214 Lugo-Perez and Lloyd: Ecological Implications of Organic Mulches breaks (Matson and Waring 1984; Oren et al. 1985; Waring et al. 1987). This hypothesis was based on studies that examined moun- tain pine beetle outbreaks (Dendroctonus ponderosae). In these studies, stand wood production per unit of leaf area increased after thinning and fertilizing. Contrary to the GDB hypothesis, Waring and Pitman (1985) proposed that trees prioritize photosynthate al- location as follows: new foliage > new roots > storage > diameter growth > chemical defenses. Under this assumption, trees growing under dense canopy are more susceptible to herbivores because they receive lower levels of photosynthetic active radiation and overall produce less photosynthate. Therefore, the amount of pho- tosynthate available for synthesis of chemical defenses become limited and trees become more susceptible to herbivores. Waring and Pitman (1985) and Waring et al. (1992) recommended stand fertilization and thinning as pest control management strategies. Because of forest fertilization studies, fertilization has been adopted in urban landscapes as a managements practice to im- prove pest resistance properties on urban trees (see Herms 2002). This recommendation, clearly contradicts Lorio’s (1986) point of view of a physiological trade-off between photosynthate al- location to growth or to synthesis of chemical defenses. Lorio (1986) used the GDB hypothesis to explain how the synthesis of oleoresin [principle defensive chemicals against bark bee- tles (Ruel et al. 1998)] can be reduced during periods of rapid growth, creating an opportunity for bark beetle attack. Lorio’s proposition was supported by studies using a variety of bo- tanical taxa (Bryant et al. 1987, Bryant et al.1988, Hunter and Shultz 1995, Villalba et al. 2002). These studies showed that nutrient enhancement delayed long-term induced chemical de- fenses and improved the palatability of foliage to herbivores. IMPLICATIONS FOR ARBORICULTURE The International Society of Arboriculture recommends ap- plying 5 to 10 cm of organic mulches around trees to improve aesthetics and edaphic conditions in the landscape (ISA 2004). The reviewed literature in this paper supports this recommenda- tion. This study also concludes that organic mulches can directly affect soil water and N availability with indirect consequences for plant photosynthate allocation. Plant physiological response is species specific and depends on mulch material, and on soil properties such as fertility, water holding capacity and pH. How- ever, the information compiled in this paper also illustrates gen- eral physiological and ecological consequences for plants after altering edaphic conditions through mulching. These general consequences are illustrated in the conceptual model (Figure 1). It is also suggested that the controversy between Waring’s and Lorio’s points of view (i.e., photosynthate allocation prior- ity versus photosynthates trade-off between growth and chemi- cal defenses) is present in urban systems. Furthermore, that the experimental evidence supporting the use of fertilizer to enhance tree pest resistance is scarce and inconclusive. In fact, evidence suggests fertilization enhances insect performance by redirect- ing the resources allocated to defense and/or by increasing the palatability of the host plant (Herms 2002). These contradictory results reveal the necessity for additional studies that will help us understand the implication of mulching on tree physiological re- sponses that affect their ecological, aesthetic, and economic value. Acknowledgments. The authors thank Robert Tripepi, Yaniria Sanchez-de Leon and two anonymous reviewers for their suggestions. We also thank the TREE Fund (John Duling Grant), the University of Idaho’s (UI) Still- inger Grant Program, and the UI’s Department of Plant, Soil and Ento- mological Sciences for financial and logistic support. LITERATURE CITED Acquaah, G. 2004. Horticulture: Principles and practices. Prentice Hall, Upper Saddle River, New Jersey. pp. 744–760. Aerts, R., 1989. The effect of increased nutrient availability on leaf turn- over and above-ground productivity of two evergreen ericaceous shrubs. Oecologia 78(1):115–120. Aerts, R., and F.S. Chapin, III. 2000. The mineral nutrition of wild plants revised: a re-evaluation of processes and patterns. Advances in Eco- logical Research 30:1–67. Aerts, R., and H. De Caluwe. 1997. Nutritional and plant-mediated controls on leaf litter decomposition of Carex species. Ecology 78(1):244–260. Aerts, R. and H. Decaluwe 1989. Above ground productivity and nutrient turnover of molinia-caerulea along an experimental gradient of nutri- ent availability. Oikos 54(3):320–324. Appleton, B.L., J.F. Derr, and B.B. Ross. 1990. The effect of various landscape weed control measures on soil moisture and temperature and tree root growth. Journal of Arboriculture 16(10):264–268. Barrett, J.E., and I.C. Burke. 2000. Potential nitrogen immobilization in grassland soils across a soil organic matter gradient. Soil Biology and Biochemistry 32(11-12):1707–1716. Brady, N.C., and R.R. Weil. 1999. The nature and properties of soils, 12th. New Jersey, Upper Saddle River. Bryant, J.P., T.P. Clausen, P.B. Reichardt, M.C. McCarthy, and R.A. Wer- ner. 1987. Effect of nitrogen fertilization upon the secondary chem- istry and nutritional value of quaking aspen (Populus tremuloides Michx.) leaves for the large aspen tortrix (Choristoneura conflictana (Walker)). Oecologia 73(4):513–517. Bryant J.P., J. Tuomi, and P. Niemela. 1988. Environmental constrain of constitutive and long-term inducible defenses in woody plants. Chemical Mediation of Coevolution. K. Spencer. San Diego, Califor- nia, USA, Academic Press: 376–389. Buerkert, A., A. Bationo, and K. Dossa. 2000. Mechanisms of residue mulch induced cereal growth increases in West Africa. Soil Science Society of America Journal 64(1):346–358. Bussiere, F., and P. Cellier. 1994. Modification of the soil temperature and water content regimes by a crop residue mulch: experiment and modeling. Agricultural and Forest Meteorology 68(1-2):1–28. Chapin, F.S., III. 1991. Integrated responses of plants to stress. Biosci- ence 41(1):29–36. Chapin, F.S., III. 1995. New cog in the nitrogen cycle. Nature 377(6546):199–200. Christiansen E., R.H., Waring, and A.A. Berryman. 1987. Resistance of conifers to bark beetle attack: searching for general relationships. Forest Ecology and Management 22(1-2):89–106. Cook, H.F., Valdes G.S.B., and Lee H.C. 2006. Mulch effects on rainfall interception, soil physical characteristics and temperature under Zea mays L. Soil & Tillage Research 91(1-2):227–235. ©2009 International Society of Arboriculture
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