Fertilization at planting impairs root system development and drought avoidance of Douglas-fir (Pseudotsuga menziesii) seedlings

Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) seedlings ( 1 + 1 bareroot) were divided into 8 - 13 cm(3) and 23 - 35 cm(3) initial root-volume categories and either not fertilized or fertilized with 60 g Apex(R) in the planting hole at a drought-prone clearcut site in the Oregon Coast Range, USA. Regardless of initial root volume, fertilized seedlings consistently reached lower predawn xylem water potential (Psi(xylem)) values than non-fertilized seedlings during summer. Roots did not proliferate relative to fertilizer placement and mean values for lateral-root length, root tips, and root biomass were less for fertilized seedlings in each of four vertical root zones. Poor root system development of fertilized seedlings may have been associated with the rapid release of high fertilizer salt concentrations into the rhizosphere. Root-volume growth was positively correlated with Psi(xylem), emphasizing the importance of new root growth following planting for drought avoidance. Criteria for successful field fertilization must consider the anticipated drought level of the site.