Abstract
Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of extreme droughts on tree radial growth, we used a network of tree-ring width data of 1689 trees from 100 sites representing most of the distribution of two drought tolerant, deciduous oak species (Quercus petraea and Quercus robur). We first examined which climatic factors and seasons control growth of the two species and if there is any latitudinal, longitudinal or elevational trend. We then quantified the relative departure from pre-drought growth during droughts, and how fast trees were able to recover the pre-drought growth level. Our results showed that growth was more related to precipitation and climatic water balance (precipitation minus potential evapotranspiration) than to temperature. However, we did not detect any clear latitudinal, longitudinal or elevational trends except a decreasing influence of summer water balance on growth of Q. petraea with latitude. Neither species was able to maintain the pre-drought growth level during droughts. However, both species showed rapid recovery or even growth compensation after summer droughts but displayed slow recovery in response to spring droughts where none of the two species was able to fully recover the pre-drought growth-level over the three post-drought years. Collectively, our results indicate that oaks which are considered resilient to extreme droughts have also shown vulnerability when droughts occurred in spring especially at sites where long-term growth is not significantly correlated with climatic factors. This improved understanding of the role of drought seasonality and climate sensitivity of sites is key to better predict trajectories of post-drought growth recovery in response to the drier climate projected for Europe.
Highlights
The frequency and severity of extreme droughts have increased in Europe (Spinoni et al, 2018; Vicente-Serrano et al, 2014) and elsewhere in the world (Spinoni et al, 2014) leading to significant changes in resource availability and altering the overall boundary conditions that are crucial for tree performance in forests (Arneth et al, 2010; Matías and Jump, 2015; Richardson et al, 2018)
The important role of drought seasonality has been identified for Quercus species dominated ecosystems in the Mediterranean regions where spring drought had a larger impact on leaf production and photosynthesis compared to autumn drought (Misson et al, 2010a) and on radial growth compared to summer drought (Camarero et al, 2015; Gavinet et al, 2019)
The radial growth response to summer water balance was more sensitive towards the warmer, southern margin of the distribution of Q. petraea but not of Q. robur
Summary
The frequency and severity of extreme droughts have increased in Europe (Spinoni et al, 2018; Vicente-Serrano et al, 2014) and elsewhere in the world (Spinoni et al, 2014) leading to significant changes in resource availability and altering the overall boundary conditions that are crucial for tree performance in forests (Arneth et al, 2010; Matías and Jump, 2015; Richardson et al, 2018). Despite years of numerical empirical studies and meta analyses, uncertainties remain about the magnitude of influence of extreme droughts on tree growth (Gazol et al, 2017; Zang et al, 2014), post-drought patterns of tree growth recovery (Anderegg et al, 2015; Ovenden et al, 2021; Zweifel et al, 2020), and processes determining the post-drought recovery trajectories (Gazol et al, 2020; Gessler et al, 2020) These uncertainties are causing concerns given the projected increases in the frequency and severity of extreme droughts (Dai, 2012; Vogel et al, 2020). These authors showed that trees across species were in principle able to show rapid recovery to droughts that occurred in wet seasons, while this did not happen when they were growing at climatically sensitive sites
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