Document Type : Full length article
Department of physical geography, Faculty of Geography, University of Tehran
In addition to responding to human scientific curiosity, research on the past climate is important to understand the trends, quiddity, factors, and impact of these changes in the environment. One of the most widely used methods of reconstructing climate data for decades and centuries is tree rings. Trees are living climatic evidence that records the changes and fluctuations of climate change that occur with their annual growth, so by studying their annual rings, we can better understand the paleoclimate conditions. Juniperus Polycopos trees are one of the valuable species in Dendroclimatology studies due to their longevity and suitable wooden trunk, which are widely distributed in Kerman province. The Juniperus habitat we studied is located in the northern highlands of Kerman province and using tree rings, data related to the temperature of the past few centuries of the region has been reconstructed and by studying them, climate change trends have been identified.
Materials and methods
The Juniperus habitat of this study is located at a mountainous massif in the north of Kerman province, between the three cities of Ravar, Zarand, and Kuhbanan, in 31° 25’ north and 56°50’ east. Juniper trees in these heights are found as several habitats with higher density and single trees scattered in the mountains, frequently on the southern, southwestern, and western slopes of the habitat at an altitude of 2700 to 3200 meters. 200 samples of 96 trees in the habitat were taken with an increment borer. Rings counted and their width was measured by LINTAB desktop and TSAPWin software from bark to the trunk with an accuracy of 0.01 mm. The cross-dating between the growth curves of the trees, done with TSAP software, and the results of GLK, CDI, GSL, CC, and Tv statistics showed the desired quality for cross-dating of the obtained growth curves for most of the trees. Based on the obtained growth curves, the chronology of Juniperus trees in the region was made in ARSTAN software by the BiweightRobust averaging method. Then De-trending and standardized by a negative exponential curve. Finally, the Standard chronology was selected for use in studies. The length of the chronology is 517 years (1500-2017 AD) with a reconstruction confidence period of 252 years. The quality of chronology was measured by the mean correlation statistics of all habitat trees (Rbt), Expressed population signal (EPS), signal-to-error or anomaly ratio (SNR), mean sensitivity (MS), and Autocorrelation (AC1). Then, the relationship between climate and width of the rings was measured using station data of Kerman province and CRU TS4.01 climatic data for Iran-116 last years by Pearson correlation coefficient and temperature reconstruction was performed using a linear regression model.
Results and discussion
The results showed that the temperature of the months before the growing season and March at the beginning of the growing season had a positive effect on the width of the rings and the temperature of April, May and June had a negative effect. In the middle of March, when the growing season in the study area begins, we have the most positive relationship between air temperature and the width of the rings. April and May show weak negative relations with temperature. There is no significant correlation during the months of the growing season, ie from April to September. According to the described months, the annual and winter temperatures have a weak positive correlation, the spring and summer temperatures have a non-correlation and the temperatures of the multi-month periods related to the cold months of the year have a weak positive correlation. The study of chronology and temperature trends has shown the occurrence of divergence between temperature and growth rings in the last 25 years. This is the difference between the recorded temperatures and the reconstructed temperatures of the width of the rings. Reconstruction of the past temperature in divergent chronologies leads to an overestimation of the reconstructed temperature. In this paper, the divergence problem is solved by using the long period of CRU data and obtaining the temperature-width correlation of the rings from 1996 onwards. The reconstructed temperature of habitat shows an increase of about 1.5 degrees over the last 2 decades compared to the long-term average (680 years). Other periods obtained include a relatively colder period, close to the average in the 8 decades of the twentieth century, with a short-term increase in the 1950s, an increase in temperature of about 0.5 degrees compared to the average in 1840, 1850, and 1870. Period of 0.5-degree decrease in 1760-1820 AD, period of 0.5 to 1-degree increase from 1720 to 1760 AD, long period of temperature drop from about 1700 to 1500 AD with different rate of decrease from 0.5 to 1 degree, noted. In general, the reconstructed temperature, except for a warm period from 1700 to 1760, generally showed a decrease of 0.5 to 1.5 degrees in the period 1500-1830 AD, coinciding with the event of the Little Ice Age in Europe for the study area.
Results show that the temperature before the growing season and especially in March at the beginning of the growing season has a direct effect on the width of the rings. The positive correlation between March temperature and ring width is due to the beginning of cambium activity at the early of growing season. Higher temperatures in March can cause the growing season to start earlier, resulting in a wider ring in the target year. In the warmer months of the year, the width of the rings shows a weak negative relationship with temperature, which can be due to the occurrence of water stress for trees with rising temperatures and increased evapotranspiration. Although the start of cambium activities at the beginning of the growing season depends on the increase in average air temperature, in arid regions such as northern Kerman province, moisture is a much more important factor and affects the annual growth of the tree. Therefore, increasing the average temperature increases the evapotranspiration of the tree decreases soil moisture, and shows its negative impact on annual growth. In general, it can be said that air temperature in the study area in the months before the growing season has a positive effect and in the months of the growing season has a negative effect on the width of growth rings. The temperature reconstruction results also show two important periods, initially, there was a steady drop in temperature between 0.5 and 1.5 degrees Celsius between 1830 and 1500 AD, which coincided with the Little Ice Age and coincided with studies in other parts of the world, including continental Europe. This indicates that the study area is also affected by some differences in the temperature decrease of the Little Ice Age. Another period is the sharp rise in temperature in the last two decades compared to the long-term average, which indicates that the study area is affected by global warming.