عنوان مقاله [English]
Background and objectives: The formation of soil humus is shaped by both biological and non-biological elements. Biological factors include vegetation type and soil organisms, while non-biological factors encompass climate, topography, and bedrock. Therefore, soil humus form serves as a crucial indicator in forest ecosystems. Previous studies have established a clear correlation between changes in humus form and variations in elevation, yet the connection between humus form and topographical morphology remains uncertain. Hence, this research seeks to elucidate the relation between local-scale changes in humus forms and topographic morphometric indices.
Materials and methods: Variables were derived from 510 humus profiles, categorizing the main and secondary humus forms based on the European classification method for terrestrial humus forms, which is specifically designed for temperate mountain forests. The data was collected along an altitude gradient spanning from 200 to 1000 meters within a 1700-hectare area in district one of ShastKalate forest. Primary and secondary topographic attributes were extracted from a Digital Terrain Model (DTM) with a spatial resolution of 10 meters. Discriminant and regression analysis were employed to establish the relationships between variables.
Results: The results indicated that dividing the region into two altitude ranges had a significant impact on the accuracy of humus form classification. In the low altitude range (below 599 meters), the overall accuracy coefficient for the main humus form increased by 55%, and for the secondary humus form by 83%. In the middle altitude range (above 600 meters), the overall accuracy coefficient increased by 14% for the main humus form and by 58% for the secondary humus form. Additionally, the Kappa coefficient increased by 112% and 49% for the main humus form in the low and middle-altitude regions, respectively. Regarding humus form abundance, the MULL form was more prevalent in the low altitude zone, while the MODER and AMPHI forms were more abundant in the middle zone. In terms of secondary humus forms, Eumoll was most abundant in the low altitude zone, while Eumoder dominated the middle zone. Furthermore, the frequencies of Eumacroamphi, Eumesoamphi, and Leptoamphi forms were higher in the middle zone compared to the low altitude zone. Contrary to previous research findings, it was observed that altitude had a negative relationship with changes in primary and secondary humus forms in the middle altitude range, while it had a positive relationship in the low range. Similarly, the slope had an opposite effect on humus form changes. Profile curvature and plane curvature exhibited a positive relationship with primary and secondary humus forms in both the low and middle altitude ranges. This implies that an increase in domain concavity along the slope direction and convexity perpendicular to the slope leads to the presence of all humus differential horizons and the formation of a large biological soil structure in the semi-organic Ah horizon. This, in turn, explains the presence of Eumull and Leptoamphi forms. Moreover, the topographic wetness index displayed a positive relationship with changes in both main and secondary humus forms in the low and middle altitude zones. This suggests that as soil moisture increases, facilitated by higher organic matter content in the soil surface horizons, the main humus form tends to shift towards MODER.