Seasonal variation in the nutrient content of Aspen foliage.

Chemical composition of aspen (Populus tremuloides) leaves, collected on three sampling dates during the growing season of 1966, varied greatly. Content of calcium, magnesium, sodium, and fat increased as the season progressed, whereas total nitrogen, phosphorus, and potassium content decreased significantly. Browse quality therefore changed with season. Aspen leaves contain most calcium and magnesium late in the season; this would help to maintain desirable soil properties when leaves drop. Quaking aspen is a widely distributed and important tree species in North America. Smith (1952:151) indicates that aspen is one of the 10 most important summer browse species for mule deer (Odocoileus hemionus) in the West. However, Gastler et al. (1951), comparing the composition and palatability of 11 plant species browsed by deer during the winter in South Dakota, found aspen browse was low in importance as a deer food. Ullrey et al. (1964) compared the digestibility of white cedar (Thuja occidentalis) and large-toothed aspen (Populus grandidentata) browse being fed to white-tailed deer (0. virginianus). Cedar browse was more digestible than aspen browse and deer clearly preferred it. Short et al. (1966:223-224), in Colorado, determined seasonal changes in the chemical composition of nine plant species believed to be important food sources for mule deer. Protein content of aspen twigs varied from 6 to 7 percent in winter to a high of 17 percent in the spring. Phosphorus increased from 0.18 percent in winter to 0.49 percent in the spring. Crude fat reached a maximum of 15 to 19 percent in winter but declined in the spring. Important seasonal changes occur in the chemical composition of leaves as well as of twigs. Nitrogen, phosphorus, and potassium decrease with the advance of the growing season whereas calcium and magnesium usually increase in many native This content downloaded from 157.55. 39.92 on Wed, 22 Jun 2016 06:48:28 UTC All use subject to http://about.jstor. org/terms 476 Journal of Wildlife Management, Vol. 34, No. 2, April 1970 Table 1. Average and range for the chemical content (in percent dry weight) of leaves from eight aspen stands on three dates during 1966. JUNE 9 JULY 20 SEPT. 7 COMPONENT Average Range Average Range Average Range Ca 0.59 0.32-1.09 1.47 0.93-1.89 2.32 1.54-4.18 P 0.36 0.29-0.47 0.22 0.16-0.27 0.21 0.16-0.28 Ca/P 1.76 0.71-3.51 6.94 3.58-9.45 11.5 7.12-14.9 K 1.39 1.14-1.63 1.18 0.94-1.37 0.99 0.70-1.30 Mg 0.05 0.03-0.08 0.11 0.07-0.15 0.16 0.09-0.25 Na 0.04 0.03-0.05 0.05 0.04-0.07 0.08 0.06-0.13 Protein 17.0 13.4-20.9 13.2 11.9-14.6 11.8 10.1-14.6 Fat 7.28 6.34-8.06 7.79 7.20-8.43 9.92 7.50-12.3 Ash 5.08 4.23-6.27 6.24 5.12-8.30 7.53 5.85-11.7 H2Oa 66.3 59.7-74.1 a In percent of fresh weight. species (McHargue and Roy 1932). Until now, little information about seasonal changes in aspen leaf composition has been published. In Utah, adjacent stands of aspen vary greatly in date of leafing, leaf coloration, date of leaf fall, size and shape of leaves, tree morphology, and resistance to disease. This heterogeneity of characteristics may indicate a variation among aspen stands in their ability to extract various nutrients from the soil and to incorporate them in leaves. If so, the result would be a difference in browse quality during the growing season and in the quality of litter when leaves fall. Some stands may therefore be more desirable than others for retention as a source of browse and as a means of maintaining soil fertility in mixed stands. The research reported here determined the variation in chemical composition of aspen foliage during one growing season.