Arboriculture & Urban Forestry 35(5): September 2009 whereupon excess N accumulation diminishes both the photosyn- thetic capacity and the activation state of Rubisco (Manter et al. 2005). The elevated concentration of N in the foliage of trees re- ceiving the 4.27 g N application rate suggests an enhanced capac- ity for growth, although there is no published data to indicate if the observed concentrations approached a physiological threshold. Despite the treatment-induced effects, foliar total [N] declined throughout the study period. Foliar total [N] values 90 days after fertilization at Lisle, IL were 41%, 35%, and 53% lower than val- ues observed at 14 days for trees receiving 0, 1.42 g N, and 4.27 g N, respectively. Similarly, at Arlington, WI, foliar total [N] at 90 days for trees receiving 0, 1.42 g N, and 4.27 g N were 59%, 71%, and 59% lower than maximum values at 14 days. Seasonal declines in foliar total [N] are the product of N dilution resulting from an increase in leaf area due to cellular expansion, the accumulation of secondary metabolites associated with leaf maturation, and the ex- port of N prior to leaf senescence (Escudero and Mediavilla 2003). The rate of application did not affect total N in the current season stem wood at either location during any of the harvest periods. Maximum total [N] in current stem wood was observed prior to fertilization and decline substantially through 60 days after treatment (data not shown). At Arlington, WI, total [N] in the current season stem wood at 60 days was 36%, 28%, and 25% lower than pre-treatment values for trees receiving no fertilizer N, 1.42 g N, and 4.27 g N, respectively. At Lisle, IL, declines of 30%, 31%, and 42% were observed over the same period of time for current season stem wood. At both locations, total [N] in the current season stem wood increased between 60 and 90 days after treatment. Total [N] increased by 28%, 9%, and 17% between 60 and 90 days at Arlington, WI in trees re- ceiving no fertilizer N, 1.42 g N, and 4.27 g N, respectively. At Lisle, IL, current season stem wood increased by 25%, 11%, and 19%, respectively, between 60 and 90 days after treatment. The rate of application significantly affected total [N] in stem wood 90 days after treatment at Arlington, WI only. At 90 days, total [N] in stem wood of trees receiving the 4.27 g N application rate was 19% and 36% higher than trees receiving no fertilizer N and those receiving the 1.42 g N treatment, respectively (Figure 2). There was no difference between the control trees and trees receiving 1.42 g N. At Lisle, IL, the rate of application did not affect total [N] in stem wood tissues during any of the harvest periods. Independent of treatment level, total [N] in stem wood declined substantially at both locations during the period of time between pre-treatment sampling and the 14 days after fertiliza- tion harvest period. Such early season declines have been shown to be the result of the remobilization of stored N, primarily from perennial tissues, to support early season growth (Neilsen et al. 2001). Total [N] in stem wood at Lisle, IL was substantially and consistently lower than those observed at Arlington, WI, sug- gesting stored N may have been further depleted to support the second flush of foliage. These observations are consistent with findings surrounding changes in N partitioning and acquisition patterns following defoliation. In red oak (Quercus rubra L.) trees, Lovett and Tobiessen (1993) observed that the sources of N to support growth after defoliation included both acquiring soil derived N and remobilizing N stored from stem and root tissues. The trend in the data for total [N] in current season stem wood and stem wood are consistent with theories surrounding the resorp- tion and storage of N removed from the leaf prior to senescence. Gomez and Faurobert (2002) found that resorbed foliar N was 255 Figure 2. Total [N] in stem wood of common hackberry at (A) Ar- lington, WI, and (B) Lisle, IL in 2002. Symbols represent the mean (n = 5) ± adjusted standard error. Asterisk indicates significant difference at P ≤ 0.05. stored, to a great extent, as low molecular weight proteins within the radial parenchyma and phloem in the stem wood of peach (Pru- nus persica L. Batsch) trees. However, the extent of the increases in total [N] during the early stages of dormancy does not appear to be correlated to the rate of application. This lack of consistency among treatment levels may have been the result of N dilution associated with using the entire volume of the stem to prepare a sample for analysis or it may be an artifact associated with the 90 days sampling period. Werner (2008) observed continued and sub- stantial increases in total [N] through 150 days after fertilization in a complimentary field study using common hackberry trees. Total [N] in root tissues of trees receiving 1.42 g N was 55% and 31% higher than control trees 30 and 90 days after treatment, respectively, at Arlington, WI (Figure 3). During these same har- vest periods, root total [N] in trees receiving 4.27 g N increased by 70% and 41%, respectively, over values observed in control trees. The observed increases in root total [N] of fertilized trees during these harvest periods suggest either a surplus of N that was stored in the roots and/or the incorporation of fertilizer N into the growth and development of new roots (Pregitzer et al. 1990). Hackberry’s indeterminate growth pattern, the almost complete abscission of leaves at 90 days after treatment in Ar- lington, WI, and a continued decline in total [N] in the root tis- sues of control trees, suggests fertilization resulted in a surplus of ©2009 International Society of Arboriculture
September 2009
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