Journal of Arboriculture 31(2): March 2005 99 CARBON STORAGE BY URBAN TREE CULTIVARS, IN ROOTS AND ABOVE-GROUND Andra D. Johnson and Henry D. Gerhold Urban trees can favorably affect factors underlying global warming by storing carbon and by reducing energy needs for cooling and heating buildings. To estimate carbon stored in roots and aboveground portions of trees, data were collected consisting of whole tree sampling of Amelanchier, Malus, Pyrus, and Syringa cultivars. Roots were excavated using an Air-Spade®. Regression analysis resulted in two equations for predicting total carbon storage based on height and diameter of trees up to 20 cm dbh: Y = 0.05836 (dbh2 Y = 0.0305 (dbh2 × h)0.9499 ) for root carbon storage, and for aboveground carbon storage, explaining 97% and 96% of the variation, respectively. Average carbon stored in roots of various cultivars ranged from 0.3 to 1.0 kg for smaller trees, those 3.8 to 6.4 cm dbh, to more than 10.4 kg for trees 14 to 19.7 cm dbh. Average total carbon stored by cultivars ranged from 1.7 to 3.6 kg for trees less than 6.4 cm dbh to 54.5 kg for trees larger than 14 cm. The data from these equations apply mainly to trees in nurseries and recently transplanted trees. Comparisons showed that aboveground estimates from previous studies using a sampling technique overestimated values obtained from actual above-ground weights. (Urban For. Urban Green. 2003. 2:065–072) NICHES IN THE URBAN FOREST: ORGANIZATIONS AND THEIR ROLE IN ACQUIRING METROPOLITAN OPEN SPACE Jane A. Ruliffson, P. H. Gobster, R.G. Haight, and Francis R. Homans As a response strategy to minimize the impacts of urban sprawl, public and private organizations are striving to acquire open lands that will contribute to a resilient and multifunctional urban forest. In the Chicago metropolitan region, we interviewed representatives of 15 organizations to understand the land acquisition process—the structures and functions of groups involved, their acquisition goals, and the cooperation among groups as they work to build metropolitan green infrastructure. Our findings reveal strength in diversity—a variety of groups working at different levels with complementary goals can help meet the complex challenges of land protection in rapidly urbanizing areas. J. For. 2002. 100(6):16–23) THE EVOLUTIONARY IMPLICATIONS OF EXPLOITATION IN MYCORRHIZAS Keith N. Egger and David S. Hibbett Some views of mutualism, where the fitness of two symbiotic partners is higher in association than when apart, assume that they necessarily evolve toward greater benefit for the partners. Most mutualisms, however, seem prone to conflicts of interest that destabilize the partnership. These conflicts arise in part because mutualistic outcomes are conditional, depending upon complex interactions between environmental, developmental, and genotypic factors. Mutualisms are also subject to exploita- tion or cheating. Although various compensating mechanisms have been proposed to explain how mutualism can be main- tained in the presence of exploiters, none of these mechanisms can eliminate exploitation. In this paper we explore various compensating mechanisms in mycorrhizas, examine the evidence for exploitation in mycorrhizas, and conclude that mycorrhizal mutualisms exhibit characteristics that are more consistent with a concept of reciprocal parasitism. We propose that researchers should not assume mycorrhizas are mutualis- tic based upon structural characteristics or limited functional studies showing bilateral exchange and should view mycorrhi- zas as occupying a wider range on the symbiotic continuum, including commensalism and antagonism. We recommend that comparative studies of mycorrhizas incorporate other types of root associations that have traditionally been considered antagonistic. (Can. J. Bot. 2004. 82(8):1110–1121) HOST PLANT QUALITY AND FECUNDITY IN HERBIVOROUS INSECTS Caroline S. Awmack and Simon R. Leather Host plant quality is a key determinant of the fecundity of herbivorous insects. Components of host plant quality (such as carbon, nitrogen, and defensive metabolites) directly affect potential and achieved herbivore fecundity. The responses of insect herbivores to changes in host plant quality vary within and between feeding guilds. Host plant quality also affects insect reproductive strategies: Egg size and quality, the allocation of resources to eggs, and the choice of oviposition sites may all be influenced by plant quality, as may egg or embryo resorption on poor-quality hosts. Many insect herbivores change the quality of their host plants, affecting both inter- and intraspecific interactions. Higher-trophic level interactions, such as the performance of predators and parasitoids, may also be affected by host plant quality. We conclude that host plant quality affects the fecundity of herbivorous insects at both the individual and the population scale. (Annu. Rev. Entomol. 2003. 48:505–519) ©2005 International Society of Arboriculture
March 2005
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