Re-evaluating the Significance of the First-order Lateral Root Grading Criterion for Hardwood Seedlings

—Numerous authors have reported on the importance of the number of firstorder lateral roots (FOLR) when evaluating the morphological quality of nursery hardwood seedlings. Studies have shown that seedlings with a greater quantity of FOLR outperform seedlings with a lesser quantity of FOLR in the field. However, the FOLR measure may be limited in its ability to quantify root system morphology. Additionally, the number of FOLR is correlated with other morphological characteristics, and few studies have directly evaluated the importance of FOLR relative to other variables for predicting outplanting success. A trial in southern Indiana compared the ability of the initial number of FOLR to predict field performance for three hardwood species (black cherry, white oak, and northern red oak) relative to four other initial morphological variables (shoot height, stem diameter, root volume, and whole plant fresh weight). Regression analyses indicated that regardless of species, FOLR tended to be among the least effective predictors of total height and diameter after one field season. These results indicate that FOLR may provide a less accurate indicator of hardwood seedling morphological quality than other easily-measured variables. The success of reforestation and afforestation operations may be improved if nurseries produce seedlings with target morphological and physiological characteristics that have been quantitatively linked with outplanting success (Rose et al. 1990). Morphological characteristics may be easily assessed by both nursery workers and field foresters. As such, these characteristics are often used to help evaluate seedling quality. Commonly-measured morphological characteristics include shoot height, stem diameter, and root system size (Rose et al. 1990). Because shoot growth following planting is most limited by water availability (Burdett et al. 1984), roots must rapidly extend through the soil profile to re-establish root-soil contact and absorb water to minimize transplant shock (Sands 1984). The capacity of roots to do this is likely related to both seedling physiological quality (Ritchie and Dunlap 1980) and root system morphology. Various measurements of nursery seedling root system morphology have been proposed, including root mass, root volume, number of first-order lateral roots (FOLR), root length, and root area index. Many measurements of root system morphology are either destructive, tedious and time consuming, or both. This generally limits their application to only selective research projects. Two root system measurements that are relatively rapid and non-destructive, and thus may have application in operation, are root volume and number of FOLR. Root volume is measured using the water displacement method (Burdett 1979). Nursery seedling root volume has been directly correlated with reforestation success for Douglas-fir (Pseudotsuga menzeiseii (Mirb.) Franco) and ponderosa pine (Pinus ponderosa Dougl. ex Laws.) in the western United States (Rose et al. 1991a; Rose et al. 1991b; Rose et al. 1997). Carlson (1986) reported that root volume helps determine the potential for water uptake prior to new root growth in loblolly pine (Pinus taeda L.). Little research, however, has examined the importance of initial root volume in eastern deciduous tree species. Instead, the majority of research involving root morphology of these species has focused on FOLR (i.e., the number of first-order lateral roots exceeding 1 mm in diameter at junction with the tap root). Numerous studies have reported that hardwood seedlings with more FOLR perform better in the field than those with less FOLR (e.g., †Assistant Professor of Forest Regeneration (DFJ), Hardwood Tree Improvement and Regeneration Center, Department of Forestry and Natural Resources, Purdue University, West Lafayette, IN 47907-2033; and Extension and Research Forester (JRS), Hardwood Tree Improvement and Regeneration Center, Department of Forestry and Natural Resources, Purdue University, PO Box 216, Butlerville, IN 47223. DFJ is corresponding author: to contact, call (765) 494-3608 or e-mail at [email protected].