| The effects of stocking rates on plant population reproductive characteristics, resource allocation, nutrient allocation and stoichiometric characteristics of Stipa breviflora, Artemisia frigida and Cleistogenes songorica in Inner Mongolia desert steppe were measured and analyzed in 2008 and 2009, through the method of combining the fixed sampling quadrat randomly outside with analysis inside. The optimal stocking rate in desert steppe was analysed synthetically in order to provide theoretical basis for the grazing management in desert steppe in the future.The experimental site had been grazing for over 7 years, and was designed with random blocks completely. The experimental treatments were composed of 4 stocking rate levels (0, 0.91, 1.82, and 2.71 sheep hm-2half year-1), and denoted control (CK), light grazing (LG), moderate grazing (MG), and heavy grazing (HG), respectively. Each stocking rate level included three replications. The experimental livestock was two years old Mongolian wether with fuzz.The population reproductive characteristics of Stipa breviflora included absolute height, bunch density, bunch area, quantity and length of forage and reproductive branch, basal diameter of forage branch, characteristics of mature seed (length, diameter and thousand seed weight), awn characteristics (length of awn and pubescence, knee number and knee bending point position), root characteristics (root number, average length and diameter, root length density, specific root length and root area index) and root biomass. The resource biomass allocation including caryopsis biomass (mature seed and its awn, green seed & awn biomass), stem biomass (stem of forage and reproductive branch and total stem biomass), leaf biomass (leaf of forage and reproductive branch and total leaf biomass), total biomass of forage and reproductive branch, aboveground biomass and root biomass (taproot, lateral root and total root biomass).The population reproductive characteristics of Artemisia frigida and Cleistogenes songorica included absolute height, bunch density, bunch area, quantity and length of forage and reproductive branch, basal diameter of forage branch, root biomass (taproot, lateral root and total root biomass). The resource biomass allocation including aboveground (stem, leaf, and total biomass of stem & leaf) and root biomass (taproot, lateral root, and total root biomass).The nutrient allocation of Stipa breviflora, Artemisia frigida and Cleistogenes songorica were composed of total N and P contents, and stoichiometric rate included the characteristics of C?N, C?P and N?P in each module.The main results were as following:(1) In the peak growing season(July and Auguest), with the stocking rate increasing, the absolute height and total root biomass of Stipa breviflora, Artemisia frigida and Cleistogenes songorica decreased, and branch density increased. Meanwhile, the forage branch density and specific root length of Stipa breviflora increased, and reproductive branch density, root number and root area index decreased. However, the forage branch density and indefinite root density of Artemisia frigida increased with the stocking rate increasing.(2) Grazing treatment negatively affected on the seed characteristics mature in June significantly(P<0.05), such as length, diameter, thousand seed weight, and awn weight, while had no significant influence on the seed characteristics mature in September (P>0.05). However, the peak value of seed awn occurred in LG plots involving both mature in June and in September. And the mature seed awn length, pubescence length, number, and density which mature in September were significantly higher than that of mature in June (P<0.05).(3) As for Stipa breviflora mature seed, the second awn knee point in June was significantly higher than the first awn knee point in September and significantly higher than the first awn knee point in June (P<0.05). And grazing treatment decreased the first and second awn knee point in June, but had no significant effect on the first awn knee point in September (P>0.05).(4) With the stocking rate increasing, the resource allocation proportion of aboveground, forage branch, green seed & awn resource allocation increased, and reproductive branch biomass allocation, mature seed and awn resource allocation proportion decreased; and the biomass allocation proportion from bunch, forage and reproductive branch to leaf increased, that of to stem decreased. The resource allocation proportion of forage branch was significantly higher than that of reproductive branch in bunch (P<0.05).(5) Grazing treatment accelerated the stem resource allocation of Artemisia frigida and Cleistogenes songorica, but diminished the leaf resource allocation, so the ratio of stem to leaf increased. Grazing treatment also reduced the taproot and lateral root resource allocation of Artemisia frigida, however, increased that of Cleistogenes songorica, and the peak value of lateral root resource allocation proportion appeared in LG plots, so LG accelerated the root growth.(6) As for Stipa breviflora, the total N content of each module were peaked in LG plots. And the total N and P content were highest in seed mature in June, the total N content in taproot was secondary, and total P content in reproductive branch was least. The total N and P in stem and leaf of forage branch were higher than that of reproductive branch, and that of stem were higher than that of leaf. The total N and P in seed and taproot were higher than that of awn and lateral root, respectively. As for Artemisia frigida, the total N and P content in leaf were higher than that of stem, and than that of taproot, and than that of lateral root. As for Cleistogenes songorica, the total N and P content in leaf were higher than that of stem, and than that of lateral root, and than that of taproot.(7) It was limited by N that the stem and leaf of forage branch and seed matured in June & September of Stipa breviflora. The stem of reproductive branch was limited by N in CK and LG plots, and limited by P in MG and HG plots, and the leaf was limited by P. The stem, taproot and lateral root of Artemisia frigida were limited by N, and the leaf was limited by P in CK, while limited by both N and P in LG, MG and HG plots. The stem and taproot of Cleistogenes songorica were limited by N, and the leaf was limited by N and P, and the lateral root was limited by P in CK, while limited by both N and P in LG, MG and HG plots.In a word, so far as reproductive resource allocation and nutrient allocation be concerned, LG was the feasible stocking rate in Inner Mongolia desert steppe. According to reproductive resource allocation pattern, the capability of pattern of Stipa breviflora was leaf>stem>caryopsis>root, aboveground>root, forage branch>reproductive branch, leaf and stem of forage branch>that of reproductive branch taproot higher>lateral root, respectively. The capability of the allocation pattern of Artemisia frigida and Cleistogenes songorica were both stem>taproot>leaf>lateral root and aboveground>root.The aboveground of Stipa breviflora was limited by N in CK, LG, and MG, and limited by P in HG. And the root was limited by P. The whole population of Stipa breviflora was limited by N and P in CK and LG plots, and limited by P in MG and HG plots. The whole population, aboveground and root of Artemisia frigida and Cleistogenes songorica were limited by N. |
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