234 Arnold and McDonald: Groundcovers, Mulches, and Surfaces Affect Tree Establishment aid of the manufacturers to reduce variation in physical and chem- ical properties, aside from color, to the greatest extent possible. Irrigation was applied as explained in the previous study. Soil moisture potential was measured weekly from April through September using tensiometers (Model 2725 JetFill Tensiom- eters, Soil Moisture Equipment Corp., Santa Barbara, CA). Pre-dawn and midday xylem water potentials were determined monthly during the growing season for each tree using a pres- sure chamber as previously described. Tree height, trunk diam- eter at 15.2 cm above the soil surface, and foliar stress ratings were recorded monthly during the growing season. Temperatures in the top 10.2 cm of soil or sand, and in the atmosphere at 1.3 cm (0.5 in) above the surface of the soil or bricks, were measured as previously described during July 9–13, 2001. Reflected vis- ible light (LI-185B and LI-210SP photometric sensor, LI-COR, Lincoln, NE) and atmospheric temperatures at 5.1, 15.2, 30.4, and 90 cm (2, 6, 12, and 36 in, i.e.; from ground level to just below the canopy) were recorded for three plots from each treat- ment between noon and 2:00 pm on July 6, 2001. In these cases, sensors were aimed downward facing the soil surface at the ap- propriate heights to measure reflected and horizontally diffused light, rather than skyward when measuring ambient sunlight. The experiment was discontinued after the first year due to exces- sive loss of redbud trees in the brick treatments near the end of the year. Data was analyzed as described in the first experiment. Baldcypress Responses On November 28, 2001, the brick-on-sand experiment was rees- tablished utilizing Taxodium distichum as a test species, due to its ability to better withstand adverse conditions in the region (Arnold 2008). Temperatures in the top 10.2 cm of soil or sand, in the at- mosphere at 2.54 cm (1 in) above the surface of the soil or bricks, were measured as previously explained from November 15–24, 2002. Growth, stem xylem water potentials, and soil water poten- tials were monitored as previously described, except that an entire stem tip was used for xylem water potential determinations with baldcypress in place of the single leaves used with the Texas red- buds. Additionally, in December 2002, bricks, sand, and soil were excavated to a depth of 20.3 cm (8 in) and roots sifted from the substrates outside of the originally planted root ball. These roots were dried [three days at 50°C (122°F)] and a mean regenerated root density (mass per unit volume) was calculated for each plant- ing pit. Data were analyzed as described in the first experiment. RESULTS AND DISCUSSION Study 1: Organic Mulches, Inorganic Mulches, and Living Groundcovers Impact Eastern Redbud Establishment Survival was 100% during the first year, while five trees were lost during the second year, one each in the bare soil, ornamental ag- gregate, and T. asiaticum plots, while two from the brick-on-sand plots were lost. Also, a third tree had nearly defoliated by autumn in the brick-on-sand plots. Interestingly, all of the soil surface cov- ers resulted in a greater height and trunk diameter growth for sur- viving trees of C. canadensis than the bare soil treatment (Table 1), except brick-on-sand which had similar height growth as the bare soil treatment. This was the case even with those treatments where living groundcovers were used that might have competed with the trees for water and nutrients. Pine bark mulch resulted in lower percentages of the canopy of C. canadensis exhibiting fo- liar stress symptoms in comparison to recycled paper or brick-on- sand treatments (Table 1). While all trees survived during the first growing season after transplant; on average, 90% of the canopy of trees in brick-on-sand plots exhibited symptoms of foliar stress (e.g., chlorosis, marginal necrosis or abscission). Litzow and Pellett (1983) reported better growth of Fraxinus pennsylvanica Marsh., with several types of organic mulches compared to bare soils. Trees grown in pine bark mulch also had less negative mean midday xylem water potentials throughout the season than those grown in bare soil, or with recycled paper, St. Au- gustinegrass, decorative gravel, or brick-on-sand treatments (Table 1). However, all except the trees in brick-on-sand treat- ments recovered from the prior day’s maximal midday water Table 1. Effects of soil surface covers on tree growth, physiological measures, and soil moisture potential. Growth of Texas redbuds were compared in simulated 1.1 m x 1.8 m (3.5 ft x 6 ft) planting pits with surfaces covered with bare soil or one of seven combinations of organic, inorganic or living groundcovers. 1st Year Tree Soil surface treatment Bare soil Pine bark mulch Recycled paper Asian jasmine St. Augustinegrass Rotated annuals Decorative gravel Brick-on-sand height (cm) 183 bvu 207 a 209 a 200 ab 223 a 215 a 208 a 185 b Trunk diameter (mm) 28.6 bvu 34.7 a 33.6 a 33.8 a 35.4 a 34.5 a 35.3 a 34.7 a w Initial soil density on the site was 1.17±0.05 g·cm-3 v Values represent mean of 5 observations. Foliar stress symptoms (% of canopy) 42 bctu 30 c 76 ab 66 abc 52 abc 38 bc 54 abc 90 a (0.42±0.002 lb·in-3 ). u Means followed by the same letters within a column are not different at P < 0.05 level using least squares means procedure. t Values represent mean of 40 observations. s Values represent mean of 15 observations. Xylem water potentialz Midday (MPa) -2.35 btu -1.99 a -2.30 b -2.14 ab -2.34 b -2.21 ab -2.27 b -2.28 b z Values are means of eight monthly observations from each of five plots per treatment. y Transpiration was measured on October 21, 1999. x Initial soil pH on the site was 6.84±0.06. Predawn (MPa) -0.50 abxu -0.41 a -0.43 a -0.44 a -0.44 a -0.45 a -0.46 a -0.60 b Soil waterz potential Transpirationy (kPa) (mol·m-2 -15.4 detz -13.3 cd -16.2 de -9.9 b -13.1 bcd -17.9 e -10.8 bc -6.6 a ·s-1 3.778 abcsu 4.560 ab 3.473 bc 2.885 c 4.856 a 4.718 ab 3.702 abc 3.670 abc pHx H2 O) 6.80 ab 6.81 ab 6.67 b 6.92 a 6.91 a 6.85 a 6.89 a Soil (pH units) (g·cm-3 6.88 av Bulk densityw ) 1.14 bv 1.20 ab 1.12 b 1.10 b 1.31 a 0.94 c 1.14 b 1.28 a ©2009 International Society of Arboriculture
September 2009
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