Assessing a trees’ stem water potential is essential to evaluate the tree’s performance during drought. Particularly for mature trees, it is, however, extremely hard to monitor water potential throughout the growing season because of the difficulty of canopy access. Daily variations of stem diameter (SDV) could provide a powerful alternative. SDV have been shown to incorporate both radial growth and the diurnal shrinkage and swelling of bark tissue, which is caused by daytime transpiration and nighttime refilling, respectively. During dry periods, bark tissue that is depleted in water cannot entirely refill at night, leading to a progressive overall shrinkage of the tree’s stem diameter and allows calculating values for tree water deficit (TWD). Comprehensive tests, if SDV-based values for TWD reflect values for a tree’s stem water potential are yet missing for mature trees. As such, TWD has not yet been fully established as a simple and continuous proxy for a trees’ water status. Using a canopy crane we calculated an SDV-derived TWD for six Central European forest tree species during one moist (2014) and one exceptionally dry (2015) growing season and compared these values to the trees’ stem water potential. For this purpose, we employed six different mathematical approaches.
Results/Conclusions
We found a tight overall relationship between stem water potential and TWD in all six species. Most of the six approaches that we used to calculate TWD yielded TWD values that resulted in significant relationships with stem water potential (p < 0.001) for the different species. However, only one TWD variable had the highest explanatory power (R2) across the six species and both years (up to 86 % explained variation). Intriguingly, this variable does not account for radial growth during periods of shrinkage in its calculation indicating that plastic growth is impeded in such times. The relationship between TWD and stem water potential can best be explained by a logistic function. We propose that this function in combination with SDV measurements can be employed to estimate stem water potential of trees for an entire growing season. We conclude from our data that TWD is tightly correlated to the stem water potential of mature tree species and can, thus, be used to describe continuous seasonal variations in a tree’s water status. Because of its relatively easy application and deployment, there is high potential for this method to play a major role in future investigations of tree water relations.