Arboriculture & Urban Forestry 37(2): March 2011 Table 1. biomass and β, the index used to measure vertical root distribution. Contrasts 1. Control versus 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. Root Abundance A comprehensive understanding of root dynamics was obtained by contrasting treatment effects on the abundance of roots of varying diameters at different soil depths. Some general trends, irrespective of root diameter class, were evident throughout the abundance data. First, within-treatment abundance gener- ally increased in each successive 5 cm soil increment through- out the uppermost 15–20 cm (depending on treatment); below this, root abundance decreased abruptly (Figure 3). Within each class of root diameter size, treatment effects were present only in the top 20 cm of soil. Abundance of roots from 20–30 cm depth was statistically similar across all treatments (Table 2). In the uppermost 20 cm of soil, all pavement treatments altered root abundance and distribution relative to control plots (Table 2, contrast 1). Vertical root distribution fol- lowed a similar pattern in both fine and medium diameter roots. From the soil surface to 10 cm depth, control plots had significantly fewer roots than paved treat- ments (Table 2, contrast 1). From 10–15 cm depth, root abundance in control plots increased, whereas mean root abundance for paved treatments remained stable. As a consequence, no significant difference was found between controls and paved plots at this depth. Finally, between 15–20 cm depth, a significant difference reemerged; however, this time control plots had a great- er abundance of roots than all paved plots. A truncated version of this pattern ex- isted for coarse roots. No difference ex- isted between controls and paved plots in the uppermost 10 cm, but then between 10–15 cm depth, greater root abundance was exhibited beneath paved plots. The converse was true from 15–20 cm, as root abundance was greater in control plots. Whereas root abundance beneath pavement treatments was greatest be- tween 10–15 cm deep, maximum root abundance in control plots did not typi- cally occur until 20 cm depth (Figure 3). This difference in root distribution was corroborated by β values, which sug- gested relatively shallow allocation of roots in paved plots and relatively deep allocation in control plots (Figure 2). The effect of pavement profile design was seen in all three root diameter classes, but only in the layer 10–20 cm beneath the soil surface (Table 2, contrast 2). In each 47 -values for single degree-of-freedom contrasts comparing the effect of pavement type and profile design on root df 1 1 1 1 pbiomass 0.062 0.013* 0.047* 0.022* pβ < 0.001* 0.002* 0.243 0.976 root diameter class, the mean root abundance for IP and PP plots exceeded that for IP+ and PP+ plots. Thus, there were a greater number of fine, medium, and coarse roots in the 10–20 cm soil depth under pavements without a compacted subgrade and gravel base. Alone, pavement type never affected root abundance (Table 2, contrast 3), implying that mean root abundance was similar beneath porous and impervious pavements. Further inspection revealed an interaction between pavement type and profile design, which affect- ed fine roots 5–15 cm beneath the soil surface (Table 2, contrast 4). Without a compacted subgrade and gravel base, porous pavements yielded significantly greater fine root abundance than impervious pavements. Conversely, when pavement profiles were designed to include a compacted subgrade and gravel base, the abundance of fine roots was greater beneath impervious pavement (Figure 3). Figure 3. The effect of pavement type and profile design on mean abundance of roots by soil depth. Top: fine roots (< 2 mm); Middle: medium roots (2–5 mm); and Bottom: coarse roots (> 5 mm). Note: the scale of the y-axis differs between plots. ©2011 International Society of Arboriculture P
March 2011
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