| The tolerance of plants to herbivory reflects the degree to which a plant can regrow and reproduce after damage from herbivores,mechanisms of tolerance are linked with the relative growth rate of the plant species,biomass allocation,the compensatory growth and ability to shunt carbon stores.Kobresia humilis meadow lies in a very cold area,the haberages that growing in alpine meadow contain high nutrient and is the important material foundation to develop the grassland agriculture of the the Qinghai-Tibetan Plateau.Kobresia humilis and Elymus nutans is the dominant species and important companion species in alpine K.humilis meadow on the Qinghai-Tibetan Plateau.In this paper,we study the grazing tolerance mechanism of the two clonal plants K.humilis and E.nutans in an alpine Kobresia humilis meadow through setting grazed/ungrazed plots and measuring their biomass allocation,compensatory growth and total non-structural carbohydrates after grazing defoliation,which provide scientific basis for the protection,updates and sustainable utilizing of alpine pastures.Four habitats with different disturbance factors,disturbance intensity and resource availability were selected in the study,i.e.pen pasture,pass pasture,artificial pasture and control.The main results as follows:(1) The soil resources was higher than the other habitats in pen pasture;the total nitrogen in pass pasture was higher than in artificial pasture and control pasture;organic matter,total phosphorus,nitrate nitrogen and phosphorus pentoxide were no significant difference between artificial pasture and control pasture,there is lower nutrient availability.Compared with the control pasture,the community defoliation was highest in pen pasture,lower in pen pasture,lowest in artificial pasture;the community coverage was highest in pen and control pasture,lower in pass pasture,lowest in artificial pasture.(2) The defoliation of K.humilis in the pen pasture,the pass pasture and the control pasture was 68.90%,42.36%and 0.00%,respectively.K.humilis exhibited over-compensation in pass pasture, and full-compensation in pen pasture.Besides the biomass allocation of stalks+inflorescences in June,no significant differences were found between grazed and ungrazed treatments.In June,the biomass allocation of leaves decreased and the biomass allocation of rhizomes increased with the increasing of defoliation;while only the biomass allocation of stalks+inflorescences was significantly different in August 2007,which was highest in pass pasture,there was no significant difference between pen and control pasture.The total non-structural carbohydrates contents of different parts of K.humilis ramet were highest in pass pasture in June;and the total non-structural carbohydrates contents of different parts of ramet in pen pasture was lower than in pass and control pasture in August,there was no significant difference between pen and control pasture.In addition, the total non-structural carbohydrates contents of different parts of ramet in pass and control pasture were twice that in June 2007.(3) The independent variable that enters the equations can explain the compensation of K. humilis ranged from 62.7%to 99.6%.The defoliation of K.humilis was negatively related to the compensation of aboveground and leaves,the water content of soil was negatively related to the compensation of different ramet parts,the absolute value ofβcoefficients of the defoliation and the water content of soil were lowest,which showed that it was not the most important impacting factors of the compensation.The compensation of aboveground and leaves was positively related to total nitrogen and negatively related to organic matter and nitrate nitrogen,the compensation of inflorescences was positively related to total nitrogen and negatively related to ammoniacal nitrogen. The regression equations also showed that total nitrogen,organic matter,nitrate nitrogen and ammoniacal nitrogen had significantly influenced the compensation of different parts of K.humilis ramet,thus,nitrogen fertilizer have an effective influencing for the compensatory growth of plants and grass arrangement.(4) The defoliation of E.nutans in the pen pasture,the pass pasture,the artificial pasture and the control pasture was 42.41%,26.59%,6.29%and 0.00%,respectively.The results showed that aboveground and leaf showed over-compensation in pen and artificial pasture,and full-compensation in pass pasture;total dry weight exhibited over-compensation in artificial pasture, and full-compensation in pen and pass pasture.In June,the biomass allocation of leaves decreased with the increasing of the defoliation.In August,the biomass allocation of leaves in pen and artificial pasture was higher than in pass and control pasture,the biomass allocation of stalks+inflorescences was highest in pen pasture,lower in artificial pasture,lowest in pass and control pasture;the biomass allocation of roots was contrary to the biomass allocation of leaves.In June,the total non-structural carbohydrates contents of leaves in pen,pass and artificial pasture was higher than in control pasture; the total non-structural carbohydrates contents of roots was highest in pass and artificial pasture, lower in pen pasture,lowest in control pasture.In August,the total non-structural carbohydrates contents of leaves in pass and artificial pasture was higher than in pen and control pasture;the total non-structural carbohydrates contents of roots in pen,pass and artificial pasture was higher than in control pasture.And the total non-structural carbohydrates of contents of different parts of ramet in August were twice that in June.The relative growth rate of the aboveground biomass tended to increase with the increasing grazing density,the rament density in artificial pasture was twice that in control pasture,no difference was found between in pass and control pasture. (5) The independent variable that enter the equation can explain the compensation of E.nutans ranged from 99.4%to 99.7%,the equation showed that the defoliation of E.nutans was positively related to the compensation of different ramet parts,and the water content of soil was negatively related to the compensation of different ramet parts.The absolute value ofβcoefficients of the defoliation of E.nutans and the water content of soil were lowest,the compensation of aboveground and leaves was positively related with organic matter and negatively related with total phosphorus; the compensation of total dry weight was positively related with phosphorus pentoxide and negatively related with nitrate nitrogen and ammoniacal nitrogen.The regression equations also showed that total phosphorus,organic matter,phosphorus pentoxide,nitrate nitrogen and ammoniacal nitrogen had significantly influenced the compensation of different parts of E.nutans ramet,the phosphorus fertilizer have a significant influencing on the compensatory growth of E. nutans.To sum up,K.humilis exhibited over-compensation in pass pasture,and full-compensation in pen pasture.And,aboveground and leaf biomass of E.nutans showed over-compensation in pen and artificial pasture,and full-compensation in pass pasture.Although there is highest nutrient availability in pen pasture,K.humilis showed full-compensation because of the long-term heavy grazing and trampled,and the intense competition may have influenced the compensatory capacity of K.humilis.E.nutans have greater competitive ability,with overall plant size and earlier germination.Meanwhile,E.nutans is able to over-compensate under heavy defoliation,due to high nutrient availability in pen pasture.Nitrogen fertilizer have an effective influencing for the compensatory growth of K.humilis and grass arrangement,while,phosphorus fertilizer have a significant influencing on the compensatory growth of E.nutans.Therefore,K.humilis exhibited over-compensation in moderate grazing condition,the heavy grazing will influence the long-term growth of the population.In order to develop the artificial grassland of E.nutans and prevent from degradation,fertilization management should be strengthened. |
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