Competition And Coexistence Of Leymus Chinensis And Its Main Companion Species In The Songnen Grasslands

Leymus chinensis is a dominant grass in the meadow steppe of the Songnen Plains ofnortheastern China where it has an ability to resist drought, cold and salt-alkali stress. Therewas necessary for us to do some experiments on plant competition in light of the patchlandscape in the natural grasslands. Firstly, we tended to explore the interaction among L.chinensis and its main companion species in the term of spatial distrbution type and thedynamics of pattern intensity in ecotones. And then mowing and excluding method was usedintegrately to simulate the grazing behavior of herbivore to test whether the mowing couldhave important effects on the interactive relationship between L. chinensis and Phragmitescommunis or Kalimeris interifolia. The competitive and coexistence relationship between L.chinensis and P. communis was investigated by combing the results of experiments in thenatural grasslands and in the partially controlled conditions in order to explore themechanisms of competition and coexistence for both species. Seven main conclusions couldbe obtained from the experiments conducted during the years from 2000 to 2004 and theywere as follows.(1) There was intense competitive interaction between L. chinensis and its maincompanion species according to the results of spatial distribution types and the dynamics ofpattern intensity. The competitive interaction among species enforced the change of patterntype from aggregated to random distribution, then to regular distribution for each species. Thecompanion species invaded into communities dominated by L. chinensis by formingaggregated distribution pattern. Similarly, L. chinensis also defended the invasion of its maincompanion species by aggregated distribution. The distribution scale, type and intensity werecorrelative with both the specific species and the soil heterogeneity.(2) There was aggregated distribution for L. chinensis and P. communis in both mixedcommunity and ecotone. The aggregated intensity for L. chinensis was larger in ecotone thanthat in mixed community. However, no significant difference was found for P. communis inaggregated intensity in the mixed community and ecotone. Distribution type and intensity forL. chinensis in the mixed community were influenced by both the intra-and inter-specificcompetition, besides the biological characteristics. Whereas the distribution type and intensitywas affected by soil heterogeneity and inter-specific competition for L. chinensis in ecotones.The species characteristic of P. communis was an important factor because there was nosignificant difference of aggregative intensity in the mixed community and in the ecotone。(3) The matters stored in the root were the main source and energy for plant tocompensate growth after plants were mowed. P. communis grew faster than L. chinensis in themixed communities with P. chinensis and L. chinensis. L. chinensis was a super competitorthan K. interifolia in the mixed communities with both species. The accumulation of biomassand the relative growth rate were increased by mowing treatments, but were correlative withthe intensity of mowing. The interactive relationships between L. chinensis with P. communisor K. interifolia were asymmetry and positive to one side. The mowing treatment couldreduce the intensity of interaction and even change the direction of interaction.(4) The index of niche overlap (HO) indicated that the overlap on the soil nutrient andpH was larger than that on conductance and soil moisture dimension for L. chinensis and P.communis. The demand of L. chinensis and P. communis on soil nutrient and pH was moresimilar, but different on soil moisture and salt ion stress. P. communis favored the habitat withhigh level nutrient, therefore it was the effective user of soil nutrient. L. chinensis was moreadaptive to habitat with salt-alkali stress. Coexistence of L. chinensis and P. communisimprove the possibility of niche differentiation at the dimension of soil moisture in the naturalgrasslands. Although they both resisted salinization and alkalization by osmotic adjustment, P.communis was more sensitive to the stress of drought and salt-alkali than L. chinensis(5) More biomass was allocated to leaf to forage solar radiantion when the nutrient insoil increased for L. chinensis and P. communis. Therefore, there was intense lightcompetition for both species. The allocation of nitrogen and non-structure carbohydrate andtheir concentration indicated that P. communis had advantage over L. chinensis in foragingresources. Results of growth analysis and the concentration of chlorophyll also confirmed thatP. communis could assimilate much more light and dominated the upper layer of communityto boost the ability of absorbing the nutrient in the soil.(6) The distance of buds to the soil surface for clone species could reflect thegermination time for both L. chinensis and P. communis. Buds in the mixed communitytended to germinate early than those in the community dominated by L. chinensis or P.communis. Earlier germination would benefit P. communis by exploring nutrient early,thereby dominated in mixed community. And earlier germination of L. chinensis could reducethe negative effects of P. communis on L. chinensis during the whole growth season.(7) Results in the partially controlled experiment suggested that L. chinensis and P.communis made demand on the same resource based on the value of relative yield total equalto 1 at poor nutrient level. However, the species proportion of P. communis was larger did notimply inter-specific competition but intense intra-competition for P. communis in mixture.Whereas there was positive relationship for both species. Results in the second year alsoconfirmed that both species made same demand on the soil nutrient and coexistence couldbenefit the total yield of mixed experimental population because the competitive relationshipcould improve the utilizing efficiency of resource, or there was difference in space or time ofutilizing resource for both species.'Selection mechanism on the adaptation of plant to resource and non-resourcestress'was proposed based on the results of the experiments in five years. It is hypothesizedthat plants having high capability in foraging and turnover resource have advantages at highnutrient levels;therefore the competitive ability is increased with the productivity in livinghabitat. Plants making low demand on nutrient with low expense could win, but thecompetitive ability do not change with the productivity at nutrient lower level. When therewas non-resource stress on plants, the outcome of competition was determined by thetrade-off between the capability in foraging resource or the endurance to low nutrient leveland the endurance to the non-resource stress. 'Life history theory' proposed by Grime and'resource ratio theory' by Tilmen was integrated in the former part of this hypothese. And thelatter part of this hypothese developed the classical competitive theory by adding the effect ofnon-rsource enviroental factors on the competitive ability of plant.