Abstract: Warming induced advancements of leaf-out timing has outpaced the slower advance of stochastic occurrences of freezing events in many parts over Europe. As a consequence, many tree species have shifted their spring phenology into a period where freezing temperatures are likely to exceed species-specific resistances, increasingly causing complete and large-scale canopy defoliations –visible even on satellite images. Here we examine the penalty of damaging spring frost events on saplings as well as their recovery potential in relation to different leaf-out timing.
In February 2019 we placed 240 saplings of Prunus avium, Carpinus betulus. Quercus robur and Fagus sylvatica in a cooling chamber to control and manipulate leaf-out timing. Transferring cohorts of saplings to a warm climate chamber resulted in 4 leaf-out dates per species, spanning/covering the potential range of natural leaf emergence. In addition, another cohort of saplings was exposed to ambient conditions. Shortly after leaf-unfolding each cohort was exposed to 2 intensities of a simulated frost aiming at partly or fully killing the leaves, before planted outside under a shading net simulating forest understory conditions at the research facility near Zurich. Re-greening, biomass increment, NSC-reserves and senescence in relation to unfrozen control saplings were assessed and analysed.
Most species survived the simulated frost event except for ~30% of Carpinus saplings. Short term recovery in terms of canopy regreening occurred within a month and reached ~40% (Fagus, Carpinus) to 80% (Quercus, Prunus) of the leaf area of unfrozen control saplings. Long-term recovery showed a clear priorization of NSC-recovery over growth, resulting in barely any (Carpinus) to ~50% of the biomass increment of control saplings during the first growing season.
Recovery after frost can be seen as part of a species’ strategy to time leaf-out. While Prunus and Quercus, despite of contrasting freezing resistances, were able to quickly recover their canopy foliage, Fagus and Carpinus, poorly recovered in terms of leaf area and growth increment with Carpinus eventually failing at all. Strategies to cope with freezing damages observed here include the deployment of reserve buds, the ability to resprout from the stem base, lower C/N-ratio in recovering leaves and overcompensation of NSC-reserves to be used in the following growing season. Such mechanisms may provide competitive advantage and reshuffle the cards of evolution with increasing frequency of damaging frost events and other stressors.
