Root growth potential: its development and expression in forest tree seedlings.

This review presents a synthesis of current knowledge on Root Growth Potential (RGP) of nursery-grown forest seedlings. RGP, the measure of a transplanted seedling's ability to rapidly produce new roots, is a key indicator of seedling vigour and survival after planting. Physiologically, RGP appears to be closely linked to bud dormancy. It peaks when the chilling requirement for dormancy release is fulfilled, then declines, presumably reflecting the internal allocation of current photo-assimilate to shoot growth. RGP can be manipulated in the nursery through practices that induce dormancy, increase root fibrosity, and enhance carbohydrate reserves. Autumn and winter nursery temperatures influence RGP through their effect on dormancy release. Date of lifting establishes the dormancy status of seedlings when they enter storage, and temperature and duration of storage further influence the dormancy status at the time of planting. Improper handling and misplanting can reduce RGP expression, as can low soil temperature, low soil water potential and soil compaction on the planting site. INTRODUCTION Initial survival of planted trees depends in large measure on their physiological readiness to rapidly produce new roots and thereby re-establish intimate contact with the soil (Smith, 1962). This ability is sometimes referred to as the "Root Growth Potential," or RGP, and many authors have stressed its importance as a critical ingredient of seedling quality and subsequent reforestation success (e.g., Gilmore, 1962; Zaerr, 1967; Larson, 1970; Larson and Whitmore, 1970; Lathrop and Mecklenberg, 1971; Etter and Carlson, 1973; Farmer, 1975; Day and MacGillivray, 1975; W. Webb, 1975a, b, 1977; von Lupke, 1976; Day et al., 1976). While it has been difficult to establish a clear cause-effect relationship between RGP and seedling survival after planting, a compelling body of evidence (summarised in Table 1) indicates that the two are often very closely correlated. Thirty years ago Wakeley (1948) emphasised that seedling morphological grades were inadequate indicators of seedling performance, and that seedlings that survived and performed well apparently did so due to their superior physiological grade. He concluded, however, "How to recognize physiological grades before planting the seedlings and observing their success or failure remains to be discovered. " Some twenty years later, E. C. Stone and co-workers demonstrated that a seedling's ability to grow roots in a test environment could be used as a measure of seedling physiological grade or overall seedling vigour (Stone, 1955; Stone and Schubert, 1959b, c; Stone and Jenkinson, 1971). Furthermore, the period during which seedlings exhibit high RGP coincides very closely with the period during which they are most tolerant to desiccation and physical damage (Hermann, 1962, 1964, 1967; Mullin, 1978; Lavender and Wareing, 1972) and therefore are more able to survive the rigours of lifting, handling, storing and outplanting. The RGP level can thus be used, in effect, as an index of seedling resilience (Stone, 1955; Stone et al, 1963; Stone and Jenkinson, 1971; Day et al, 1976). N.Z. J . For . Sci. 10(1): 218-48 (1980). No. 1 Ritchie and Dunlap — Root Growth Potential in Tree Seedlings 219 TABLE 1. Relationship between Root Growth Potential and field survival. (Correlation ratings are authors' interpretations.) Species R G P vs. SURVIVAL Lodgepole pine (Pinus contorta) Loblolly pine (P. taeda) Ponderosa pine (P. ponderosa) Ponderosa pine (January planting) Ponderosa pine (March planting) Jeffrey pine (P. jeffreyi) Douglas fir (Pseudotsuga menziesii)