جستجوی مقالات مرتبط با کلیدواژه « distribution pattern » در نشریات گروه « کشاورزی »

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.

Quantification of forest structure is one of the most important factors in sustainable management. The aim of this study was to compare the survey method and the effect of area and shape of sample plots in quantifying the structure of oak forests in the middle Zagros. To that end, an area of 30ha in the forests of Qhale-Gol Khorramabad was identified and a complete inventory was conducted. The data were entered into the geographic information system (GIS) environment in the form of information layers, and then, based on the objectives of the research, a regular random grid with specific dimensions of 100x100m and with an intensity of 10%, 25 sample plots (circle and square shape) were taken. In addition, to investigate the effect of the area of the sample plots on the structural characteristics of the forest, square plots of 25, 50, 75 and 100 were randomly taken with an intensity of 10%. According to the Mann-Whitney test, the shape of the sample plots had no significant effect on the results, and the dominance and differentiation indices in all methods had the same results. However, the area of the plots and their distribution had an effect on the results of the two indices of Mingling and Uniform angle. For the Mingling of 50 to 100 are plots and according to the Uniform angle index, 12 and 25 are plots had a significant difference from the average of the society. Nevertheless, no apparent difference was observed in the range of values and classes of any of the structural indices. according to this, all examined methods and sample plots have an excellent ability to describe the structure. In the case of more sensitive indicators such as Uniform angle and Mingling, the error can be reduced by applying corrective methods such as changing the number of neighbors.

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.

Chaetognaths (arrow worms) are strong predaceous mesozooplankton; which feed on other more abundant zooplankton’s groups; their most important preys are Copepods and Appendicularians. So they are considered as one of the most important groups among mesozooplankton communities, especially in the pelagic food web, also widely distributed in the neritic zone of all the oceans. Considering the fisheries activities and ecological aspects of economically and traditionally important ecosystems of the northern coastline of Bushehr, this study aimed to identify, introduce and assess the temporal and spatial distribution patterns of the Chaetognaths in the Persian Gulf, restricted to coastal waters of this area. Chaetognaths were collected during seven sampling programs, from Frake river-estuary, three sites of large creek-estuary of the Shif, one site in the sea, and one site in the Lashkari creek-estuary, at the middle and last day of each season. During all surveys, the depth of the sampling location, Secchi depth, water temperature, pH, and salinity were measured. Finally, 4 species were identified, Flaccisagitta enflata, Aidanosagitta neglecta, Aidanosagitta regularis, and Zonosagitta pulchra, with relative abundances of 56.56%, 29.42%, 11.25% and 2.87% respectively. The highest total density was observed in the sea station at the cold season, for two species of F. enflata and A. regularis. A. neglecta and Z. pulchra reached to the highest density in coastal stations and shif creek at summer. Based on dbRDA (distance based Redundancy Analysis), the depth of sampling location explained effectively the distribution pattern of F. enflata and A. regularis, with a positive correlation,   but salinity demonstrated a negative effect. Also based on the results of dbRDA, depth of the habitat is the most important factor which explains negatively the distribution pattern of A.neglecta and temperature was the correlated factor with the distribution pattern of Z. pulchra in our study area.

Quantitative methods to the evaluation of vegetation are the basis for describing and analyzing the plant communities and density as one of the main characteristics plays an important role in evaluating the performance (short and long effects) of rangeland ecosystems and management decisions. Therefore, the efficiency of plotless plant density methods in different density intensities and distribution patterns have been examined using a simulated population based on the real density of Astragalus albispinus (control method) in a semi-arid region of Marjan, Boroujen. Simulation schemes in the three density intensities and three distribution patterns within eight 40×100 m sampling units were designed. The predictive precision and accuracy of seven distance-based methods in various density intensities and distribution patterns were investigated comparing with the control method and the ideal point error index. Overall, angel order method in regular distribution pattern and point centered quarter method in random distribution pattern were the best plotless density methods of plant populations in low density. Third closest individual method was the most accurate and precise method of density among the studied methods in other different density intensities and distribution patterns. Therefore, in selecting the most appropriate distance method for estimating the density, the type of distribution pattern and in some cases the intensity of plant species density should be considered to achieve an unbiased estimation of plant populationʼs density, accordingly, the different habitats of this plant species can be monitored and evaluated in terms of management and utilization.

