Indicators of deciduous shade tree development useful for timing chemical defoliation and harvest

Abstract

In the mild climate of the Pacific Northwest, the harvesting of deciduous shade trees in wholesale nurseries is prolonged from October to January because of autumn growth and prolonged leaf retention. Economically, it is advantageous to defoliate nursery trees at the earliest possible date. Premature defoliation, however, can cause die back or predispose trees to over-wintering damage or poor performance the subsequent year. The purpose of this study was to identify and characterize the physiological stage of development at which leaves can be removed from deciduous trees without adversely affecting survival and regrowth. This physiological stage was termed "interfronoce," Interfronoce could not be reliably predicted from visual growth phenomena such as leaf coloration, leaf fall, and terminal bud formation. Preliminary data showed that stems and roots of plants unharmed by defoliation were less hydrated and contained higher concentrations of soluble carbohydrates than less mature plants which were damaged. It was hypothesized that osmotic potential of cells and the ionic status of cytoplasm were considerably different in interfronoce vs. pre-interfronoce plants, and that these differences could be used to predict interfronoce. The hypothesis was tested by studying the influence of defoliation on uniform Cornus stolonifera Michx. plants grown under controlled SD or LD conditions for 10 wks, and 2 years of study on field-grown nursery stock. In the controlled study, xylem water potential and turgor pressure of SD-10 hr plants in interfronoce were generally lower than in pre-interfronoce plants grown at LD-16 hr days. Sequential hand defoliation showed that 60 days of SD treatment induced interfronoce in C. stolonifera. The xylem water potential also reached its lowest point after 60 days of SD exposure. Defoliation prior to interfronoce caused bud and stem die back and stimulated refoliation. In 1974, phenological observations were made on 59 cultivars of 25 species. Observations were made of leaf expansion, autumn leaf coloration, and terminal bud formation. On 12 cultivars of 5 species, measurements were made of xylem water potential and osmotic potential on terminal shoots. Turgor pressure was calculated. The timing of these phenological and physiological changes was correlated with weather data. Data on each species were grouped for a multiple discriminant function analysis to determine the most reliable interfronoce indicators. Response to hand defoliation was used as the standard for establishing when plants entered interfronoce. A combined index of interfronoce based on xylem water potential, osmotic potential, and turgor pressure predicted the onset of interfronoce with a high degree of accuracy in Acer, Malus, Quercus, and Sorbus. In terms of subsequent performance (growth), the time of interfronoce ranged from Sept 16 to Nov 11 in the species studied. Seasonal changes in water status of 38 cultivars of 22 shade tree species were determined under field conditions in 1975 since certain changes in water status appeared to coincide with onset of interfronoce in 1974. Predawn xylem water potential, osmotic potential, and turgor pressure values (water status) were determined at 2 wk intervals from July 26 to Nov 21. A multiple discriminant function analysis indicated that stem water status was reliable for measuring interfronoce in cultivars of Acer, Betula, Carpinus, Crataegus, Fraxinus, Gleditsia, Malus, Platanus, Quercus, Sorbus, and Tilia studied. Chemical defoliants were safely used to hasten leaf abscission by 4-8 wk when water status was used to predict interfronoce. Comparatively low xylem water potential measurements effectively timed the application of chemical defoliants. Relationships between interfronoce, dormancy, and the rest period and quiescence phases of dormancy are discussed and illustrated diagrammatically.