The Comparison Of Relative Competitive Ability And Its Drought Resistance Characterristics Among Co-existing Species In Old-field Communities Of Loess Hilly Region

There are two main mechanisms of community maintenance and succession,the differences of relative competition ability of community co-existing species and the response to the main limiting factors.Soil moisture is an important determining factor responsible for vegetation restoration in loess hilly region,so it has important theoretical and practical implications for elucidating community succession mechanisms and regional vegetation restoration to study the physiological ecological response mechanism under drought stress and changes of relative competition ability.In this paper,10 co-existing species in old-field communities of Loess Hilly Region were studied.They are Artemisia scoparia,Artemisia sacrorum,Artemisia giraldii,Artemisia argyi,Oxytropis glabra,Lespedeza dahurica,Agropyron cristatum,Bothriochloa ischaemum,Cleistogenes songorica and Sonchus arvensis.The experiment was carried out under two treatments,soil moisture and planting density.There are three levels of planting density?10,20 and 30 individuals/m²?which was determined by the Community Density Series?CDS?method.The treatment of soil moisture included invariant and changed water treatments.Invariant water treatment was carried out under severe?50%FC?,moderate?65%FC?and mild?80%FC?water deficit levels and changed water treatment was carried out under mild water deficit level?80%FC?and kept no watering for 50 days to severe water deficit level?50%FC?.In order to provide a certain theoretical basis for vegetation restoration and ecological environment improvement in loess hilly region,we measured the suitability index,drought physiological index,photosynthesis and other physiological characteristics of co-existing species under different water conditions to studied the responses of relative competition ability and competition hierarchies to the changes of soil moisture,compared the drought resistance and water use efficiency,analyze its changing trend in succession sequence,discuss the mechanism of vegetation succession plant competition and environmental regulation.The results showed that:?1?Impacts of soil moisture variation on the competition among co-existing species in old-field communities of loess hilly regionThe competition effect of community co-existing species based on the relative biomass indicated that,when the water conditions changed,the competitive hierarchy of community co-existing species was not exactly the same and the larger the difference of water conditions are,the greater the difference in the competitive hierarchy of community co-existing species are.In the case of medium density treatment,the competitive hierarchy of co-existing species in high and low water conditions is not the same?Spearman,P=0.701?.At the same moisture gradient and different density conditions,the competitive hierarchy of community co-existing species was different?P>0.05?.This may be due to changes in the light competition conditions that are accompanied by the density.The competition effect of community co-existing species based on the relative proportional growth indicated that,under the same water gradient and different density conditions,the competitive hierarchy of co-existing species was basically the same?P<0.05?,and under different water conditions,it was not the same and the corresponding changes occurred?P>0.05?.For example,under medium density treatment,the correlation of competitive hierarchy of co-existing species between the high and low water conditions was r=-0.588 and P=0.074.For the changed water experiment,there is a positive correlation between individual size variation caused by the competition in water-welled period and mortality rate in drought period?P=0.039,r²=0.525?.According to the two-stage hypothesis of resource dynamics,it showed that the community co-existing species competition was an important factor for the community formation in this experiment.As mentioned above,the effects of competition and water stress treatment on the size of individuals of different coexisting species are different and can cause different population effects.Using the CDS method to deal with and unify the extended biological/non-biological environmental impact assessment formula,the corresponding accumulative community effects of plant competition and environmental impact are:under different water and density conditions,the competition effects on the growth performance of most co-existing species in the community were greater than the drought effects.Community competition effects based on the relative biomass and relative proportional growth had a higher value?1.58 and 0.10?than the community drought effects?