Constraints to tree growth imposed by urban soil alkalinity.

Soil alkalinity is commonly a limitation to tree growth in urban soils of the Chicago region. Associated chlorosis may sometimes be successfully treated, but longer term modification of the root environment is desirable. Selec- tion and use of trees tolerant of soil alkalinity (calciphytes) are urgent needs. Planning a harmonious tree/site system re- quires simultaneous consideration of the limitations of both elements. Alkalinization of urban soils from runoff from con- crete and limestone surfaces appears to be widespread. Resume. L'alcalinite du sol est communement une limitation a la croissance de I'arbre dans les sols urbains de la region de Chicago. La chlorose qui en decoule peut quelquefois etre traitee avec succes, mais des modifications a long terme de I'environneme nt racinaire sont souhaitables. La selection et I'utilisation d'arbres tolerants a I'alcalinit6 du sol sont ardemment dSsirees. L'alcalinisation des sols urbains caused par I'ecoulement des surfaces de beton et calcaire paratt etre repandue. The selecting of trees for urban planting in the Chicago region requires attention to two serious limitations—soil alkalinity and clayiness. This region has extensive areas of soil derived from alkaline glacial till. Urbanization often brings about a mixing of surface and deeper material producing a tree-root environment scarcely conducive to proper growth. The following discussions deal mainly with situations in the Chicago area, but in- formation and ideas are thought to have ap- plicability to urban areas elsewhere. It has been stated that more than 80 percent of urban tree problems begin in the soil (7). This premise is the basis for giving special attention to understanding how root systems can grow satisfactorily in adverse urban soils. Underground problems for trees planted in urban situations are attributable to poor drainage and root drowning, summer dehydration of roots, roots outgrowing the soil volume available to them, increasing of water and nutrient needs, and alkalinization or salinization of the soil (12). Hightshoe (2) lists ma- jor urban soil limitations as droughtiness, coarse textures, claypans, wetness, acidity, shallow depth, nutrient imbalances, compaction, toxicity, salinity, and alkalinity. All of these limitations in- dicate environmental extremes of one kind or another. Proper performance of urban trees re- quires that environmental conditions, especially those of soil environments, have qualities of eveness in which detrimental extremes rarely oc- cur. Attaining a harmonious urban tree/site relation- ship requires two interdependent and concurrent considerations: the selection of the kind of tree to be planted and the selection and/or preparation of the site to accomodate the special requirements of the tree chosen. Initially, the selection of the kind of tree should be based upon assessment of the general qualities of the site and the feasibility of modifying and ameliorating site limitations. Site selection or preparation should take into account the functional capabilities of the chosen kind of tree to tolerate site adversities. By matching and meshing the functional attributes of the tree with the topographic and soil qualities of the site, a har- monious tree/site system can be produced. The selection of the right tree for the right place should be based upon possibilities for survival and endur- ing health. Attractive trees can be selected from lists of rugged, durable, and ecologically ap- propriate trees, but lists of aesthetically appealing trees provide little basis for selecting trees with good survivability (10).