Evaluating the Effects of Solar Radiation Parameter on Radial Growth of Quercus castaneifolia in Astara Forests

 
Introduction
Climate change happens due to abnormal changes of climate within the earth’s atmosphere and their consequences. One of the most significant effects of climate change is the irreversible damage to some floral species and forest ecosystems. Tree species reflect the impact of climate changes on their ring width. Generally, tree rings vary in size each year depending upon the environmental conditions or climate variables that the tree experiences. In fact, tree rings are severely dependent on the changes of climate variables. It is considered a basis for “dendrochronology” which is the scientific method of dating based on the analysis of patterns of tree rings or investigation of climate variables effects on tree ring width. Many national and international studies have been conducted on dendrochronology. However, the national researches investigated the effects of temperature and rainfall changes on radial growth of trees. Unfortunately, due to lack of long-term solar radiation data in the most of meteorological stations in Iran, the effect of this parameter on the radial growth of trees has not been studied so far. Meanwhile the solar radiation has a significant effect on the width of tree rings-especially in tolerant shaded species like oak. One of the most important species of trees in Iran is the Oak tree (Quercus castaneifolia), due to its being spread in different areas of the country and its high economic and environmental value. Unfortunately, in recent years, because of climate changes and droughts, and also overcutting of Quercus castaneifolia for wood and live-stokes food supply, this precious species has been subjected to serious threats. In this regard, the role of climate in addition to improper management is significant. To protect Quercus castaneifolia, the first and most important step is the scientific planning of forest management, through knowing the effective parameters of climate change on the growth and physiology of the trees. Therefore, implementation of more research on the oak species and utilizing the results is essential for their protection.    
By considering the above mentioned facts, to solve the problem related the lack of solar radiation data, in this study, an appropriate methodology was introduced based on empirical formulas for calculating the solar radiation. Thereafter, the incoming solar radiation in the Astara Forests was calculated. In the next step, by using seven discs harvested from Quercus castaneifolia trees, the chronology of this species for each disc in two perpendicular directions was obtained by using LINTAB tool coupled to TSAPWin software. Finally, the relationship between tree ring widths and climate variables was determined.
 
 
Materials and Methods
In this research the seven discs were harvested from Quercus castaneifolia trees with mean tree ages of 35 to 78 years in the Astara Forests located in north of Iran. In addition, the climate variables were collected from the synoptic meteorological station of Astara. However, due to financial, technical or institutional limitations, the solar radiation data are absent, incomplete or inaccessible in most areas of the world especially in the developing countries such as Iran. This seriously hinders the progress of forestry researches such as chronology studies that require solar radiation as a key driving input. Therefore, various methods have been explored in order to estimate, with reasonable accuracy, the solar radiation from other available meteorological data. One kind of widely used method is based on empirical relations between solar radiations and commonly measured meteorological variables. Therefore, in this research, Angstrom-Prescott model (Eq. (1)) was selected to estimate the monthly and yearly global solar irradiation (R_s) in the Astra Forests.





(1)

 




In the next step, the chronology of each disc in two perpendicular directions was obtained by using LINTAB tool coupled to TSAPWin software. To evaluate the obtained chronology for each disc, the Gleichläufigkeit (GLK), Crossdate Index (CDI), and Signature GLK (GSL) were used.
 
Results
According to published results on determination of a and b parameters of Angstrom-Prescott model for different regions of Iran, 0.404 and 0.204 were selected for Astra Forests district, respectively. Afterwards, using recorded meteorological parameters in Astara Synoptic Station, the monthly and annual global solar irradiation were estimated for Astra Forests during 1985 to 2005 (Fig. 1).
 
 
Fig. 1: Annual and Monthly Average of R_s
 
In the next step, the chronology of species for each disc in two perpendicular directions indicated that statistical indexes had appropriate values. Findings showed that the average of chronology for each disc derived from two vertical directions was significant. In the other words, three useful parameters for evaluating the chronology’s quality of all seven discs i.e., GLK, CDI, and GSL were more than 73, 46, and 99.9%, respectively. Thereafter, the average sizes of two perpendicular directions for each of the seven discs were calculated and the chronology of the average values was carried out. Only four of the discs showed proper results according to their indices. Using the chronology of the four discs, the regional chronology of Quercus castaneifolia trees in Astara Forests was calculated (Fig. 2). Ultimately, a statistical relation was determined between the R_s and the chronology of these trees (Fig.3).
 
 
Fig. 2: Regional chronology of Quercus castaneifolia trees in Astra Forests
 
 
Fig. 3: Quercus castaneifolia trees ring widths vs R_s
 
Discussion and Conclusion
Quercus castaneifolia tree ring widths vary with R_s by a second order polynomial function. The R value of this function (-0.71) is proper, which is an indicator of a strong relationship between R_s and Quercus castaneifolia tree ring widths. The negative value of R shows an inverse relationship, an expected fact which obviously reflects the shade intolerant quality of oak species. According to the similarity of temperature and R_s and also to the previous studies on the effect of temperature on tree ring widths, which resulted in negative R values, it can be concluded that the findings of the present research are reliable and justified