SEASONAL BRANCH AND FINE-ROOT GROWTH OF JUVENILE LOBLOLLY-PINE 5 GROWING SEASONS AFTER FERTILIZATION

In 1989, we established two replications of two fertilization treatmen ts in a 10-year-old loblolly pine (Pinus taeda L.) plantation. Between March and September 1993, branch internode and needle fascicle expans ion in the upper and lower third of crowns were measured weekly on thr ee south-facing branches of each of four trees, and new root initiatio n and elongation were measured at 10-day intervals in three vertical r hizotrons per plot. In one replication, soil water content was measure d daily. Fertilization significantly increased the expansion of first flush internodes in the upper crown and first flush needle fascicles i n the upper and lower crown. New root growth was stimulated by fertili zation in the second half of the growing season. The timing of root gr owth responses to fertilization corresponded to branch phenologies in the upper and lower crown that were conducive to increased basipetal t ransport of photosynthate. We conclude, therefore, that new root growt h was linked to source-sink activities in the crown. Root initiation w as greater in the upper than in the lower part of the soil profile; ho wever, as the growing season progressed and water deficit increased, t his relationship was reversed. The effect of soil depth on seasonal ro ot growth was closely associated with water availability, suggesting t hat root initiation deep in the soil profile is critical for the conti nued production of new roots in environments subjected to short-term, but relatively severe, water deficits.