Seasonal branch and fine root growth of juvenile loblolly pine five growing seasons after fertilization.

In 1989, we established two replications of two fertilization treatments in a 10-year-old loblolly pine (Pinus taeda L.) plantation. Between March and September 1993, branch internode and needle fascicle expansion in the upper and lower third of crowns were measured weekly on three south-facing branches of each of four trees, and new root initiation and elongation were measured at 10-day intervals in three vertical rhizotrons per plot. In one replication, soil water content was measured daily. Fertilization significantly increased the expansion of first flush internodes in the upper crown and first flush needle fascicles in the upper and lower crown. New root growth was stimulated by fertilization in the second half of the growing season. The timing of root growth responses to fertilization corresponded to branch phenologies in the upper and lower crown that were conducive to increased basipetal transport of photosynthate. We conclude, therefore, that new root growth was linked to source-sink activities in the crown. Root initiation was greater in the upper than in the lower part of the soil profile; however, as the growing season progressed and water deficit increased, this relationship was reversed. The effect of soil depth on seasonal root growth was closely associated with water availability, suggesting that root initiation deep in the soil profile is critical for the continued production of new roots in environments subjected to short-term, but relatively severe, water deficits.