Effect Of Simulated Decline In Soil Depth And Water On Growth And Reproduction Of Two Grasses In Karst Region

The karst is widely distributed in the global scope.About 15%-20% of the land areas in the world are highly sensitive and fragile karst ecosystems,among which the karst landform in the southwest of China has the widest distribution area and the most complete development.The special geological background and long-term palaeo-karstification leads to shallow soil and discontinuous soil distribution.With the special double layer structure in karst area,the soil leakage in this area is serious.Karst drought occurs from time to time.Excessive interference of human activities leads to serious vegetation degradation,soil erosion and rocky desertification in karst areas.Shallow soil and karst drought are key limiting factors affecting plant survival,growth and reproduction in karst regions.In addition,the overall precipitation in southwest China has been decreasing in recent years,which brings greater challenges to plant.Three different soil depth(bottom covers an area of 100 square centimeters,height,respectively,to 10 cm,40 cm and 70 cm)and three water treatment based on the precipitation pattern in the past 30 years in Chongqing were setted up to simulate the karst habitats.Xanthium sibiricum Patrin ex Widder and Bidens pilosa L.were as the plant materials.The plant morphological characteristics,biomass accumulation and allocation pattern,the number and size of the reproductive organs were investigated to explore the response strategies of plants to the decrease of water supply,soil depth and water supply +soil depth.The results were as follows:(1)With the decline of water supply,the soil moisture significantly decreased;With the decline of soil depth,there was no significant change in soil moisture content;With the decline of water supply+soil depth,the soil moisture significantly decreased.(2)With the decline of water supply,soil depth and water supply + soil depth,the total biomass accumulation of the vegetative organs of X.sibiricum and B.pilosa decreased significantly,but the biomass allocation pattern of the vegetative organs of the two species was different.With the decline of water supply,the biomass ratio of roots and leaves of X.sibiricum had no significant change,while the biomass ratio of roots and leaves of B.pilosa showed an increasing trend,and the root shoot ratio,the root length,specific root length and specific leaf area of both species had no significant change.When soil depth and water supply + soil depth decreased,the root biomass ratio and root shoot ratio of both species increased significantly,the root length and leaf area of both species decreased significantly and the specific root length increased significantly,but the specific leaf area of X.sibiricum increased significantly or did not change significantly while the specific leaf area of B.pilosa decreased significantly.(3)With the decline of resources level(water supply,soil depth and water supply+soil depth),X.sibiricum and B.pilosa had no earlier flowering,and the timing of flowering was delayed with the intensification of resource stress.With the decline of water,the two sepcies had no shorter fruting,but the fruiting of two species was shortened with the decline of soil depth and water supply +soil depth and the life span of two species was shortened with the decline of water supply +soil depth.With the decline of all resources level,the total number and biomass of fruit and seeds of X.sibiricum decreased significantly,while the total number and biomass of fruit of B.pilosa decreased significantly and the total number and biomass of seeds of B.pilosa only decreased with the decline of soil depth and water supply +soil depth.There was no significant correlation between the number and size of infructescence of the two species.However,with the soil depth treatment,there was a significant positive correlation between the seed number and the average seed size of X.sibiricum and B.pilosa.Meantime,there was a significant negative correlation between the seed number and seed size of X.sibiricum with water treatment,and there was a significant negative correlation between the seed number and the average seed size of B.pilosa with water supply + soil depth treatment.(4)With the decline of all resources(water supply,soil depth and water supply+soil depth),the total biomass of X.sibiricum and B.pilosa decreased significantly,and the reproductive allocation of X.sibiricum decreased significantly,meanwhile biomass ratio of above-ground vegetative organs of X.sibiricum had no significant changes and biomass ratio of underground vegetative organs of X.sibiricum increased significantly with the decline of soil depth or water supply +soil depth.For B.pilosa,the reproductive allocation decreased significantly with the decline of soil depth,but there was no decreasing trend with the decline of water supply and water supply+ soil depth.With the decline of all resources,the biomass ratio of above-ground vegetative organs of B.pilosa decreased significantly,while the biomass ratio of underground vegetative organs increased significantly with the decline of soil depth and water supply +soil depth.There was a positive correlation between absolute investment in reproductive organs and vegetative organs and total biomass,as was the investment in reproductive organ and vegetative organs.However,there was no significant correlation between the reproductive allocation and vegetative investment of X.sibiricum,while there was a significant negative correlation between the reproductive allocation and vegetative investment of B.pilosa with water treatment and water + soil treatment.The decline of resources had an inhibitory effect on the growth and reproduction of X.sibiricum and B.pilosa.When resources were reduced,both species reduced individual size to reduce survival risk.And the two species produced different response strategies by trading off the relationship between different vegetative organs,the relationship between vegetative growth and reproductive growth,and the relationship between the number and size of plant traits.The life focus of X.sibiricum is vegetative growth,and more resources are invested in vegetative growth to relieve the environmental pressure.However,the reproductive characteristics of B.pilosa are more malleable and reproductive adaptability is higher than that of X.sibiricum,and it is more inclined to escape the environmental pressure through reproduction.