Zhenmin Tang, Chambers, Jimmy L., Young, Patricia J.
There are 215 million acres of timberland in the southern United States and nearly 30 percent are southern pine forests. Loblolly pine is the most extensively planted commercial pine species in the South. In Louisiana, the growing volume of loblolly pine forests is nearly 7 billion cubic feet. Timber revenues of Louisiana exceed the total revenue of all other plant and animal products. In 2003, forestry contributed $3.7 billion, including value-added, to the state’s economy, according to the LSU AgCenter's most recent Ag Summary.
The availability of resources such as light, mineral nutrients and water affects forest growth and yield. For example, soil fertility is generally low in pine plantations throughout the southern United States. On sites of poor fertility, nutrient deficiencies substantially reduce the timber production of southern pine plantations. Low light availability can limit branch expansion and trunk volume. Forest cultural practices such as thinning (the partial removal of trees), fertilization and weed control are commonly used to increase site quality and plantation productivity. Understanding the effects of forest management on resource availability and timber production requires extensive assessment of the relationship between cultural treatments and environmental constraints to tree growth. With rising demands for timber and other wood products in the South and the nation as a whole, forest managers and researchers continue their efforts to enhance the growth and yield of southern pines through improved management techniques.
In 1988, the U.S. Department of Agriculture's Forest Service began examining the long-term effects of cultural practices on loblolly pine productivity on the Kisatchie National Forest in central Louisiana. In cooperation with the Forest Service, scientists at the LSU AgCenter use advanced technologies to investigate tree growth and yield under different thinning and fertilization treatments. Levels of thinning include no tree removal or precommercial tree removal at ages 8 and 14 years. Levels of fertilization include no fertilization or broadcast application of diammonium phosphate at 667 pounds per acre in late 1988 and 179 pounds urea, 18 pounds triple superphosphate and 45 pounds potash per acre in 1995. This study provides valuable information about trunk volume, leaf area production, crown physiology and root system expansion in response to forest cultural treatments. Results help interpret how site factors and intensive management interact to influence the timber production of loblolly pine plantations in Louisiana.
The tree crown is a complex structure that regulates the radiant dynamics, energy budget, and water and carbon balance of forest ecosystems. Crown characteristics determine the amount of intercepted light, the most important factor affecting the physiology and growth of individual trees. Other site factors such as temperature, relative humidity and mineral nutrients also affect tree volume. Presently, how these factors interact to affect tree trunk size is poorly understood. The study of large trees is generally lacking because of problems accessing tree crowns and developing instrumentation for environmental measurements. With the recent progress in measurement technologies, LSU AgCenter researchers are attempting to evaluate how site factors affect tree crown processes, what crown characteristics can be culturally manipulated to increase trunk height and diameter, and when intensive management needs to be prescribed for maximum trunk volume.
Steel tower systems were built to provide access to the live crowns of large loblolly pine trees for ecological and physiological measurements. Sensors were installed in the upper and lower crown to monitor microenvironmental change continuously. Data acquisition systems consisting of data-loggers and fiber optic cables were used to transfer environmental data to a remote computer for analysis and summarization. Growth of shoots and roots was measured intensively during several growing seasons. Foliage biomass, leaf area and physiological factors, such as net photosynthesis and tree water status, were also monitored intensively.
In dense, unthinned stands, tree growth was limited mainly by low light intensity throughout the crown. Thinning increased light penetration into the lower crown and reduced competition for available growing space, nutrients and water. Trunk diameter and volume rose considerably for four years in the thinned stands relative to the unthinned stands. We found that branch size and retention, leaf area and photosynthesis increased in response to thinning. Six years after the initial stand density reduction, however, the annual volume growth response decreased to the level before treatment, suggesting that more trees must be removed to maintain the growing space needed for high rates of growth.
At nutrient-deficient sites in Louisiana, fertilization is increasingly applied to improve soil fertility. Our findings indicate that tree trunk volume increased 30 percent to 45 percent in response to fertilization. Maximum increases in tree volume growth occurred in the third and fourth years following fertilization. The combination of fertilization and thinning was the best treatment for timber production.
Greater nutrient availability enhanced branch growth and leaf area in the fertilized stands. However, the positive effects of fertilization on the volume growth no longer existed six years after fertilizer treatment. Thus, reapplication of fertilizers was needed.
In early 1999, water exclusion treatments were applied. Two levels of water exclusion included normal rainfall or rainfall exclusion. Tree trunk height, diameter and physiology were measured intensively throughout growing seasons. The latest sap-flow monitoring technology was used to monitor water movement through trees continuously. The data of water movement allowed estimation of tree-level and stand-level water consumption. This study provides insight into the links between tree physiology and trunk volume growth under a combination of fertilization and rainfall exclusion treatments, which is essential to better understand the effects of cultural practices on the carbon exchange, water use and timber production of loblolly pine plantations in a changing environment.