Evaluating Seedling Quality: The Basics By Diane Haase and Robin Rose, Winter 2004

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1 Evaluating Seedling Quality: The Basics By Diane Haase and Robin Rose, Winter 2004 Each year, several million forest seedlings are outplanted. However, many do not survive or grow well. There are many factors that can contribute to poor field performance such as weather patterns, planting technique, site conditions, and seedling quality. This article covers aspects of seedling quality that can be measured prior to outplanting. By establishing and measuring seedling quality parameters, both the nursery and the seedling buyer can gain a better understanding of the crop on a site-specific and speciesspecific basis. This allows the customer to shop around and to have confidence in the product. It also helps the nursery manager to improve future crops by identifying any problem areas. Morphological Quality Morphology is defined as the form or structure of an organism. Seedling shoot height and stem diameter are the most common measures used for growing and grading standards in forest nurseries. Other parameters can be assessed as well. Morphological standards vary greatly by species, seed zone and stocktype. No single factor has been shown to provide a perfect prediction of outplanting success, but each has been linked with seedling performance potential in some way. Shoot height Shoot height is the distance from the cotyledon scar to the base of the terminal bud on dormant seedlings or to the tip of the shoot on growing seedlings. Because height is correlated to the number of needles on the shoot, it is a good estimate of photosynthetic capacity and transpirational area. Taller seedlings may have a competitive advantage on sites with severe weed competition and may be indicative of superior genetics. On the other hand, taller seedlings with greater transpirational area may have a disadvantage on dry sites, and exceptionally tall seedlings may be difficult to plant, out of balance (poor shoot:root ratio), or subject to wind damage. Stem diameter Stem diameter is defined as the diameter of the main stem at the cotyledon scar. Diameter is often considered the best single predictor of field performance and has been shown in various studies to be highly correlated with growth and survival. Height:Diameter is a unitless ratio indicative of seedling sturdiness. Figure 1 shows height and diameter data and the corresponding sturdiness ratios.

2 Bud length Bud length is correlated with the number of needle primordia and thereby gives an indication of seedling vigor and field growth potential. Root and shoot volume Root volume includes all root mass below the cotyledon scar while shoot volume includes all shoot mass above the cotyledon scar. Studies show that seedlings with adequate root volume consistently grow and survive better than those with poor root volume. Root volume is generally well correlated with stem diameter, suggesting that nurseries should provide large stem diameters as well as incorporate root grading into the sorting process. It s important to note that root volume does not necessarily equate with root fibrosity since a seedling with many fine roots can have the same volume as a seedling with a large tap root. Weights Fresh or dry weight is commonly measured on the whole seedling or on root, shoot and foliage separately. Since water content in the tissue can vary, dry weight tends to provide a more consistent measurement than fresh weight. There is a strong correlation between seedling dry weight and stem diameter and thus it correlates to field survival and growth similarly. Shoot-Root Ratio Shoot:Root is a unitless ratio calculated from root and shoot weights or volumes. The ratio indicates the balance between the transpirational area (shoot) and the water absorbing area (root) of the seedlings. Figure 2 shows volume data and the corresponding ratios for the same sample of seedlings used for Figure 1. Form and Damage Existence of multiple shoots, stem sweep, poor color, root deformity, stiff lateral roots, physical damage, and any other noticeable characteristics that can affect seedling performance are also important morphological factors to evaluate. Physiological Quality Cold Hardiness

3 Cold hardiness is defined as a minimum temperature at which a certain percentage of a random seedling population will survive or will sustain a given level of damage. LT 50 (lethal temperature for 50 percent of a population) is commonly used to define the cold hardiness level. Changes in LT 50 are strongly linked to the seedling dormancy cycle and stress resistance and are influenced by seed source, nursery practices and environment. Studies have shown that LT 50 at lifting correlates well with first-year survival and shoot growth. LT 50 is measured by freezing seedling samples in a programmable freezer and then evaluating them for damage to buds, cambium and needle tissues. Four or more target temperatures are chosen at a given sampling period based on their expected ability to bracket the LT 50 temperature at a given time in the dormancy cycle (for example, if the LT 50 is expected to be approximately -9 degrees C, then selected temperatures might be -4 degrees, -7 degrees, -10 degrees, and -13 degrees in order to capture damage ranging from percent). After freezing, seedlings are placed into a greenhouse with adequate moisture and ambient temperatures. Bud, cambium and foliar damage are assessed after seven days. If the cambium is dead in the lower half of the seedling or if greater than 50 percent of the buds are damaged, then the seedling is considered nonviable. The foliage is only a determining factor when cambium or bud damage is borderline. The LT 50 is then determined by plotting percent survival against temperature (Figure 3). Root Growth Potential Root growth potential (RGP) is defined as the ability of a tree seedling to initiate and elongate roots when placed into an environment optimal for root growth. RGP is measured in late winter or early spring by either potting a sample of seedlings or placing them into a hydroponic tank. Seedlings are kept in a greenhouse environment for two to four weeks and then new roots (distinguishable by their white tips) are counted and measured. RGP can predict actual field performance when trees are dead or when water uptake is dependent on new growth. However, since RGP only represents the potential to grow new roots, that potential may or may not be expressed when the seedling is outplanted because soil temperatures at the time of outplanting are usually below optimal for RGP to be fully expressed. Bud Dormancy The number of days to budbreak under favorable growth conditions is an index of a seedling s dormancy state and stress resistance. Days to budbreak is dependent on the number of chilling hours ( 5 C) a seedling is exposed to in the nursery. Mitotic index (MI) is another measure of bud dormancy and is defined as the percentage of cells in mitosis at a given time. Douglas-fir buds are considered fully dormant

