Pre-Plant Storage and Handling of Difficult to Transplant Ash Alternatives

The loss of ash trees in the midwestern United States due to emerald ash borer has emphasized the need for more diversity in urban and community forests. However, nursery growers report that many ash alternatives are difficult to transplant. Understanding the physiological reasons why one tree species has superior transplantability over another is an important factor in increasing production of ash replacements. In this study, we examined four difficult to transplant ash alternatives; hackberry (Celtis occidentalis), swamp white oak (Quercus bicolor), red oak (Quercus rubra), and baldcypress (Taxodium distichum). Trees were assigned at random to one of four treatments: 1) control, 2) cold storage, 3) sweating, or 4) heeled-in.  All treatments except control were maintained for three weeks. Trees in Cold storage were placed in a 4^oC (40^oF) walk-in cooler. Tree roots were covered with straw and watered as needed to prevent desiccation. The sweating treatment was designed to replicate a common nursery practice used to stimulate growth on recalcitrant trees. Trees were placed on the floor of a greenhouse and were alternated with layers of burlap and then covered with a large sheet of white plastic.  The heeled-in treatment was designed to simulate a common nursery practice of storing trees outdoors prior to planting. We stood the trees up and covered the roots with a pine bark-peat moss (80:20; v:v) mix. After treatment, trees were assigned at random to one of three evaluations: root growth potential (RGP) test, container planting or field planting. We collected data regularly from May to October. Species and pre-plant treatments affected survival, stem caliper growth, and shoot terminal growth.  Sweating trees before planting decreased stem caliper growth and increased shoot die-back. Root growth potential was very low for oak trees and varied by treatment for hackberry.  Baldcypress trees had a relatively high root growth potential in all treatments. Stem water potential measured immediately before and after pre-plant treatments suggest that oak trees and baldcypress trees were able to rehydrate during sweating while water stress levels in hackberry trees remained high (stem water potential < 2.0 MPa). The results suggest that poor transplanting success reported for oak trees may be related to low root growth potential, whereas poor success of hackberry trees may be due to shoot desiccation during storage and handling.