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.
Highlights
Predicted climate change in temperate regions raises concerns about the ability of forest ecosystems to cope with longer and more severe summer drought periods
The purpose of this paper is to examine the impact of severe summer droughts on the survival of first year seedlings of three different sessile oak provenances, sourced in the same region in Belgium but with a variable origin, and on the phenology of the surviving seedlings in this vulnerable phase of their life
Seedlings that shared a pot displayed a lower probability of survival in the drought stressed condition (Table 3; Figure 3)
Summary
Predicted climate change in temperate regions raises concerns about the ability of forest ecosystems to cope with longer and more severe summer drought periods. Among important European tree species, oaks are well-known to be tolerant to drought (Leuschner et al, 2001), having a xeromorphic leaf structure and an adapted root structure that can cope with temporal and spatial variability in soil water and nutrient availability, and displaying an ability to rapidly resume assimilation after periods of water deficiency (Kubiske and Abrams, 1993; Galle et al, 2007; Kuster et al, 2013a,b) For this reason, oaks are put forward as promising candidate tree species to replace drought sensitive species such as beech (Fagus sylvatica) or spruce (Picea abies) on warm and dry sites in Europe (Leuschner et al, 2001). A reduced aboveground growth pattern with a diminished biomass production, together with a shift toward below-ground root growth, are wellestablished responses to drought in oak species (Broadmeadow and Jackson, 2000; Thomas and Gausling, 2000; Arend et al, 2011; Spiess et al, 2012; Kuster et al, 2013b) whereas the effects on phenology are less thoroughly examined and mainly described as an earlier stop of height growth under dry growing conditions (Jensen and Hansen, 2010; Spiess et al, 2012), visible in an earlier cessation in secondary (radial) growth (Pflug et al, 2015) and which may show an after effect in the subsequent spring by an advanced bud burst (Kuster et al, 2014)
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