Response Of Water Conservation Function To Stand Density Of Pinus Yunnanensis Forest In The Arid Valley Area Of Anning River

The dry and wet seasons in the arid valley of the Anning River are distinct,with evaporation greater than precipitation,and the ecological environment is fragile.The large area of Pinus yunnanensis forest in the area is not under reasonable management control,resulting in poor water retention.Therefore,to clarify the differences in water content function of Pinus yunnanensis forests in the arid valley of the Anning River at different stand densities,and to determine the reasonable retention density of Pinus yunnanensis forests in the area,we provide a theoretical basis for water conservation,ecological environment improvement and soil and water conservation in the area.In this study,typical Pinus yunnanensis forests in the arid river valley of the Anning River were used as the study area,and Pinus yunnanensis forests in the study area were classified into five different forest density classes through fieldwork,namely,800～1 640plants/hm²（M₁）,1 641～2 480 plants/hm²（M₂）,2 481～3 320 plants/hm²（M₃）,3 321～4 160 plants/hm²（M₄）,and 4 161～5 000 plants/hm²（M₅）.We investigated the differences in water-holding functions of Pinus yunnanensis forests at different stand densities and revealed how stand density changes stand structure and thus affects the water-holding functions of Pinus yunnanensis forest communities.The main findings are as follows:（1）Water conservation capacity of the vegetation layer,litter layer and soil layer of Pinus yunnanensis forests in the dry river valley of the Anning River:（1）The water conservation capacity of the canopy layer of Pinus yunnanensis forests in each site ranged from 2.04 to 5.69 t/hm²,showing an increasing trend with increasing stand density.The water conservation capacity of the canopy layer ranged from 0.33 to 1.19 t/hm²,showing a trend of decreasing with increasing stand density.The canopy layer occupies the main position in the total water conservation capacity of the vegetation layer,and its proportion increases with the increase of stand density.（2）The total accumulation of dead litter in the understory ranged from 2.97 to 7.86 t/hm².The accumulation of dead litter in the semi-decomposed layer was greater than that in the undecomposed layer at all densities,and the decomposition of dead litter in M₁ was the highest;the total water holding capacity of dead litter in medium and high stand densities was greater than that in low stand densities,but the water holding capacity of dead litter in low density classes was better than that in high density classes;the water absorption rate of dead litter in the sample plot was the highest 0.5 h before immersion,and decreased rapidly thereafter.（3）The mean range of soil bulk weight for each density was 1.08～1.20 g/cm³,the mean size range of non-capillary porosity was 1.40～4.72%,and the mean range of total porosity was 38.90～49.77%;the non-capillary water holding capacity and saturated water holding capacity of M₃ were the highest,94.45 t/hm² and 995.38 t/hm²,respectively,and its soil non-capillary water holding ratio The saturated water holding capacity was also the highest at 9.49%.The initial infiltration rate of M₂ soil was the largest（14.79 mm/min）,and the steady infiltration rate of M₁ soil（5.70 mm/min）was the largest.overall,the soil infiltration of low and medium density stands was better than that of high density stands,and the soil infiltration rate and time could be fitted as the relationship equation:y=ax^b.（2）Based on the field conditions in the study area and the results of the pr evious study,and combined with the research results of others,we selected cano py water conservation capacity（Y₁）,irrigation and grass water conservation capac ity（Y₂）,litter storage（Y₃）,litter effective water conservation rate（Y₄）,litter effec tive water conservation capacity（Y₅）,soil saturation water conservation capacity（Y₆）,soil effective water conservation capacity（Y₇）,soil initial infiltration rate（Y₈）,soil stabilization infiltration rate（Y₉）as the comprehensive evaluation indexes.They were used as the comprehensive evaluation indexes of water conservation function of Pinus yunnanensis forest,and the water conservation function model of Pinus yunnanensis forest in Anning river was obtained:Z=0.106Y₁+0.110Y₂+0.115Y₃+0.113Y₄+0.092Y₅+0.124Y₆+0.128Y₇+0.104Y₈+0.119Y₉.The combined water conservatio n capacity of Pinus yunnanensis forest was ranked from largest to smallest:M₃（0.89）>M₂（0.83）>M₁（0.73）>M₄（0.72）>M₅（0.71）.（3）We compared the spatial and non-spatial structure of Pinus yunnanensis stands at different stand densities and used stepwise regression to analyze the effects of these differences on the water conservation function of Pinus yunnanensis stands.The results showed that:（1）For vegetation layer,differences in crown density had a significant positive effect on canopy water conservation capacity,while the amount of litter storage had a significant negative effect on irrigation water conservation capacity;（2）For litter layer,densities and irrigation cover had a significant positive effect on effective water conservation capacity of litter,while species diversity index and mixing degree of irrigation layer had a significant positive effect on effective water conservation capacity of litter.（3）For the soil layer,the mean diameter at breast height,the biomass of the scrub layer and the cover of the scrub layer will have a significant positive effect on the saturated water conservation capacity of the soil,the accumulation of dead sediment and the cover of the scrub layer will have a significant positive effect on the effective water conservation capacity of the soil,the cover of the scrub layer will have a significant positive effect on the initial infiltration rate of the soil,and the crown density will have a significant negative effect on the steady infiltration rate of the soil.