62 Bryan et al.: Transplant Season, Irrigation, and Planting Depths on Landscape Establishment DISCUSSION in baldcypress and sycamore, and survival in sycamore. Bald- cypress performed better when planted with their root collars at grade or below grade. Planting sycamore root collars below grade resulted in increased mortality and also reduced height and trunk diameter when compared to planting root collars at grade or 7.6 cm above grade. Arnold et al. (2007) reported that planting with the root collar 7.6 cm below grade adversely af- fected the growth of crapemyrtle (Lagerstroemia indica L. × Lagerstroemia fauriei Koehne. ‘Basham’s Party Pink’), green ash (Fraxinus pennsylvanica Marsh.), oleander (Nerium olean- der L. ‘Cranberry Cooler’), sycamore (P. occidentalis), and vi- tex (Vitex agnus-castus L. ‘LeCompte’), although the severity varied depending on species. Planting above grade was benefi- cial for sycamore when compared to planting at or below soil surface grade (Arnold et al. 2007). Arnold et al. (2007) reported that various ornamental trees planted at grade or above grade outperformed trees planted below grade, however in this study baldcypress performed better when planted with their root collars at grade or below grade. Baldcypress is reported to be tolerant to low oxygen/anaerobic soil conditions (Kozlowski and Davies 1975) and may also form adventitious roots in response to con- tinuously saturated soils (Davidson et al. 2006). This may explain why baldcypress was not negatively affected by planting the root collars below grade. Planting baldcypress with root collars above grade in the present study was detrimental to overall growth, perhaps due to a wicking effect on the exposed portion of the root ball which may have decreased root ball moisture content. Wells et al. (2006) suggested that when Yoshino cherry (Pru- Planting depth significantly affected RGRheight and RGRdiameter nus × yedoensis Matsum.) trees were planted with their root col- lars below grade they may have experienced reduced water infil- tration or insufficient access to shallow mineral nutrient pools, causing nutrient deficiencies and increasing tree mortality. Arnold et al. (2005) suggested that in denser soil, planting the root collar deep may result in poor growth, possibly as a result of decreased soil moisture and/or oxygen. Planting deeply in heavy soils can cause the roots to experience prolonged flooding conditions after a period of heavy rainfall, resulting in hypoxic conditions. Dur- ing drought periods, tree roots planted deeply in heavy soils may have a reduced ability to extract water as stronger adhesive forc- es of smaller pore spaces decrease the soil water potential, thus reducing the amount of plant available water compared to less heavy soils. Kozlowski and Pallardy (1997) reported that stomata in sycamore readily close under anoxic soil conditions resulting in reduced respiration and growth. However, Tang and Kozlowski (1982) and Tsukahara and Kozlowski (1985) observed the forma- tion of swollen lenticels on stems of sycamore seedlings in re- sponse to anoxic conditions, which may enhance absorption and translocation of oxygen to the roots. Thus, sycamore trees may be able to adapt to periodically flooded soils. Arnold et al. (2007) reported that planting sycamore with root collars 7.6 cm below grade resulted in 50% mortality when planted in a fine sandy loam underlain at a depth of 15 to 30 cm by a hard clay pan. In the pres- ent study, planting young sycamore trees with their root collars below grade may have caused the roots to suffer periods of hypox- ic conditions interspersed with periods of drought while the root system was not developed yet, resulting in 53% mortality within 12 months, even in the well-drained sandy loam at this study site. . The study authors suggest this may have been a result of an unusually wet spring/summer in 2006 (24 mm above the 30 year average from March to September, Office of the Texas State Climatologist), and that winters are fairly mild in the southern United States. Richardson-Calfee et al. (2007) reported that with proper maintenance of soil moisture, autumn and spring trans- planting resulted in similar root regeneration as summer planting/ transplanting of sugar maple (Acer saccharum Marsh.) (Virginia site). In addition, Harris et al. (2001) reported that, with proper irrigation, autumn or spring planting/transplanting resulted in similar growth (height and diameter) and rate of root length ac- cumulation in Turkish hazelnut (Corylus colurna L.) (Virginia site). In contrast, Shoemake and Arnold (1997) reported that autumn transplanting resulted in better growth and survival of sycamore than spring transplanting, which in turn was better than summer transplanting (Central Texas site). Low survival of autumn-transplanted seedlings was related to a low physiological potential for root regeneration at that time of year (Larson 1984, Ohio site) and the inability of new transplants to grow roots in cold soils (Jenkinson 1980, Western Sierra Nevada site). How- ever, these varied and contrasting results may depend on plant species/ecotype and/or geographic factors, including climate. In this study, sycamore growth was not significantly affected RGRdiameter Transplanting season did not affect baldcypress RGRheight by irrigation treatment. This could be due to wetter than normal conditions during the growing season (2006), as temperature did for the 2007 growing season. It is unlikely that excessive tem- peratures would explain the difference in growth between years. Temperature-wise, the 2006 and 2007 growing seasons (March to September) were not unusual, and were on average only 0.8°C (1.5°F) and 0.06°C (0.7°F), respectively, warmer than average (Office of the Texas State Climatologist, Department of Atmo- spheric Sciences, Texas A&M University, College Station, TX). The trees experienced wetter than normal conditions during June (2007) and July (2006, 2007), when drought conditions would normally be likely to occur. Total precipitation during the grow- ing season (March–September) was 24 mm above average in 2006, and 77 mm above average in 2007 (Office of the Texas State Climatologist, Department of Atmospheric Sciences, Texas A&M University, College Station, TX). From May to mid July 2006, there were large rainfall events followed by prolonged drought, while in 2007, individual rainfall events were smaller, but distributed evenly during the summer season (Figure 1). It is possible that excessive amounts of water during the growing sea- son in 2007 periodically reduced oxygen availability to the roots and produced hypoxic conditions that slowed down plant growth, but it did not stop plant growth altogether. Alternatively, trees could have been established by the second year, or in a growth lag phase. Given the general lack of an irrigation effect in this study, the growth responses of sycamore to planting depth during the first year were likely due to factors other than soil moisture. not deviate much from the normal averages. Average RGRheight and RGRdiameter CONCLUSION Relative growth rate of height and trunk diameter of baldcypress was significantly affected by planting depth, but not by transplant season. Planting trees with root collars at soil grade or slightly be- low soil grade (7.6 cm) in a sandy loam soil produced taller trees ©2010 International Society of Arboriculture were greater for the 2006 growing season than or
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