Ecology is the study of the relationship between organisms, each other and their environment. Bark is a highly visible part of a tree. By observing the color, thickness and function of the bark we can learn about the environmental factors that affect trees. These may include food production, fire, water availability, sunlight, wind, weather, and temperature.
Food production occurs through photosynthesis in tree cells that contain chlorophyll. These cells are most often associated with green leaves, but many trees with smooth, thin, or peeling bark have these cells in the cork skin layer immediately below the outer bark layer. Sunlight can penetrate thin bark even on days where temperatures are below freezing. Warmth generated by sunlight on south or southwest sides of a tree will start photosynthesis. Trees growing in habitats with less than ideal growing conditions due to too much shade, a shorter growing season, or higher altitudes can depend on this secondary source of food production.
Bark’s outer layer is made of dead cells filled with air. When combined with thickness, color, and density, these factors determine the effects of temperature fluctuations occurring from several sources. In winter, melting water from ice or snow can flow into any opening in the bark. When temperatures fall, rapidly drying and cooling bark refreezes the water causing it to expand and put pressure on the bark to create or enhance cracks or to pry the bark loose from the tree. Normally this promotes additional bark growth, but when repeated often over a short period of time, the tree’s appearance will become rougher with an increasingly uneven surface. Similarly, lightning changes the moisture content of bark into instant steam, splitting and blowing off sections of bark. Smooth bark species are much less affected as the energy follows the sheet of water down the smooth bark and into the ground.
Sunlight is another factor both positively and negatively affecting a tree’s appearance. In winter, southward facing bark exposed to direct sunlight can heat to over 70 degrees. As the sun sets, dropping temperatures can cause bark to shrink around a still warm and expanded inner core resulting in stretching or cracking. This may also occur in trees subjected to hot sun and cool nighttime temperatures after bark has been dried out due to drought, reducing its ability to stretch. Some species with thin bark will reflect sunlight rather than absorbing it. Bark of the quaking aspen rubs off in powdery form that reflects sunlight well enough that Native Americans used the powder as sunscreen.
Fire is a familiar environmental hazard. The best protection from fire comes from bark composed of several layers. Ridges, scales, plates, and furrows also contribute to keeping temperature fluctuation small on the inside of the tree. They have an uneven surface that retains moisture and makes them more resistant to the effects of fire. Trees growing in hotter, drier climates and more exposed to fire grow bark faster than other species. Pitch pine inhabits drier areas of the northeast U.S. and has bark that thickens at an early age and contains epicormic buds that sprout into new branches after exposure to fire. Some species have a different strategy for fire. Quaking aspen has thin bark that burns easily, but the trees resprout quickly and often from old rootstock. Paper birch also burns easily, but puts out a much greater quantity of seed annually than other species in the same habitat.
The environment places many stresses on the external layers of trees. Form, function, and appearance is initially determined by DNA, but is continually being modified by location and external forces. As you get outside or look out your window, try to identify the environmental factors affecting your trees. Next week, our final look at bark will deal with impacts from other organisms on trees.
