Repeated Summer Drought and Re-watering during the First Growing Year of Oak (Quercus petraea) Delay Autumn Senescence and Bud Burst in the Following Spring

Abstract

Climate change predicts harsher summer droughts for mid-latitudes in Europe. To enhance our understanding of the putative impacts on forest regeneration, we studied the response of oak seedlings (Quercus petraea) to water deficit. Potted seedlings originating from three locally sourced provenances were subjected to two successive drought periods during the first growing season each followed by a plentiful re-watering. Here, we describe survival and phenological responses after the second drought treatment, applying general linear mixed modeling. From the 441 drought treated seedlings 189 subsisted with higher chances of survival among smaller plants and among single plants per pot compared to doubles. Remarkably, survival was independent of the provenance, although relatively more plants had died off in two provenances compared to the third one with mean plant height being higher in one provenance and standard deviation of plant height being higher in the other. Timing of leaf senescence was clearly delayed after the severe drought treatment followed by re-watering, with two seedlings per pot showing a lesser retardation compared to single plants. This delay can be interpreted as a compensation time in which plants recover before entering the subsequent developmental process of leaf senescence, although it renders seedlings more vulnerable to early autumn frosts because of the delayed hardening of the shoots. Onset of bud flush in the subsequent spring still showed a significant but small delay in the drought treated group, independent of the number of seedlings per pot, and can be considered as an after effect of the delayed senescence. In both phenological models significant differences among the three provenances were detected independent from the treatment. The only provenance that is believed to be local of origin, displayed the earliest leaf senescence and the latest flushing, suggesting an adaptation to the local maritime climate. This provenance also displayed the highest standard deviation of plant height, which can be interpreted as an adaptation to variable and unpredictable weather conditions, favoring smaller plants in drought-prone summers and higher plants in more normal growing seasons.

Taxon-Independent and Taxon-Dependent Responses to Drought in Seedlings from Quercus robur L., Q. petraea (Matt.) Liebl. and Their Morphological Intermediates

“…The delayed leaf senescence upon re-watering after a severe water deficit, the delayed subsequent bud burst, and the advanced leaf senescence in the following growing year were found both in the here-described experiment with offspring of Q. petraea, Q. robur, and morphological intermediates, as well as for different provenances of Q. petraea [18,43]. For the different taxa sourced in the same provenance (here-described experiment), no dependency on the taxon of the mother tree could be detected, whereas phenological responses in a similar experiment with different provenances of Q. petraea were dependent on the provenance [18]. These dissimilar responses for the provenance and taxon can likely be related to recent genomic findings.…”

“…Provenance-specific and taxon-specific loci have been discovered in the genomes of Q. robur and Q. petraea, with "species discriminant" loci representing genome regions affected by directional selection maintaining species' identity, and "provenance-specific" loci representing genome regions with high interspecific gene flow and common adaptive patterns (e.g., phenological responses) to local growth environment [42]. The delayed leaf senescence is suggested to be caused by a compensation time upon re-watering after a severe drought in which physiological repair mechanisms act before the seedlings enter the next developmental phenological phase [18]. The observed reduction in height growth in the second growing season likely caused an earlier growth stop, resulting in an advanced leaf senescence in this year.…”

Drought Treated Seedlings of Quercus petraea (Matt.) Liebl., Q. robur L. and Their Morphological Intermediates Show Differential Radial Growth and Wood Anatomical Traits

“…Remarkably, the drought exposed seedlings in 2015 display a further advanced ring closure than the control group, indicating a higher capacity of water transport at the start of the growing season. Re-watering after severe water limiting conditions in oak seedlings can result in a retarded leaf senescence, which can be interpreted as a type of re-activation to compensate for the lost growing time during the drought period itself [30]. Moreover, in beech, a stimulation of the net-photosynthesis after drought and re-watering was suggested to compensate the restraint of photosynthesis during the drought treatment [55].…”

“…Several studies have already monitored effects of water limitation in oak seedlings [28][29][30][31]. Here, we aim to improve our knowledge on the radial growth and the wood anatomical variation in the stems of seedlings from the interfertile Q. petraea and Q. robur and from their morphological intermediates, after water limiting treatments.…”

Does Drought Stress on Seedlings Have Longer Term Effects on Sapling Phenology, Reshooting, Growth and Plant Architecture in Quercus robur, Q. petraea and Their Morphological Intermediates?

“…Bud burst was advanced in the spring following a growing season with drought stress [22]. A delayed autumnal senescence, together with a delayed bud burst in the subsequent spring, were observed upon re-watering after drought stress in late summer [23].…”