Arboriculture & Urban Forestry 35(5): September 2009 stress to similar predawn water potentials (Table 1). While the more negative predawn xylem water potentials of C. canaden- sis in brick-on-sand treatments may have contributed to the poor survival rates and foliar stress ratings relative to the oth- er treatments, it was not likely attributable to a lack of avail- able soil moisture as the brick-on-sand soils had less negative soil water potentials than any of the other treatments (Table 1). Asian jasmine plots had the next, least negative soil water potential, while rotated annuals, recycled paper, and bare soils had the most negative soil water potentials (Table 1). This might be expected for bare soils which would likely have higher rates of evapotranspiration from the uncovered soil than those with a surface cover. The plots with herbaceous annuals might also be expected to have more negative soil water potentials as the annuals would also compete for available moisture with the trees. This study observed that the recycled paper mulch tended to form a crust-like surface as it dried after wetting which may have impeded penetration of precipitation or surface applied ir- rigation. Although soil moisture was available for trees in the Asian jasmine plots, these trees had the lowest transpiration rates when measured near the end of the study (Table 1), sug- gesting some competition may have been underway between the trees and Asian jasmine for the available soil moisture. It was interesting to note that while St. Augustinegrass may have—on average across the growing season—induced some water stress relative to pine bark treatments, by the time tran- spiration was sampled in autumn (Table 1), C. canadensis in St. Augustinegrass plots had one of the greater transpiration rates. This may have been due to the slower growth of St. Augustine- grass, which is a warm season turfgrass (Duble 1996; Arnold 2008), during the cooler temperatures of autumn which would have minimized its competition for soil moisture. Increased tran- spiration and stomatal conductance of various cultivars of crape- myrtle (Lagerstroemia indica L.) in containers on Bermudagrass turf (Cynodon dactylon L.) surfaces were reported in comparison to pine bark mulched surfaces or bare soil (Zajicek and Heilman 1991); however, this represented only effects of canopy modi- fications of light or heat reflectance and/or relative humidity as no root interactions were possible between the turf and trees in their study. Green and Watson (1989) report increased trunk di- ameter and crown growth of Acer saccharinum Marsh., grown in 2.4 m (8 ft) mulched circles of composed leaves and wood chips relative to trees with an unspecified turfgrass growing up to the trunks. They also report reduced tree root density under the turfgrass relative to mulched surfaces. Growth of F. pennsyl- vanica during establishment was reported to increase in propor- tion to the surface areas of Bermudagrass turf controlled either manually, chemically, or with pine bark mulch beneath the tree canopy (Arnold and McDonald 2008). These results concurred with the results of the previous two studies in respect to mulch- es generally promoting increased tree growth; however, the St. Augustinegrass in our study did not impede the growth of C. canadensis or reduce tree transpiration (Table 1). This might be due to species differences in the trees or turfgrasses, differences in irrigation practices among the studies, or perhaps due to the time of year in which transpiration measurements were collected. Asian jasmine achieved nearly 90% soil surface coverage within three months, and St. Augustinegrass maintained a high degree of coverage (88% to 100%) until late in the season, from 71% to 72%, when temperatures cooled. Surface coverage by ro- 235 tated herbaceous annuals was variable (20% to 95%) as seasonal color changes were removed and new ones established. Herba- ceous annual color coverage was greatest in the spring transition season (95% in April with P. × hybrida) and latter portion of the summer annual season (72% to 74% from August to October with C. roseus). The extensive mass of herbaceous tissues on the an- nuals during these periods likely contributed to the drying of the soils in these treatments noted earlier (Table 1). Despite the ne- cessity of shallow soil disturbance when removing and installing color changes, no adverse effects on tree growth or transpiration were observed (Table 1). Installation of color changes was done by digging small holes as close as possible to accommodate the herbaceous plants’ root balls and these were typically small plants [six-cell packs, 158 cm3 (9.64 in3 ) in each cell]. A greater effect might have been seen on tree growth if more root zone disturbance were to have occurred if the plots with herbaceous annuals were tilled, or if herbaceous plants with larger root balls were planted. Weed cover on control plots was greatest in April (7.8% of surface area), May [19%, mostly spurge (Euphorbia L.) and dal- lis grass (Paspalum dilatatum Poir.)], and September [33%, most- ly spurge and yellow nutsedge (Cyperus esculentus L.)]. Weed cover on control plots was greater (P ≤ 0.05) than that of most other surface covers throughout the season, with the exception of June and July when weed growth was less than 2.8% on all treat- ments. Decorative gravel had the second most extensive weed invasion in April (4%), May (6.2%), and September (13.2%), but only differed statistically (P ≤ 0.05) from the other soil surface covers (aside from bare soil) in September. It is interesting to note that of surviving C. canadensis, the lowest percentage increas- es in trunk diameter were associated with the two soil surface treatments which permitted the most weed growth, even though weed growth was very low in most treatments throughout the growing season. Aside from those instances enumerated above weed coverage was between 0.2% and 5.6%, in all treatments. Gilman (1997) suggests that weed control was second only to irrigation as a cultural factor in initial establishment of landscape trees. Downer and Hodel (2001) reported that the primary benefit of mulching palms (Arecaceae C.F. Schultz) where water deficits occur might be the control of turfgrasses as competing weed spe- cies. Skroch et al. (1992) reported that several organic (pine bark, hardwood bark, cedar chips, and pine needles) and inorganic (polyethylene or polypropylene sheets) mulches were effective in reducing or eliminating weeds relative to bare soils, but they did not test recycled paper, gravel, rotated annuals or masonry surfac- es. Weed suppression may be one reason that most any of the trees with soil surface covers, aside from brick-on-sand pavers, tended to grow better than those on bare soil. However, if this were the case one would expect to see more competition effects from the living groundcover treatments or rotations of herbaceous annuals. Initial soil pH on the site prior to installation of the soil sur- face covers was 6.84±0.06 and initial soil density was 1.17±0.05 g·cm-3 (0.042±0.002 lb·in-3 ). Soil pH changed little among treat- ments during the course of the study (Table 1), varying only 0.25 pH units among all treatments, and were well within ranges suit- able for most landscape plants (Foth 1990). Bulk densities did vary significantly (P < 0.05) by the end of the study with soils under the brick-on-sand and St. Augustinegrass turf exhibiting the greatest bulk densities compared to all other treatments except pine bark (Table 1). Some tamping of the sand beneath the bricks at instal- lation and pressing in the St. Augustinegrass sod may explain the ©2009 International Society of Arboriculture
September 2009
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