Abstract
Abstract. The reconstruction of droughts is essential for the understanding of past drought dynamics and can help evaluate future drought scenarios in a changing climate. This article presents a reconstruction of summer droughts in France based on annually resolved, absolutely dated chronologies of oxygen isotope ratios (δ18O) in tree ring cellulose from Quercus spp. Samples were taken from living trees and timber wood from historic buildings at two sites: Fontainebleau (48°23′ N, 2°40′ E; 1326–2000 CE) and Angoulême (45°44′ N, 0°18′ E; 1360–2004 CE). Cellulose δ18O from these sites proved to be a good proxy of summer climate, as the trees were sensitive to temperature and moisture availability. However, offsets in average δ18O values between tree cohorts necessitated a correction before joining them to the final chronologies. Using the corrected δ18O chronologies, we developed models based on linear regression to reconstruct drought, expressed by the standardized precipitation evapotranspiration index (SPEI). The significant correlations between the SPEI and cellulose δ18O (r ≈ −0.70), as well as the verification of the models by independent data support the validity of these reconstructions. At both sites, recent decades are characterized by increasing drought. Fontainebleau displays dominantly wetter conditions during earlier centuries, whereas the current drought intensity is not unprecedented in the Angoulême record. While the δ18O chronologies at the two studied sites are highly correlated during the 19th and 20th centuries, there is a significant decrease in the correlation coefficient between 1600 and 1800 CE, which indicates either a weaker climate sensitivity of the tree ring proxies during this period, or a more heterogeneous climate in the north and the south of France. Future studies of tree ring isotope networks might reveal if the seasonality and spatial patterns of past droughts can explain this decoupling. A regional drought reconstruction based on a combination of the two sites shows good agreement with historical records of grape harvest dates in France, which provide another proxy of summer climate.
Highlights
Droughts can have severe impacts on ecosystems and on human activities (Büntgen et al, 2010; Seneviratne et al, 2012; Diaz and Trouet, 2014), as they influence water availability, groundwater recharge, algal blooms (Paerl et al, 2011), forest productivity (Allen et al, 2010), carbon sink saturation (Nabuurs et al, 2013), crop yields, forest fires, soil degradation, building subsidence and insurance costs (Corti et al, 2011, 2009), as well as human health (Haines et al, 2006; OBrien et al, 2014)
Using the corrected δ18O chronologies, we developed models based on linear regression to reconstruct drought, expressed by the standardized precipitation evapotranspiration index (SPEI)
The isotopic composition of the source water is principally controlled by the δ18O of local precipitation, it can be modified by evaporation of water from the soil and the mixing of water from different seasons, which depends on water residence times in the soil (Tang and Feng, 2001; Brooks et al, 2010)
Summary
Droughts can have severe impacts on ecosystems and on human activities (Büntgen et al, 2010; Seneviratne et al, 2012; Diaz and Trouet, 2014), as they influence water availability, groundwater recharge, algal blooms (Paerl et al, 2011), forest productivity (Allen et al, 2010), carbon sink saturation (Nabuurs et al, 2013), crop yields, forest fires, soil degradation, building subsidence and insurance costs (Corti et al, 2011, 2009), as well as human health (Haines et al, 2006; OBrien et al, 2014). More intense and longer meteorological droughts have already been identified in southern Europe (Sousa et al, 2011; Vicente-Serrano et al, 2014) and southern France (Giuntoli et al, 2013) in past decades. I. Labuhn et al.: French summer droughts since 1326 CE sources and challenging the sustainability of the current agricultural system (Itier, 2008; Levrault et al, 2010). (Vidal et al, 2010) have provided a homogenized reference framework for French droughts since the 1950s based on high resolution meteorological reanalyses. Prior to the instrumental period, little is known about the past intensity, return period and spatial extent of drought events, mostly based on historical sources (Garnier, 2011). A reconstruction of the temporal and spatial extent of drought in the past would constitute a valuable basis for expanding the knowledge on past droughts using natural climate archives
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