January 20, 2009
Pictured here is a stately American Elm (Ulmus Americana), a particularly hardy tree that was nearly devastated by a beetle-borne fungal disease ("Dutch Elm disease") accidentally introduced from Eurasia to North America in the 1940s.
In the vicious competition for life, all successful species have evolved defensive strategies to protect themselves from becoming food. Trees cannot run, hide or fight, so they need different strategies to survive and thrive, and the evolution of tree bark has played a big role. The outer bark (rhytidome), the part we see and touch, is a tough layer of dead cells, continually replaced from beneath, that keeps out water, prevents desiccation, insulates against cold and heat, resists invasion by fungi or bacteria or insets or animals, and disposes of metabolic wastes. If tree bark had not evolved, it’s possible that trees would either not exist or they would be very different from their appearance today. The American Elm is the state tree of North Dakota, Ohio and Massachusetts. Photo taken in March of 2008 from Cambridge, Massachusetts.