How ecosystems adapt to climate changes depends in part on how individual trees allocate resources to their
components. A review of research using tree seedlings provides some support for the hypothesis that some tree
species respond to exposure to drought with increases in root[ratio ]shoot ratios but little change in total root
biomass. Limited research on mature trees over moderately long time periods (2–10 yr), has given mixed results
with some studies also providing evidence for increases in root: shoot ratios. The Throughfall Displacement
Experiment (TDE) was designed to simulate both an increase and a decrease of 33% in water inputs to a mature
deciduous forest over a number of years. Belowground research on TDE was designed to examine four hypothesized
responses to long-term decreases in water availability; (1) increases in fine-root biomass, (2) increases in fine
root[ratio ]foliage ratio, (3) altered rates of fine-root turnover (FRT), and (4) depth of rooting. Minirhizotron root
elongation data from 1994 to 1998 were examined to evaluate the first three hypotheses. Differences across
treatments in net fine-root production (using minirhizotron root elongation observations as indices of biomass
production) were small and not significant. Periods of lower root production in the dry treatment were
compensated for by higher growth during favorable periods. Although not statistically significant, both the highest
production (20 to 60% higher) and mortality (18 to 34% higher) rates were found in the wet treatment, resulting
in the highest index of FRT. After 5 yr, a clear picture of stand fine-root-system response to drought exposure
has yet to emerge in this forest ecosystem. Our results provide little support for either an increase in net fine-root
production or a shift towards an increasing root[ratio ]shoot ratio with long-term drought exposure. One possible
explanation for higher FRT rates in the wet treatment could be a positive relationship between FRT and nitrogen
and other nutrient availability, as treatments have apparently resulted in increased immobilization of nutrients in
the forest floor litter under drier conditions. Such hypotheses point to the continued need to study the interactions
of water stress, nutrient availability and carbon-fixation efficiency in future long-term studies.