| The grazing-tolerance of plants refers to the compensatory capability of which under grazing to the injured tissue. The compensatory growth and relative growth rate increases and biomass allocation changes under herding are important mechanism of increased grazing-tolerance. Kobresia humilis meadow, where rich in nutrient and resistant to herbivory, plays an important role in the grassland agriculture of the Qinghai-Tibetan Plateau. In order to reveal the compensatory mechanisms and the grazing-tolerance ability of dominant species of the meadow, to test validity of the hypothesis of compensatory growth and find the reason of Kobresia humilis meadow being a grazing disclimax, an field manipulating experiment with split-pot design was conducted with clipping treatment on the whole plot (stubbled1cm, stubbled3cm and unclipped) and both fertilizing (fertilized Urea+DAP and unfertilized) and watering (watered and non-watered) treatments in subplots at the Haibei Research Station of the Chinese Academy of Sciences. Our objectives were to examine the compensatory growth of four species, i.e. Elymus nutans, Stipa aliena, Kobresia humilis and Carex scabrirostris, including ramet height, ramet density, compensatory aboveground biomass per ramet and relative growth rate (RGR). The main results as follows:(1) The compensatory height of E.nutans and S.aliena was significantly reduced with the clipping intensity increased. The compensatory height of which under heavy clipping was reduced by27.35%and34.46%compared with no-clipping; K.humilis and C.scabrirostris in heavy clipping were reduced by17.02%and16.33%, but had no significant difference between moderate clipping and heavy clipping. It confirmed that Gramineous plant was more vulnerable to clipping, especially S.aliena. The compensatory height of E.nutans, S.aliena, K.humilis, C.scabrirostris was significant increased with fertilizing treatment, the increased amount was38.08%,19.31%,50.11%and71.86%, respectively. It indicated that promotion of compensatory growth in fertilizing was greater on the two Cyperaceae plants.(2) Only ramet density of S.aliena was significant increased (45.01%) after clipping, ramet density of E.nutans, K.humilis and C.scabrirostris showed no significant statistical variation among different clipping treatments, but compared with no-clipping, ramet density of E.nutans was reduced by12.93%under heavy clipping, and ramet of the two Cyperaceae plants was increased by30.12%,69.64%respectively. It indicated that Gramineous plant was more vulnerable to clipping when it comes to ramet density, and Cyperaceae plant was steady increased relatively. Ramet of E.nutans was increased by35.02%in fertilizing, but the other three plants showed no variation, indicated that response of ramet density to fertilization among different species are variable.(3) The compensatory aboveground biomass of E.nutans and S.aliena was reduced by41.97%,52.19%respectively under heavy clipping, K.humilis and C.scabrirostris didn’t show significant variation. The compensatory aboveground biomass of all of the four species were significant increased in fertilizing, the increased amount was138.6%,20.69%,145.6%and62.52%respectively. It indicated that compensatory aboveground biomass of Gramineous plant was more vulnerable to clipping, and promotion of the four species in fertilizing were variable, effect in E.nutans and K.humilis was more evident relatively, C.scabrirostris was the second, and the K. humilis was the weakest.(4) The RGR of height of the E.nutans, S,aliena, K.humilis and C.scabrirostris under heavy clipping were increased by626.71%,703.28%,640.35%and408.98%respectively, on the whole, Gramineous plants was higher than Cyperaceae plants; and it increased by44.08%,21.80%,77.34%and77.85%in fertilizing, as a whole, Gramineous plants was lower than Cyperaceae plants. But it increased by47.11%,28.13%,75.73%and68.76%under interaction of heavy clipping and fertilization treatment, the two Cyperaceae plants was significantly higher than the two Gramineous plants. It indicated that fertilization could be a great promoter to the plant which under clipping, especially Cyperaceae plants. The great improvement of fertilization to RGR of height might be an important mechanism of compensatory growth of Cyperaceae plants.(5) The RGR of the compensatory aboveground biomass of E.nutans was increased by98.17%under heavy clipping when compared with no-clipping treatment, while S.aliena was reduced by83.18%, and the two Cyperaceae plants showed no significant changes; It increased by81.74%of the E.nutans which in fertilizing, and C.scabrirostris was reduced by51.91%; The interaction of fertilization and clipping had no significant effect on S.aliena and K.humilis, the RGR of the compensatory aboveground biomass of C.scabrirostris was reduced by68.76%. It indicated that clipping had more significant effect on RGR of the compensatory aboveground biomass of the two Gramineous plants, and the effect were different between the two Gramineous plants; Fertilization played a limited role on the RGR of the compensatory aboveground biomass of the plants which under clipping and the effect were different among different species.