DiscussionTop

Tree-ring data 

The slight negative skew, the low excess kurtosis, and the results of the Anderson-Darling test (Table 1) justify the goodness of fit of the used tree-ring data with a normal distribution. The average correlation among all individual radii (0.29) and the explained variance in the first principal component of the series over the common interval (31%), which represent the strength of their common climatic signal (Touchan et al., 2014), is the lowest registered from all the sites used for climate reconstruction in Algeria so far (Touchan et al., 2008a, 2010, 2016, 2017; Kherchouche, 2012, 2013; Slimani, 2014). This reflects the moderate common growth signal among our sampled trees, which is mainly due to the relatively wet conditions of the study area. The mean sample segment length indicates that our chronology is suitable for investigating high and mid-frequency climate variability on interannual to decadal time-scales. Moreover, the EPS statistics shows that by the 1830s, individual chronologies are sufficiently well replicated to capture the population growth-signal.

Precipitation reconstruction

Fig. 2 highlights the importance of March-June precipitation as the most obvious controlling factor on the growth of TAK Atlas cedar forest. The results also showed the importance of an increase in mean monthly temperature during the cold and rainy period. May is the only month to be significant with both monthly total precipitation (positively) and average temperature (negatively). This indicates that during the beginning of the hot period, an increase in mean temperature is efficiently attenuated by an increase in total precipitation. In contrast, a hot May associated with a rainfall deficit affects negatively tree growth. Negative impacts of rainfall deficit, especially during the growing season, are observed in many aspects of forest health including fine root biomass reduction, as a consequence of reduced transpiration and respiration rates, seedling recruitment, productivity and mortality of larger/mature trees, susceptibility to pathogen or insect attack, and vulnerability to damage from fire (Zhao & Running, 2010; Reichstein et al., 2013 ; Brunner et al., 2015).

Seascorr identified March-June precipitation as the most appropriate seasonal predictand for reconstruction (Table 2), which accounts for about one-fourth of the total annual rainfall. The highest p-value for the test of null hypothesis confirms the stability of the correlation from early to late period. These key months for precipitation influence on tree growth have also been identified for the eastern Mediterranean (Touchan et al., 2003, 2005, 2007, 2014; Akkemik et al., 2005, 2008; Griggs et al., 2007; Köse et al., 2011).

978–2013 generated positive validation RE statistics, indicating a good level of predictive skill. Split-sample validation supports temporal stability of the tree-ring signal for precipitation but validated stronger on the second half of the reliable instrumental data (Table 3). This is due to the fact that the early period experienced lower precipitation variability than the late period: the reconstructed data exhibited seven extreme events (four wet and three dry) from 1978 to 1995 and 14 extreme events (seven wet and seven dry) from 1996 to 2013. In addition, the PRESS procedure showed a small drop from adjusted r2 to predicted r2 , which suggests that the final model has not been overfitted. Moreover, the highly significant correlation between actual and estimated precipitations and their very low mean difference, as well as the fact that the residuals are not correlated with the predictand variable confirm the robustness of the used statistical model.

Analysis of extreme dry and wet periods

The 10-year moving average of the reconstruction reveals high interannual to decadal variation in March-June precipitation, which correspond to high and mid-frequency climate signals (Fig. 4). This trend is confirmed by the low mean intervals between two successive wet (2.9 years) or dry (2.8 years) March-June periods, which promote the occurrence of prolonged extreme events.

The reconstructed data is relatively equally distributed among wet, dry and normal events. It includes 86% of the wet events and all the dry events reported by Slimani (2014) for an October-June precipitation reconstruction from the Atlas cedar forests of the Djurdjura massif for the period 1898-2011. Less common extreme events (29% of the wet and 61% of the dry years) have been observed between our reconstruction and that reported by Slimani (2014) in the Aurès region. It is noteworthy that on their common period (1830-2009), all the shared wet events between these two reconstructions occurred during the last four decades and the best synchronicity in dry events was registered during the last decade. This reflects the substantial increase in the common growth response between these distant forests to macroclimatic conditions from the middle part of the 20th century. Similar results have been noted between the Atlas cedar forests of the Tellian Atlas and the Saharan Atlas (Slimani, 2014). Stronger growth-climate relationships due to climatic restrictions have also been reported by many authors for the European forests, from the Mediterranean to Siberia (Andreu et al., 2007; Shestakova et al., 2016; Del Rio et al., 2021).

