Carbohydrate Reserve Theory: What You Learned Might Be Wrong

For years, managers used the carbohydrate reserve theory to decide when to graze plants on rangelands to maintain healthy and desirable plants. The carbohydrate reserve theory states that the soluble carbohydrates stored in the roots and crowns of plants indicate plant health and ability to regrow after grazing. During the early vegetative stage of plant growth, carbohydrate “reserves” are low, so plants should not be grazed. During late vegetative and early reproductive stages of growth, carbohydrate “reserves” are higher, and plants can better tolerate grazing (see figure below). Over the years, researchers produced carbohydrate concentration curves for different grasses, forbs, and shrubs, like the one illustrated below. Unfortunately, carbohydrate reserves in plants are not good indicators of its ability to regrow after grazing for several reasons: 1. Carbohydrates are typically measured as concentrations that change only a small amount during the year, but fluctuate widely throughout the day. 2. Concentrations don’t reflect the total amount of carbohydrate available for regrowth. To accurately measure the total amount of carbohydrates, the concentration of soluble carbohydrates in different plant tissues (roots, crowns, leaves, stems) must be multiplied by the weight of those tissues. Most early studies only analyzed roots and crowns, but stems in grasses and forbs and twigs in shrubs are also important storage sites for soluble carbohydrates. 3. Carbohydrate reserves, whether expressed as concentrations or as total amounts, are not correlated with the ability of a plant to regrow after grazing. The rate and amount a plant can regrow without light are also not correlated with either concentrations or total amounts of carbohydrate reserves stored in the roots or crowns of the plant (Richards and Caldwell, 1985). 4. The carbohydrate reserve stored by bunchgrasses is very small, equal to about 1 to 2 days of photosynthesis during the summer.