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Spring 2011

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

June 7, 2011

Earlywood’s over, latewood begins

As summer approaches, pin oaks stop producing spring wood, the layer now just under the bark. These earlywood vessels began developing well before the tree's leaves were fully formed. Their cells were fed not by sugars from this year's leaves but by starches stored in the wood produced last summer, so-called latewood.

"The condition of earlywood is more of a function of last year's growing season than this spring's climate," says National Forest Service plant physiologist Kevin T. Smith. Sugars produced by the leaves that are out now will be used to build this season's latewood vessels, which are smaller and structurally stronger than earlywood. A complicated combination of hormonal, genetic and moisture factors triggers the growth of latewood vessels, says Smith.

When hot weather arrives, leaves become inefficient at making sugars, and oak trees curtail production of wood tissue.

"They have about an eight-week window when they grow the bulk of their annual wood," says Richard C. Murray, author and principal arborist at Shannon Tree and Landscaping in Silver Spring. "When temperatures soar into the 90s, the growing process slows dramatically."

At this point, trees begin to alter the contents of their woody cell walls, forming a cementlike structural component called lignin. They also begin to convert sugars to starch for storage.

Cold weather and short days will halt the green growing season, but trees are never fully dormant. In the fall and winter they grow non-woody roots and regulate stored starches for cell metabolism. "Trees are on 24/7," says Murray. "They are always up to something."

Cross section through a Pin Oak branch.
Cross section through a white pine Cross section through a hickory

As an arborist, Richard
C. Murray
has spent a lifetime slicing through wood, but he seems less interested in chain saws than in what they can reveal about the nature of trees.

In his recently revised "Tree Biology Notebook," Murray provides detailed descriptions of the structure and growth of wood and its reaction to damage, stress and pathogens.

Pictured here are a couple of examples collected by Murray:

Whorled branch cores look like spokes inside the trunk of a white pine, top. The cores were resistant to the rot that consumed the center of the tree, which walled off the damage and continued to grow new wood for more than 20 years.

When this 65-year-old hickory wood, bottom, was 28 years old, a sapsucker chipped out a series of holes in the bark, seen here as a dark brown ring. The bird returned to feed on sap and insects drawn to the wounds.

Photos from Richard C. Murray