The driest single March-June period was recorded in 1858, while all data from previous reconstructions in Algeria reported the driest event in 2002 (Touchan et al., 2008a, 2010, 2016, 2017; Kherchouche et al., 2012, 2013; Slimani, 2014). In our reconstruction, the 2002 March-June precipitation is only the ninth-driest year. Actually, this year has been one of the rainiest in the instrumental data, with total annual precipitation of 1021.5 mm, while the average annual precipitation is 781.1 mm. Despite this, the cored trees produced a narrow ring. This is due to the very irregular precipitation in 2002, with a severe deficit in spring and a rainy summer in form of heavy storms. Likewise, the year 2012 registered a total annual precipitation over the mean (895.6 mm) but also generated a narrow ring. Examination of the daily data also showed a very high precipitation irregularity during this year, with a very high number of stormy days (37), and revealed that 61% of the winter-spring precipitation fell in February (320 mm), for which the monthly precipitation exceeded the average by 350%. Moreover, during that year, from December to June, only April did not register a rainfall deficit. These results highlight that rainy years characterized by very high irregular distribution and spring precipitation deficit can result in agricultural droughts, because too much water is not efficient as it falls outside the growing season or in form of heavy rain and, consequently, is not used by the plants (Mannocchi et al., 2004).

The first two decades of the reconstruction were dominated by wet events. Half of the years from 1830 to 1847 registered a March-June precipitation above the wet threshold (1830, 1832-1833, 1838, 1842-1843 and 1845- 1847). Touchan et al. (2008b) also reported 1833 as a wet year for the northwestern Tunisia. Nicault et al. (2008) noted that the nineteenth century in the Mediterranean region was dominated by wet conditions although with a relatively dry period around 1860. Emerit (1972) reported that the popular uprise led by shaykh El-Mokrani and shaykh Aheddad in 1871 in the region of Kabylie was mainly due to the miserable situation of the population, which, at the end of the Second French Empire, suffered various calamities such as epizootics, locust outbreak, disease outbreaks, drought, poor harvest and severe famine. Indeed, during the two decades preceding this revolt (1851-1870), our reconstruction exhibited 11 events of drought, among which four events lasted two years (1851-1852, 1855-1856, 1858-1859 and 1869-1870). Touchan et al (2008a) reported the year 1867 as the second single driest event since the middle of the 15th century in northwestern Africa. Emerit (1961) noted that the biggest economic crisis in Algeria in that period has been that of 1867-1868, during which a part of the country was subjected to a dreadful famine. The author reported that in 1867 the drought was so severe that seeds could not germinate and herds died in droves from lack of food, and, enfeebled by privation, the population became an easy prey to typhus and cholera. The same information have been reported by Kitouni (2013), who added that 500,000 persons, representing about a fifth of the total Algerian population at that time, died of hunger and hunger-related diseases. Nonetheless, it must be mentioned that in the reconstructed precipitation, 1867 was identified as a dry event, but does not figure among the driest ones. In this regard, Emerit (1972) pointed out that the revolt broke out in the regions less affected by starvation. Indeed, receiving higher rainfall than the interior of the country, the Kabylie region suffered less from famine (Emerit, 1961; Kitouni, 2013). It is noteworthy that in our reconstruction this year is followed by a wet event. Similar result was reported by Touchan et al. (2008b) in Tunisia where a severe drought occurred in 1867 and was followed by the single wettest year (1868) registered during the period 1771–2002.

From 1873 to 1880, the reconstructed data contained five dry events (1873, 1874, 1875, 1877 and 1879) and only two normal March-June precipitations (1876 and 1878). This result is consistent with those mentioned by Kherchouche et al. (2012, 2013) and Slimani (2014) who noted sustained dry conditions for the Aurès region during this period. Boudy (1950) in Abdessemed (1981) noted that an exceptional drought could have occurred between 1875 and 1880, which could have destroyed many stands of Atlas cedar in the Aurès region. Dieback of entire stands of Atlas cedar, associated to severe drought and/or severe winters, especially those of 1879 and 1880, has also been reported by Lapie (1909, 1928) for the Aurès and Kabylie regions. In northwestern Africa, Touchan et al. (2008a) reported a six-year drought from 1876 to 1881, where every consecutive year was below the long-term reconstructed Palmer Drought Severity Index (PDSI) mean. The year 1879 has also been marked by a May-July rainfall deficit in Cyprus (Touchan et al., 2014). In Ankara (Turkey), Kuniholm (1990) reported that a severe drought event occurred during the period 1873-1874, which resulted in the death of nearly a third of the human population as well as large number of herd animals.

This period of drought ended with a wet event that lasted two years (1880-1881) and the following four decades, from the 1880s to 1910s, were dominated by wet conditions, especially during the period 1888-1901, which did not record any dry event and where eight out of the fourteen years recorded a wet March-June period.

