Arboriculture & Urban Forestry 35(5): September 2009 237 Figure 3. Summer temperatures in the root zone of Cercis canaden- sis var. texensis grown with soil (solid line) or pale blond tan (light, dotted and dashed combination line), red-brown (medium, dotted line), or dark charcoal (dark, dashed line) brick-on-sand paving surfaces. Mean high and low atmospheric temperatures for the period were 36.2°C (97.2°F) and 23.9°C (75°F), respectively. Midday xylem water potentials increased during the sea- son across treatments (data not presented), with xylem water potential differing substantially among the treatments only for C. canadensis from light colored brick treatments in August. At that time, midday xylem water potentials for trees with light bricks were more negative [-3.42 MPa (-34.2 bars)] than trees with medium or dark bricks, or bare soil [-2.1 MPa (-21 bars), -1.93 MPa (-19.3 bars), or -1.98 MPa (-19.8 bars), respectively]. Pre-dawn xylem water potentials followed a bit different pat- Figure 2. Photosynthetically active radiation (PAR) (A) and atmo- spheric temperatures (B) from near ground level into the canopy above bare soil (solid lines), or pale blond tan (light, dotted lines), red-brown (medium, dashed lines), or dark charcoal (dark, dashed and dotted combination lines) paving bricks; n = 3; brackets in- dicate standard errors of the means at each height. High, mean, and low atmospheric temperatures for July 6, 2001 were 35.6°C (96°F), 30°C (86°F), and 22.8°C (73°F), respectively. Kjelgren and Montague (1998) report substantially elevated leaf temperatures of F. pennsylvanica and Acer platanoides L. grow- ing over asphalt surfaces compared to turfgrass. Mulches may buffer soil temperature fluctuations, but mulches may also in- crease atmospheric temperatures above them by reradiating more heat or reflected light relative to soils covered with turfgrasses and various trees and shrubs respond differently to these con- ditions (Montague et al. 1998; Montague and Kjelgren 2004). Cercis canadensis grown in bare soil had greater height growth, trunk diameter increase, and a lesser proportion of the canopy exhib- iting stress symptoms than the light and medium brick surface trees (Table 2). Height growth and stress symptoms were also adversely affected by dark brick surfaces (Table 2). Although the mean soil water potential for the bare soil and dark brick treatments were more negative than the light brick treatments, differences were modest, averaging from -4.9 to -6.4 kPa (-0.049 to -0.064 bars) indicating that insufficient soil moisture alone may not be responsible for the growth differences found for C. canadensis among treatments. tern. Averaged across the season (Table 2), pre-dawn xylem water potentials indicated that the best recovery (least negative water po- tential) from the previous day’s water stress was with trees grown in bare soil, while the light colored brick trees had the poorest recovery. Pre-dawn water potentials for trees with medium and dark bricks was intermediate (Table 2). Soil water potentials were actually more negative under trees in bare soil than under those with brick-on-sand treatments, but the trees with bare soils still exhibited better pre-dawn recovery (less negative xylem water po- tentials) than those with brick-on-sand treatments (Table 2), sug- gesting that some factor other than soil moisture availability was likely involved in the responses. It is of interest to note that only trees in the bare soil treatments survived into the fall with low levels of canopy stress symptoms. All C. canadensis in the brick treatments were either dead or severely declining when the study was terminated in late fall. Thus, no quantitative measurements on the root systems of C. canadensis were made as the study was terminated. However, poor root growth out of the planted root ball was observed as the trees were removed from the plots to prepare for the next experiment (Figure 4). Results of this study suggested that any reductions in root zone heating with lighter col- ored brick pavers were insufficient to off-set the effects of greater light / heat reflection into the canopy by the lighter colored pav- ers. However, due to the early loss of the trees, these results were tentative. Thus, the entire study was repeated using a more urban tolerant tree for our region, Taxodium distichum (Arnold 2008). ©2009 International Society of Arboriculture
September 2009
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