جستجوی مقالات مرتبط با کلیدواژه « Distribution pattern » در نشریات گروه « آبخیزداری، بیابان، محیط زیست، مرتع »

Rangeland plants undergo continuous changes due to various disturbances, with fire being a significant environmental factor impacting plant communities. In many rangelands, the frequency and extent of fires are increasing, altering the composition and structure of vegetation communities in fire-prone environments. However, limited studies have focused on the effects of fire on the quantitative analysis of shrub plant stands in rangelands, hindering our understanding of fire's impact on structural changes in these plants. This study aimed to analyze the quantitative structural changes of Astragalus microcephalus Willd. over an 8-year period following a fire.

One control area and eight fire-affected areas, each covering one hectare and sharing similar environmental conditions and plant communities, were selected. Vegetation sampling was conducted along six 30-meter transects, with 24 points spaced 5 meters apart selected using a systematic-random method. Structural characteristics of the stands were measured by completely harvesting all shrub plant bases using the distance method. Spatial distribution patterns were investigated using Johnson and Zimer's indices, C index, Hopkins, Eberhart, uniform angle, Clark and Evans indices, Mingling index for species admixture, and Berger-Parker index for crown differentiation.

The results revealed a clustered distribution pattern in both the control area and the fire-affected area, as indicated by the Clark-Evans indices and uniformity angle. The C index and Hopkins distribution pattern indicated uniformity, which remained consistent between the control and fire-affected areas, suggesting no alteration in distribution pattern due to fire. The Johnson and Zimer's Index showed varying values for the control and fire-affected areas across different years. The Mingling index indicated weaker mixing in the fire-affected area compared to the control area. The Berger-Parker index indicated increased dominance of Astragalus microcephalus after the fire. Crown differentiation analysis revealed a greater difference in crown size between the control and fire-affected areas in the early years following the fire. One-way analysis of variance showed significant differences between different years after the fire in terms of Clark-Evans, Johnson and Zimer's, and Berger-Parker indices at a confidence level of one percent (p<0.01).

The clustered distribution pattern of Astragalus microcephalus Willd. in Khalkhal rangelands remains unchanged after 8 years following a fire, likely due to factors such as post-fire regrowth from the collar surface, seed distribution type, and reproductive strategy. Fire has led to increased dominance of Astragalus microcephalus and reduced vegetation composition. Understanding changes in structural characteristics of shrub species is crucial for assessing disturbance impacts, studying spatial distribution patterns, species dominance, and dimension differences following disturbances in mountain pastures, and planning for habitat management and restoration.

Intensification of natural disasters make it important to know the time variations of rainfall for wet and dry spell management and adaptation strategies. Measuring and predicting of precipitation anomalies using statistical methods is an important tool for climate studies. Seasonal changes determine the distribution of precipitation occurrence over a year period.

The purpose of the present study was to investigate the seasonality variations of monthly rainfall data using circular statistics method at 28 rain gauge stations in a 30-years period. According to the results, the seasonality index of monthly rainfall, standard deviations and standard deviations were calculated in the monthly time scale. In the circular statistics method, the time of occurrence of the event is determined based on the angle on the scale of the studied event interval.

According to the results, the value of the seasonality index (r) in the 8 rain gauge stations are greater than unity, which includes Pol Almas, Kousetopragi, Nir, Lay, Sarein, Siyahpoosh, Neur and Yamchi, meaning that the occurrence of precipitation in these stations occur completely with seasonal natureAccording to the results, there is no correlation between the mean amount of rainfall and the values of the seasonality index (r). The highest Seasonal index (r) is calculated for the Lay station with a value of 1.53 and the minimum value of the seasonality index (r) is assigned to the time series of Totoonsin station with zero SI value. At most of the studied stations, it is observed that the value of the SI value (r) is close to 1 which indicate the seasonal nature of the precipitation occurrence.

Based on the values of the seasonality index, wet periods and the time distribution of rainfall in different months of the year can be determined. Also, the seasonality index can be used to determine the duration of droughts. The numerical calculation of the index over a long period of time can provide a more comprehensive understanding of precipitation distribution. Most of the rain gauges with seasonal rainfall pattern located in the east and west part of the Ardebil province. In conclusion, determining the distribution of precipitation in the study area showed that the precipitation occurrence in the Ardabil province occurs seasonal with distribution of precipitation in two to three months of the year.