Seedling root growth as a function of soil density and water content

Compaction caused by some intensive forest management practices can reduce tree growth, but growth reduction is the result of complex interactions between soil properties and tree physiological processes, which may differ by species. We used a 7 by 7 factorial greenhouse experiment to create a matrix of bulk density (rho(b)) and volumetric water content (theta(v)) to better understand soil compaction effects on seedling growth of: (i) ponderosa pine (Pinus ponderosa var. ponderosa Dougl. ex Laws) grown on Dome and Cohasset soils; (ii) shortleaf pine (Pinus echinata Mill.) on a Clarksville soil; and (iii) loblolly pine (Pinus taeda L.) on an Argent soil. Models of root length density (RLD) were developed using multiple regression. The general model of RLD = b0 + b1theta(v) + b2 rho(b) + b3theta2v described rooting response for the Clarksville-shortleaf and Argent-loblolly soil-species combinations (p = 0.005). However, the ponderosa pine RLD response on Cohasset soil was linear and there was an interaction between theta(v) and rho(b) in the Dome soil. Shoot mass of seedlings growing within the least limiting water range (LLWR) was greater than those growing outside the range for all soil-species combinations except the Argent-loblolly pine (p = 0.05). The loblolly pines had greater shoot mass at theta(v) above the upper LLWR limits (aeration limiting). Least limiting water range has potential as a soil quality indicator, but seedling response was not always associated with LLWR. Root length density (RLD) response surface models in conjunction with seasonal site water data have potential for determining compaction-induced soil limitations for tree growth, but need to be field tested and calibrated for both soil and species.