Effect of canopy gap size and ecological factors on species diversity and beech seedlings in managed beech stands in Hyrcanian forests

We studied the species diversity of the herb layer and ecological factors in harvest-created gaps in beech stands under a single-tree selection system in Northern Iran. To determine diversity, the number of beech seedlings, and other ecological factors, 16 gaps were selected and subplots of 5 m2 were positioned at the centre and at the cardinal points of each gap. Species richness and Simpson diversity index increased with increasing gap area as did numbers of seedlings. With increasing humus layer thickness, species richness declined but the Hill evenness index increased. Species richness increased with increasing light availability. There was no relationship between crown radii of beech trees and diversity indices. Correlations between environmental factors and numbers of individuals of some species in the herb layer were not significant except in a few cases. The results help explain the effects of man-made gaps on the dynamics of managed beech stands and this benefits evaluation of silvicultural operating plans.

Symbiotic nitrogen fixation in black locust (Robinia pseudoacacia L.) seedlings from four seed sources

We conducted a greenhouse experiment to investigate the role of seed source in growth and symbiotic nitrogen fixation of black locust （Robinia pseudoacacia L）. Seeds from different sources were planted in the same environmental conditions and inoculated with a suspension of mixed Rhizobium. We used the modified ^15N isotope dilution method to estimate biological nitrogen fixation of Robinia trees. Different Robinia seed sources differed significantly in terms of tissue dry weight （50.6-80.1 g）, total N （1.31-2.16 g） and proportion of nitrogen derived from the atmosphere （ 0-51%）. A higher nitrogen fixation rate of Robinia trees was associated with higher dry weight. Moreover, the leaves of Robinia proved to adequately represent the nitrogen fixation capacity of entire plants. Our results confirmed that assessment of seed sources is a useful way to improve the nitrogen fixation capacity and therefore the growth rate of Robinia.

The effect of single-tree selction system on soil properties in an oriental beech stand of Hyrcanian forest, north of Iran

A case study was conducted in beech forests of northern Iran to determine the effect of the created gaps on some soil properties in beech stand. Changes of soil properties in small （60-150 mE）, medium （151-241 mE）, large （242-332 m^2） and very large （333-550 m^2） gaps, as well as under closed stands were studied eight years after-the gap creation. Soil samples were taken from three depths, 0-10, 10-20 and 20-30 cm. The gaps were different from their around undisturbed stands in terms of the following soil parameters： Mg^＋2 concentration of 0-10 cm at medium gap size, bulk density of 10-20 cm at very large gap size as well as K^＋ and Ca^＋2 concentrations at 20-30 cm at small and large gap sizes, respectively. Furthermore, the size of the gaps had no effect on soil characteristics through the whole profile. Water saturation percent （Sp %） at 0-10cm as well as P and Mg^＋2 at 20--30 cm was different amongst undisturbed stands around different gap sizes. The center and the edges of the gap were different only in terms of organic carbon at the depth of 10-20 cm. Significant differences were observed between gaps and closed canopy regarding P and Ca^＋2 at depth 0--10 cm and 10-20 cm, respectively. It can be concluded that applied silvicultural system for harvesting trees which created these gaps might be suitable for conservation and forest management in the region.

The influence of environmental variables on distribution patterns of Irano-Turanian forests in Alborz Mountains, Iran

Topographic and edaphic variables are the main ecological factor determining species spatial variability on mountainous forests. A field study was performed in central Alborz to investigate how the edaphic and topographic parameters can affect the tree and shrub communities. Initially, 27 forest stands were identified and the homogeneous units were separated regarding physiognomy. In each single homogeneous unit, one random sample plot(1000 m^2) and totally 43 plots were established. In each plot, the presence and abundance of all trees and shrubs were recorded and four soil samples were taken from depths of 0-5 and 5-20 cm. Concerning classification results, eight different forest communities were identified. The lowest and highest soil p H values were observed in Malus orientalis and pistacia-Amygdalus communities, respectively. The water saturation percent of pure- and mixed Juniperus excelsa and Rhus coriaria was the highest amongst communities. The clay content was the highest in pure J.excelsa. The 0-5 organic matter and Nitrogen content in mixed J. excelsa were significantly higher than pure J. excelsa and other communities. The CCA(Canonical Correspondence Analysis) results indicated that the altitude, precipitation, pH, EC, SP, clay and CaCO_3 are the most important factors determine the distribution of trees and shrub in central Alborz.

The effects of fire and seasonal variations on soil properties in Juniperus excelsa M.Bieb.stands in the Alborz Mountains,Iran

This study was conducted in Juniperus excels a stands on the southern slopes of Iran’s Alborz Mountains,to determine the effects of fire and seasonal variations on soil physicochemical properties and enzyme activities.A total of 64 composite soil s amples were randomly collected in the spring and fall from two burned and unburned sites at depths of 0-10 and 10-20 cm.The results of a two-way ANOVA analysis indicate that fire increased the organic carbon(OC),total nitrogen(TN),and available phosphorus(P_(ava)) contents of the soil by 16%,59%,and 53%,respectively.Similarly,when burned sites were compared to unburned sites,the activities of acid phosphatase(ACP) and urease enzymes increased by 73% and 12%,respectively.Nevertheless,fire did not affect soil texture,bulk density(BD),pH,electric conductivity(EC),exchangeable potassium(Kexc),or the activities of alkaline phosphatase(ALP) and dehydrogenase.According to two-way ANOVA results,OC,TN,P_(ava),K_(exc),and EC values were significantly higher in the fall,whereas pH and ALP values were significantly higher in the spring.Additionally,phosphorous and dehydrogenase activity were significantly different in selected soil factors at 0-10 and10-20 cm depths.The interaction of fire,season,and soil depth were significant for phosphorous and urease.Moreover,the activity of ALP correlated well with pH(r=0.68),P(r=-0.74),OC(r=-0.53),and TN(r=-0.37),whereas the activity of ACP correlated significantly with OC(r=0.64) and TN(r=0.71).Two years after the fire,soil properties in J.excels a stands had either improved or remained unaffected,returning to pre-fire levels.Additionally,soil chemical properties varied significantly across sampling seasons,which should be considered when comparing and interpreting soil data in future research.