Developmental And Distribution Drivers And Ecological Value Assessment Of Biocrusts In Mu Us Sandy Land

Biocrusts perform a variety of ecological functions in desert ecosystems,and the vegetation canopy often provides a good developmental site for them.Currently,there is a paucity of research on the environmental drivers of sub-canopy and inter-canopy development and the quantitative assessment of the ecological value of biocrusts at the regional scale.In this study,44 typical sample plots were evenly distributed across a semi-humid and semi-arid climate zone（42,612 km²）in the Mu Us Sandy Land to investigate the distribution and development of sub-canopy and inter-canopy moss and algal crusts and various environmental factors.The main environmental drivers were identified by canonical correspondence analysis（CCA）and rank correlation analysis（Sperman）,and a predictive model for both sub-canopy and inter-canopy crusts was constructed using categorical regression trees（CART）.Secondly,194biocrust distribution sites were recorded across the area,and the distribution pattern and area of biocrusts in the area were predicted based on a maximum entropy model（Max Ent）using climate,vegetation,topography and soil raster data.Thirdly,nine sample sites were set up along the rainfall gradient in the area,and the nutrient accumulation per unit area of biosolids was analysed and calculated,while the amount of biosolids per unit area of the area for wind and sand control,water conservation and carbon sequestration and oxygen release was obtained through literature surveys.The ecological values were calculated based on the predicted biosolids area.The main conclusions are as follows:（1）On a regional scale,vegetation cover,slope and slope orientation are the main drivers of the development and distribution of moss and algal crusts under and between canopies.Under-canopy and inter-canopy crust cover are positively correlated with vegetation cover,while inter-canopy crust cover is negatively correlated;moss crust tends to be distributed under the cool and moist canopy,while algal crust also grows well between canopies;inter-canopy moss crust cover,under-canopy and inter-canopy algal crust cover are positively correlated with soil nutrients,while under-canopy moss crust cover is negatively correlated;under-canopy and inter-canopy crust cover are negatively correlated with grazing disturbance.On sunny or semi-sunny slopes with high gradients,the coverage and thickness of sub-canopy and inter-canopy algal crusts and sub-canopy moss crusts were higher,while on shady or semi-shady slopes,the coverage and thickness of inter-canopy moss crusts were higher;the coverage of sub-canopy and inter-canopy algal crusts and the thickness of sub-canopy moss crusts were positively correlated with the average annual maximum temperature.The two types of crust cover under the canopy were positively correlated with the mean annual wind speed,while the inter-canopy was negatively correlated.Climatic water gain/loss is positively correlated with inter-canopy crust cover and moss crust thickness,but negatively correlated with inter-canopy crust thickness.（2）The models for predicting the distribution of moss and algal crust development under and between canopies at the regional scale were constructed.The models were validated by the coefficient of determination（R²）and the root mean square error（RMES）,and the results showed that the models had high prediction accuracy in general.Vegetation cover contributed most to the prediction models of sub-canopy moss crusts and inter-canopy moss crusts and algal crusts;slope contributed most to the prediction models of shear strength of sub-canopy algal crust and inter-canopy moss crusts;organic matter content and mean annual maximum temperature contributed most to the prediction models of thickness of sub-canopy and inter-canopy moss crusts.（3）A model was constructed to predict the spatial distribution of biocrusts at the regional scale.The prediction results were:the total area of biological crust in the Mu Us Sandy Land was 13,636km²,accounting for 32%of the whole area,and the largest area was distributed in the southeast,followed by the central,northeast and southwest,and the lowest in the northwest,southwest and northeast edges,with a general increasing trend from northwest to southeast.Soil type,vegetation cover,water vapour partial pressure,average annual wind speed,slope direction,snowmelt depth,soil cation exchange,actual evaporation dispersion and soil moisture were the nine dominant factors affecting the spatial distribution of biological crusts.（4）The annual amount of nutrient accumulation,wind control and sand fixation,water retention and carbon sequestration and oxygen release per unit area of biological crust in the Mu Us Sandy Land was obtained through experiments and literature review,and then the market value method was used to calculate the ecological value of each biocrust per unit area.The total ecological value of the four functions of the biocrusts in the Mu Us Sandy Land was estimated to be 1.247 billion yuan·a^-1,with an ecological value of 914.75 yuan·a^-1 per hectare,of which the service value of wind protection and sand fixation was the highest at 616 billion yuan·a^-1,accounting for 49.32%of the total ecological value,followed by the service values of water conservation and carbon sequestration and oxygen release at 326 billion yuan·a^-1 and 306 billion yuan·a^-1,respectively.The service value of nutrient accumulation is relatively low,at RMB 131,000 yuan·a^-1,accounting for only 0.01% of the total ecological value.