They were early arriving, eager to learn how to become trained volunteers with Tree Stewards of Arlington and Alexandria. Five interns who began classes last winter joined 25 new students to fill the training room at Fairlington Community Center on Tuesday, Sept. 24. Through this fall, part of winter and into next spring, they will learn what trees need to be the life-giving oxygen generators that will help mitigate some of the effects of climate change.

Lesson One was an overview of tree anatomy delivered by Lara Johnson of the Virginia Department of Forestry. Among the surprises: The heartwood of a tree is dead and dark with toxins. The coloration is what makes a walnut table darker than maple. Only the bark and cambium are alive, so watch those weed whackers, and don’t carve your initials on a tree. Damage to the bark can allow diseases and destructive insects to enter, sicken the tree and possibly kill it.

Cambium contains two highways that feed the tree. Xylem moves water and soil nutrients skyward to the top of the tree, where leaves use those and sunlight to produce sugars during photosynthesis. Phloem transports the sugars low underground to the tree’s roots, which are far more extensive than might be imagined, extending two or three times the width of the tree canopy, or drip line. (See Rooting Around.)

Tree anatomy is significantly more complicated, and Johnson said trainees will learn about that in future classes.

Next up: Dan Schwartz of the Northern Virginia Soil and Water Conservation District describes urban soils and their impact on trees. Slide deck presentations can be found under the Training Materials tab.