The Relationships Between ?¹³C And Ecological Stoichiometry Of Pinus Sylvestris Var. Mongolica And Environmental Factors In Hulunbuir

Pinus sylvestris var.mongolica is one of the main greening tree species in the"Three North Shelterbelt"system in China,and it is introduced from Hulunbuir to other regions in China.So far,the phenomenon of its withering or death has taken place in some of the introduced places,such as Horqin Sandy Land,although its growth in Hulunbeier is relatively healthy.It is thus necessary to study the ecological adaptation mechanism of Pinus sylvestris var.mongolica to the environment.Previous studies show that the contradiction between demand and supply of water and nutrient is mainly responsible for its decline.However,a complete and detailed understanding on its ecological adaptation mechanism is lacked,coupling relationship between environmental factors is unclear,and the processes and modes for the impact of environmental factors on its photosynthetic physiological process are also unclear.Hulunbuir is located in the transitional zone of semi-humid and semi-arid.The enrionmental conditions controlled by water among sites in Hulunbuir are obviously different,providing us an ideal study area for the study of the ecological adaptation mechanism of Pinus sylvestris var.mongolica.This study will provide a scientific basis for its ecological management under the background of global climate change.Photosynthesis and transpiration are key physiological processes affecting plant growth.Leaf?¹³C is the key index to understand these physiological processes and environmental impact processes.In different regions,no consistent conclusions have been obtained about the influence processes and modes of environmental factors on leaf/pollen?¹³C.As reproductive organs and photosynthetic organs of plants,pollen and leaf have their special ecological significances.Their?¹³C can effectively integrate the spatiotemporal differences of environment and comprehensively reflect the biogeochemical processes of plants.The study of modern process of Pinus sylvestris var.mongolica will also provide a theoretical basis for the reconstruction of local paleoclimate.In this paper,the variation of?¹³C values in leaf and pollen were studied,the effects of environmental factors on leaf?¹³C were identified,the relationship between the?¹³C values in leaf and ecological stoichiometry,and the fractionation between?¹³C values in leaf and pollen of Pinus sylvestris var.mongolica were determined.219 leaf samples were collected from different trees in the study area,and 102 pollen samples with the same source as some leaf samples were collected.Besides,the?¹³C values in leaf and pollen,other indexes including the contents of carbon,nitrogen,phosphorus,potassium and proline in leaves were also tested.51 soil samples from 13soil profiles under Pinus sylvestris var.mongolica forest were collected,and soil physical and chemical indices were analyzed.With the help of SPSS software,statistical methods of analysis were used in this paper,including linear correlation analysis,ANOVA and path analysis.The main conclusions are as follows:(1)In different years,different sampling sites,the mean values of?¹³C in leaves of Pinus sylvestris var.mongolica were significant different.The mean value of?¹³C in leaves of Pinus sylvestris var.mongolica in 2017 was-31.5‰,and that in 2019was-27.3‰.Effects of sampling sites in different years on leaf?¹³C value were significant.Leaf?¹³C values are signicantly negatively correlated with lantitude and longitude,implying that leaf?¹³C values are associated with water and heat conditions.(2)Leaf?¹³C of Pinus sylvestris var.mongolica was significantly correlated with climate factors.It was positively correlated with temperature,sunshine hours and vapor pressure deficit(VPD),negatively correlated with precipitation and relative humidity,especially with these climatic factors in flowering season(p<0.001).This fact indicated that leaf?¹³C could capture not only the short-term effects of climate factors on physiological process of Pinus sylvestris var.mongolica,but also its long-term characteristics of temperature and precipitation.(3)Except soil water content,there were less significant negative correlation between the leaf?