Effects of paclobutrazol and fertilizer on the physiology, growth and biomass allocation of three Fraxinus species

Fraxinus spp. (ash) trees are widely found in urban areas across the U.S. where they experience varying degrees and types of stress. 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 growth but increase root: shoot ratios. In 2010, 63 Fraxinus spp. trees, (21 each, Fraxinus americana, Fraxinus quadrangulata and Fraxinus mandshurica) 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 F. quadrangulata had lower foliar N (1.94% ± 0.086), than F. americana (2.59% ± 0.050) and F. mandshurica (2.69% ± 0.063). In 2011, fertilizer increased F. quadrangulata foliar N (2.19% ± 0.085) compared to control trees (1.72% ± 0.128). Foliar N did not differ between F. americana or F. mandshurica fertilized or control trees or between PB and control trees of any species. 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.