The Study Of Shrub-Grasses Positive Interaction And Its Driving Factors In Arid Desert

The interaction among plants is the crucial issue in modern ecology.It is generally believed that the physical environment and individual competition are the key factors to determine plant individual fitness,population dynamics,community assembly and ecosystem function.However,the role of positive interaction among plants in improving species diversity and community stability under stresses cannot be ignored.The research,that on the facilitative effect among plants in arid desert region,will help us to understand ecological responding models,population development trends and species composition variations under the impact of global climate change,which provides scientific accumulation and theoretical basis for revealing the formation process of vegetation pattern,ecological restoration strategy and species diversity management in arid desert ecosystem.However,the study on positive interaction in arid desert areas of China is scarce.We chose desert plants as research materials,and conducted experiments using the methods of natural observation,removing or transplanting neighbor plants and simulating drought stress.We try to answer the following questions:（1）is the plant-plant positive interaction prevalent in our study area?（2）How do both vegetation growth and plant-plant interaction respond to the changes of shrub size?（3）How does shrub species modulate fine-scale environmental conditions and affect the understory plants?（4）How does the net interaction between species change during growing season?（5）What is the change pattern of net plant-plant interaction at the extreme end of drought gradient?Through above empirical studies,we finally attempt prove that whether the expectation of SGH is valid.We found:1.Positive interactions were widespread in our studying area.In the harsh environment of arid desertification grassland,positive plant interactions were very common（RII>0）,which was consistent with the prediction of stress gradient hypothesis.Yet,the responses to the removal of plants were species-specific and trait-specific.In terms of biomass,the positive response of Salsola collina to its neighbors was significantly higher than that of Bassia dasyphylla,Corispermum mongolicum,Chloris virgata,Caragana Korshinskii and Nitraria shphaerocarpa,whereas Agriophyllum squarrosum were not affected by the removal of neighbor plants.The response of specific leaf area was mainly reflected in the direction of interaction.For specific leaf area,S.collina,C.mongolicum and C.virgata displayed negative responses to their neighbors,whereas B.dasyphylla,C.Korshinskii and N.shphaerocarpa showed positive responses.Compared with biomass and specific leaf area,the influence of neighbor plants on plant height was weaker.Among them,only the plant height of B.dasyphylla,S.collina,C.Korshinskii and Nitraria shphaerocarpa showed a significant positive response.Meanwhile,the importance of positive interactions diminished gradually with the improvement of water conditions.For C.mongolicum,under simulated drought conditions,the RII value of understory vegetation biomass and density was significantly higher than that of water adding treatment.After water addition,the RII values of understory vegetation biomass and density were significantly lower than those of natural environment.There was no significant difference in species richness at different water environments.2.With the development of shrub patch sizes（increasing）,the net interaction between shrubs and their understory plants changed from facilitation to competition.Through natural observation and the removal of shrub canopies in N.sphaerocarpa dominated communities,we explored the effect of shrub patch sizes.Resulting from fertile island effect,the micro-climates and nutrient conditions beneath shrubs were considered to be superior to that of open aera,and gradually improved with the development of shrub patch sizes.However,the vegetation within the shrub does not gain more growth advantages with the improvement of the environment during the increasing of the shrub patch sizes.The above ground biomass,density,richness and coverage of understory plants in small shrubs had the best performance,while those under large shrubs were the least.The small N.sphaerocarpa showed significant positive effects on the biomass,density and coverage of understory plants,while N.Sphaerocarpa with medium size had no significant effect on the understory plants,or even negative effects for some understory species.The species richness under shrubs was not significantly affected by the size of N.sphaerocarpa.With the increasing of shrub patch sizes,the net interaction between N.sphaerocarpa and their understory species changed from positive（RII>0）to negative(RII<3.The multiple effects of shrubs on the microhabitat and understory plants are species-specific at a fine scale.The multiple impacts of shrubs on microhabitat showed species-specificity at the fine scale:the soil