Climatic Limitations for Tree Growth Potential in Southern Forests

CLIMATIC LIMITATIONS FOR TREE GROWTH POTENTIAL IN SOUTHERN FORESTS Chelvadurai Manogaran* The role of the southern United States as a supplier of forest products will have to be even greater in the future than at present although this region possesses the most rapidly growing trees in the nation. Pines form the predominant part of the pulpwood, plywood, and lumber production of the southern states. The forest resources of the South produce more than 63 percent of the nation's plywood and about 17 percent of the United States' total plywood production. (1) A 50 percent increase in southern pine lumber production is expected as a result of the urgent need for the construction of 26 million houses during the next eight years. (2) If forest production is not increased beyond the present level, harvest will catch up with growth by 1990. Although field experiments relating to the role of irrigation in forestry have yielded significant findings , the results have application limited to certain localities. These studies do not provide estimates of how much more growth would be realized on a regional scale if adequate moisture were made available to trees during the growing season. Potential growth of trees is not realized in many areas of the southern United States because of the prevalence of soil moisture deficits in summer; the effect of this environmental feature has been largely overlooked. The purpose of this paper is to develop a model of tree growth that estimates the depressing effect of drought. Potential growth might be estimated empirically if appropriate environmental factors were selected to predict tree growth. Such methods consider energy, moisture , and drought as factors of the environment that work together, and not independently as in a laboratory or controlled experiment, in promoting or retarding potential growth of trees. If the model predicts growth under specified conditions of soil moisture deficit, the same model could be utilized to estimate potential growth under conditions of no soil moisture deficit, which irrigation could assure, and naturally occurring moisture deficits. The additional growth, estimated as the difference between these conditions, is considered the quantity of growth that would result from the use of irrigation in forestry. A regional evaluation of additional growth resulting * Dr. Manogaran is assistant professor of geography at the University of WisconsinParkside in Kenosha. This paper was accepted for publication in March 1973. 72Southeastern Geographer from the use of irrigation in forestry is possible since the independent climatic data are available for the entire area where forests grow in the South. DERIVING A MODEL OF CLIMATE'S POWER FOR FOREST GROWTH. An equation which employs climatic variables and growth data is produced so that two kinds of projections can be made: 1) the growth under actual weather conditions which results from variable limitations due to moisture deficit; and 2) the potential growth that could occur if moisture deficit were prevented by irrigation. Data Utilized to Generate a Predictive Model. Diameter increments recorded during 1960 and 1961 on naturally occurring loblolly pines by Bassett and Moehring in southeast Arkansas and northeast Louisiana, respectively, are utilized to develop the model of tree growth. (3) (4) The pines were at maturity and varied in age from 30 to 41 years. Although the trees sampled by Bassett were homogenous with respect to age and species, the sample plots varied in stand density. The trees sampled by Moehring varied greatly with respect to age and stand density. The plots consisted of mixed stands of hardwoods and pines. The trees sampled in these two areas were combined to determine the extent to which the factors of climate explain the monthly growth of loblolly pines. One hundred seventy observations on the diameter growth of pines were utilized to generate the predictive model. The data on climate and growth were used to compute a sequence of multiple linear regression equations in a stepwise manner to identify the climatic variables that are most significantly correlated with tree growth. The readily available data on temperature and precipitation for the study areas were transformed into water budget parameters utilizing the water budget procedure developed by Thornthwaite. (5) Thornthwaite's water budget is an accounting of the continuous budget of energy's demand...