Comparison of some Soil Physico-Chemical and Microbial Characteristics in Relation to Oak Decline in Different Elevation Classes in Southern Zagros Forest

The decline of trees has been raised as one of the main problems in arid and semi-arid regions. Understanding the type and extent of the relationships among different environmental factors – e.g. soil characteristics as the basis for growth and development of plant species - and natural disturbances can be facilitated the ways of sustainable forest management and a useful tool for monitoring them. This study aimed to compare some soil physical, chemical and microbial properties in relation to the oak decline in the southern Zagros forest in Malekshahi county in Ilam province. The two-way analysis of variance showed that the effect of elevation, as well as the interaction between oak vitality classes and elevation on soil salinity, organic carbon, nitrogen, potassium and lime were significant. Among soil chemical properties, the factors of organic carbon (OC), nitrogen (N), Potassium (K) and electrical conductivity (EC) of the understory of trees were significantly affected by elevation, oak vitality classes and the elevation by vitality interaction. Higher values of these characteristics observed under the healthy oak trees in highland, which indicated a decreasing trend with increasing rates of drought in this elevation class. The effect of elevation and oak vitality and their interaction on sand, clay and bulk density was significant. The results showed that the soil saturated moisture content was significantly lower under dead trees (33.9%) in middle land and semi-dried trees (32.28%) in lowland than other trees. The percentage of sand was higher under dead trees than semi-dried and healthy trees in lowland and midland. A minimum amount of soil bulk density was found at all three classes of elevation under the semi-dried trees showed. The interaction between elevation and vitality was significant on basal respiration (BR) and substrate-induced respiration (SIR). The highest values of BR and SIR observed under healthy trees in highland (71.22 and 97.32 mgCO2-C kg-1 day-1) which a decreasing trend has been found with the increasing intensity of tree dieback in the study area. The amount of N, OC, and K showed the highest value under healthy oak trees in highland. According to the obtained results, it can be concluded that the soil chemical and biological properties can be able to predict the decline of trees along the elevation gradient.