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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.
