of the Hubbard Brook forest.

        1.  Sugar maple abundance in the forest community
        2.  Sugar maple regeneration
        3.  Sugar maple decline
        4.  Sugar maple tree rings
        5.  Sugar maple cross-section
 
 
 
 

1.  Sugar maple abundance in the forest community

        American beech, sugar maple, and yellow birch are the three major hardwood canopy species of the Hubbard Brook forest.  Yellow birch is slowly disappearing as the larger trees die and are replaced by beech and sugar maple.  Beech bark disease (BBD) swept through the area in the mid 1970s, damaging and even killing many of the larger beech trees.  Despite the disease, however, beech has remained the codominant canopy species, along with sugar maple.  Since the appearance of BBD, the sapling beech population has exploded.  As illustrated in the figure, beech saplings now make up three quarters of all the stems in the 2 to 9 cm diameter class. We suspect that damage to the aboveground portion of the larger trees by BBD stimulated sprouting from the roots.  The root sprouts have gradually grown to sapling size and have come to dominate the understory, reducing light levels at the forest floor, and making it difficult for the saplings of less shade tolerant species, such as sugar maple, to survive.  It appears that beech will continue to dominate the understory for some time to come, and it may also eventually dominate the canopy if sugar maple regeneration continues to decline.
 
 
 

2. Sugar maple regeneration

Sugar maple readily regenerates under the shade of a forest canopy and every year the mature maples in a forest produce seed.  In some years they produce a bumper crop of seeds and the next spring the forest floor is covered in germinants.  Even after non exceptional seed years, germinants are commonly seen in the hardwood zone of the Hubbard Brook forest.  In some areas, the seedlings survive for several years, creating a carpet of young sugar maple (as in this photo).  Some of these healthy seedlings eventually grow into saplings, but sugar maple saplings are becoming increasingly rare in the Hubbard Brook forest.  They are more common at lower elevations and there is often an abundance of sugar maple regeneration in large gaps.  At the higher light levels found in these gaps, sapling sugar maple can grow faster than sapling beech and therefore has a competitive advantage.  In the rest of the forest, it appears that sugar maple regeneration cannot survive being shaded out by the very vigorous beech understory.

        A study of sapling population has been conducted on W6 since 1965.  In that year there was a profusion of sugar maple seedlings (<50 cm tall), perhaps the result of a single exceptional seed year.  In subsequent surveys, the seedling density has fluctuated but never been as high as the 1965 density.  The graph below shows beech and sugar maple stem densities for saplings between 1.5 and 9.5 cm dbh.  In 1965, sugar maple and beech saplings were present at similar densities in all size classes.  By the next survey in 1982, sugar maple sapling density had dropped precipitously in the smaller size classes.  In subsequent surveys, sugar maple sapling densities continued to decline while beech sapling densities exploded.

        Beech is very effective at intercepting light in the understory, and we believe it is the shading effect of beech that is the cause of this decline in sugar maple regeneration.  Canopy damage from the ice storm of 1998 may provide a chance for the sugar maple regeneration to recover--sugar maple can grow more quickly than beech at higher understory light levels.  However, understory beech at Hubbard Brook is extremely vigorous and seems to be poised to take full advantage of any extra light reaching the understory.  It may not be until this cohort of beech replaces the canopy that sugar maple regeneration has another chance to thrive.
 

Note:  The stems/ha scale for sugar maple is only 20% of the beech scale. Also, note the steady decline of sugar maple in most diameter classes between 1965 and 1997.

 
 
 

3. Sugar maple decline
 

        Sugar maple across the northeastern US and eastern Canada have been exhibiting an increased incidence of decline in recent years.  Sugar maple decline is not a disease with specific causal agents, but a series of events that eventually leads to the death of the tree.  It has been around for decades but has not been considered a widespread problem until recently.  The exact causes of sugar maple decline seem to vary with site are therefore hard to pinpoint. Symptoms include reduced foliage and reduced twig growth, and then dieback of branches in the upper canopy.  The photos above show a sugar maple with crown dieback and a close-up of dead branches in the crown.

        In a forested environment, sugar maple decline is believed to result from the stresses of a changing climate, often combined with several successive years of insect defoliation.  In some cases, where soils are naturally poor in base cations and therefore "acid-sensitve" (as at Hubbard Brook), acid rain may also play a role in sugar maple decline by further stripping the soil of important nutrients.  Weakened by multiple stresses, sugar maples become more susceptible to pathogens.  Three fungi that commonly attack sugar maple are Armillaria mellea (root rot), Nectria cinnabarina (branch canker) and Steganosporium ovatum (twig blight).  It is the activity of these secondary pathogens on an already weakened tree that eventually leads to the death of the tree.  Sugar maple decline is apparent in some areas of the Hubbard Brook forest, however it is not yet a widespread problem here.
 

Sugar maple decline links:

• Cornell University Plant Disease Diagnostic Clinic
• University of Rhode Island Landscape Horticulture Program

 
 

4. Sugar maple tree rings

        This core was taken from the bole of a sugar maple. Each ring represents a year. By measuring the widths of the rings and multiplying by two, we can estimate the amount of diameter growth in any given year.  These rings are fairly easy to distinguish, but the smaller rings of slower growing trees can be very difficult to read.
 
 
 

5. Sugar maple cross-section

        This cross section was taken from the base of a tree that had previously been tapped for the production of maple syrup.  Sugar maples often have darker wood in the center of the bole, and where maples have been tapped or otherwise scarred there is further staining of the wood.  We have found that dark wood has a much higher concentration of calcium than light wood, leading us to conclude that some older, damaged sugar maples may be a large sink for calcium in the forest.  (The large holes were drilled for analysis of wood chemistry.)
 

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Web page created August 2001
by Ellen Denny and Thomas Siccama