"Divergence problem" in estimating temperature based on tree rings (Case study: Juniper mountainous habitats in northern Kerman province)

The study of tree rings is one of the most widely used methods of climate reconstruction for centuries and millennia, but the occurrence of climatic anomalies such as global warming in recent decades has caused divergence problem in the series of tree rings in some areas. Which challenges the ability of this proxy to regenerate the climate. The “divergence problem” is the difference between the recorded temperatures and the regenerated temperatures using tree rings in the forests of the northern high latitudes. While the data recorded by thermometers show a significant warming trend in the late twentieth century, many of the chronologies obtained from the sites mentioned do not show a similar change in ring width. Reconstruction of past temperatures in divergent chronologies overestimates past reconstructed temperatures. The present study tries to investigate the occurrence of "divergence problem" in low latitudes using juniper growth rings in the north of Kerman province in Iran and provide a solution for temperature reconstruction according to the chronologies with this problem.

The study area of this study is one of the lesser-known habitats called “Tangal Ravar”, located in the northern highlands of Kerman province in Iran at the 31°25' of North and 56° 50' of East. In the spring of 2018, more than 200 tree ring samples taken of juniperus trees. Chronologies created in ARSTAN software by BiweightRobust averaging method, and then de trending and standardized by negative exponential curve method and 20-year spline, Climatic data were prepared from two synoptic stations of Kerman and Zarand to investigate the relationship and correlation of climatic factors with the ring width index and reconstructions. For better comparison of these two stations, CRU TS4.01 data with the statistical period of 1901- 2016 for Iran, also used. In this study, two methods used to reconstruct the annual temperature. In the first method, first the temperature threshold (according to CRU temperature data) is determined and then by dividing the years, from data above the threshold to construct a regression relationship of the divergence period with inverse correlation and from data below Thresholds were used to construct the regression relationship of the pre-divergence period with direct correlation. In the second method, multivariate regression used between temperature data, ring width and precipitation for the statistical period of 1901-1995. The reconstructed precipitation used for multivariate reconstruction until the end of the chronology.

The results of comparison between temperature data and tree rings chronology index of the studied habitat showed that the trees of this habitat have been suffering from divergence since the mid-1990s. Initially, the results of correlation between station temperature data and average chronology of the region showed a generally negative relationship and it concluded that air temperature throughout the year has a negative correlation with the growth of juniper trees in the region and in general, this factor is a limiting factor. , But this is not consistent with most research on the subject. Temperature reconstruction in this case showed an inverse function of the chronology chart trend due to the negative relationship. Therefore, using a negative relationship to reconstruction temperature over a long period led to meaningless estimation during periods of severe reduction in rings width. However, with the removal of the 1996-2016 period from the data series, the correlation results underwent extensive changes to positive. The results showed that in years with an average temperature between 17 to 17.5 degrees and sometimes from 16.8 to 17.8 degrees, there is no significant relationship between the width of the rings and the temperature, and this temperature range can be considered without regard to taking the role of precipitation, the optimal growth temperature. At temperatures below 17 or possibly 16.8 degrees there is a direct correlation and at temperatures above 17.5 or possibly 17.8 degrees there is an inverse relationship between growth and temperature. In addition, comparison of chronology with climatic trends showed that precipitation has a more decisive role in the radial growth of juniper trees. In this regard, the reconstruction results showed that the use of bivariate regression and the application of precipitation for the second variable, despite the smoother data estimation than the real data, in general show better results than data segmentation based on temperature threshold.

Examination of station temperature data with chronology obtained from the region showed that due to the increase in temperature along with the relative decrease in precipitation in recent decades, temperature data with the chronology of ring width have diverged. Therefore, in order to reconstruct the temperature and study the climate / growth relations in the region, it is necessary to carefully study the relationship between This divergence for the study area has clearly started in 1996 and has continued until the end of chronology in 2017, for more than the last 2 decades. The time difference between the occurrence of the "divergence issue" in the study area and the northern areas is about 40 years. This issue can be related to the occurrence of later increase in temperature averages in the study area compared to high latitudes and the greater impact of juniper trees in arid regions such as the north of Kerman province from moisture. So that the increase in air temperature alone is not able to limit the radial growth of juniper trees and the creation of moisture stress is necessary for this issue, this stress has obviously occurred since 1996 with successive droughts in the region. The results showed that due to the location of the study site in a dry area, the use of precipitation as a second variable gives better results for temperature reconstruction. Finally, the regenerated temperature shows an increase of about 1.5 degrees in global warming in recent decades. In addition, except for an increase in the decades 1740-1750-1760, a decrease of 0.5 to 1.5 degrees of temperature compared to the long-term average from about 1820 to 1520 AD can be considered the reaction of juniper trees in the region to the Little Ice Age.