Root Distribution Relationships to Soil Properties in Missouri Oak Stands A Productivity Index Approach

This study examined the feasibility of adapting a soil-based productivity index (PI) to predict root distribution in Missouri oak (Quercus spp.) stands. Roots and soil were sampled to 150 cm in 58 plots in the Missouri Ozarks and Missouri River hills. Soil pH, bulk density, and moisture holding capacity were determined by horizon, and the data were used in equations designed to quantify "sufficiency" for root growth. The resulting sufficiency values were used to calculate individual and combined soil property indices for each plot. First, sufficiency values for combined indices were calculated in three ways-multiplicative, geometric, and arithmetic. Then, the indices were derived by multiplying individual or combined sufficiency values by a root index factor, which this study found to be representative of proportional root distribution in "ideal" soil, and then summing the weighted values. Of the individual indices, soil pH explained 30% of fine root length variability and 29% of the total root length variability. Neither soil bulk density nor soil moisture explained root length variability. In fact, these properties actually reduced predictability when included in combined indices. Of the combined indices, the multiplicative-based index explained a greater percentage of root length variation than either the geometric- or arithmetic-based indices. However, no combined index accounted for more than 11% of the variation in root length distribution. This suggests that, to be usable in forestry, the soil-based productivity index needs to be refined and/or expanded to include variables such as nutrients and climate.