Arboriculture & Urban Forestry 39(2): March 2013 the size of the N pool and the capacity to store N removed from senescing tissues increases as tree biomass accumulates with time/age (Nielsen et al. 1997). Therefore, the study authors sus- pect that observed increases in N accumulation within the current season stem wood of young hackberry trees, many of which die during the winter dormant season, diminishes the overall capac- ity of young trees to store N. The combination of this diminished N storage capacity and a comparatively fast rate of growth in young trees would, therefore, increase the demand for N from the soil and/or other external sources to meet future demands for N. Conversely, the enhanced N storage capacity and comparatively slow rate of growth in mature trees would appear to decrease the demand for soil derived N, with the resulting deficit being offset by increases in the use of previously assimilated, stored N to meet the annual demand for N within developing and/or photosynthet- ic tissues. The combination of total % [N] and the percentages of NDFF within the transient and perennial tissues of young and mature trees support this conclusion. With one exception, total % [N] within the N demanding foliage of mature trees was not significantly different from values observed in young trees; yet, the percentage of NDFF in the foliage of young trees was almost 200% greater than that of mature trees. The preferential use of un- labeled, stored N over the isotopic enriched soil N in mature trees appears to account for much of the discrepancy in the percent- age of NDFF within these tissues. These findings are consistent with other studies (Nielsen et al. 1997, Weinbaum and Van Kessel 1998) and would appear to indicate that as trees age the demand for external sources of N, including fertilizer N, diminishes. Over the course of the 2001 study period, the rate of appli- cation did not exert strong influence over total % [N]. In those instances where it was significant, it primarily occurred at the highest rate of application and generally within the tissues of young trees. Typically, the percentage of NDFF increased with an increasing rate of application in both growth phases. How- ever, increases in the percentage of NDFF within the tissues of young trees had a tendency to be proportional to the increase in the rate of application and substantially lower in mature trees. These findings suggest that fertilizer N uptake relative to the rate of application was higher in young trees. The observation that total % [N] in the fruit was functionally equivalent to that of the foliage throughout much of the growing season indicates the fruit of mature trees is without doubt a substantial sink for N; one that could potentially offset the reduction in the amount of fertilizer N allocated to other tissues and increase whole plant fertilizer N up- take. In this study, however, the amount of fertilizer N partitioned to the fruit of mature trees would have to be more than double what was allocated to the leaves to make up for the observed foliar differences in the percentage of NDFF between young and mature tree. The data would seem to suggest that despite the additional sink represented by the fruit, the demand for external N in mature hackberry trees is less than that of young hackberry trees, with observed differences being offset through the internal cycling of previously acquired N. These findings and conclusions are consistent with Sanchez et al. (1992) who observed simi- lar declines in the use of fertilizer N in mature ‘Comice’ pears (Pyrus communis) trees and linked the reductions to the preferen- tial use of previously acquired/stored N to meet new foliar needs and/or the competition for newly acquired N by fruit tissues. CONCLUSION The combination of total % [N] and the percentage of NDFF in the aboveground tissues of mature hackberry trees, suggests an age-related change in the extent to which these trees acquire, utilize, and partition N. Mature trees utilized internal stores of previously assimilated N to a greater extent than young trees to meet the needs of N demanding tissues. Furthermore, the evidence for the preferential use of stored N in mature trees appears to be independent of the N application rate. In other words, adding more fertilizer N did not override the tendency for the preferential use of previously assimilated N. Thus, the dependence upon soil-derived sources of N does not appear to grow exponentially with increasing canopy spread associated with age/size. The chronic fertilization of mature trees that pro- duce low or infrequent fruit mass, particularly at the high end of the ANSI A-300 rates, could result in substantial amounts of fertilizer N moving into un-targeted sinks (e.g., ground water, atmosphere). It would appear from this data that nutrient man- agement regimes extrapolated from recommendations developed for young trees may not be entirely appropriate for trees of all ages. Continued research is needed, for it is unclear if mature trees will increase uptake at lower rates of application or under multiple application scenarios. Additionally, there is a need for research that incorporates destructive harvesting, a wider range of fertilizer formulations, tree species, and landscape conditions. Acknowledgments. This research was fund- ed by the International Society of Arboricul- ture’s TREE Fund, the Wisconsin Arborist Association, and a University of Wiscon- sin System Consortium grant. Use of trade names in this publication does not imply endorsement by the University of Wisconsin- Madison of products named, nor criticism of similar ones not mentioned. Technical assistance of Drs. Brian Yandell and Jun Zhu, Mr.’s. William Schmitt, Armand Krueger, and Peter Crump are gratefully acknowledged. Special consideration is given to the City of Waukesha, Wisconsin, and Mr. David Liska, City Forester, for providing research locations, equipment, and support personnel necessary to com- plete this study. From a dissertation submitted by L.P. Werner in partial fulfillment of the requirements for the Ph.D. degree. LITERATURE CITED Aerts, R. 1996. Nutrient resorption from senescing leaves of perennials: Are there general patterns? Ecology 84:597–608. American National Standards Institute (ANSI). 2011. A300 (Part 2) Soil Management a. Modification, b. Fertilization, and c. Drainage. Tree Care Industry Association, Inc., Manchester, New Hampshire, U.S. Atkinson, D., M.G. Johnson, C.M. Crisp, G. Maude, and E.R. Mercer. 1987. Variation in the natural enrichment of 15 N in apple trees and soils. Journal of the Science of Food and Agriculture 38:19–22. Cooke, J.E.K., and M. Weih. 2005. Nitrogen storage and seasonal nitrogen cycling in Populus: Bridging molecular physiology and eco- physiology. New Phytologist 167:19–30. Deng, X., S.A. Weinbaum, and T.M. DeJong. 1989. Use of labeled nitrogen to monitor transition in nitrogen dependence from storage to current-year uptake in mature walnut trees. Trees Structure & Func- tion 3:11–16. Diaz, C., T. Lemaître, A. Christ, M. Azzopardi, Y. Kato, F. Sato, J.F. Morot-Gaudry, F. Le Dily, and C. Masclaux-Dabresse. 2008. Nitro- gen recycling and remobilization are differentially controlled by leaf ©2013 International Society of Arboriculture 91
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