Numerous hypotheses have been proposed concerning how forest tree root systems might respond to increases or decreases in precipitation (and evapotranspiration) that may accompany global climate changes. Changes in root biomass production, root:shoot ratios, rooting depth, root density, and rates of fine root turnover have all been considered. A review of the literature on “long-term” effects of changes in soil moisture regimes on tree seedlings and young tree plantations (30+ studies) reveals that indeed most tree species examined are somewhat plastic with respect to net C allocation to root systems. Root:shoot ratios and/or the rooting depth of woody plants may increase with reductions in soil moisture, and either or both may decrease with enhancements. Results of our own five-year study on the impact of reductions and increases in precipitation on root-related variables in a mature upland hardwood forest are only partially consistent with this literature. Many limitations to the plasticity of root systems exist and need to be considered when attempting to predict long-term forest changes to climate change. Both the evidence in these studies and the limitations of their application to forest ecosystems will be discussed.