Impact of a damaging spring frost followed by a summer drought on saplings of four temperate species

Abstract

Climate warming leads to earlier leaf-out which may put trees at higher risks of late spring frost (LSF) damage. Moreover extreme droughts in summer are increasing in frequency and magnitude. The probability that a damaging LSF and an extreme summer drought occur in the same year will therefore increase. Although the impact of LSF and extreme drought on tree vitality has been in depth investigated separately, the response and recovery of trees after a combination of these two stresses remain largely unknown, yet it might be crucial for tree persistence in the future.Here, we exposed 2-year-old saplings of four species (Quercus petraea, Quercus robur, Fagus sylvatica and Acer campestre) to an artificially LSF (trees exposed to -5.5℃ for 3 hours) shortly after leaf emergence (at the beginning of May). Then we applied a 2-month summer drought treatment from early July to end of August (well-watered vs. drought, 50% reduction of water). During the entire growing season we measured seedling growth, gas exchange and nonstructural carbohydrates (NSC) to examine how trees respond to and recover from single and double stress.The artificial SLF severely damaged 90% of Quercus robur, 70% of Fagus sylvatica, 40% of Quercus petraea and 20% of Acer campestre. After 1.5 months, biomass and specific leaf area (SLA) of late-frost treated trees still differed significantly from the control in Quercus petraea, Fagus sylvatica, Acer campestre. Assimilation (A) was lower in late frost treated trees before the drought traetments for all four species.LSF strongly damaged leaves and reduced tree growth. The lag effect of frost damage may interact with the following summer drought damage, and we expect stronger impact of the drought when following an SLF than alone. However, it is also possible that reduced canopy size due to previous LSF mitigates the drought damages. Data analysis is ongoing that evaluates the lag effect of late-spring frosts on the resilience of trees to drought.