Below-ground hydraulic constraints during drought-induced decline in Scots pine
Below-crown hydraulic resistance, a proxy for below-ground hydraulic resistance, increased during drought in Scots pine, but larger increases were not associated to drought-induced defoliation. Accounting for variable below-ground hydraulic conductance in response to drought may be needed for accurate predictions of forest water fluxes and drought responses in xeric forests.
Hydraulic deterioration is an important trigger of drought-induced tree mortality. However, the role of below-ground hydraulic constraints remains largely unknown.
We investigated the association between drought-induced defoliation and seasonal dynamics of below-crown hydraulic resistance (a proxy for below-ground hydraulic resistance), associated to variations in water supply and demand in a field population of Scots pine (Pinus sylvestris L.)
Below-crown hydraulic resistance (rbc) of defoliated and non-defoliated pines was obtained from the relationship between maximum leaf-specific sap flow rates and maximum stem pressure difference estimated from xylem radius variations. The percent contribution of rbc to whole-tree hydraulic resistance (%rbc) was calculated by comparing stem water potential variations with the water potential difference between the leaves and the soil.
rbc and %rbc increased with drought in both defoliated and non-defoliated pines. However, non-defoliated trees showed larger increases in rbc between spring and summer. The difference between defoliation classes is unexplained by differences in root embolism, and it is possibly related to seasonal changes in other properties of the roots and the soil-root interface.
Our results highlight the importance of increasing below-ground hydraulic constraints during summer drought but do not clearly link drought-induced defoliation with severe below-ground hydraulic impairment in Scots pine.
KeywordsDrought Forest mortality Hydraulic resistance Rhizosphere Roots Xylem pressure
The authors would like to thank J. Barba, P. Meir, M. Mencuccini, H. Romanos, A. Palomares, Y. Salmon and T. Hölttä for their help with the experimental design and fieldwork. We are also very grateful to the staff from the Poblet Forest Natural Reserve for their support to our research at ‘Barranc del Tillar’.
This work was funded by competitive grants CGL2010-16373, CGL2013-46808-R and CGL2014-55883-JIN from the Spanish Ministry of Economy and Competitiveness. D.A. was funded by a FPU predoctoral fellowship (AP2010-4573).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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