Summer rainfall variability in European Mediterranean mountains from the sixteenth to the twentieth century reconstructed from tree rings

Abstract

Since the end of the last glacial period, European Mediterranean mountains have provided shelter for numerous species of Eurosiberian and Boreal origin. Many of these species, surviving at the southern limit of their range in Europe and surrounded by Mediterranean ones, are relatively intolerant to summer drought and are in grave danger of loss, as a result of increasingly long and frequent droughts in this region. This is the case of the Scots pine (Pinus sylvestris) and the Austrian pine (Pinus nigra ssp. salzmannii) which are found on Central Iberian Peninsula at the edge of their natural range. We used a tree ring network of these two species to reconstruct past variations in summer rainfall. The reconstruction, based upon a tree ring composite chronology of the species, dates back to 1570 (adjusted R 2 = 0.49, P < 0.000001) and captures interannual to decadal scale variability in summer precipitation. We studied the spatial representativeness of the rainfall patterns and described the occurrence rate of extremes of this precipitation. To identify associations between macroclimatic factors and tree radial growth, we employed a principal component analysis to calculate the resultant of the relationship between the growth data of both species, using this resultant as a dependent variable of a multiple regression whose independent variables are monthly mean temperature and precipitation from the average records. Spatial correlation patterns between instrumental precipitation datasets for southern Europe and reconstructed values for the 1950-1992 period indicate that the reconstruction captures the regional signal of drought variability in the study region (the origin of this precipitation is convective: thermal low pressure zones induced in the inland northeastern areas of the Iberian Peninsula). There is a clear increase in the recurrence of extreme dry events as from the beginning of twentieth century and an abrupt change to drier conditions. There appears to be a tendency toward recurrent exceptionally dry summers, which could involve a significant change for the Eurosiberian refugee species.

Changes in Foliar Functional Traits of S. pyrenaicus subsp. carpetanus under the Ongoing Climate Change: A Retrospective Survey

“…Under cooler conditions, plants can foster the development of bigger leaves to heighten their boundary layer thickness, allowing a faster attainment of the propitious temperatures for photosynthesis [36]. The greater LA observed in conserved specimens of S. carpetanus that experienced continual cold, wet growing seasons between 1956 and 1973 (Figures 1b and 2a; [27]) contrasts the response of those specimens collected in years with drier, hotter growing seasons. This suggests the ability of S. carpetanus to adjust its leaf macro-morphology to potentially maximize photosynthetic returns in temperate growing seasons through improved heat exchange capacity as an outcome of the greater boundary layer thickness in bigger leaves [36].…”

“…The latter given the reduced SS found in individuals collected under growing seasons with rainfall below normal values, expressly in 2009 and 2011 registered as two of the driest growing seasons in these mountains within the time of our study. The smaller SS was particularly notable for individuals from the 90s and onwards which displayed a significant reduction in SS in response to the overall decline in annual precipitation regimes observed in Sierra de Guadarrama [27]. Moreover, the higher temperatures befalling the active vegetation period in recent years also had a significant effect over the variation in SS (p value = 0.02).…”

“…In the high-mountains of Mediterranean-type climate, such as the Sierra de Guadarrama in Central Spain, the greatest constraint for plant growth is summer drought, co-occurring with high temperatures and irradiance during the short active growing season of vegetation. Climate records have advocated for a general annual shift towards drier conditions since the 1990s, coupled to an increase in summer warming overlapping with the brief active-growing season [27,28]. In turn, physiological and morphological adaptations are impending in plants inhabiting these areas to counteract the constraining environmental conditions befalling the brief window for plant activity.…”

Two millennia of Main region (southern Germany) hydroclimate variability

“…Calibration was conducted between the TRW chronology of the living oak trees and the DPS record. Correlation analysis was performed using a MATLAB ® (MathWorks, 1994(MathWorks, -2008 script (Schönbein, 2011) which aggregated DPS data for each year, altering the length of the data interval (from 31 to 361 d in steps of 10 d) and the start date (between 1 January and 15 December). Statistical significance was attained for α = 5 %.…”

“…Statistical significance was attained for α = 5 %. For details about the running the script, we refer to Schönbein (2011) and Land et al (2017).…”