Does visually assessed sapwood depth lead to an underestimation of whole tree water use calculations for sessile oak?

Forest ecosystems face considerable long-term risks in the context of escalating drought and rising temperatures. Consequently, studies examining the water balance of individual trees and entire forest stands are imperative to assess potential impacts and explore potential silvicultural strategies to mitigate the effects of climate change.

A range of methods have been employed to measure tree sapflow density (SFD), including TD (thermal dissipation), TTD (transient thermal dissipation), HRM (heat ratio method), and HFD (heat field deformation). Each method comes with advantages and disadvantages. However, when calculating the total water use of trees, two additional variables to SFD must be considered. The first is the conducting sapwood depth, and the second is the xylem sapflow profile, which represents the decrease in SFD from the outer to the inner part of the conductive sapwood. In the case of oak, the literature suggests that the conducting sapwood area/sapwood depth has primarily been determined using the light transmission method in combination with coloration due to the ring-porous properties and the formation of different colored heartwood.

In this study, the HFD method was used to measure the SFD at 1 cm intervals up to a total depth of 7.5 cm in Quercus petraea (Matt.) Liebl. (sessile oak). In addition, the sapwood depth of each tree was assessed via the light transmittance method in combination with heartwood coloring and yielded an average sapwood depth of 2.7 ± 0.6 cm for the measurement trees within a diameter range from 31.5 cm to 42.0 cm. These results are consistent with the results on sapwood depth reported in the literature for various oak species, which indicate and were interpreted with more or less zero sap flow with the beginning of the heartwood. However, the HFD data showed that xylem sapflow extended to an average depth of 5.5 cm with a steep logarithmic decline, but resulting in relative sapflow rates still around 30% at the visual axis between sapwood and heartwood.

Furthermore, both the actual measured sap flow profile and a sapwood depth-based model were used to calculate the whole tree water use. Within a range of daily water use per tree of 6 to 60 L d^-1, the calculation based on the measured sap flow profile was on average 19.3 ± 0.6 % higher than with the sapwood-based profile. This daily offset of about 20 % is particularly important when calculating the water use of trees and stands under good weather conditions with high sap flow rates.