375 Stock Quality of Black Walnut (Juglans Nigra) Seedlings as Affected by Half-Sib Seed Source and Nursery Sowing Density D.F. Jacobs, K.E. Woeste, B.C. Wilson and J.R. McKenna Hardwood Tree Improvement and Regeneration Center Department of Forestry and Natural Resources Purdue University West Lafayette, Indiana United States of America Keywords: seedling quality, root volume, shoot height, stem caliper, genetic identity, bareroot nursery production Abstract Morphological quality of black walnut (Juglans nigra L.) seedlings largely dictates their capacity to establish well in plantations and their performance potential for use as rootstocks in grafting. Larger seedlings and rootstocks, attained from propagation at relatively low nursery sowing densities, have comparatively more expansive root systems for exploitation of soil resources and greater nutrient/carbo- hydrate reserves to support aboveground growth. The relative role of genetic selection in dictating morphological quality of nursery-grown black walnut seedlings, however, has not been thoroughly documented. We collected seed from three half-sib families (A, B, C) of black walnut in Indiana, USA which were sown as sprouted seeds into bareroot nursery beds at three sowing densities (L, 11.2 m -2 ; M, 24.2 m -2 ; H, 29.4 m -2 ). Sowing density significantly affected mean shoot height (H > M > L) and root volume (L > M > H), but not stem diameter. Shoot height was increased by 14.4 cm (43%) in H vs. L and root volume was increased by 32 cm 3 (39%) in L vs. H. Seedling response to family selection was significant for height, root volume, and stem diameter (C > B > A). The magnitude of the difference in response was greater for family selection compared to nursery sowing density, as height and root volume were increased by 15.2 cm (46%) and 44 cm 3 (60%) in C vs. A, respectively. Further, family C had greater mean stem diameter and root volume in H than did family A in L; mean height for these two families were about equal between H and L. Our results suggest that identification of superior genetic sources may be relatively more important than sowing density in nursery propagation of high quality black walnut seedlings. INTRODUCTION Black walnut (Juglans nigra L.) is an important component of forests in the Central Region of the USA, producing valuable timber and nut resources (Williams, 1990). Black walnut is among the most common species grown in forest tree nurseries in this region (Jacobs et al., 2004), with seedlings being used for afforestation plantings and to a lesser extent as rootstock for grafting of genetically improved germ plasm. Planted hardwood seedlings in this region often grow poorly or fail to survive (Jacobs et al., 2004), which may be partially attributed to variable nursery stock quality. Morphological attributes such as shoot height and stem caliper are easily measured and often used to evaluate nursery stock quality (Thompson, 1985). In addition, large root system morpho- logy appears to provide a useful assessment of potential hardwood seedling performance (Schultz and Thompson, 1996; Jacobs and Seifert, 2004) presumably due to increased access of a more expansive root system to soil resources, as well as supplying greater carbohydrate and nutrient reserves to promote aboveground growth. Benefits to using high quality nursery stock are likely to be realized following seedling planting, as well as when using rootstock for grafting. Control of black walnut nursery stock quality is largely achieved through cultural practices conducted in the nursery (Jacobs, 2004). Among the most important nursery cultural variables is bed sowing density, with larger seedlings consistently being produced Proc. V th Int. Walnut Symp. Eds. M.E. Malvolti and D. Avanzato Acta Hort. 705, ISHS 2006