Sapwood-Heartwood isotopic composition in four forest species: implications for isotopes studies

Obtaining reliable estimates of isotopic composition of xylem water transported in tree stems is crucial for ecohydrological studies. In most tree species xylem consists of two physiologically different parts, sapwood and heartwood. The former functions as the flowpath for sap flow, whereas the latter does not conduct water and provides mechanical support to the stem. However, some studies highlighted that water stored in heartwood might sustain transpiration by providing water during dry periods. Therefore, assessing how the isotopic composition in sapwood and heartwood compartments changes over time is critical to explain tree hydraulic.

Typically, studies rely on wood cores from the tree trunk to isotopically characterize xylem water in order to assess water sources for tree use (e.g., soil water from different depths, groundwater, stream water or a mixture of those), and only few studies specified which functional portion of the wood was sampled. There is currently a lack of knowledge on the possible isotopic difference between heartwood and sapwood potentially leading to uncertainties on the origin of the extracted water.

In the present study, we investigate four forest species characterised by different xylem anatomy, wood density, and timing of physiological activity to evaluate the degree of differentiation in isotopic composition between sapwood and heartwood.

We carried out biweekly sampling campaigns over one growing season (March-October 2020) in a central European forest (Luxembourg) to assess sapwood and heartwood isotopic composition and water content of European beech (Fagus sylvatica), sessile oak (Quercus petraea), douglas fir (Pseudotsuga menziesii), and spruce (Picea abies).

Preliminary results showed a temporal variation in isotopic composition both in sapwood and heartwood for all investigated species. In conifers, we found a stronger difference in isotopic composition and water content between sapwood and heartwood compared to broadleaved species suggesting a larger degree of compartmentalization. Heartwood displays consistently heavier oxygen delta values compared to sapwood in all species whereas sapwood shows heavier hydrogen delta values compared to heartwood only in conifers. These preliminary results suggest that the occurrence of potential mixing between older water stored in the heartwood and newly uptake water flowing in the sapwood should be taken into account in tree water uptake studies.