(6) The effect of watering treatment was not significant in our study, it may be indicated that water is not a limit factor for compensatory growth. Interactive effects of fertilization and watering has to be seen only in the compensatory aboveground biomass of K.humilis. The compensatory aboveground biomass of the K.humilis will reduce on watering with no-fertilizing. Therefore, the improvement of the compensatory aboveground biomass is mainly due to fertilization.(7) The order of the compensation index of all the four species was K.humilis> C.scabrirostris>E.nutans>S.aliena. It suggested that tolerance ability of the two Cyperaceae plants was better than the two Gramineous plants. Under different treatments, the two Cyperaceae plants showed full compensation or overcompensation, the average compensation index ranged from0.957to1.632; the two Gramineous plants showed undercompensation or full compensation, the average compensation index ranged from0.838to0.940.When compared with the grazing-tolerance of the four plants, K.humilis is the greatest, and S.aliena is the weakest. Fertilization can significantly improve the tolerance ability of plants, but it diversed among different species. The average amount of compensation of K.humilis, C.scabrirostris, E.nutans and S.aliena in fertilizing were increased by58.85%,51.74%,97.98%and25.23%respectively, the promotion of compensation growth on E.nutans was the greatest, K.humilis and C.scabrirostris were the second, and S.aliena was the weakest. Watering treatment have no significant effect on the tolerance ability of the four plants, or even weaken it. Interactive effect of fertilization and watering have certain effect of enhancing the tolerance ability on the four plants.(8) According to correlation analysis among the compensatory biomass, relative growth rate and the compensation index, it shows that the tolerance ability of the four plants has significantly positively correlated relationship with compensation growth of plant height, ramet density and ramet compensatory aboveground biomass, but the tolerance ability of E.nutans and K.humilis didn’t show a significant correlation with relative growth rate, promotion effect of relative growth rate on tolerance ability reflected only in S.aliena and C.scabrirostris, suggesting that the intensity of grazing-tolerance have a more direct dependent relation with the size of compensation growth, and the correlation with relative growth rate may vary by species.In summary, the tolerance ability of plants were different, not only is related with plant shape and growth type, also associated with grazing intensity and soil resource availability. Fertilization will effectively enhance the compensation growth ability of plants, alleviate or eliminate the bad effect of heavy clipping, but the effect may diverse among different species. The study confirmed the antagonistic interaction between compensatory growth and interaction of fertilization and clipping, also consistent with the prediction of CRH and LRM. At the same time, it suggested that the enhancement of RGR under grazing or clipping is not the necessary and sufficient conditions of overcompensation, or GRM may not apply to all species. The study confirm that long run of grazing is the reason of Alpine K.humilis meadow to become a grazing disclimax from the perspective of compensatory growth changes of four dominant species under different utilizing intensity and pedogenesis nutrient gradient, and support it from the angle of population. The two Gramineous plants are prone to sustain population recession in heavy grazing because of weak tolerance ability, therefore, we should fix a moderate grazing intensity (about50%) in the management of artificial grass and protect it by adequate fertilization. In the study area, the grazing intensity and soil resource availability are prior in influencing tolerance ability of plants, thus, we should control grazing intensity reasonably in the use of natural alpine meadow, and improve the content of instant nutrients in the soil by fertilization, then removing the restrictive effect of plant growth and reproducing the because of lack of soil nutrients, which is good for overcompensatory, hence the alleviation of over grazing, also preventing grassland from degenerating. |
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