بازسازي دماي بيشينه بهار- تابستان در يال غربي زاگرس مياني با استفاده از يك گاه‌شناسي منطقه اي (2010-1750)

Reconstruction of Spring-Summer Maximum Temperatures Based on a Regional Chronology (1750-2010) in the Western Ridge of Central Zagros, Iran

حلقه هاي درختي مي‌تواند به‌عنوان يك شاخص اقليمي مفيد جهت مطالعه نوسانات ماهانه و فصلي متغيرهاي اقليمي طي سده‌هاي گذشته به‌كار روند. براي به‌دست آوردن يك گاهشناسي منطقه اي، تعداد 54 نمونه از دو گونه درختي بلوط ايراني و بلوط مازودار در رويشگاه دالاب در استان ايلام و رويشگاه شينه در استان لرستان برداشت شد. در اين پژوهش پهناي حلقه هاي رويشي سالانه درختان توسط دستگاه LINTAB5 مجهز به نرم افزار TSAP مورد اندازه گيري گرفت و تطابق زماني منحني‌هاي رويشي درختان دو رويشگاه به دو شيوه چشمي و آماري توسط نرم‌افزار مذكور به انجام رسيد. با تعيين ميزان EPS، مطمين‌ترين بخش از گاهشناسي منطقه‌اي (2010-1840) براي بازسازي مورد استفاده قرار گرفت. داده‌هاي دماي بيشينه ماهانه و فصلي ايستگاه هاي هواشناسي ايلام (2010-1987) و خرم‌آباد (2010-1951) براي واسنجي گاهشناسي منطقه اي در طول دوره آماري مشترك به كار گرفته شد. نتايج پژوهش نشان داد كه دماي بيشينه در فصل رويشي و فصل قبل از آن بر روي رويش درختان بلوط اين منطقه تاثير منفي دارد. دماي بيشينه بهار- تابستان در منطقه مورد مطالعه طي دوره بازسازي شده (2010-1840) روند صعودي را نشان مي دهد. دماي بيشينه طي دوره بازسازي شده در سال هاي 2010، 2000، 1984، 1966، 1958، 1955، 1952، 1945-1944، 1933، 1929 و 1904 افزايش شديد داشته است.

Introduction Long-term climate records are valuable for environmental planning, and tree rings allow records to extend to the time before the establishment of weather stations. For a better understanding of the past climate fluctuation and to mitigate the effect of climate fluctuation, dendroclimatic reconstructions have been applied in many parts of the world. Inter annual variations of climate have an intensive effect on water resources, agriculture, human settlements and regional ecosystems. The central Zagros region of Iran experiences different climatic situations over the seasons of a year. In recent years, dramatic changes in regional climate have damaged both local forests and agriculture. Weather stations cover a short time span of instrumental data in Iran. To overcome this challenge, tree rings can be used to put recent climate trends in a long-term context of climate variability. The central Zagros Mountains are one of the most important sites of oak forest in west of Iran. However, tree-ring based climate reconstructions are still scarce for this country. In this study, we present a reconstruction of temperature variations in the central Zagros Mountains using Quercus infectoria and Quercus persica tree rings over the last 250 years (1750-2010). Material and Methods The study sites are located in the central Zagros Mountain range in the west of Iran. Due to the climatic regime, there is a clear distinction between a dry (summer) and a wet (winter-spring) season in this region. Our sampling sites are covered by old-growth oak forests and have the shortest distance to meteorological stations. In this study 54 cores from two species of Quercus persica in Dalab site (Ilam province) and Q. infectoria Olive in Shineh site (Lorestan province) were extracted using an increment borer. After the sample preparation, annual ring widths were measured with a LINTAB5 measuring system. The TSAP-Win software was used for cross dating and correlations between the growth corves. The growth corves were standardized with ARSTAN program to remove none climatic effects. The Expressed Population Signal (EPS; Wigley et al, 1995) was calculated for the regional chronology (RC). Monthly and seasonal maximum temperature from Ilam (1987-2010) and Khoramabad (1951-2010) weather stations were used to calibrate the regional chronology (1750-2010) during the common period (1951-2010). Based on a linear regression model, seasonal maximum temperature of spring-summer was reconstructed over the 1840-2010 period. Results and Discussion The results of the study show that the two site chronologies are strongly correlated with each other (p < 0.05) for the common period 1834–2010, indicating common inter-annual and decade growth variations that show the influence of common regional environmental forcing factors. Maximum temperature has a negative effect on oaks tree growths in the region during the pre-growing and growing seasons. The negative effect of maximum temperature on the trees’ growth in the growing season is more severe than in the per-growing season. Spring-summer Maximum temperature of the region shows a rising trend during the reconstructed period (1840-2010). Severe warm years during the reconstructed period occurred in 2010, 2000, 1984, 1966, 1958, 1955, 1952, 1945, 1944, 1933, 1929 and 1904. High maximum temperature might accelerate respiration, transpiration and evapotranspiration rates of trees and simultaneously decrease carbohydrate storage in the stem (Bao et al, 2012). The sampling sites are located in a wind exposed location of a semiarid region, therefore wind exposure can further increase the advert effect of maximum temperature on tree growth. Increasing maximum temperature reduces water availability of trees due to enhanced evaporation as well. Conclusion We developed a tree ring-width regional chronology from two oak sites in the central Zagros Mountains in the west of Iran. Correlation coefficients indicate that the trees are sensitive to temperature variations, and maximum temperature has a negative effect on tree growth during the per-growing and growing seasons. Due to the correlation coefficient between regional instrumental records and the regional chronology, Spring–Summer maximum temperature of the growth year was used for reconstruction over the last 170 years (1840-2010). Severe warm years during the reconstructed period occurred in 2010, 2000, 1984, 1966, 1958, 1955, 1952, 1945, 1944, 1933, 1929 and 1904.