Limitations of bioassays for monitoring forest soil productivity: Rationale and example

Sustaining forest soil productivity is a common goal of forest landowners in the Southeast, but determining the effects of forest management practices on soil and site quality has not been easy, An unbiased measurement is needed to monitor sustained soil and site productivity. Growth cycle comparisons, traditional measures of forest productivity, are not very timely, are often biased, and do not always account for changes in soil quality due to management. This was demonstrated by comparing growth curves of two successive loblolly pine (Pinus taeda L.) rotations, and by comparing growth curves influenced by two site preparation treatments within the last rotation. The growth cycle comparison between rotations was not a definitive indicator of sustained soil quality because stand response could have been confounded by differences in genotype and vegetation management from the first to the second rotation. Stand responses due to soil treatment in the last rotation were confounded by the influence of different levels of competing vegetation, Because stand response is a function of many confounding non-soil factors as well as soil quality, soil sustainability and the effects of management should be determined by measuring soil properties and processes directly.