-10.90 and-5.54?,indicating that the competition has a larger impact on community biomass accumulation and plant growth,and the accumulative effects?2.02 and 5.57?were greater than the sum of the two individual effects?9.33 and 4.54?,which means,the competition and water stress had synergistic effects on the community,showing that biological and non-biological environment has a synergistic effect on the shaping of the community.With low density as the contrast,the community effect of competition under high density conditions is generally higher than medium density.Under three water conditions,the community effect caused by the competition of co-existing species with relative biomass as the index was the largest under the moderate water condition?RCCI=3.86±1.79?,and the mild water condition was the next?RCCI=-0.03±5.63?;Community effect caused by co-exist species competition based on the relative proportional growth was highest under severe water conditions?RCCI=6.65±1.33?,the next was under mild water conditions?RCCI=6.09±1.01?.The changes of competitive hierarchy under different water conditions indicated that environmental changes will cause changes in the corresponding community status of the coexisting species and thus cause community succession.Competition and environment affect the formation,maintenance and succession of communities at the same time.The change of community effect was:the greater the density is,the greater the community effect of competition is;the greater the degree of water stress is,the greater the effect of environmental community shaping is.?2?Activities of active oxygen scavenging enzymes?SOD,POD,CAT,and APX?,Osmotic adjustment substance Pro content,and membrane damage marker MDA content of co-existing species under different water stressDuring the growing season of July,August and September,samples were taken from8:00 to 9:00 in the morning.Functional leaves with good growth and disease-free in the middle of each plant were selected to measure each physiological index,and measurements were repeated 3 times.By analysis of variance and multiple comparisons,the results showed that POD,CAT,APX activity and Pro content responded significantly to water treatment,which showed an increase with water stress;MDA content was higher under moderate and severe water stress conditions than the mild water stress.However,there is no significant difference in SOD activity under the three water conditions,and the specific reason needs further study.Community co-existing species had significant differences in six physiological indicators.The activities of SOD and APX of Cleistogenes songorica,the POD activity of Oxytropis glabra and Cleistogenes songorica,the CAT activity of Agropyron cristatum,the Pro content of Artemisia scoparia and Oxytropis glabra were all at a relatively high level.The MDA content of Agropyron cristatum and Cleistogenes songorica was relatively low,indicating that Agropyron cristatum and Cleistogenes songorica have relatively strong drought resistance.In the growth month,there were no significant effects on the six physiological indexes.There was a significant negative correlation between CAT activity and MDA content?P=0.04,r=-0.229?.That is,the higher the CAT activity,the lower the MDA content,and the stronger the drought tolerance of the species,such as Agropyron cristatum.The sensitivity of the co-existing species to drought stress decreased with the increase of its successional niche positions by-0.054.That is to say,plants in early succession stage were more sensitive to drought stress,and drought tolerance was relatively weak,but plants in late succession stage were more tolerant to drought,and not sensitive to drought stress.?3?Effects of soil moisture variation on the characteristic parameters of photosynthesis of community co-existing speciesDifferent plants have different daily photosynthesis trends.Diurnal variation of net photosynthetic rate of Artemisia scoparia,Artemisia giraldii,Oxytropis glabra,Bothriochloa ischaemum,Cleistogenes songorica and Sonchus arvensis showed a single-peak curve,while Artemisia sacrorum,Artemisia argyi,Lespedeza dahurica,and Agropyron cristatum showed a double-peak curve and there is a clear"midday break"phenomenon.The daily average net photosynthetic rate of community co-existing species was the largest(1.81 mol·m^-2·s^-1)under mild water condition.The daily average transpiration rate(2.16 and 2.24 mmol·m^-2·s^-1)and the daily average water use efficiency(1.47 and 1.53mumol·mmol^-1)showed no significant difference under mild water condition and moderate water deficit.Under severe water deficit,the average daily transpiration rate significantly reduced(1.22 mmol·m^-2·s^-1),the reason is that the stomatal conductance of the plants was reduced under drought conditions,and the plants reduce the loss of water by reducing the transpiration;and the daily average water use efficiency is significantly increased(2.39mumol·mmol^-1),indicating that a certain degree of drought can increase the species water use efficiency.The daily average net photosynthetic rate and transpiration rate of co-existing species increased by 0.382 and 0.644 respectively with the increase of its succession niche positions,while the daily average water use efficiency decreased by-0.150 with the increase of succession niche positions.The sensitivity of daily average net photosynthetic rate,transpiration rate and water use efficiency decreased with the increase of succession niche positions at-0.041,-0.174 and-0.087,respectively.The results showed that the plants in late stage of succession were not sensitive to drought stress,the drought resistance of plants in the early stage of succession is is relatively sensitive to drought stress.