A return to dry conditions started in the 1920s and reached high drought frequencies around the mid-20th century. Similar results were reported for the Aurès region by Kherchouche et al. (2012, 2013) and Slimani (2014) and for the northwestern Africa by Touchan et al. (2008a). The last dry event recorded in the 1920s in our reconstruction occurred in 1928. Touchan et al. (2014)

mentioned this event as the driest single year during the last six centuries in Cyprus. This period was followed by a two-year wet March-June period ending with the fourth wettest event of the reconstructed data (1930). Touchan et al. (2005) reported similar results in the spring/summer precipitation reconstructions for the eastern Mediterranean. Half of the years over the period 1938-1949 registered dry events (1938, 1939, 1942, 1944, 1947 and 1949). Similar trend was reported in Algeria by Kherchouche et al. (2012, 2013) and Slimani (2014). OSS (2008) mentioned the droughts of the decade 1940s, during which several weather stations recorded until eight successive dry years, as the worst of the 20th century in Algeria. According to Tinthoin (1946), the period 1938- 1945 was marked by low crop yields and herds suffered heavy losses because of drought, which became more and more pronounced, especially between 1942 and 1945. In his literature, Camus (1947, 1948) also wrote about the 1940s as a period of continuing concern with respect to drought, plague, and famine. Touchan et al. (2008a) have also noted these substantial and sustained dry conditions during this decade over the northwestern Africa. Et-tobi et al. (2009) reported that during this period a severe drought caused one of the worst forest dieback episodes in Morocco. In southern Turkey, Touchan et al. (2003) noted the lowest five-year spring mean precipitation in the instrumental record (1931-1998) during the period 1941-1945. In Cyprus, Touchan et al. (2014) mentioned the year 1949 as the driest in the instrumental data for the period 1917-2010.

In our reconstruction, the longest period of drought lasted four years and occurred between 1956 and 1959. However, these events are not among the most severe ones. Nonetheless, Meddour-Sahar et al. (2008), listed the years 1956, 1957 and 1958 among the worst Algerian forest fires statistics.

No dry event occurred throughout the period 1966- 1989, during which was recorded one of the wettest 10- year reconstructed periods (1976-1985) and one of the three wet periods that lasted three years (1976-1978). Similar result has been reported by Slimani (2014) in the massif of Djurdjura over the period 1962-1993, while one of the worst dry events recorded in the Aurès region occurred between late 1970s and early 1980s and resulted in a severe episode of forest dieback (Kherchouche et al., 2012, 2013; Slimani, 2014). Kettab et al. (2004) reported that on a temporal scale, the Algerian coastal area experienced a wet period from 1954 to 1986. In Cyprus, Touchan et al. (2014) noted that the period 1967-1976 was the wettest reconstructed decade over the six last centuries.

After the mentioned extended wet period, our study area experienced its maximum precipitation variability over the last decades, which were marked by two main contrasting intervals. The first period is the decade 1993- 2002, which recorded the maximum number of dry events and contained the third lowest late winter/early summer precipitation (1999). This shift towards drier conditions from the 1990s to early 2000s has been mentioned in Algeria, for the Kabylie and Aurès regions (Kherchouche et al., 2012, 2013; Slimani, 2014, Slimani et al., 2014), in Tunisia (Touchan et al., 2008b), in Morocco (Esper et al., 2007; Linares et al., 2011) and over northwestern Africa (Touchan et al., 2008a, 2010), and was directly associated to the last forest dieback episode that occurred in the Mediterranean Bassin (Allen et al., 2010). In addition, Meddour-Sahar et al. (2008) reported a substantial increase in fire occurrence in Algeria between 1992 and 2000. During this period, seven out of nine years, which were registered over the two periods of successive years 1992-1994 and 1997-2000, largely surpassed the yearly average fire frequency: among these years, 1993, 1994, 1997, 1999 and 2000 have been identified as dry events in our reconstruction. Moreover, Touchan et al (2008a) pointed out that the 1999-2002 droughts in North Africa appear by several metrics to perhaps be the worst since at least the middle of the 15th century. However, a scan of Figs. 5 and 6 reveals that in our study area this period is marked by the highest frequency of drought, while the most severe events occurred in the mid-19th century. After this last dry interval, the last decade of the reconstruction (2006-2015), registered the maximum frequency in wet events and contained the third wettest March-June period (2010). In their investigation on the evolution of temperatures and rainfall over the Algerian coastline, Nouaceur et al. (2013) noticed that after a very dry period of about 15 years, wet and hot conditions prevailed during the decade 2003-2012, which was characterized by heavy rainfall involving severe storms. In this respect, Morsli & Habi (2012) noted that floods in Algeria are rarely due to large-scale weather disturbances and are mainly caused by localized and short-lived but devastating torrential rainstorms. During the last decades, such flooding episodes, like those of Bab El Oued (Algiers) in 2001 and of Ghardaïa in 2008, caused considerable material damages and killed 772 and 43 people respectively (Nouri et al., 2016).