Effects of Paclobutrazol on the Physiology, Growth, and Biomass Allocation of Three Fraxinus Species

Fraxinus spp. (ash) trees are widely found in urban areas across the United States where they experience varying degrees and types of stress.  The arrival of emerald ash borer (Agilus planipennis Fairmaire) into North America in the early 1990s dramatically altered the future of urban Fraxinus trees in North America. To date, this invasive phloem-feeding pest has killed tens of millions of Fraxinus trees and is found in 24 states and two Canadian provinces.  Fertilizer and paclobutrazol (PB) are marketed as a means to alter growth and enhance tree vigor.  Fertilizer typically increases radial growth but may reduce root:shoot ratios.  Paclobutrazol, a gibberellin inhibitor, may reduce radial and shoot growth but increase root:shoot ratios.  We evaluated effects of fertilization or PB applications on two North American species, Fraxinus americana L and Fraxinus quadrangulata Michx., and one Asian species, Fraxinus mandshurica Rupr., grown in a common garden location and under similar conditions.  These three species are common in urban landscapes, are evolutionarily diverse and have varying levels of inherent resistance to emerald ash borer.  In 2010, 63 Fraxinus spp. trees, (21 of each of the above species) growing in a plantation were assigned to one of three treatments:  fertilization, PB application, or untreated control.  Gas exchange, foliar nutrient concentration, radial growth, and biomass allocation were assessed to evaluate tree response to treatments.  Throughout the two-year study, gas exchange was not affected by species or treatment.  In 2010, there were no treatment effects on foliar N but differences among species were apparent and in 2011, fertilizer increased F. quadrangulata foliar N (2.19% ± 0.085) compared to control trees (1.72% ± 0.128).  In 2010, there were no treatment effects on growth.  In 2011, radial growth of fertilized trees was 20% and 40% more than control and PB trees, respectively.  In October 2011, we harvested trees to assess treatment effects on biomass allocation.  Root:total biomass ratios of PB trees were 9% or 10% higher compared to control or fertilized trees, respectively, when species were combined.  Biomass allocation was rarely different between fertilized and control trees.  Results indicate the three Fraxinus species differ considerably in growth, foliar nutrients, and biomass allocation and their response to PB and fertilizer.