Arboriculture & Urban Forestry 37(1): January 2011 3 Table 1. Single degree-of-freedom contrasts comparing the effect of pavement type and profile design on total stem height and diameter growth, as well as aboveground biomass. Contrasts 1. Control vs. all other treatments 2. Main effect (pavement profile design) 3. Main effect (pavement type) 4. Interaction (pavement profile design x pavement type) * P < 0.05 Diameter Growth Stem diameter growth was also dependent upon treatment (P < 0.001). Unlike height growth results, the mean diameter growth for all pavement treatments exceeded that for control trees (Table 1, contrast 1). While the effect of pavement profile de- sign and pavement type were both significant, diameter growth depended on their interaction (Table 1, contrast 4). Diameter growth gains provided by porous pavement were limited to plots without a gravel base and subgrade compaction, as pave- ment type did not effect change in IP+ or PP+ plots (Figure 2). Biomass Aboveground biomass was dependent on treatment (P < 0.001) (Fig- ure 3). Mean biomass for control trees was lower than the mean of treated trees (Table 1, contrast 1). While both pavement type and profile design main effects were significant, these factors exhibited a strong interaction (Table 1, contrast 4). Pavement type had no effect in plots with a compacted subgrade and gravel base, but trees in PP plots had significantly greater shoot biomass than trees in IP plots (Figure 3). DISCUSSION Within the context and limitations of this experiment, the results show: 1) impervious pavements alone do not re- strict or compromise tree growth, relative to control plots; and 2) relative to control plots and impervious pave- ments, porous pavements can improve tree growth, but only in the absence of a compacted subgrade and gravel base. Effect of Pavement on Tree Growth It’s important to deconstruct pavements into their primary constitu- ents, the pavement surface, and the underlying structural layers in- cluding a subbase, subgrade, and base. Many of the problems faced by street trees are generally ascribed to pavements without distin- guishing between the surface or underlying structural layers. In the 1970s, it was commonly believed pavement surfaces caused reduc- tions in soil moisture by precluding infiltration (Roberts 1977); how- ever, more recent research has indicated that street trees may suffer from too much, rather than too little water (Berrang et al. 1985), though it’s unclear whether this is due to the surface or underlying soil compaction. Pavement surfaces have also been said to increase air (Whitlow and Bassuk 1988) and soil (Graves 1994; Celestian and Martin 2004) temperatures above levels for optimal physio- logical function. Furthermore, soil compaction beneath pavements has been linked to poor plant performance (Smiley et al. 2006). As pavements are associated with soil moisture extremes, excessive soil and air temperature, and root-limiting soil compaction, it is easy to understand why decline in street trees is ascribed to pavements. In spite of this, none of the pavement treatments in this experi- ment negatively influenced tree growth relative to controls. In fact, df 1 1 1 1 Pheight 0.542 0.083 0.033* 0.046* Pdiameter 0.009* 0.001* 0.041* 0.015* Pbiomass 0.007* 0.0003* 0.004* 0.001* Figure 2. The effect of pavement type and profile design on mean trunk diameter growth of Platanus orientalis relative to control plots characterized by bare soil. Values represent total growth following two growing seasons. Error bars represent one standard error. Figure 3. The effect of pavement type and profile design on mean shoot biomass of Platanus orientalis relative to control plots char- acterized by bare soil. Values represent total growth following two growing seasons. Error bars represent one standard error. tree height, diameter, and aboveground biomass were equivalent, or greater, in pavement-treated trees relative to control trees (Table 1, contrast 1). This suggests trees do not necessarily suffer from reduced growth and vigor as a direct result of overlying pavements. It is not disputed that street trees in paved areas are often associ- ated with reduced growth rates and low survival, as this is well established (Gartner et al. 2002). However, pavements themselves are not necessarily the direct cause of tree decline. An alternative explanation is that street trees suffer from the compounding stress- es often associated with pavements, such as restricted soil volume (Kopinga 1991), soil compaction (Philip and Azlin 2005), physi- cal injuries to the stem and branches (Fostad and Pedersen 1997), air pollution (Su and Sun 2006), soil pollution via salt or other ©2011 International Society of Arboriculture
January 2011
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