What we often see as a pest issue or deem an annoyance is in reality a function of Mother Nature’s food chain. For a variety of big and small animals living in our forests, bark and wood simply equals food!

Animals that eat bark and wood (herbivores) all have a digestive system that allows them to extract nutrients from the woody material — something that humans don’t have the ability to do. To truly understand how and why bark and wood serve as important food sources for a variety of animals, you need to have some insight into the cell wall structure of trees, be familiar with the anatomy of bark and wood, know a little about the movement of nutrients within a tree, and finally, appreciate the workings of the digestive system of a herbivore.

Cell wall structure of plants

First, let’s look at the cell wall structure of plants. The cell walls of plants contain cellulose, hemicellulose and pectin. All of these cell wall components are potential food sources if the proper enzymes are present in the digestive system of the animal that will allow them to break them down into their basic sugar and organic acid components. For example, cellulose is a pure carbohydrate and has as much food value as starch. However, only animals that have colonies of microorganisms in their gut that produce the enzyme cellulase can digest it and convert it to food. Pectins, when broken down, contain galacturonic acid (a sugar acid) and the sugars arabinose and galactose.

Bark and wood anatomy

Although there are some variances between tree species, all bark contains sugar, starch, cellulose, hemicellulose and minerals. However, it is the inner bark with its living cells (the phloem and cambium layers), that transport and store high concentrations of nutrients, that are the primary target of bark-eating animals.

The outer bark has fewer nutrients and is less palatable because the cell walls contain higher levels of lignin. Lignins contain phenols (often used in disinfectants) that make the outer bark less digestible. This is why you often see animals such as white-tailed deer target the younger twigs and smaller branches as these parts of the tree have a higher proportion of inner to outer bark.

From an animal perspective, the nutrient value of the wood (xylem) in a tree as a food source lessens with age. When older xylem cells die (heartwood) they cease to transport water or store nutrients and begin to amass resins, phenols and terpenes. The only nutritional value of these cells is in the lignified cell walls.

However, in the living part of the xylem (sapwood) the cells are actively transporting water and dissolved minerals between the roots and the crown of the tree and, to a lesser extent, storing starch. Sapwood contains higher concentrations of nutrients and smaller amounts of non-digestible ingredients such as lignin. As a result, the sapwood is the most preferred part of the xylem for wood feeders as a food source.

Digestive systems of herbivores

Strict herbivores who consume bark and wood as food must have a digestive system that will break down huge macromolecules (proteins, starch, cellulose and hemicellulose) into smaller molecules (amino acids, sugars, etc.) to allow them to be absorbed into the circulatory system.

This requires a complex digestive system, one that is able to process large masses of foodstuffs (wood and bark) with low nutrient levels, while also being able to break down the plant cell walls to gain access to the proteins and carbohydrates inside.

This is accomplished through a combination of mechanical grinding and an enzymatic process that begins in the mouth and then continues in the digestive system of the animal. Although the digestive system will vary between herbivores, they all depend on microorganisms inhabiting special compartments of the stomach, a cecum (an out-pocketing of the small intestine), an exceptionally long intestine or an enlarged colon.

The digestive system of herbivores can be divided into two basic systems – a single compartment stomach (monogastric) or one that consists of several chambers (digastric).

In monogastric herbivores (e.g., rodents, rabbits) protein is digested and absorbed in a single stomach chamber, and digestion of cellulosic material is aided through the fermentation in the intestine (generally with a cecum or an enlarged colon). To ensure there is sufficient time for digestion and nutrient absorption, the intestines in these animals are often more than 25 times their body length. Monogastric animals are referred to as hindgut feeders.

Because the first pass through the digestive system of some hindgut feeders doesn’t provide sufficient time for microorganisms to digest coarse food material, a number of these animals will eat 25–60 percent of their feces to increase the digestion rate and nutrient absorption of their food.

Disgastric herbivores (e.g., deer, moose) have a stomach that is divided into four chambers.  They partially chew their food as it is gathered, store it in the rumen (first stomach compartment) and later will regurgitate and re-chew it. This cycle is repeated until the food is broken down to allow it to pass into the next stomach chamber. Digastric animals are referred to as foregut feeders.

A few last words

Now you have the whole story. Bark and wood are important food sources for a number of wildlife species that inhabit Ontario’s diverse forests. Perhaps with this new information you will be a little more understanding when the rabbits or deer begin to browse on your newly planted seedlings or, perhaps not, and they will be remain relegated for eternity in the pest category.

Acknowledgements

This article is based on information from the report Why do Animals Eat the Bark and Wood of Trees and Shrubs? FNR-203. Purdue University.

Photo Credits

¹ Kenneth M. Gale, Bugwood.org

This article was featured in a past edition of the S&W Report, the newsletter of the Ontario Woodlot Association. 

© Ontario Woodlot Association, 2011

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