Experiment A
Fundamental Controls of Growth and Productivity

Much information exists on the production potential of intensively managed forest plantations. However, there is still much to be learned about processes driving growth. Factors of interest include carbon gain, water use and nutrient acquisition. Information on root acquisition soil resources is especially limited. The USDA Forest Service and the Department of Energy initiated a large-scale short rotation woody crop (SRWC) research project on the Savannah River Site near Aiken, SC. The study aim is to gain greater understanding of the factors driving growth and production in SRWC plantations with specific emphasis on measuring resource acquisition above and belowground at an equal level of detail.

Study Description: The study is located on a level, deep sand site characterized by limited native water and nutrient soil resources. Five tree genotypes (two cottonwood clones, sycamore, sweetgum, and loblolly pine) were planted in spring 2000 in a set of 95 half-acre plots. The trees are growing with a range of fertilizer and irrigation treatments in a unique experimental design that allows for testing of treatment interactions and evaluating optimal nutrition.

Objectives:

• Characterize environmental conditions including evaporation, soil nutrients and soil moisture for each treatment plot.
• Identify species differences in productivity and belowground allocation when grown with a range of resources availability.
• Measure root and shoot carbon and nitrogen mass balance of dormant tissues at regular intervals through the rotation and characterize species on their resource use efficiency.
• Monitor fine-root and leaf tissue production and turnover and relate it to resource acquisition.
• Measure whole-tree water use (transpiration) and relate it to absorbing (root) and evaporating (leaf) surfaces.
• Measure light interception, carbon and water gas exchange of leaves and relate it to canopy development.
• Measure root nitrogen uptake and relate it to changes in root uptake surfaces and whole plant nitrogen mass balance.

Status: After three growing seasons, each tree species responded differently to varying combinations of water and nutrient and water additions. All species responded positively to increased nutrient availability, but they varied in dependence on water availability. Canopy development is directly related to stem productivity, but root development only responds positively to high availability of both water and nutrients. Leaf nutrient content and gas exchange were unaffected by the silvicultural treatments. Water use, carbon gain and nutrient uptake differed among resource availability treatments because of changes in acquisition surface, not the rate of uptake. Shifts in nutrient mass balance and response to treatments are expected to change as stands continue to develop.