EVALUATION OF PHYSIOLOGICAL TESTS AS PREDICTORS OF YOUNG TREE ESTABLISHMENT AND GROWTH

Journal of Arboriculture, Mar 2004 by Percival, Glynn C

Abstract. High mortality rates of bare-root slock can occasionally occur post-planting due to poor plant vitality at the time of planting. Consequently, physiological tests of plant vitality, such as stem electrolyte leakage, root growth potential (RGP), and chlorophyll fluorescence, are valuable because of their ability to identify low-vigor or damaged plants that will perform poorly when planted into the landscape. The survival, growth, and foliar damage of three tree species commonly used in landscape plantings was accurately predicted by stem electrolyte leakage, RGP, and chlorophyll fluorescence 17 weeks post-planting under field conditions following freezing damage. Of the three physiological tests, correlation of field performance with stem electrolyte leakage had the highest mean r2 values, which, based on results of this investigation, was a marginally better predictor of future plant performance compared to chlorophyll fluorescence and RGP. Chlorophyll fluorescence measurements taken from woody tissue of birch (Betula spp.), a deciduous tree species, were highly predictive of subsequent survival, foliar damage, and height increment 17 weeks post-planting, indicating that chlorophyll fluorescence may have applications as a predictor of plant vitality and future growth in deciduous ornamental trees. The practical advantages and disadvantages offered by each plant vitality test to professionals involved in urban tree management are discussed.

Key Words. Chlorophyll fluorescence; electrolyte leakage; root growth potential; freezing damage; holly; evergreen oak; birch; Ilex aquifoliutn', Quercus ilex', Betula pendula.

Occasionally, bare-root ornamental trees and shrubs transplanted in urban landscapes show poor establishment and high mortality rates during the following growing season. At the time of planting, visible differences in plant vitality such as leaf yellowing and/or crown dieback, are rarely detectable, especially in the case of deciduous stock for which no leaves are present during fall and winter when planting of bare-root stock is recommended. Consequently, depending on the time of planting, it is not until many months later during the spring that such symptoms as leaf yellowing and stem dieback become visibly apparent.

Physiological tests of plant vitality are valuable because of their ability to identify low-vigor or damaged plants that will perform poorly when planted into the landscape (Ritchie and Dunlop 1980; Lindqvist 2001; Lindqvist and Asp 2002).

Shoot electrolyte leakage has been shown to indicate post-planting needle damage and first-year survival of conifer seedlings following heat damage (Binder and Fielder f 995), be highly correlated with incipient damage and growth potential of lour conifer species following warm and cold storage (Colombo 1997; Maki and Colombo 2001), and correlate strongly with post-freezing survival rates of fruit trees (Ketchie et al. 1972).

Root growth potential (RGP), a performance test of the ability of nursery stock to initiate and elongate roots in a standardized environment favorable for root growth, has been used extensively to evaluate the effects of environmental and physiological stress on conifer seedlings and is a useful indicator of seedling vitality (McCreary and Duryea 1987; Simpson and Ritchie 1997). However, its use as a predictor of held growth and survival has been an area of debate, emanating primarily from concern about the value of a test performed under ideal conditions predicting performance under field stresses (Mohammed et al. 1997).

Chlorophyll fluorescence, an indication of the fate of excitation energy in the leaf photosynthetic apparatus, has been used to provide a rapid and nondestructive diagnostic system of detecting and quantifying physiological injury in tree leaves and needles (photosynthetic organs) under low temperatures, salinity, and water stress conditions (Palta 1992; Sestak and Stiffel 1997). Chlorophyll fluorescence, especially Fv/Fm (ratio of variable to maximum fluorescence), has proved particularly useful in screening programs because in many instances the effects of environmental stress can be detected prior to visible signs of deterioration (Jimenez et al. 1997; Percival and Fraser 2001; Percival and Sheriffs 2002). Previous work by Mohammed et al. (1997) demonstrated that measurements of chlorophyll fluorescence (Fv/Fm) post heal stress of several stocktypes of jack pine (Pinus banksiana) and black spruce (Picca manana) were closely correlated with stem volume increment over time, concluding that Fv/Fm measurements could be used to forecast field growth performance of the two test species. Recent work has also shown that chlorophyll fluorescence values were highly predictive of growth, foliar damage, and survival rates in white and red pine (Pinus strobus and P. resinosa) and white and black spruce (Picea glauca and P mariana) seedlings at week 7 post-planting following prolonged exposure of seedling material to elevated storage temperatures (Maki and Colombo 2001).