Yellow birch (Betula alleghaniensis)  in the Hubbard Brook forest:

        Yellow birch 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 has intermediate shade intolerance and can live to be 300 years old or more.  The largest yellow birches in the forest were young trees left standing during logging of the valley in the late 19th and early 20th centuries.  These trees are slowly dying out and are replaced only in spots were a treefall lets enough light into the understory for a yellow birch sapling to grow into the canopy.

        1.  Yellow birch abundance in the forest community
        2.  Yellow birch regeneration
        3.  Yellow birch leaves
     4.  Yellow birch bark
 
 
 

1. Yellow birch 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 mostly by beech and sugar maple.  Yellow birch saplings are common but not abundant in the understory.  Since yellow birch has only intermediate shade tolerance, most of these saplings will weaken and eventually die in the low light of the understory before making it to the canopy.  However, at higher elevations and along stream courses, where windthrow disturbance is more prevalent, it seems that there is more yellow birch replacing gaps in the canopy.
 
 
 

2. Yellow birch regeneration

Every year there are from a few to hundreds of yellow birch germinants per square meter on the forest floor.  Yellow birch produces abundant seed which is dispersed by wind beneath the canopy.  Those seeds that land in a favorable spot on bare soil or a rotting log will germinate.  With enough light and moisture the germinant will develop further into a seedling.  Under a full forest canopy, however, very few will survive to their second growing season.

        Although yellow birch is the most shade tolerant of the birches, it is still less tolerant than both beech and sugar maple.  A gap in the forest canopy encourages yellow birch seedlings to grow into saplings, but the gap is usually closed by the lateral canopy growth of neighboring trees or by advance regeneration of beech or sugar maple long before the yellow birch sapling could reach the height of the canopy.  At that point the sapling languishes in the shade of the overstory.  Another canopy gap would allow another growth spurt and a very large gap might even allow yellow birch to move into the canopy, but generally speaking, yellow birch saplings under a forest canopy will not survive.

        Yellow birch also propagates by root sprouting, most commonly from the base of a broken off tree.  In this situation, the healthy roots nourish shoots that can very quickly grow to sapling size in the gap left by the original crown.  In a large enough gap, these shoots could grow into the canopy.

        A study of sapling population has been conducted on W6 since 1965.  Beech sapling populations have exploded since 1965 with a corresponding decrease in the sugar maple sapling population.  The yellow birch sapling population has shown no dramatic pattern and appears to be unaffected by the increase in beech stem density.
 

 
 
 
 

3. Yellow birch leaves

Yellow birch leaves are generally paired with two leaves on short spur branches.  (This can be observed in the photo upon close inspection.)  The leaf shape is ovate and rounded or subcordate (slightly heart shaped) at the base.  White birch is also common in the Hubbard Brook Experimental Forest at higher elevations, and its leaves are similar but more dramatically heart shaped at the base.  Birches commonly hybridize and where yellow and white birch grow in the same place, certain individuals display traits of both species.
 
 
 

4. Yellow birch bark

Yellow birch bark is a very striking silvery yellow and peels off in small strips that curl up on the bole.  On saplings, however, the bark is a shiny reddish brown and looks identical to the bark of white birch saplings.  The two species can be distinguished by a wintergreen flavor present in the twigs of yellow birch, but not in those of white birch.

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