The Arghavan Valley of Binalood county located at Khorasan Razavi province, is a habitat for valuable and almost rare plant community in eastern Iran which is dominated by Cercis griffithii Boiss. shrubs. This habitat is very important for environmental aspects and combat desertification. It has documented the place where arid and semi-arid shrublands are destroyable, highly unrestorable. The Arghavan valley is located at geomorphological faces of mountainous which contains two distinct geomorphic facies of regular slopes and irregular ones that appear as rock outcrops with debris. Apparently, this shrub has a kind of distribution pattern and biological power which is related to the geomorphic facies. Then, to survey  the density of the shrubs in soiled slopes and rock outcrops, five sites were selected in such a way that enjoy the most similarities with each other to minimize the impact of other environmental factors. By counting the number of the shrubs and determining the area of ​​each site using Google Earth software, density of the shrubs determined and statistical differences of facies were compared. In order to evaluate and compare vigority of the shrubs, 30 plants were selected randomly and their canopy cover was calculated. 15 plants were then selected and leaf areas of 12 leaves were measured at each plant. Two attributes of leaf area and canopy cover were determined and compared statistically. Based on the results, at two geomorphic facies, mean density and leaf area were significantly different at 95% level, and the difference in mean canopy cover of the shrubs was significantly at 99% level. The shrub, therefor, has higher density on rocky outcrop with debris but more vigor on regular slopes. Favorable micro-climatic conditions of rocky outcrops for seed and seedlings establishment may cause higher density at these facies. More possibility of root development, water and nutrition absorption at soiled slops caused more vigority at these geomorphological facies.

The evaluation as well as data banking of distribution patterns are considered as the most important management action for the conservation of biodiversity. Iran is considered as one of the most important of diversity centers of Onosma L. and includes a high rate of endemism. Due to the lack of adequate data on conservation and distribution patterns of the genus at local scale, the current study was conducted in Central Alborz, as an important diversity and endemism centers in Iran, based on field and herbarium assessments, using Geographic Information System. Sampling was performed as a survey of reported points as well as other parts of the region. For physico-chemical analysis of soils, pH, nitrogen (N), phosphorus (P), potassium (K), organic carbon (C) and soil texture of each habitat were measured from the surface to the depth of plant roots. Results indicated that studied species are mainly distributed at the elevation range of 1257 to 3060 m.a.s.l. Members of Haplotricha subdivision are scattered at an altitude of 307 to1257 m and Heterotricha subdivision at an altitude of 2880-1589 m. Dominantly, the studied species are distributed in semi-arid and Mediterranean climates. However, some species are distributed in semi-humid climate. Generally, most of the species are distributed in habitats with an average annual rainfall of about 250 to 500 mm, on sedimentary, sedimentary-volcanic, igneous and rarely on quaternary deposits. Soils` pH ranged between 7.13 (O. gaubae) and 8 (O. dichroantha). Studied species are often on loamy clay sandy soils. Less than 50% of the populations of various Onosma species, across the study area, are located in protected areas. In addition, O. gaubae is classified as a rare taxon with a priority for conservation, due to its very small population size and geographical range. The current study has also analyzed the distribution patterns of Onosma in Central Alborz region, based on geo-botanical factors. Findings of this study can be used as an important step to improve the distribution patterns of the genus in Iran and the southwest Asia.

Tree marking can cause changes in forest. This study aimed to assess tree marking operation and to predict its effect on stand structure of Persian ironwood-hornbeam stands at Bahramnia forestry plan, north of Iran. Therefore 98 trees that were marked in compartment 9 of the mentioned forest in 2017 based on selection system and according to the recent laws of Forests, Range and Watershed Management Organization of Iran were measured before cut. Characteristics such as tree species, diameter, height, volume, decay condition and standing or fallen situation of the trees were recorded.  Independent samples t-test was used to compare the mean diameter, height and volume between marked trees and stand trees. To obtain the mean diameter and height of the stand trees 16 circular sample plots with the area of 1000 square meters were considered in which the diameter and height of all trees were recorded. The results showed that the majority of marked trees, based on both tree numbers and volume, belong to hornbeam species while the dominant tree species of the compartment is Persian ironwood tree. This result shows that the marking operation acted to change the species composition of the stand in favor of Persian ironwood species. In comparison of the mean diameter, height and volume between marked trees with stand trees a significant difference were observed in terms of diameter and volume, but in case of trees height no significant difference was found. Studying the distribution pattern of marked trees indicated a cluster distribution pattern. Totally it can be concluded that tree marking operation according to the recent laws of Forests, Range and Watershed Management Organization, which emphasize on marking and cutting of fallen and broken and decaying trees, in long term will result in loss of dead and decaying trees in forest ecosystem and will decrease ecological capacity of these forest ecosystems.