4 when mitotic activity in the bud cells is zero, a condition that generally occurs from December through February. Measuring MI consists of placing the shoot tips in a fixative for 24 hours then squashing and staining the apical meristem on a microscope slide. Actively dividing cells are identified and counted as a percentage of the total number of cells within a counting grid. Foliar Nutrient Concentration Nutrients are well known to play a crucial role in determining plant quality and performance. Many studies have linked foliar nutrient concentration with subsequent growth. By also measuring foliar dry weight of a specific unit (e.g. 100 needles), nutrient content can be calculated. Foliar dry weight, nutrient concentration and nutrient content can then be examined in an integrated graphic format (vector analysis). This is useful for examining plant responses to various silvicultural treatments and in comparing differences between plants based on existing conditions (Figure 3). Plant Moisture Stress Plant moisture stress (PMS) is an important physiological measure of a seedling s water status. PMS reflects the water potential (sum of osmotic and turgor potential) in a plant and is often used to schedule irrigation and monitor water stress during lift and pack operations. PMS measurements with a pressure chamber are rapid and simple. Conclusion Measurement of one or several seedling quality parameters is useful in many situations. For example, if a forester believes that his crop is top-heavy coming out of the nursery, s/he may choose to measure shoot:root ratio in addition to the standard height and stem diameter. Or, one could measure cold hardiness on a crop that set bud late in order to select a lift date when the seedlings are most stress resistant. Another example would be to order nutrient testing on a crop that has poor foliar color. By obtaining seedling quality data, one can narrow down the cause of poor field performance. If the seedling quality data is favorable, then the forest manager can explore other factors such as site quality and handling. If the seedling quality data is unfavorable, then the forest manager can choose another nursery, add more criteria into the growing contract, or work with the nursery manager to improve the subsequent year s crop. Ultimately, seedling quality data can only add to the ability to understand and manage a young plantation. Diane L. Haase is associate director of the Nursery Technology Cooperative, College of Forestry, Corvallis, Ore. She can be reached at diane.haase@oregonstate.edu or Robin Rose is

5 director of the Nursery Technology Cooperative and director of the Vegetation Management Research Cooperative at the OSU College of Forestry. He can be reached at or Sidebar Testing facilities Many seedling growers and buyers do in-house seedling quality evaluation while some companies perform no tests at all. There are only a couple testing facilities in the Northwest. The Roseburg Forest Products facility in Lebanon, Ore., ( , mjalbrecht@msn.com) offers morphology, RGP, cold hardiness and other tests. Seedling Quality Evaluation Services at OSU ( , sqes@oregonstate.edu) offers morphology and cold hardiness testing. For nutrient analyses, there are several labs available in the Northwest. These testing facilities are available to anyone on a fee basis. Contact them directly for prices and seedling sample sizes necessary for specific tests.

6 FIGURES Figure 1. Morphological quality of a sample of Douglas-fir seedlings. This graph shows height and diameter. The diagonal axes indicate the height:diameter sturdiness ratio. B) shoot and root volume, the diagonal axes indicate shoot:root ratio.

7 Figure 2. Morphological quality of a sample of Douglas-fir seedlings. This graph depicts shoot and root volume, the diagonal axes indicate shoot:root ratio.

8 Figure 3. Cold hardiness graph indicating freeze damage at four temperatures and the corresponding LT50.

9 Figure 4. Vector diagram showing nitrogen concentration, content and foliar dry weight responses to fertilization treatments relative to an unfertilized control.