VARI as an indicator of site productivity of Pinus taeda L.: soil, litter, and plant nutrition

In commercial Pinus taeda stands, the appearance of needle chlorosis has intensified in slow growth areas. This study aimed to verify if Visible Atmospherically Resistant Index (VARI) captures heterogeneity of Pinus taeda development and to determine what specific factors are associated with occurrence of chlorotic symptoms. We evaluated a 14 year-old reforested P. taeda stand that had already undergone two thinnings; the study area encompassed 18.42 ha on soil derived from acidic igneous parent material. A VARI map was used to delineate the area into four classes: Very Low, Low, Medium, and High. In each class, tree growth was evaluated, and samples of tree tissue (needles, trunk cross sections, and fine roots), litter, and soil were collected for evaluation. There were no differences in soil classification and fertility between classes for this acidic low fertility soil. However, compared to the lower VARI classes (Low and Very Low), tree wood volume (without bark) of the High class was 0.20 m³ higher per tree. Higher VARI classes showed greater concentrations of Ca and Mg in needles, bark, and fine roots. The High class also showed higher concentrations of Ca and Mg in litter, but with less litter accumulation. Therefore, VARI efficiently captured tree growth heterogeneity with a direct relationship noted between VARI and tree growth (height, DBH, volume, and trunk mass). In addition, chlorosis symptoms (yellowing) at needle tips and tree crown loss were more likely indicative of Mg deficiency, which probably caused reduced tree development.