Appendicularians are the most important group among zooplankton communities, widely distributed in all the oceans and they are considered as one of the most important sources of productivity and food supplies for secondary consumers. So this study aimed to identify, introduce and assess the temporal and spatial variations of the Appendicularians in the Persian Gulf, restricted to coastal waters of northern part of Bushehr. Appendicularians were collected during seven sampling programs, from Frake river-estuary, three sites of large creek-estuary of the Shif, one site in the sea, and one site in the Lashkari creek-estuary, at middle and last day of each season, midsummer of 2016 to spring of 2017. During all surveys, depth of the sampling location, Secchi depth, water temperature, pH, and salinity were measured. Finally, 4 species were identified, Oikopleura dioica, Oikopleura longicauda Appendicularia sicula and Fritillaria sp. The highest total density was observed in the sea station at March, with dominance of O. dioica and a relative abundance of 80%, relative abundance of O. longicauda, A. sicula and Fritillaria sp. were calculated 18.9%, 1.06% and 0.04% respectively, so Fritillaria sp were introduced as rare species. Two species of O. dioica and O. longicauda demonstrated completely different spatial and temporal distribution pattern, and based on dbRDA (distance based Redundancy Analysis), transparency explains effectively the distribution pattern of O. longicauda, and also depth and salinity are the most important factors which explain the distribution pattern of O. dioica and temperature was correlated factor with abundances of A. sicula, in our study area.

This study was carried out in Noyjian forests of Khorramabad, the destructed and less destructed stands were investigated in terms of three structural aspects: species diversity, spatial pattern and dimension variation have been compared. For this purpose, at first based on the available information and see the forest closely, two classes of crown dense 35% and more than 35% were bounded. Then in each of this part, 10 (one-acre) sample plots were taken with the similar physiographic conditions. According to the results, the diversity in these stands is low, but in general, the diversity in the less destructed stands is more than the sparse stands. Among the species diversity indices, the Margalef index shows the most difference. In relation to the pattern of distribution, the mean aspect and uniform angle indices show similar results based on random distribution of most species in both stands. According to the dominant index these species (Quercus brantii, Acer monspessulanum and pears) are dominant and other species are defeated. In general, the mean of the differentiation index shows a large difference between canopy coverings in different stands, which is more than that in sparse stands. According to the result, in general, the structure of less- destructed stands is in a more favorable position in different aspects. Since the physiographic conditions of these stands are similar, human factors can be considered as the main cause of the apparent difference between these stands. Therefore, it can be concluded that in the reduction of these factors, protection and support of destructed stands, the condition of this destructed stands will be more desirable.

Recognition of spatial distribution pattern of plants is useful in describing the ecosystem stability, designing management plans and protective rehabilitative measures. In order to determine the distribution pattern of certain plant species in Ghamishloo National Park, six safe regions were selected. Three hundred sixty quadrates with the size of 1×1 m2 were established by using random-systematic method along six transects and the number of individuals of each species was recorded. In this study, dispersion indices such as clustering, Green, Lloyd and Morisita were used. Since a large number of plants with different densities were selected, the results of indices showed that density had no effect on distribution pattern. The values of Green's index showed a pattern between random and non-random (clumped) distribution pattern. Therefore, it is recommended to use maximum clumped values to determine the exact type of distribution pattern. Among the study indices, no specified maximum value was defined for Lioyd and Morisita regarding the clumped pattern. For this purpose, two formulas were defined to determine the maximum clumping and the results of indices were interpreted according to these formula.

The distribution pattern is one of the most important features of plant communities which refers to the condition and distribution of a species within a plant community. This study aims at investigating the distribution pattern of Orchard grass (Dactylis glomerata) assuming that the density of the plant is procedural. The study area was located in Northern Khorasan province, 30 km from the south of Dargaz. A systematic random sampling method was used to collect samples from three Northward ranges. Each domain was divided into three sub-domains, including high density, medium density, and low density (from high to low range). In each sub-domain, the random sampling was carried out from five plots (2×1) and one 30-meter transect which contained 10 randomly selected spots. A total of 45 plots and 9 transects were used to sample the region. Indices used in this study included Johnson-Zimmer distance index and the quadrate indices of Eberhart and the standard Morisita. The experiment carried out in Shekarab region of Tandoureh National Park revealed that this plant has a clumped distribution pattern based on Johnson-Zimmer and Eberhart indices. Morisita indices and standard Morisita showed different distribution patterns.

