Evaluation of the Space-For-Time approach to study of the effects of temperature increasing on the leaf litter decomposition

Background and objectives: In forest ecosystems, the decomposition of leaf litter is one of the most important ways of entering nutrients into the soil, and the availability of soil nutrients is largely due to the dynamics of nutrients and leaf litter decomposition in the forest. Decomposition of dead leaves enables important ecological functions and affects the composition of the internal nutrients of trees. Decomposition processes influence the formation of soil organic carbon stocks. It is necessary to understand how climate change and global warming affect the decomposition process and thus the amount of carbon stored in the soil. The space-for-time (SFT) approach makes it possible to compare litter decomposition in current conditions and at different altitude levels, and to investigate the effects of increased temperature on litter decomposition. The present research was conducted with the aim of investigating the effects of increasing or decreasing temperature on the decomposition rate of leaf litters in pure and mixed states.
Methodology: In the present research, by using the space-for-time approach, the decomposition rate of leaf litters (in two pure and mixed states) in three tree species (Quercus brantii, Celtis australis and Pistacia atlantica) was carried out in three altitude classes of 750, 900 and 1050 meters in the forests of Dareh Shahr. The forests of this region have relatively severe topography and have mountains and hills with steep and smooth slopes, and there are rocky outcrops in many parts of it. The average height of the area is 1200 meters and the average slope is 15%. The average annual temperature is 21.40°C and the average annual precipitation in this region is 426.3 mm. The climate type of the studied area is semi-humid. To achieve the objectives of the research, the leaf litter of the studied species were randomly and manually collected from the forest floor in the fall season of 2020. After preparing litter bags, a number of 243 single-pocket and double-pocket litter bags were installed on the mineral soil at the litter collection sites and during 180 days with intervals of 30, 60 and 180 days and the decomposition rate of the leaf litters were measured. In order to measure the decomposition rate of the leaf litters, the primary and secondary weight of the leaf litter was recorded and the weight loss of the leaf litter was calculated through the weight relationships. To determine the initial quality of leaf litters, the nutritional elements of leaf litters such as nitrogen, carbon, phosphorus, calcium, potassium and magnesium were measured.
Results: Based on the results, it was found that the studied leaf litters are similar in terms of magnesium, phosphorus and nitrogen concentrations. Quercus brantii leaf litters had the highest amount in terms of potassium and carbon concentrations. Also, Celtis australis leaf litters had the highest amount in terms of calcium concentration. The quality ratio of C:N in Celtis australis litter was low, this issue shows the high quality of this leaf litter. The results showed that in the studied time periods, litter type, height above sea level (except for the period of 180 days) and the interaction effects of litter type and height had significant effects on decomposition rate, so that increasing the altitude above sea level has led to further decomposition process. Based on the findings of the research, it was found that in the period of 30 days, the highest amount of decomposition related to the combined treatments of Celtis australis (Celtis australis+Pistacia atlantica) and Quercus brantii (Quercus brantii + Celtis australis) (14.61% and 14.40% respectively) at the height of 1050 meters and the lowest was related to the treatment of Pistacia atlantica (4.52%) at the height of 750 meters. In the period of 60 days, the highest amount of decomposition rate was observed in the combined treatment of Celtis australis (Pistacia atlantica + Celtis australis) at the height of 1050 meters (15.41%) and the lowest amount of mass loss was observed in the treatment of Pistacia atlantica at the height of 750 meters (5.31%). Also, at the end of the incubation period, the highest amount of decomposition rate belongs to the combined treatment of Celtis australis (Celtis australis+Pistacia atlantica) at the height of 1050 meters (16.12%) and the lowest is related to the pure leaf litter of Pistacia atlantica at the height of 750 meters (10.12%).
Conclusion: In general, based on the findings of this research, with the increase in altitude above the sea level, the rate of mass loss not only did not decrease, but also increased significantly (especially in the first two months of incubation) compared to the lower altitude classes. Also, at the end of the study period, only the interaction of height and type of litter was able to affect the amount of decomposition of litter.