Arboriculture & Urban Forestry 38(1): January 2012 sistent at one liter per day, while lysimeters with mulch was consistent at 0.5 L per day after the first day (Table 1). The net cumulative evaporation through Day 2 was not statistically dif- ferent between mulched and non-mulched treatments (Table 1). Cumulative evaporation through the third dry day after irri- gation was 0.5 L greater from non-mulched 360 L lysimeters. Mulched or not, only about one liter (range 0.5 to 2.0 L, de- pending on the day) evaporated daily from the surface of the con- tainer substrate- or native soil-filled lysimeters in each of the first three days after irrigation. However, water loss from drainage and transpiration, if trees had been growing in the lysimeters, by the end of the first day after irrigation would have required replace- ment irrigation or substantial rainfall to maintain tree vigor com- parable to that at transplanting (Gilman et al. 1998). An 8 cm cali- per maple would transpire about 28 L per day under non-stressed conditions during the late spring and early autumn in USDA Har- diness Zone 9 when this experiment was conducted (Beeson and Brooks 2008). In addition, irrigation needs of container-grown trees often increase after planting to the landscape compared to needs in the nursery because the perched water table is eliminated when the container is removed and the tree is planted (Gilman et al. 1996; Beeson and Brooks 2008). In Florida, recently-planted trees could become fatally water stressed by the third or fourth day without irrigation (Gilman 2001). Since about one liter evaporat- ed (Table 1) and an average of about 28 L could have transpired if a tree was present (Beeson and Brooks 2008), the vast majority (96%) of water would have left the root ball of a well-irrigated tree in a given day by transpiration, not evaporation. Medina et al. (2005) found similar results for ten tree species grown in contain- ers in Ohio, U.S. Comparable evapotranspiration to Beeson and Brooks (2008) was reported for five tree species of similar size in a different climate (Edwards 1986; Steinberg et al. 1990). Even if mulch could prevent all evaporation from the root ball, evaporat- ed volume appears negligible compared to the volume transpired, and is unlikely to the impact irrigation frequency or volume re- quired to maintain tree vigor and health after planting. Altland and Lanthier (2007) also showed that organic mulch applied to the surface of small (3 L, 15 cm × 15 cm) containers with Hydran- gea macrophylla Thunb. had little or no influence on evaporation. Gilman and Grabosky (2004) demonstrated that some of the negative impacts of mulch application could be attributed to in- terception of irrigation and rainfall by mulch placed on top of the root ball resulting in tree water stress from a dry root ball. Others have also observed more water stress in mulched trees of certain species in field studies (Arnold 2005). This phenomenon was confirmed by the increased evaporation from mulch-covered lysimeters (Table 1) compared to non-mulched the first day fol- lowing irrigation. This may have caused at least some of the reduced survival and growth rates in mulched plots of the cited studies. Watson and Kupkowski (1991a) also showed that roots readily grow up into mulch applied over an existing root system. This upward root growth could encourage formation of stem girdling roots on young recently planted trees and might have contributed to the death of Prunus trees from stem girdling roots in the longest study of its kind described by Wells et al. (2006). This study showed that application of mulch only resulted in about one liter less evaporation from the surface of a 360 L plug of soil or container substrate over a three-day period following irrigation. Simple extrapolation from this data suggests that much less would evaporate from a smaller container. Combined with 21 the disadvantages listed above, this makes a case for keeping the root ball surface relatively free of mulch on recently planted trees. Mulch interception of water may be more of a problem with light applications of irrigation or rain than when more volume is ap- plied, and when thick layers of certain mulches that mat togeth- er form a barrier impenetrable to water (Gilman and Grabosky 2004). Mulch made of pine needles and perhaps other materi- als with a low density and a low water-holding capacity may al- low for better water infiltration, but this has not been studied. The lysimeter protocol used in this study was not a real landscape simulation because water was only allowed to drain out the bottom, and no trees were in the lysimeters. Water in a real landscape would have been able to flow horizontally away from the root ball, which may have increased rate of water loss from the root ball and reduced evaporated water volume since the root ball could have drier quicker. The current study prob- ably better simulated a well-drained soil type and might not directly apply in soils with slow percolation rates. The pres- ence of a tree in the lysimeter would have shaded the root ball surface perhaps altering the evaporated: transpired water loss ratio, and the transpirating tree would have more rapidly dried the root ball. Water would have been held more strongly by the soil colloids which would have reduced evaporation. Fur- ther studies would need to be conducted to determine if this could have completely eliminated any of the small differences in evaporation between mulched and non-mulched lysimeters. Acknowledgments: Thanks go to The TREE Fund, GreatSouthernTreeConference.org, and the Florida Nursery Growers and Landscape Association for partial funding. LITERATURE CITED Altland, J., and M. Lanthier. 2007. Influence of container mulches on irrigation and nutrient management. Journal of Environmental Horticulture 25:234–238. Arnold, M.A. 2005. Planting depth and mulch thickness affect estab- lishment of green ash (Fraxinus pennsylvanica) and Bougainvillea goldenraintree (Koelreuteria bipinnata). Journal of Arboriculture 31:163–170. Arnold, M.A., and G.V. McDonald. 2008. Surface area and method of weed control surrounding green ash trunks affects landscape estab- lishment. Proceedings Southern Nurseryman Association Research Conference 58:391–393. Arnold, M.A., and G.V. McDonald. 2009. Groundcovers, organic and inorganic mulches, and masonry surfaces differentially affect estab- lishment and root zone characteristics of urban trees. Arboriculture & Urban Forestry 35:232–240. Asworth, S., and H. Harrison. 1983. Evaluation of mulches for use in the home garden. HortScience 18:180–182. Barajas-Guzman, M.G., J. Campo, and V.L. Barradas. 2006. Soil water, nutrient availability and sapling survival under organic and poly- ethylene mulch in a seasonally dry tropical forest. Plant and Soil 287:347–357. Beeson, R.C., Jr., and J. Brooks. 2008. Modeling actual evapotranspira- tion of Acer rubrum from a rooted cutting to an 8 m tall tree. Acta Horticulture 792:91–97. ©2012 International Society of Arboriculture
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