Abstract
Research highlights: The susceptibility of oaks to late spring and early autumn frosts is directly related to their leaf phenology. Drought may alter the leaf phenology and therefore frost tolerance of oaks. However, the effects of drought on oak leaf phenology and frost resistance have not been thoroughly studied. Background and objectives: One of the consequences of climate change is an increase in the frequency of dry episodes during the vegetation period. Pedunculate oak (Quercus robur L.) is an economically and ecologically important forest tree species that prefers humid habitats. Therefore, knowledge of the impact of drought on this species is of great importance for the adaptation of forestry strategies and practices to altered environmental conditions. The aim of this study was to determine the impact of drought on leaf phenology and spring frost susceptibility in nine provenances. Materials and methods: One-year-old saplings originating from nine European provenances were used in the trial. The saplings were exposed to experimental drought and then re-watered in two subsequent years. Spring and autumn leaf phenology were scored. The trial was impacted by a late spring frost in the third year, and the resulting leaf frost injury was scored. The effects of drought treatment on the phenology and frost susceptibility of plants from the provenances were analysed. Results: Leaf phenology of plants from most of the studied provenances was significantly influenced by the drought treatment (p < 0.001). Drought induced a carry-over effect on flushing phenology, which was observed as delayed bud burst (from 0.6 to 2.4 days) in the second year and as advanced bud burst (from 0.1 to 6.3 days) in the third year. Therefore, opposite shifts in flushing phenology may be induced as a result of differences in the time span when plants sense water deficits. In contrast to flushing, autumn leaf phenology was unambiguously delayed following the drought treatments for all studied provenances (from 2.1 to 25.8 days). Differences in late frost susceptibility were predominantly caused by among-provenance differences in flushing phenology. However, the drought treatment significantly increased frost susceptibility in the plants (the rate of frost-injured plants per provenance increased from 3% to 78%). This higher susceptibility to spring frost was most likely caused by the advanced flushing phenology that resulted from the drought treatment in the previous year.
Highlights
The hazards associated with climate change, including rising temperatures, decreasing precipitation, and an increasing frequency of extreme climatic events, are expected to intensify [1,2,3].In general, the productivity of forest ecosystems is severely impaired by water availability, and droughtForests 2019, 10, 50; doi:10.3390/f10010050 www.mdpi.com/journal/forestsForests 2019, 10, 50 may induce episodes of large-scale tree decline in temperate forests [4]
The effect of drought on autumn leaf senescence was unambiguous, the mean bud burst dates were shifted in opposite directions in the two analyzed years
Our study supports the hypothesis that opposite shifts in bud burst dates occur due to the different times in the growing season at which plants sense water deficit signals
Summary
The productivity of forest ecosystems is severely impaired by water availability, and drought. Forests 2019, 10, 50 may induce episodes of large-scale tree decline in temperate forests [4]. Most likely, such declines will substantially increase the necessity for artificial regeneration of temperate forests. The increasing severity and frequency of droughts may impact forest regeneration, causing unacceptably high seedling mortality rates. Late spring frost is an additional abiotic factor that strongly limits forest regeneration [5]. It is necessary to enhance our understanding of the mutual impacts of drought and frost on forest reproductive material
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