How do pine trees guard against drought?

Do young pines build up food reserves at the expense of growth to enable them to survive longer in the event of a drought? This controversial hypothesis is refuted by a new study carried out by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL). In the experiment, the trees swiftly adapted to an artificial drought and were equipped to combat it the following year.

Climate change could make life uncomfortable for conifers in some parts of Switzerland, with experts expecting such trees to be more likely to die during dry spells. Indeed, many pines in the Canton of Valais died in the wake of droughts in 2003, 2011 and 2016. One possible reason could be starvation, the idea being that when the trees close the stomata in their needles to reduce water loss, this also makes them unable to absorb their 'food', carbon dioxide (CO₂).

For this reason, forest managers and foresters are wondering whether they should plant replacement trees that are better adapted to drought, for example Mediterranean species.

In an experiment, a WSL research team led by Christoph Bachofen, joined by colleagues from the ETH Zurich and the University of Basel, tried to 'kill two birds with one stone', first by testing how two-year-old Scots pines and black pines of different origins, ranging from the Alps to the Mediterranean, cope with lengthy droughts. For two summers, the trees were given no water between June and September – a scenario climate models suggest will become more frequent in the future.

Second, the scientists tested the controversial hypothesis that pines actively store carbohydrates at the expense of growth in order to stave off starvation when stressed by drought. This stemmed from the repeated observation that, when under drought stress, pine trees store more carbohydrates in the form of starch. However, according to the WSL's researchers, foregoing growth to guard against drought only makes sense in regions where lengthy droughts can be expected virtually every summer, as in the Mediterranean. In temperate zones, neighbouring trees that did not restrict their own growth would outgrow any more stunted competitors.

Does more CO₂ arm trees better against drought?

To find out, the researchers fed the most dry-stressed pines additional carbon in the form of CO₂. If the trees were actively storing starch at the expense of growth, these starch reserves should clearly increase and thereby improve the trees’ chances of surviving a drought.

Yet the research team now reports in the Journal of Ecology that neither did the pines fill their starch reserves with the additional CO₂ , nor were the trees' chances of survival found to depend on the extent of those reserves. Furthermore, pines of southern origins also showed no 'growth versus starch storage' trade-off. In fact, they both stored more starch and grew more robustly than pine trees in humid regions. "Black pines from dry regions generally coped better with drought", explains Thomas Wohlgemuth, Team Leader of the WSL's Disturbance Ecology research group.

"Our results contradict the theory that pine trees actively lay down carbon reserves to draw on during droughts", the researchers explain. Nonetheless, they did observe a change in the young trees: pines of southern origin and northern pines both formed shorter needles after the first year of drought. This way the trees reduced evaporation, and all of them survived the second dry year. "The conclusion to be drawn here is that, up to a point, young pines can acclimatise to resist a moderate increase in summer drought", the researchers write.