Potential effect of stand structure on belowground allocation

Stand structure affects two key variables that affect biomass allocation to the stem: leaf area and height to the center of the crown. By translating wind forces into a bending moment, these variables generate bending stress within a stem. The uniform-stress axiom of stem formation can be used to ca lculate current stem mass for a given bending moment and stem allocation fo r changes in bending moment over a time period. Stem allocation probably af fects allocation to fine roots since the stem precedes the root system on t he chain of carbohydrate sinks, and total net primary production is a linea r function of leaf area. This study indirectly supports this link between s tand structure and belowground allocation, A regression model based on the relationship between bending moment and stem allocation explained 98% of th e variation in stand-level stem production of a 12-yr-old loblolly pine (Pi nus taeda L.) plantation subjected to factorial combinations of irrigation and fertilization. Furthermore, relative fine-root allocation was inversely related to relative stem allocation and increases in apparent bending stre ss. Corresponding associations between bending stress and relative allocati on between the stem and fine roots appear to exist for other species, provi ding additional support for the proposed link.