Arboriculture & Urban Forestry 35(4): July 2009 Arboriculture & Urban Forestry 2009. 35(4): 211–217 211 Ecological Implications of Organic Mulches in Arboriculture: A Mechanistic Pathway Connecting the Use of Organic Mulches with Tree Chemical Defenses Javier Lugo-Perez and John E. Lloyd Abstract. In addition to the aesthetic and practical benefits of mulching, studies have shown indirect benefits of organic mulch- es to tree establishment and growth. These indirect benefits are associated with direct improvements on soil water and nutri- ent availability by mulches. The generalization of the organic mulches benefit to soil and trees has been questioned by several studies showing contradictory results under different experimental conditions and mulching materials. In addition, overall ben- efits for trees may be overlooked by focusing studies on some aspects of plant performance (e.g., plant growing rate) while ig- noring others (e.g., plant chemical defense). This paper reviews studies showing how organic mulches can directly affect plant resource availability in the soil, presenting evidence from the literature that illustrates the influence of organic mulches on plant resource availability can also affect tree photosynthate allocation dynamics with direct consequences on plant chemi- cal defenses. Based on the reviewed literature, presented here is a mechanistic pathway to illustrate how organic mulches can influence plant resources in the soil, and in turn how that can affect tree physiology and tree-insect interactions in urban areas. Key Words. Growth-Differentiation Balance; Nitrogen Availability; Photosynthate Allocation; Water Availability. Organic mulches are extensively used in urban systems to improve plant establishment and development (Acquaah 2004). Mulch is defined as “any material such as straw, sawdust, leaves, plastic film, and loose soil that is spread on the surface of the soil to protect the soil and plant roots from the effects of raindrops, soil crusting, freezing, and evaporation” (Brady and Weil 1999). This definition is based on mulch-substrate physical properties that benefit mostly soil structure and water content. Beside the physi- cal benefits of mulching, this practice can increase or decrease the amount of inorganic N in the soil depending on the carbon (C) to nitrogen (N) ratio (Melillo et al. 1982; Herms et al. 2001; Lloyd et al. 2002; Erhart and Hartl 2003). These alterations to the soil, that directly affect tree growth, can also affect the synthesis of chemi- cal defenses and as consequence plant-herbivore interactions. This paper proposes a mechanistic pathway to explain how the impact of organic mulches applied to the soil surface can influ- ence patterns of tree resource uptake, C assimilation and C al- location between growth and chemical defenses. Present here is a model that illustrates pathway connections and identifies the three most likely affected processes: (i) soil resource availability (water and N), (ii) plant C and N uptake, and (iii) plant C alloca- tion between growth and the production of secondary compounds (Figure 1). The objective of this paper is to compile scientific evidence, pertinent for arboriculture, showing direct and indirect relationships among mulch application, soil resource availabil- ity and plant physiological responses. This review should in- crease awareness among arborists and scientists in the tree care industry to improve the utilization of organic mulches and pro- mote further experimental studies in urban landscape scenarios. CONCEPTUAL FRAMEWORK Effect of Organic Mulches on Resource Availability Organic mulches can affect the soil availability of two important resources for plants: water and N. Benefits of mulches to soil water content have been associated with the physical properties of mulches to buffer soil temperature (Mbagwu 1991; Bussiere and Cellier 1994). During days with high temperatures, a mulch layer protects soil surface from direct solar radiation. As a con- sequence, soil temperature remains cooler than air temperature and less water evaporates from the soil. Iles and Dosmann (1999) reported that 2–10 cm (0.79–4 in) of organic mulches (pine bark, pine wood, and hard wood chips) were more effective in reduc- ing soil surface temperature [by ~2ºC (3.6ºF)] and increasing soil gravimetric water content [by ~0.2 g kg-1 (3.2x10-3 oz lb-1 )] compared to inorganic mulches (pea gravel, lava rock, river rock and crushed red bricks) or bare soil. Furthermore, Appleton et al. (1990) found that, during summer days, when soil covered with fabric or plastic film to protect them from weed establishment were also mulched, soil temperature was lower and water con- tent was higher than nonmulched soils independently of the weed control treatment. Although the influence of mulching on soil water content is frequently associated with reduction in soil tem- perature, De Vleeschauwer et al. (1980) showed that mulching can also improve water content by enhancing biological activity of soil macrofauna (e.g., earthworms) that increases soil porosity. Despite the numerous studies reporting positive benefits of organic mulches on soil water content, other studies have also ©2009 International Society of Arboriculture
July 2009
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