238 Arnold and McDonald: Groundcovers, Mulches, and Surfaces Affect Tree Establishment Table 2. Effects of soil surfaces on tree growth and soil water potential. Growth of Cercis canadensis var. texensis and Taxodium distichum were compared in simulated 1.1 m x 1.8 m (3.5 ft x 6 ft) planting pits with bare soil to surfaces covered with paving bricks of three colors (pale blond tan, red-brown or dark charcoal) on a 5.1 cm (2 in) base of builder’s sand. Cercis canadensis var. texensis Trunk Soil treatment Bare soil Pale blond tan bricks Red-brown bricks Dark charcoal bricks Heightz (cm) 235 awv 215 bc 200 c 219 b Diameterz (mm) 24.8 awv 21.3 b 21.6 b 24.3 a Portion of the canopy with stress symptomsy (%) 28 cwu 44 ab 56 a 42 b z Shoot and root growth measures are values from the end of the study. y Canopy stress symptoms and predawn xylem water potentials are means of five sample dates during growing season. x Soil water potentials are means of 14 sample dates during growing season. w Means followed by the same letters within a column are not different at P < 0.05 level using least squares means procedure. v Values represent mean of 5 observations. u Values represent mean of 25 observations. t Values represent mean of 70 observations. Alternatively, the differential water deficits between the bare soil plots and those with brick-on-sand pavers may have been too small to have induced differential growth responses, since the measured soil moisture deficits indicated reasonably moist soil conditions across treatments as would be expected given the irrigation regime. Taxodium distichum was more tolerant of the brick-on-sand treatments than was C. canadensis (Table 2) which is consistent with its purported better tolerance to urban sites (Gilman 1997; Arnold 2008). Several species native to swampy locations, like T. distichum, have been reported to have correla- tions among their ability to tolerate flooding and their response to supraoptimal root-zone temperatures as measured by growth, biomass accumulation, and survivorship (McLeod et al. 1986). Temperature patterns during late autumn, after leaf drop, in Figure 4. Examples of root balls of Cercis canadensis var. texen- sis removed from plots in bare soil (left) compared to root balls from brick-on-sand plots (right). Baldcypress Responses Surface treatments by date interactions were not significant (P ≤ 0.05) for tree height, trunk diameter, or root density, nor were the main effects of soil surfaces on tree height or trunk diameter growth for T. distichum (Table 2). No significant (P ≤ 0.05) differences among soil surface treatments, nor interactions over time, were found in predawn or mid-day water potentials for T. distichum (data not presented). However, there was a signifi- cant main effect of the soil surface treatments on root density and soil water potentials (Table 2). Soil under the lightest and darkest colored bricks had significantly (P ≤ 0.05) less negative water potentials than the bare soil treatment, but trees in these treat- ments did not grow larger. Despite drier soils in the control plots T. distichum grew similarly to those with brick-on-sand treat- ments. A possible explanation for this may be that trees in bare soil plots had a greater ability to access the available moisture in soil outside the original planting hole, as the control’s trees had greater root density outside of the planted root ball than the dark- est brick-on-sand treatment (Table 2). In general, there appeared to be a declining root density with darker brick colors (Table 2). ©2009 International Society of Arboriculture the soils under T. distichum were very intriguing, revealing the important impacts of sun exposure to the surfaces (Figure 5). Un- der overcast skies on the first three days, almost no differences in soil temperatures were apparent among the various brick-on-sand treatments and bare soil (Figure 5). However during the remain- ing days represented in the graph (Figure 5), sunny skies and still nights prevailed resulting in as great or greater warming effects in soils of brick-on-sand treatments in comparison to bare soil as was observed for summer months in soils (Figure 3), under C. canadensis. Temperature modification objectives using vari- ous soil surface covers under trees might be quite different de- pending upon the climate. For instance, Litzow and Pellett (1983) report increased growth of F. pennsylvanica under mulches which raised soil temperatures in a cold climate (Minnesota), whereas in our work the opposite appears to be the case in a warm cli- mate (Texas) with soil surface covers that raise the temperature. These experiments indicate most organic and living soil sur- face covers were preferable to bare soils for tree establishment; however, some inorganic surface covers which are frequently uti- lized in suburban and urban environments could be detrimental to tree growth and survival relative to bare soil. The apparent lack of strong competition effects among living groundcovers and trees in this study may have been attributable to the well-watered and fertilized conditions, whereas living groundcovers may have ex- erted a greater competition effect under more resource limiting conditions. Paving surfaces, such as brick-on-sand pavers can ad- versely affect shoot and/or root growth, but the magnitude of such Soil water potentialx (kPa) -6.4 awt -5.5 bc -4.9 c -5.8 ab Predawn xylem water potentialy (MPa) -0.26 cwu -0.83 a -0.48 b -0.55 ab Trunk Heightz (cm) 291 awv 269 a 289 a 297 a diameterz (mm) 70.0 awv 71.0 a 71.3 a 68.8 a Root densityz (mg@cm-3 1.11 awv 0.76 ab 0.68 ab 0.56 b ) Taxodium distichum
September 2009
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