118 Symes and Connellan: Water Management Strategies for Urban Trees Soil Surveys Comprehensive soil surveys (Van Rees et al. 1993) are also an important part of the landscape planning process. Soils are the foundation of existence for so many life forms, and yet often they are taken for granted, or poorly studied or understood, in the urban landscape. In many landscape projects, the emphasis is on plan- ning the hard landscape structures, services, and infrastructure, but when it comes to soil analysis and design, planning is inade- quate or sometimes non-existent. It would not bear contemplation to request a civil engineer to avoid measuring the bearing capacity of a soil for a building, or cutting corners in safety specifications for structural integrity under varying conditions. The same impor- tance must be placed on soils. Performance specifications, struc- tural integrity, and long-term sustainability are also the language of robust landscape soils, and this is best informed by soil surveys. Soil properties that should be considered primarily in relation to water management of trees include: a. bulk density and soil strength b. porosity, total water holding capacity, and plant available water (including soil moisture release curve) c. particle size analysis (to determine risk of compaction) d. soil texture and structure e. infiltration rates and hydraulic conductivities (for both topsoil and subsoil) f. sodium absorption ratio (to determine risk of soil particle dispersion (poor drainage and aeration) from water sup- plies containing more sodium. g. electrical conductivity (to determine risk from the use of more saline water supplies) Although very challenging, developing a better under- standing of the biochemical and physical characteristics of the site soil are crucial for informed tree management. Tree Selection for Dry Sites Any assumptions about taxa adapted to periods of aridity need to be reassessed against projected climatic changes. For example, in Australia, there is a strong interest in Mediterra- nean flora on the assumed basis that these species are drought tolerant due to months of very minimal rainfall, particularly over the summer, in their natural habitat (Dallman 1998; Peel et al. 2007). However, Mediterranean climates are usually char- acterized by significant winter precipitation (Dallman 1998; Peel et al. 2007), which may also recharge groundwater and subsoil moisture levels. Phreatophytes are plants that either rely on or access ground water for their needs (Sommer and Froend 2011) and can be found in Mediterranean climates both in Australia (Sommer and Froend 2011) and California (Mahall 2009). Californian oaks, such as Quercus agrifolia (coast live oak) and Q. lobata (valley oak), are considered to be phreatophytes (groundwater-using) that have the capacity to tap groundwater for survival over drought periods (Mahall 2009). Specifically, Quercus lobata has been reported to access moisture from depths as great as 24 meters (Howard 1992). In European Mediterranean climates, David et al. (2007) studied Quercus ilex ssp. rotundifolia (holm oak) and Q. suber (cork oak) in southern Portugal and found that more than 70% of the trees’ transpiration was sourced from groundwater at 4–5 m ©2013 International Society of Arboriculture depths. Projections of climatic changes for Melbourne indicate a significant reduction in winter-spring rainfall (CSIRO 2008), which can increase the risk of reducing subsoil and groundwa- ter moisture reserves for Mediterranean-climate-adapted trees. In terms of current water management for trees, it can be useful to use simple graphing techniques to compare the range of trees that are within or outside typical annual precipitation ranges. Graphical summaries of a study of the annual precip- itation requirements of some trees growing in the RBG Mel- bourne showed a significant rainfall deficit between the annual minimum rainfall requirement and the mean rainfall during 1999–2011, of 544 mm, for the site. Figure 3 shows the rainfall deficiency, minimum annual rainfall requirements compared to mean rainfall, for a selection of 34 eucalypts growing at the site. Figure 4 shows the deficiency, graphed in increasing annual rainfall requirement, for more than 80 Australian native species. The difference may be up to 750 mm for some individual spe- cies. While some of this deficit is currently being met by artifi- cial precipitation (irrigation), the RBG has set an upper baseline target of 900 mm per year for combined rainfall and irrigation amounts. It is unlikely that this could be sustained into the long term against current climatic projections and resource availabil- ity. As a baseline, tree selection should incorporate water require- ments that are within the typical annual rainfall requirements for the proposed site including some variation for climatic change and low rainfall years such as decile 1, or lowest 10% events. LANDSCAPE PLANTING WATER DEMAND Trees and Landscape Planting Water Demand Estimation Evolution of irrigation scheduling in urban landscapes has progressed from time-based programming to a more sophis- ticated application of a greater spread of inputs, such as cli- matic data, evapotranspiration estimation methodologies, soil moisture sensing, and increasing knowledge of plant per- formance. However, plant water use in the urban landscape is still considered to be inadequately understood (Symes et al. 2008). Furthermore, a greater emphasis on water use efficiency and the insecurity of water supply presented by greater regulation and restrictions has increased the interest in priority setting of water allocation. This preferential irri- gation is usually based on the perceived values or expecta- tions of quality given to different areas or components of the urban forest. The setting of subjective quality standards in urban horticulture has generally been a vexing and conten- tious dilemma, let alone linking these standards to irriga- tion scheduling for various landscape performance levels. There are various methodologies for estimating plant evapotranspiration (ETc). Two terms that are com- monly used are Crop Factors (CF) and Crop Coeffi- cients (Kc) (Allen et al 1998; Connellan and Symes 2006). Plant water demand expressions use a reference evapora- tion value together with the crop adjustment factor to esti- mate the water use rate. The following expressions are used: [1] ETc = Crop Coefficient (Kc [2] ETc = Crop Factor (CF) × Pan Evaporation (Epan ) × Reference Evapotranspiration (ETo ) )
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