Effect Of Decline In Soil Depth And Water Resource On The Growth,Physiology And Competition Of Two Grasses With Different Root Types Under Mixed Plantation In Karst Region

Karst hosts a unique and vulnerable ecosystem.The special geological background and long-term palaeo-karstification leads to shallow soil and discontinuous soil distribution.Karst drought is still the most obvious feature of karst ecosystem because of the unique dual hydrological structure,bare rocks,shallow soil,poor soil water storage and retention capacity.The phenomenon of rocky desertification has become more serious,which causes karst soil to be scarcer and shallower.In addition,under the change of global rainfall pattern,the climate in SW China appears to have the trend of high temperature,low rainfall,and frequent occurrence of the continuous extreme drought events,which exacerbates the frequency and intensity of karst drought.Shallow soil and karst drought are key limiting factors affecting plant survival,growth and reproduction in karst regions.The change of the soil depth and water resource will directly affect plant's growth and competition.The reduction of soil depth and water resource may inhibit plant growth,and increase competition between adjacent individuals of the same species or different species.Shallow-rooted plants mainly use water and nutrients from the top layers of the soil,whereas deep-rooted plants can use water and nutrients from the deeper soil layers.Therefore,when deep-rooted plants and shallow-rooted plants coexist,they might alleviate the inhibitory effect of resource limitation on plant growth and reduce completion because of their root niche complementary.The gramineous perennial grass,Festuca arundinacea Schreb.,is a deep-rooted plant as the root can be up to 40 cm depth in soil,while Lolium perenne L.,with root depth up to25 cm,is a shallow-rooted plant.These two species are both introduced as pioneer grasses for re-vegetation in karst regions due to their high drought resistance,barren-tolerance,strong ecological adaptability,high yield and good economic benefit.They effectively improve the local ecological environment and help to alleviate rural poverty.Therefore,we chose these two species to study the effects of decline of soil depth and/or water on the growth and morphological plasticity,photosynthetic physiology,stomatal and vein traits,and nutrient elements of two grasses under mixture by controlled experiment.In our study,three factors,namely soil depth(D_CK:15 cm,and D_S:5 cm),water(W_CK:120 mL/3 d and W_D:36 mL/3 d),and planting pattern?monoculture and mixture?were designed in a full three-way randomized block design.The key results were as follows:?1?Total biomass,aboveground biomass,root biomass and total root length of F.arundinacea and L.perenne decreased with the reduction of resources?soil depth,water,and soil depth plus water?either in mixture or in monoculture.At high resource level,i.e.,control group(CK:D_CKW_CK),for F.arundinacea,the root biomass,root mass fraction and total root length were higher in mixture than in monoculture,but the total biomass and aboveground biomass were not affected by planting patterns?monoculture or mixture?.However,they were almost the same between in monoculture and in mixture at three low resource levels,i.e,shallow soil group(S:D_SW_CK),drought group(D:D_CKW_D),and shallow soil plus drought group?SD:D_SW_D?.In contrast for L.perenne,whether at high or lower resource levels,all the above traits had similar values between monoculture and mixture.?2?As the whole,net photosynthetic rate?P_n?,stomatal conductance?G_s?,and transpiration rate?T_r?of F.arundinacea and L.perenne decreased and water use efficiency?WUE?of L.perenne increased with the decline of all resources both in mixture and in monoculture.Moreover,the range of decrease or increase was lower in mixture than in monoculture;WUE of F.arundinacea increased in monoculture,while decreased in mixture.At high resource level,no significant difference was found in P_n,G_s,T_r and WUE of F.arundinacea between mixture and monoculture;P_n,G_s and T_r of L.perenne was lower in mixture than in monoculture,while WUE was higher in mixture than in monoculture.At low resources level,P_n,G_s and T_r of F.arundinacea was higher in mixture than in monoculture,while WUE was lower in mixture than in monoculture;no significant difference was found in P_n,G_s and T_r of L.perenne between mixture and monoculture,while WUE was higher in mixture than in monoculture.?3?For F.arundinacea,the stomatal density and vein density increased,stomatal area and stomatal conductance decreased,but stomatal area fraction remain constant both in mixture or in monoculture with the decline of all resources.In contrast,for L.perenne,all the above traits did not change or slightly decreased.At high resource level,for F.arundinacea,the stomatal density and vein distance were higher,but stomatal area and stomatal conductance were lower in mixture than in monoculture.However,they were almost the same between mixture and monoculture at low resource levels.In contrast,for L.perenne,whether at high or lower resource levels,all the above traits had similar values between monoculture and mixture.The stomatal density and vein density of F.arundinacea were negatively,but stomatal area and stomatal conductance positively correlated with total root length both in mixture and in monoculture.As for L.perenne,only stomatal area showed positive and vein density negative correlations with total root length in monoculture.?4?The C and N contents in aboveground part,root,and whole plant of both species increased?except shallow soil?,while the P content and their accumulations decreased with the decline of all resources both in monoculture or in mixture.The root nutrient distribution ratio of F.arundinacea increased in monoculture but decreased in mixture with the all resources decline.The root nutrient distribution ratio of L.perenne increased with the decline of soil depth,but decreased with the decline of water.?2?The C content in aboveground part,P content in root,and their accumulations of each part and root nutrient distribution ratio of F.arundinacea were significantly higher in mixture than in monoculture at high resource level,while there was no-significant difference between mixture and monoculture for above traits of F.arundinacea at low resources levels.?3?At all resource levels,there was no significant difference between monoculture and mixture for all the above traits of L.perenne.With the declined of soil depth and water resource,the growth,photosynthesis,the development of vein and stomatal traits,element accumulation and distribution of F.arundinacea?deep-rooted?and L.perenne?shallow-rooted?was limited strongly.Deep-rooted and shallow-rooted plant species could coexist in karst regions under current climatic regimes.However,with the change of global rainfall pattern and seriousness of karst rock desertification,competition,especially root competition between deep-rooted and shallow-rooted plants might increase in the future.F.arundinacea?deep-rooted?could reduce root growth and biomass allocation,improve photosynthesis,and reduce the differences between monoculture and mixture of stomatal and vein traits,and nutrient elements for better coexistence,while L.perenne?shallow-rooted?might maintain a constant strategy of these traits in response to competition.