Knowledge of species distribution patterns in each area is the basis of vegetation measurement. Determining the species distribution pattern helps us select the appropriate methods for measuring quantitative properties such as species density and abundance. In order to study the distribution pattern of three shrub specie, a part of Kurdistan rangelands was selected as the study area. A random systematic sampling method was used along four transects of 300 meters. The first point on each transect was randomly selected and the rest (14 points) were selected along each of the transects with 20-meter intervals from each other. In each vegetation type, criteria such as density and abundance were measured in plots, and the distance from a random point to the nearest plant and to the nearest neighbors was determined along the transects. In addition, the distribution pattern of Acantholimon bracteatum, Astragalus gossypinus and Acanthophyllum mucronatum was determined using distance indices (Eberhardt, Hopkins, Johnson and Zimmer) and the quadrate indices (Green, Morisitas index, Morisitas standard, and Lioyd). The results of analysis showed that the distance indices had more accuracy than quadrate indices with the same performance.

A forest stand of 25 ha in Tol-e Gah region nearby Yasuj (Kohgiluyeh and Boyer Ahmad province) was selected to assess the spatial distribution pattern of its dead trees of Brant`s oak (Quercus brantii Lindl). The fixed area plot samples and adaptive cluster sampling method were employed to collect qualitative and quantitative information of the forest stand. Using sixty-five, 10-Ar sample plots, quantitative characteristics of all dead trunks together with those for the healthy trees in the sample plots were recorded in a way that each dead trunk found in each of the plots was considered as the center of another sample plot. In addition to determining the geographical coordinates of the dead trunks, diameter at breast height, tree height, and crown diameter of trees were measured. The Johnson-Zimmer, Eberhart, and Pielou indices (based on the distance of the sampling spot to the individual trees) and the variance to mean ratio, Morisita index, and standardized Morisita indices (efficient in using plot samples) were applied to quantify the distribution pattern of dead trees. All sample plot-related indices indicated a clumping distribution pattern for the dead trees. Among the distance indices, the Johnson-Zimmer and Eberhart indices also showed a clumping and a uniform pattern of dead tree distribution, respectively.

In order to investigate the effect of crop rotation on relation of seed bank with nightshade (Solanum nigrum L.) weed seedling population during the growth season of corn, a field experiment was carried out in Dasht-Naz Crop Company in Sari, during 2012. At the first, farm was divided into 2×2 m networks and the cross points was signed and all sampling were taken from those points. Sampling of seed bank and nightshade seedlings were done in two and three stages, respectively. Spatial distribution maps at the end of cropping season indicted that patches with high density of weed seed increased in corn-faba bean-corn rotation while decreased in soybean-wheat-corn rotation. There were strong and medium correlations for nightshade weed seeds at all sampling stages. Also, two crop rotations had a significant difference in terms of seeds averages and emerged seedlings. Since, seeds averages and emerged seedlings were lower in corn-faba bean-corn rotation than corn-wheat-corn rotation; therefore, faba bean was more appropriate in rotation with corn to reduce nightshade weed density not only in the field but also within seed bank.

Since the roads are considered as one of the indicators of sustainable development and much of the rural dusty roads are passed through the rangelands thus, the effect of roads has a very importance effect on rangeland ecosystems. Near-road rangelands were selected from the edge of the road until 5 meters distance and far-road rangelands were used in 5 to 20 meters from the edge of the road. For measuring canopy cover, production, density, relative importance, richness, evenness and diversity of plants used to 50 m transects and 1 m2 plots were used in each study areas. 4 transects and 40 plots randomly were put along three kilometers of road of Pishkouh Ali-Abad rangelands in the Yazd. Results showed that the road can changes the vegetation composition, production, and cover, therefore,there is reducation in production, canopy cover of near – road. The importance value of shrub species of near road has declined while the importance value of grasses has increased. Richness, evenness and diversity indices were reduced and distribution of plants, moved to the clamps. These factors indicated the site of near- road is less stable and plant community has crossed boundaries of ecological threshold. Therefore, it is suggested that making the rural dusty roads should be implemented with detailed studies and suitable reclamation may be done with paying attention to ecological condition of the study area.