water content in the core area of the shrubs was higher than that in open area at the interspace between shrubs for C.mongolicum and Haloxylon ammodendron（Bunge.）,while for N.sphaerocarpa,the soil water content in the inner periphery of shrub patch was the highest,which was significantly higher than that in open area at the interspace between shrubs.The soil organic carbon decreased gradually along the core area,inner periphery and outer periphery,and the soil organic carbon at the shrub core area was significantly greater than that in open area at shrub interspaces.The soil salt contents(e.g.,Na⁺,K⁺,Cl^-,and SO4^2-)of the four fine scale areas of C.mongolicum and N.sphaerocarpa were not significantly affected by the shrubbery,but the soil salt contents(e.g.,Na⁺,K⁺,Cl^-,and SO4^2-)of H.ammodendron decreased gradually along the core area,the inner periphery,the outer periphery and the open area at shrub interspaces,which led to different spatial patterns of understory vegetation in the fine scale.It can improve the growth of understory vegetation by improving the soil water content,increasing the soil nutrient and seed density for C.mongolicum.The growth of understory vegetation at the outer periphery of N.sphaerocarpa patch was better.While H.ammodendron（Bunge.）mainly plays a competitive role.4.The change of plant-plant positive interaction is species-specific and environmental context dependent.In the growing season,the interactions between C.mongolicum and the understory herbaceous plants were positive.With the growing season,the precipitation decreased and drought stress intensified,and the intensity of the positive interaction increased.However,the interaction between N.sphaerocarpa and the understory plants decreased.During the growing season,the positive interaction changed to negative interaction for N.sphaerocarpa.Herbaceous plants with different functional traits also respond differently.During the growing season,positive interactions between annual plants such as Halogeton arachnoideus,C.virgata and N.sphaerocarpa were gradually weakened,and eventually disappeared.However,the canopy of C.mongolicum did not affect these two herbs,and only showed significant positive interaction with H.arachnoideus in the late growing stage.The interaction between desert shrubs and herbaceous plants was not only determined by environmental stress,but also related to the characteristics of each species.Result from low canopy biomass density,C.mongolicum positively affects their understory plants,while the influence of H.ammodendron was negative due to its high canopy biomass density.5.Under extreme drought conditions,the positive effect of shrubs on their understory communities decline or even collapse at the extreme end of drought gradientWe conducted a field experiment along a temporal gradient for five consecutive years,in order to investigate interactions in a shrub-herbaceous plant community,more specifically,investigated the effects of C.mongolicum on both abiotic environmental variables and the performance of sub-canopy plant species.We found that shrubs can improve sub-canopy water regimes,soil properties,plant biomass,density,cover and richness,and more importantly,that the positive effect of shrubs on sub-canopy soil moisture during the summer diminishes as decreases.These results provide empirical evidence that the positive effect of shrubs on understory plant communities under arid conditions may become neutral with increasing drought stress.In conclusion,positive interaction is common in the arid desert area of Hexi Corridor,with species-specific and traits-specific.The positive interaction between C.mongolicum and understory plants was significant,and its importance decreased with the improvement of soil water environment.The change pattern of plant-plant interaction with artificial water gradient was consistent with the original SGH prediction.Drought stress is one of the main causes of plant positive interaction in this area.With the growth of shrubs,the soil nutrients in the understory were improved continuously,but drought stress was aggravated.As a result,the positive interaction between N.sphaerocarpa and understory plants was enhanced,indicating that the water condition（rather than soil nutrient）was the main factor limiting the growth of vegetation.Plant-plant interaction is species-specific at the fine scale.The combined effects of water and other environmental factors（such as temperature,nutrient,salt content,and available light）determine the direction of their interaction.During the growing season,the drought stress increased,and the intensity of positive interaction increased for C.mongolicum,while the interaction changed from facilitation to competition for N.sphaerocarpa.Additionally,the interaction between shrub and grass in desert was not only determined by environmental stress,but also showed species-specific.The collapse of positive interaction under extreme drought stress supports the recent revision of Stress Gradient Hypothesis（SGH）.