Amplifying effects of recurrent drought on the dynamics of tree growth and water use in a subalpine forest

We conducted an in situ rain-exclusion experiment at a subalpine tree line forest in the Austrian Alps where we exposed the two widespread European tree species European larch (Larix decidua) and Norway spruce (Picea abies) to three years of recurrent summer drought. We studied tree growth and tree water relations using high-resolution measurements of stem diameter variations and xylem sap flow as an estimate of whole tree transpiration. We combined these measurements analyses of xylem anatomy, non-structural carbohydrates and their carbon-isotopic composition.

We found that pronounced negative effects of drought on tree growth and stem radius variations from the beginning of the experiment that were related to a sustained diurnal and periodic depletion of internal stem water storage. Most importantly, while drought effects on stem radius variations did not increase, the effects on growth strongly amplified under repeated drought. Furthermore, xylem sap flow and its response to its main driver vapor pressure deficit was not limited during the first two years of the experiment, but abruptly decreased from the beginning of the third drought. While both tree species strongly responded to drought, spruce showed more a pronounced increase in the negative effects of recurrent drought than larch, indicating a higher vulnerability of this species to repeated droughts.

Altogether, our study shows that recurrent droughts progressively increase the impacts of drought. A future climate with an increased drought frequency may therefore impose strong legacies on tree and forest functioning.

Oberleitner, F., Hartmann, H., Hasibeder, R., Huang, J., Losso, A., Mayr, S., Oberhuber, W., Wieser, G., Bahn, M. (2022). Amplifying effects of recurrent drought on the dynamics of tree growth and water use in a subalpine forest. Plant, cell and Environment 45: 2617-2635. DOI: https://doi.org/10.1111/pce.14369