Arboriculture & Urban Forestry 39(2): March 2013 Arboriculture & Urban Forestry 2013. 39(2): 85–93 85 Fertilizer Nitrogen Uptake and Partitioning in Young and Mature Common Hackberry (Celtis occidentalis) Trees L.P. Werner and L.G. Jull Abstract. Twenty-one mature and thirty-six young common hackberry (Celtis occidentalis L.) trees received a single application of 15 tope enriched ammonium-nitrate (NH4 current season stem wood, stem wood, root, and fruit tissues were analyzed for total concentration [N] and nitrogen derived from fertilizer (NDFF). Growth phase and application rate did not consistently affect total % [N], particularly the N demanding foliage. Growth phase and application rate significantly affected the percentage of NDFF. In every instance, NDFF was highest in the tissues of young trees and/or trees receiving the 1.47 kg N 100 m-2 NO3 application rate. Mature trees relied upon previously assimilated N to meet the annual demand for N to a greater extent than young trees. Key Words. ANSI A-300; Celtis occidentalis; Fertilization; Hackberry; Nitrogen; Remobilization. ) fertilizer. Application rates were 0, 0.49, and 1.47 kg N 100 m-2 N double iso- of canopy coverage, respectively. Foliage, Annual demands for nitrogen (N) by woody perennial plants are met through a combination of soil N derived from the mineral- ization of organic residues, symbiotic relationships with bacteria capable of fixing atmospheric N, wet and dry atmospheric depo- sition, and the internal cycling of previously acquired N (Schnei- der et al. 1996; Cooke and Weih 2005; Millard et al. 2006). The relative contribution from each of these sources to meet the an- nual demand for N can vary within a growing season and over time (Weinbaum et al. 1987; Deng et al. 1989; Millard and Proe 1991; Weinbaum and Van Kessel 1998). Independent of location, trees display a logistic pattern of growth in which the annual rate of growth eventually slows over time (Weiner 2001). It is unclear if temporal shifts in the rate of growth are driven by reductions in the availability of essential mineral elements or a combina- tion of genetic, environmental, and physical restrictions (Weiner 2001; Ryan et al. 2004; Johnson 2006; Ryan et al. 2006). Re- gardless of the causal agent(s), it is logical to assume there is a concomitant decline in the annual demand for resources (i.e., N) necessary to support the growth and development of new tissues. In urban landscapes, N-based fertilizers are commonly used to supplement native sources of N to promote growth in land- scape trees or to remediate identifiable N deficiencies [Smiley et al. 2002; American National Standards Institute (ANSI) 2011]. Currently, N application rates range from 0.25 to 1.96 kg N 100 m-2 of canopy coverage, and should not exceed 1.96–2.94 kg N 100 m-2 annually (ANSI 2011). The proposed range in applica- tion rates is intended to provide latitude in annual application rates, so as to account for various management objectives (e.g., maintenance, growth promoting), the type of fertilizer applied (i.e., fast release versus slow release), soil type (e.g., coarse ver- sus fine texture, low native fertility versus high native fertility), associated land management practices (i.e., turf grass fertiliza- tion), and the physiological state (i.e., health) of the tree being treated. The basis for the range in application rates appears to be consistent with studies evaluating fertilizer-induced growth responses in young trees (Neely et al. 1970; Smith 1978; Neely 1980; van de Werken 1981; Struve 2002). The combination of demonstrated declines in tree growth rate and potential changes in the relative contributions from the various sources of N used to meet the annual demand suggest there is sufficient evidence to question the efficacy of extrapolating rates of fertilization derived from studies involving young trees onto mature trees. The objectives of this study were to examine and com- pare patterns of fertilizer N uptake at rates consistent with the ANSI standards in young and mature common hackberry (Celtis occidentalis) trees growing in urban landscapes and to evaluate growth phase-related changes in the source(s) of N used to meet the current growing-season demand for N. METHODS AND MATERIALS Common hackberry, a native tree to eastern and central United States and Canada, was used to evaluate fertilizer N uptake and partitioning at eight locations in Waukesha, Wisconsin, U.S. (43°N -88°W). Common hackberry is a riparian species capable of reach- ing heights of 15–21 m at maturity and is tolerant of air pollution, a wide range of soil types, pH, and moisture regimes. As a result, it is used extensively as a street, park, and landscape tree (Dirr 1998). Waukesha is located in southeastern Wisconsin. Mean growing season (May–September) temperatures range from 15.2°C to 23.2°C, and the area averages 880 mm of rainfall annually (Wisconsin State Climatology Office 2002). Trees utilized in the study were growing in public terraces greater than 1.83 m wide in residential neighborhoods or in public parks. A split-plot design was employed where eight homogeneous plots (four mature tree plots and four young tree plots), were ©2013 International Society of Arboriculture
March 2013
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