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Abstract: When applied to climate change-related precipitation decline, the optimal partitioning theory (OPT) predicts that plants will allocate a larger portion of carbon to root growth to enhance the capacity to access and acquire water. However, tests of OPT applied to the root system of mature trees or stands exposed to long-term drying show mixed, partly contradicting, results, indicating an overly simplistic understanding of how moisture affects plant-internal carbon allocation. We investigated the response of the root system (0-240 cm depth) of European beech to long-term decrease in water supply in six mature forests located across a precipitation gradient (855-576 mm mean annual precipitation, MAP). With reference to OPT, we hypothesized that declining precipitation across this gradient would: (H1) cause the profile total of fine root biomass (FRB; roots <2 mm) to increase relative to total leaf mass; (H2) trigger a shift to a shallower root system; and (H3) induce different responses in the depth distributions of different root diameter classes. In contradiction to H1, neither total FRB (0-240 cm) nor the FRB:leaf mass ratio changed significantly with the MAP decrease. The support for H2 was only weak: the 95% rooting depth of fine roots decreased with decreasing MAP, whereas the maximum extension of small coarse roots (2-5 mm) increased, indicating contrasting responses of different root diameter classes. We conclude that long-term decline in water supply leads to only minor adaptive modification with respect to the size and structure of the beech root system, with notable change in the depth extension of some root diameter classes but limited capacity to alter the fine root:leaf mass ratio. It appears that OPT cannot adequately predict C allocation shifts in mature trees when exposed to long-term drying.Graphical Abstract: [ABSTRACT FROM AUTHOR]