Response of cambial and shoot growth in trees treated with paclobutrazol

Abstract

Three experiments were conducted to investigate the effect of paclobutrazol on shoot and cambial growth of nine tree species located in Indiana and Louisiana, U.S. In Experiment 1, white oak (Quercus alba L.) and red oak (Q. rubra L.) in Indiana were treated in April 1995 with paclobutrazol using the soil drench method. In Experiment II, paclobutrazol was applied in April 1996, using the soil injection method, to eight species growing in Indiana: white oak, red oak, sweetgum (Liquidambar styraciflua L.), black walnut (Juglans nigra L.), European black alder (Alnus glutinosa L.), yellow poplar (Liriodendron tulipifera L.), white ash (Fraxinus americana L.), and white pine (Pinus strobus L.). In Experiment III, two species, sweetgum and cherrybark oak (Q. falcata var. pagodaefolia L.) in Louisiana, were treated with paclobutrazol by the soil drench method in March 1997. At the end of the 1998 growing season, annual shoot growth and annual xylem ring increment were measured for each of the two to four growing seasons after treatment in the three experiments. The effects of paclobutrazol on cambial growth and annual shoot growth at various positions along the trunk and in the canopy varied with species and treatment duration. Paclobutrazol reduced cambial growth in white oak, red oak, cherrybark oak, sweetgum, European black alder, and white pine with the amount varying among species, vertical location in the tree, and year after treatment.