Tree Bones

by Nancy Rose
December 1, 2016

Tree Bones

We had a nicely extended autumn this year, but the arrival of colder temperatures and pre-4:30 p.m. sunsets makes it clear that winter is nigh. Several days of strong winds hastened leaf abscission and most deciduous trees in the Arboretum are now bare.

The annual loss of leafy canopies can bring on a bit of melancholy, but take a walk and you’ll quickly see the opportunity to make new observations. From now until spring leaf-out is the time to see and appreciate the “bones” of deciduous trees, from trunk base to tiniest twig tip. A light snowfall can even enhance the experience since the snow outlines tree branches, making the structure more noticeable.

snow on maple branches

Snow outlines the decurrent forms of maples in the Arboretum’s Maple (Acer) Collection.
Photo by Robert Mayer.

Different tree species have different branching patterns. While there may be variation among individual specimens, learning species’ typical forms can help greatly with identification. Temperate region trees can be divided into two major form types, excurrent and decurrent. Excurrent trees have strong apical dominance so the central leader grows more rapidly than side branches, resulting in a conical shape. Most conifers–both evergreen and deciduous species–have this form. Some broadleaf deciduous trees have excurrent growth when young, though they often become more decurrent when mature; pin oak (Quercus palustris) and sweetgum [pdf] (Liquidambar styraciflua) are good examples of this.

dawn redwood specimen displays excurrent form

The beautiful dawn redwood specimen (Metasequoia glyptostroboides 524-48*AA) across from the Hunnewell Building displays a classic excurrent form. Photo by Kyle Port.

Decurrent (also known as deliquescent) trees lack apical dominance so many of the side branches develop as rapidly as the central leader, leading to wide branching and a rounded form. Most broadleaf deciduous trees have this growth pattern; the vase-shaped American elm [pdf] (Ulmus americana) and the oval to rounded sugar maple (Acer saccharum) are classic examples. Within decurrent trees there are many species-distinctive branching habits, from the stark, non-twiggy outline of Kentucky coffeetree (Gymnocladus dioicus) to the elegant horizontal branching of flowering dogwood [pdf] (Cornus florida).

Whether you want to hone your tree identification skills or just admire the patterns of nature, bundle up and visit the Arboretum often this winter.

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