Unrevealing tree carbon allocation beyond the stress – a case study of heat and drought impacts on Pinus sylvestris

The resistance of trees to stress events has been studied intensively, however we know little on underlying processes affecting the recovery of trees following stress release. Hence, this clearly impairs our ability to project the resilience of trees and forests to an intensification of heatwaves and drought spells.

Here we studied the legacy effects of heat and heat-drought stress on carbon (C) allocation dynamics in Scots pine. We were particularly interested in how C allocation changes post heat and heat-drought stress and how this change in allocation affects tree growth. We exposed Pinus sylvestris seedlings to increasing temperatures from 30 to 40°C within 18 days either under well-watered or drought conditions and measured stem growth, leaf water potential and above- and belowground gas exchange. Two days after stress release, we conducted a ¹³CO₂ pulse-labelling experiment in custom build single tree cuvettes (n=18) allowing us to continuously monitor ¹³CO₂ shoot and root gas exchange. We then chased the fate of the newly assimilated C from leaves to roots via soluble sugars, starch and cellulose.

Our results showed that Pinus sylvestris is able to recover gas exchange following heat release immediately in the well-watered trees, while drought-treated trees recovered slightly slower. We found indications for a stress compensatory response of the previously heat-treated trees, which tended to translocate recent assimilates faster compared to the control trees as identified in the dynamics of water-soluble carbon in the phloem and root ¹³CO₂ efflux. In addition, we found larger stem growth rates in the heat-treated trees which was also reflected by a larger investment of new assimilates to cellulose. In the trees that experienced both, heat and drought stress, C allocation differed strongly from the control trees as apparent in a half as fast C translocation from leaves to root respiration and large investments of new assimilates into starch. This delayed translocation but enhanced allocation towards C storage in needle tissues was reflected in a delayed recovery of stem growth and very low detection of the ¹³C signal in twig, root and stem cellulose. We can conclude that heatwaves of 40°C have relatively moderate responses on C allocation post-stress, whereas hot drought stress clearly affects C allocation as indicated by a delayed C transport capacity and a preferential allocation towards C storage in needle tissues. This could indicate that C allocation following hot drought stress is affected by an impaired phloem functionality, which only slowly recovers post-stress.