4 Morgenroth and Visser: Aboveground Growth Response to Porous Pavements chemicals (Marosz and Nowak 2008), and soil moisture extremes (Berrang et al. 1985). These additional stresses were not measured in this experiment, except for soil compaction and soil moisture. The latter was measured as part of a larger experiment, and it was found that soil moisture beneath pavements consistently ex- ceeded those in bare soil (Morgenroth and Buchan 2009). Given adequate soil moisture and the absence of many stresses known to afflict street trees, it is understandable why pavements alone produced no negative impacts on tree growth in this experiment. Effect of Porous Pavement on Tree Growth It has been suggested that porous pavements may play a role in improving tree growth by ameliorating underlying soil condi- tions (Ferguson 2005). This experiment confirmed tree growth can be improved by porous, rather than impervious pavement, but only in the absence of a compacted subgrade and a gravel base, where trees surrounded by porous pavements were taller and had greater stem diameter and aboveground biomass than trees surrounded by impervious pavement (Table 1, contrast 4). It would be easy to suggest that differences in growth must be associated with the permeability of the porous pavement, and thus, higher soil moisture. However, Mor- genroth and Buchan (2009) found that soil moisture did not differ beneath porous and impervious pavements. Therefore, other explanatory factors must be considered. Knowing that increased growth occurred only in plots without a gravel base and compacted subgrade suggests that the benefits proffered by porous pavement are superseded by some factor associated with the profile design. One possibility is that soil compaction counteracted the effects of porous pav- ing. Soil compaction is at odds with the requirements of trees, whereby highly compacted soils are known to negatively im- pact tree growth (Smith et al. 2001). In this experiment, soil penetration resistance in IP+ and PP+ plots was 2410 kPa. In contrast, soil in IP and PP plots had mean penetration resis- tance of only 841 kPa. In soils similar to those in this experi- ment, values between 2000 kPa and 3000 kPa are sufficient to hinder root development (Sinnett et al. 2008). Thus, it is likely the compacted subgrade restricted root development, thereby negating the positive effects of porous pavement ex- hibited in plots without a compacted subgrade and gravel base. The theory that soil compaction negates the benefits provided by porous pavements could have critical implications for future porous pavement installations. The vast majority of pavements are designed to bear heavy loads and are thus underlain by highly com- pacted subgrade and base layers. Accordingly, the use of porous pavement for tree growth amelioration may be limited to areas such as sidewalks and low-use parking lots, unless steps are taken to minimize soil compaction. One way to minimize soil compaction is to use specially designed pavement profiles, whereby the pave- ments are engineered to withstand heavy loads, while avoiding soil compaction with the use of suspended pavements or accommodat- ing compaction in soil design, such as with CU-Soil™ . While these alternatives have been proven to perform as intended (Smiley et al. 2006; Buhler et al. 2007), their prevalence is presently restricted. CONCLUSION In summary, the present experiment revealed differences in tree growth resulting from differing pavement types and pro- ©2011 International Society of Arboriculture file designs. Results indicate that pavements, in the absence of other stresses, do not cause reduced tree growth, even if the profile design includes a compacted subgrade and gravel base. It was also concluded that porous pavement could improve the aboveground growth of trees relative to those grown in impervi- ous pavement settings. However, this was dependent on the ab- sence of a gravel base and subgrade compaction. This research provided a glimpse into porous pavement’s effect on Platanus orientalis, a hardy species often planted as a street tree. More research is required to determine whether the results found here are applicable in varying climates or for different tree spe- cies planted in different soil types, and surrounded by various pavement widths and configurations, such as sidewalks, roads, or plazas. Given the increased installation of porous pave- ments in the urban environment, such future research would help explain the growth and survival of trees in the urban forest. 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 Lachlan Kirk, Ni- gel Pink, Joe Cartman, Alwyn Williams, and Lisa Kulczycki for assistance with field work. LITERATURE CITED Anton, P.A. 2005. The Economic Value of Open Space. Wilder Research. ASAE Standard EP542. 2002. Procedures for using and reporting data ob- tained with the soil cone penetrometer. St. Joseph, MI. Berrang, P., D.F. Karnosky, and B.J. Stanton. 1985. Environmental factors af- fecting tree health in New York City. Journal of Arboriculture 11(6):185– 189. Brown, L.J., and J.H. Weeber. 1992. Geology of the Christchurch Urban Area. Scale 1:25,000. Institute of Geological and Nuclear Sciences Limited, Lower Hutt, NZ. Buhler, O., P. Kristoffersen, and S.U. Larsen. 2007. Growth of street trees in Copenhagen with emphasis on the effect of different establishment con- cepts. Arboriculture & Urban Forestry 33(5):330–337. Celestian, S.B., and C.A. Martin. 2004. Rhizosphere, surface, and air tem- perature patterns at parking lots in Phoenix, Arizona, U.S. Journal of Ar- boriculture 30(4):245–252. Craul, P.J. 1985. A description of urban soils and their desired characteristics. Journal of Arboriculture 11(11):330–339. Dunster, J.A. 1998. The role of arborists in providing wildlife habitat and landscape linkages throughout the urban forest. Journal of Arboriculture 24:160–167. Ferguson, B.K. 2005. Porous Pavements. Taylor and Francis Group, New York. 600 pp. Fostad, O., and P.A. Pedersen. 1997. Vitality, variation, and causes of decline of trees in Oslo center (Norway). Journal of Arboriculture 23(4):155–165. Gartner, J.T., T. Treiman, and T. Frevert. 2002. Missouri urban forest: A ten-year comparison. Journal of Arboriculture 28(2):76–83. Graves, W.R. 1994. Urban Soil Temperatures and their Potential Impact on Tree Growth. Journal of Arboriculture 20(1):24–27. Heckel, P.F. 2004. Using trees to mitigate pollution. In: Air and Waste Man- agement Association’s Annual Meeting and Exhibition. pp. 4019–4031. Iakovoglou, V., J. Thompson, L. Burras, and R. Kipper. 2001. Factors related to tree growth across urban-rural gradients in the Midwest, USA. Urban Ecosystems 5:71–85. Jim, C.Y. 1997. Roadside trees in urban Hong Kong: Part III — Tree size and growth space. Arboricultural Journal 21:73–88.
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