Arboriculture & Urban Forestry 42(3): May 2016 infiltration (Gilman and Grabosky 2004; Arnold et al. 2005; Hanslin et al. 2005), while no detrimental effects were found aſter the application of 0.45 m of a coarse mulch (wood chip) over the soil in which the roots of trees were growing (Watson and Kupkowski 1991). Among the organic material suitable as mulches, compost is an excellent candidate. In fact, composting also decreases environmental problems related to the management of wastes by their reduc- ing volume and by killing potentially dangerous organisms (Sæbø and Ferrini 2006). While green wastes from gardens and parks previously made up the largest proportion of materials for composting, recently other materials have become even more im- portant: separated household wastes, organic wastes from agriculture and the food-processing industry, among others. Thus, the use of composted waste as mulch in urban green areas may fit strong sustain- ability criteria by creating resources from waste. Urban green areas can benefit from the availability of compost products because of the large potential for improvement of growing conditions of trees and shrubs cultivated in urban soils low in organic mat- ter (Sæbø and Ferrini 2006). Potential benefits and disadvantages of mulching with compost largely depend on compost quality, stability, and maturity (soil pollution, salinity and anoxia are likely to occur if these requirements are not satisfied, for details, see Sæbø and Ferrini 2006), but also on the size of par- ticles and carbon to nitrogen (C:N) compost ratio, which greatly affect mineralization rate (Cogger 2005). In previous experiments conducted in nurs- ery plots, mulching with coarse, high C:N compost provided an effective control of weeds and positive effects on soil chemico-physical traits and plant growth (Ferrini et al. 2008a; Ferrini et al. 2008b). In a similar experiment, mulching with fine, low C:N, had positive effects of soil temperature, soil water availability, soil biological activity, and plant growth, but was less efficient in controlling weeds (weeds started to grow on the compost layer in the next growing season aſter compost was laid, and re-distribution of compost was required) (Fini and Ferrini 2011). Clearly, the fast mineralization of fine, low C:N compost provides nutrients that can trigger a fast growth rate of the weeds germinated on the mulch layer. Typically, mulching with fine compost controls weeds for no longer than one year, while using coarse compost allows a satisfactory weed control up to three years (Ferrini et al. 2008b). 193 While extensive studies on mulching with com- post have been carried out in temperate areas, lim- ited information is available for zones with warmer and/or drier climates. Similarly, the study authors are not aware of field experiences in urban sites with very dry summers, and so the purpose of the present study was to investigate the use of mixed compost as mulching material and its possible influence on growth and physiology of elm (Ulmus) trees planted in a Mediterranean urban park, with summers char- acterized by limited rainfall and high temperatures. MATERIALS AND METHODS In late winter 2009, uniform 1.5 m tall and 1.0 cm diameter cutting-propagated elm trees of clone FL634 (a hybrid elm clone selected by IPSP-CNR for Dutch elm disease resistance) (Santini et al. 2010) were planted in an urban park located in Florence, Italy (43°46N, 11°15E). Mean tempera- tures, rainfall, and ETP data for Florence over the period 1981 to 2010, and of the years when the experiment was conducted (2009–2011), were pro- vided by LaMMA Consortium (LaMMA 2014). Mean temperatures and rainfall in Florence over the period 1981–2010 were 14.9°C and 872.6 mm year-1 , respectively. However, during the years of the present experiment, an increase in mean tempera- tures were recorded (16.1°C versus 14.9°C) as well as a decrease in rainfall in the first and third year (784.4 mm and 588.8 mm, respectively) with an extended dry period with almost no rainfall from mid-June to the end of August (5 mm in 2009 and 51.2 in 2011), while 2010 was quite rainy. Similarly, the summer of 2010 was characterized by a 25% lower potential evapotranspitation (ETP) if compared to 2009 and 2011 (data not shown). Lower rainfall and higher ETP resulted in summer 2009 and 2011 being much drier than summer 2010, as also shown by the net balance between rainfall and ETP (Figure 1). The native substrate was the result of an excavation done 15 years before for the construction of buildings nearby, and it was a silty clay loam soil, according to Juma (1999), with a very low nutrient content (Table 1). Two-year-old plants, having nearly identical size characteristics, were obtained from the Institute for Sustainable Plant Protection of the National Research Council of Italy (IPSP-CNR). Trees were planted in a single row, spaced 5 m from each other. Within the row, mulching treatments were ©2016 International Society of Arboriculture
May 2016
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