Stand Density Effects on Biomass Allocation Patterns and Subsequent Soil Nitrogen Demand

Growth and yield data from a loblolly pine plantation in southeastern Louisiana were obtained yearly from 1993 to 1996 on numbered trees within two stands initially planted on a by spacing, and two stands planted on a by spacing. Using allometric equations derived from a 1994 on-site destructive harvest, cited nitrogen concentrations of various tree tissues, and accounting for foliar nitrogen retranslocation, stand growth and soil nitrogen demands were determined. Results showed that production of all aboveground tissues increased as stand density index (SDI) increased. Annual soil nitrogen demand increased with SDI primarily as a result of an increase in nitrogen-rich foliage on the denser sites. Belowground production, as estimated from minirhizotron censuses, increased as SDI and aboveground nitrogen demands increased. More fine-root production per unit aboveground nitrogen demand was observed on less fertile plots. production per unit leaf biomass decreased with increasing SDI, and is assumed to be the result of a greater percentage of total net primary production being partitioned to fine-root production in the denser plots. The results of this study suggest that the density of forest stands may influence nutrient demands from the soil and subsequent belowground productivity through differential aboveground biomass allocation patterns and tissue nitrogen concentrations. INTRODUCTION The under ly ing mechan isms o f p lan t b iomass par t i t i on ing are of great importance to the study of forest productivity. As ga ins are made in unders tand ing the fundamenta l principles of photosynthate allocation to various tree components, the potential exists to manipulate forest stands to increase the production of merchantable wood despite constant site productivity, thereby increasing economic return for forest landowners. The ultimate objective for production foresters is to max im ize a l loca t ion o f pho tosyn tha te to merchan tab le stemwood. Strides have been made in improving yield through various silvicultural practices. In many agricultural crops, enhanced yields have resulted primarily from a shift in carbon allocation to harvestable parts, rather than an actual gain in total productivity (Evans 1976). The mechan isms fo r aboveground p roduc t i v i t y and t i ssue carbon a l locat ion are becoming more read i ly unders tood. Knowledge o f be lowground p roduc t i v i t y pa t te rns , however , lags far behind that of aboveground productivity. If enhanc ing produc t iv i t y o f merchantab le aboveground t ree components inc ludes rea l loca t ion o f ava i lab le pho tosyn tha te f rom unharves tab le be lowground s inks , then the study of carbon allocation in the whole forest stand, both aboveand belowground, is necessary. The objective of this preliminary study was to determine if there is evidence that different aboveground stand structures resulting from varying stand densities influence soil nutrient demand and subsequent be lowground produc t iv i t y . CARBON ALLOCATION Stand density is a factor that has significant influence on stand carbon allocation. Stand density is known to influence tree crown morphology (Dean and Baldwin which influences carbon allocation among stems, foliage, and branches (Ford 1982). These relations are complicated because both stand structure and productivity are associated with differences in age and site quality (Assmann 1970). However, Dean and Baldwin have shown that stand density index (SDI), a measure of growing stock that includes quadratic mean diameter and trees per hectare may be predicted solely from foliage density, mean live crown ratio, and canopy depth. There is a positive relationship between stand density and stand foliage production. For a stand of a given stand density, the amount of foliage in a closed canopy stand is a function of the site quality. However, an increase in stand density has been shown to increase leaf area index (LAI) in loblolly stands (Dean and Baldwin 1996a). Stand density has also been shown to positively influence yearly needle fall, a measure of foliage production, in other pine stands (Gho lz and o thers 1985, Gresham 1982) . There is also a positive relationship between stand density and stand production that isrelated to changes in canopy structure. Canopy structure is the result of many s imu l taneous processes inc lud ing l igh t penet ra t ion , he igh t growth, crown lifting, and intercrown abrasion (Dean and Long 1992). After the onset of competition at crown closure, foliage is driven to the top of the canopy as a result of the natural pruning of lower branches (Mar:Mohler 1947). Wind action on the crown of a tree creates a bending stress on the stem, and as the crown midpoint becomes higher, coupled with an increase in the amount of foliage associated with increasing stand density (Dean and Baldwin there is an increased load placed on the stem (Dean and Long 1986). Bending of the stem also Gilbert Fellow, School of Forestry, Wildlife and Fisheries, Louisiana State University, Baton Rouge, LA 70803; and Associate Professor, School of Forestry, Wildlife and Fisheries, Louisiana Agriculture Experiment Station, Louisiana State University Agriculture Center, Baton Rouge, LA 70803 (respectively). In: A., 1998. the ninth biennial 564 southern silvicultural conference; 1997 27; Clemson, Tech. Asheville, NC: Research increases the resistance to flow of water in the stem so that more is needed to transport the same amount of water to the foliage (Dean 1991). Therefore, as stand density and the subsequent physical load on the stem is increased, the carbon sink strength of the stem increases. The amount of stand branchwood produced, however, is negatively related to stand density. Trees in sparser stands have deeper and wider canopies than do those in denser stands (Dean and Baldwin 1996a) and, therefore, must allocate a greater percentage of total net primary p roduc t ion (TNPP) toward the p roduc t ion and ma in tenance of branches to support the equilibrium level of foliage. In summary, then, as stand density increases, stand foliage and biomass production increase, while stand branchwood produc t ion decreases . Another major sink for carbohydrates is production of the fine-root system. Indeed fine root production has been es t imated to consume 30-70 percen t o f TNPP (Santan ton io 1989) and has been shown to be inversely related to nutrient and water availability (Gower and others 1992). On sites of equal nutrition, the belowground proportion of TNPP shou ld cor re la te w i th the aboveground demand fo r nutrients and water according to the functional balance equation (Davidson which states that as aboveground nut r ien t demands increase, f ineroot production will increase to meet that demand. Nutrient demands w i l l vary dur ing on togeny ( Imsande and Toura ine 1994) with the greatest demands being placed on the nutrient reserves of a site during the early stages of stand deve lopment when the s tand is approach ing max imum lea f area (Switzer and others 1968). All else being equal, though, s tands w i th a h igher aboveground nut r ien t demand should allocate more carbon to belowground productivity to meet that demand and maintain a functional balance. Switzer and others (1968) found that the nitrogen concentration of foliar, branch, and tissue to be 1.08 percent, 0.23 percent and 0.06 percent nitrogen, respectively, in loblolly pine trees, exhibiting little change with stand age. Foliar nitrogen concentration was 18 times greater than that of stemwood, representing the greatest portion of a stands nutrient requirements, averaging 80 percent for all nutrients (Switzer and Nelson 1972). Because stand density influences the proportion of different aboveground tree tissues in a stand and those tissues vary in nitrogen concentration, stand density should a lso a f fec t nu t r ien t up take and subsequent be lowground biomass production if a functional balance exists. This preliminary study investigated the above hypothesis to determine if evidence existed to warrant a conclusive study.