¹³C values of Pinus sylvestris var.mongolica and contents of clay,organic carbon,nitrogen and phosphorus.There was a spatial coupling relationship between soil moisture,nutrient contents and precipitation.From west to East,with precipitation increasing,soil moisture,clay,organic carbon and nitrogen contents increased,but the soil p H values,the contents of sand and potassium decreased gradually.The leaf?¹³C values of Pinus sylvestris var.mongolica also showed a corresponding decreasing trend.(4)Homologous leaf?¹³C and pollen?¹³C has a similar variation and a relatively definite stable carbon isotope fractionation.It was confirmed that the pollen?¹³C was reliable in reconstructing paleoclimate.There was a significant correlation between the?¹³C values in leaf and pollen(p<0.001).The?¹³C values in leaf and pollen had a similar variation.The ratio of the?¹³C value in leaf to that in pollen was close to 0.6.The discrimination values(?_pollen-leaf)of stable carbon isotope between pollen and leaf are ranging from-5.3‰to 2.6‰.Most of the discrimination values(?_pollen-leaf)ranged from-1.0‰to 4‰,and the mean is 1.6‰.(5)The nutrient content of Pinus sylvestris var.mongolica is characterized by high carbon and low nitrogen.The mean carbon content in leaves of Pinus sylvestris var.mongolica was higher than 520 mg·g^-1,and higher than the average carbon content of forests in China.The mean nitrogen content in leaves was about 16 mg·g^-1,lower than the mean nitrogen content of forests in China,and the mean value of carbon nitrogen ratio was greater than 30.The phosphorus content in leaves was about 1.20 mg·g^-1,and the N/P ratio was 13.56,lower than 15,indicating that the growth of Pinus sylvestris var.mongolica greatly affected by nitrogen deficiency.There was no significant correlation between leaf?¹³C and ecological stoichiometry,which may be related to the limitation of nitrogen deficiency on photosynthetic process.(6)The effects of environmental factors on water use and nutrient accumulation of Pinus sylvestris var.mongolica were mainly manifested in two aspects:(1)the effects of environmental water conditions on transpiration and photosynthesis of leaves;(2)transpiration and photosynthesis in leaf are dynamic processes of nutrient accumulation.Transpiration and photosynthes affected leaf?¹³C,leaf carbon content significantly and then affected other ecological stoichiometry,altuough there was no significant linear correlation between leaf?¹³C and ecological stoichiometry.In a word,leaf?¹³C is the result of photosynthesis and transpiration under certain environmental conditions,and it is an important parameter indicating photosynthetic pathway.Air and soil moisture are the key factors affecting leaf?¹³C and the growth of Pinus sylvestris var.mongolica in Hulunbeier.Air relative humidity and soil moisture content jointly determine the air saturated water vapor pressure difference inside and outside the leaf,directly affect the dynamic process of leaves transpiration process,determine the stomatal conductance,and then affect leaf?¹³C.Light(sunshine hours)and heat(air temperature)indirectly affect leaf?¹³C through the influence of relative humidity and water vapor pressure(VPD)?Precipitation are the main climatic factors affecting VPD,although somewhat differences exist in regions with different drought degree.In the region with larger precipitation,both precipitation and VPD have significant impacts on leaf?¹³C directly.In the region with less precipitation,VPD has a significant effect on leaf?¹³C.Precipitation mainly affects air humidity,indirectly VPD,and thus leaf?¹³C.Soil water is the main source of available water for plants,and is one of the main key factors affacting leaf?¹³C value.Soil phosphorus content is a key indicator of soil water and nutrient cycling.Soil phosphorus content mainly depends on soil clay content,that is,the release process of phosphorus in clay minerals.All told,the influence mode of environmental factors on physiological process of Pinus sylvestris var.mongolica is mainly reflected in the influence of environmental factors on transpiration and photosynthesis of leaves,and then on the nutrient accumulation and growth of leaves.Leaf?¹³C value is dominanted by plant photosynthesis and transpiration under certain environmental conditions,and it is an important parameter to indicate water use efficiency and photosynthetic pathway.Air and soil moisture were the main factors affecting the leaf?¹³C of Pinus sylvestris var.mongolica in Hulunbeier.