Arboriculture & Urban Forestry 37(2): March 2011 The rooting environment for trees in IP+ and PP+ plots includ- ed a compacted subgrade with mean soil strength of 2411 kPa. Since soil strengths of between 2000–3000 kPa are known to limit root growth (Sinnett et al. 2008), the compacted subgrade in IP+ and PP+ plots may account for lower observed mean root abundance. Another effect of compaction is reducing soil aeration; this is because macropores are lost in favor of micro- pores, which are preferentially filled by water rather than air. As previously explained, low soil aeration can cause shallow root growth, which may explain the relatively lower β values, and hence shallower root distribution, for IP+ and PP+ plots. From the perspective of tree growth, pavement profile de- signs without a compacted subgrade and gravel base are pref- erable as these resulted in greater root abundance from 10–20 cm soil depth. Porous pavements were even more advantageous than impervious pavements given this profile design as they re- sulted in greater root biomass, as well as enhanced aboveground growth (Morgenroth and Visser 2011). From the perspective of minimizing conflicts between roots and pavements, the effect of porous and impervious pavements were similar; however, pavement profile designs that include a gravel base are prefer- able to those without. This is because, in this experiment, the inclusion of a compacted subgrade and gravel base resulted in fewer roots and, while those roots were significantly shallower within the soil profile, the gravel base ensured that they were never nearer than 20 cm from the underside of the pavement. CONCLUSION It is understood that although root architecture is under genetic control, the soil environment in which trees grow can influence root growth and distribution (Pritchett 1979). In this experiment, the soil environment was overlaid by pavement treatments. Previ- ous research has shown that pavements can significantly impact a soil’s physical characteristics such as soil moisture and aeration (Morgenroth and Buchan 2009), and temperature (Graves 1994; Wagar and Franklin 1994). These impacts offer a plausible expla- nation to the measured treatment effects, including greater root abundance and shallow root distribution in paved relative to un- paved plots; decreased root abundance below IP+ and PP+ plots relative to IP and PP plots; and greater root biomass beneath porous pavements relative to impervious pavements in the absence of a compacted subgrade and gravel base. Future studies should test the effect of these treatments on different tree species grown in vari- ous soil types and climates to validate and generalize these results. Acknowledgments. Funding for this research was provided by the New Zealand School of For- estry, TREE Fund, and the Auckland City Coun- cil. The authors wish to thank Graeme Buchan, Lachlan Kirk, Nigel Pink, Joe Cartman, Neil Smith, Alwyn Williams, and Lisa Kulczycki for assistance with field work. Finally, the authors are grateful to two anony- mous reviewers who greatly improved the quality of the manuscript LITERATURE CITED Barker, P.A. 1988. Proactive Strategies to Mediate Tree-Root Damage to Sidewalks. In Combined Proceedings, The International Plant Propa- gators’ Society. pp. 56–61. Crawley, M.J. 2007. Count Data. In: The R Book. John Wiley and Sons Ltd., West Sussex, England. pp. 527–548. D’Amato, N.E., T.D. Sydnor M. Knee, R. Hunt, and B. Bishop. 2002. Which comes first, the root or the crack? Journal of Arboriculture 28(6):277–282. Day, S.D., J.R. Seiler, and N. Persaud. 2000. A comparison of root growth dynamics of silver maple and flowering dogwood in compacted soil at differing soil water contents. Tree Physiology 20(4):257–263. Dittert, K., J. Wötzel, and B. Sattelmacher. 2006. Responses of Alnus glutinosa to anaerobic conditions - Mechanisms and rate of oxygen flux into the roots. Plant Biology 8(2):212–223. Gale, M.R., and D.F. Grigal. 1987. Vertical root distributions of northern tree species in relation to successional status. Canadian Journal of Forest Research 17:829–834. Gilman, E.F. 1990. Tree Root Growth and Development. I. Form, Spread, Depth and Periodicity. Journal of Environmental Horticul- ture 8(4):215–220. Graves, W.R. 1994. Urban Soil Temperatures and their Potential Impact on Tree Growth. Journal of Arboriculture 20(1):24–27. Hillel, D. 1998. Soil Temperature and Heat Flow. In: Environmental Soil Physics. Academic Press, San Diego, CA. pp. 309–339. Kopinga, J. 1994. Aspects of the damage to asphalt road pavings caused by roots. In: The Landscape Below Ground. Proceedings of an In- ternational Workshop on Tree Root Development in Urban Soils. G. Watson and D. Neely (Eds.). International Society of Arboriculture, Champaign, IL. pp. 165–178. Marini, R.P. 2003. Approaches to Analyzing Experiments with Facto- rial Arrangements of Treatments Plus Other Treatments. HortScience 38(1):117–120. Misra, R.K., C.R.A. Turnbull, R.N. Cromer, A.K. Gibbons, and A.V. La- Sala. 1998. Below- and above-ground growth of Eucalyptus nitens in a young plantation: I. Biomass. Forest Ecology and Management 106(2–3):283–293. Morgenroth, J., and G.D. Buchan. 2009. Soil moisture and aeration be- neath pervious and impervious pavements. Arboriculture & Urban Forestry 35(3):135–141. Morgenroth, J., and R. Visser. 2011. Aboveground Growth Response of Platanus orientalis to Porous Pavements. Arboriculture & Urban For- estry 37(1):1–5. Nadezhdina, N., and J. Čermák. 2003. Instrumental methods for studies of structure and function of root systems of large trees. Journal of Experimental Botany 54(387):1511–1521. Nicholson, G. 1984. Methods of soil, plant and water analysis. Forest Research Institute. 70. Nicoll, B.C., and A. Armstrong. 1998. Development of Prunus root sys- tems in a city street: Pavement damage and root architecture. Arbori- cultural Journal 22(3):259–270. Nicoll, B.C., and M.P. Coutts. 1997. Direct damage by urban tree roots: Paving the way for less damaging street trees. In: Arboricultural Prac- tice, Present and Future. J. Claridge (Ed.). Department of the Envi- ronment, Transport and the Regions, Norwich, UK. pp. 77–84. Perry, T.O. 1982. The ecology of tree roots and the practical significance thereof. Journal of Arboriculture 8(8):197–211. 49 ©2011 International Society of Arboriculture
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