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چکیده
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Disturbance is a fundamental element to maintain structural, species and functional diversity in forest ecosystems. Fagus orientalis is one of the most ecologically and economically important trees of the Caspian region, Northern Iran. These valuable ecosystems are affected by different disturbance regimes including gap formation, harvesting and cattle grazing. We examined biomass allocation of seven functional plant groups (FPG) under six different disturbance regimes. Data sampling was carried out in four beech associations; Fagetum orientalis, Carpino betulii-Fagetum oientalis, Rusco hyrcanii-Fagetum orientalis and Ileco spinigiera-Fagetum orientalis and fresh biomass for FPGs were measured in 520 sample plots of 1 m2 area. Forbs (183 gr/m2) and legumes (0.1 gr/m2) had the highest and lowest biomass, respectively. Nested GLM analysis revealed that disturbance regime only affected fern and shrub functional groups, whereas forest association only affected shrub functional groups biomass. Forward selection in RDA resulted in eight important variables out of 42 environmental variables. Among these relative light (light group), elevation, inclination (topography group), saturation moisture, potassium, pH (soil group), stocking rate and gap-grazing disturbance regime (disturbance group) had the largest impact on biomass allocation of FPGs. Selected variables explained 24 % of the total variation in the biomass data and partial RDA showed that disturbance, topography, soil and light group, respectively, explained 8.0, 7.4, 6.1 and 4.5 percent of the total variation (with spatial distribution as co-variable). Our result showed that disturbances like gap and cattle grazing in forest openings can significantly increase the biomass of ferns and shrubs. When soil moisture is high, ferns like Matteuccia struthiopteris and Pteridium aquilinum dominate the gaps and in drier gaps Rubus spp. Crataegus spp. and other shrubs will increase.
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