Effects Of Forest Gap On Metal Distribution During Precipitation Process In A Subalpine Spruce Plantation

The input of metal nutrient elements within precipitation is one of important forms of material transport processes in high-altitude forest ecosystems.Forest canopy can regulate the input input characteristic of metal elements in precipitation through interception,filtration,and absorption.Meanwhile,precipitation can also change the migration process of elements from forest canopy to soil through leaching.Due to low temperature,shallow soil,frequent geological and climatic disasters and human disturbance,Picea asperata forest in Western Sichuan,located on the eastern margin of the Qinghai-Tibet Plateau,has formed a large number of forest gaps.These forest gaps may change the spatial pattern of forest canopy,and then affect the migration of elements in precipitation process,but they lack the necessary attention.Therefore,eight metal(K,Ca,Na,Al,Zn,Fe and Mn)elements in throughfall and of typical sub-alpine spruce plantations were dynamically monitored from August 2016 to July 2017.The atmospheric precipitation input,and canopy interception were calculated,and the metal elements in the process of precipitation were analyzed.The main results are as follows:(1)During the study period,the volume of precipitation was 1049.34 mm,and the rainfall and snowfall period were 803.89 mm and 245.44 mm,respectively.After passing through the canopy distribution,the volume of throughfall in forest gap,forest edge and closed canopy accounted for 95.12%,91.25% and 86.27% of the precipitation,respectively.The interception effects of different canopy layers on rainfall and snow were as follows: closed canopy >forest edge >forest gap.The regression analysis shows that there was a significant linear correlation between throughfall and precipitation.Stemflow accounts for 5.09% of precipitation,and the linear regression equation of stemflow and precipitation(R2=0.970),and when precipitation was less than 4.67 mm,there was no stemflow.(2)The input of major metallic metal in precipitation was 177.82 kg/hm2,and 225.72 kg/hm2,174.48 kg/hm2,167.27 kg/hm2 and 31.47 kg/hm2 for forest gaps,forest edges,closed canopy and stemflow.Canopy had significant effects on K,Ca,Na and Mg input.The input major metallic elements in different forms of precipitation are K > Ca > Na > Mg.And the input of a large number of metal elements is mainly in the rainfall period, accounting for 67.79%-82.32% of the total input.After precipitation passed through the canopy and trunk,the concentration of a large number of metal elements changed,which showed stemflow > throughfall > precipitation.The enrichment level of elements under different canopy layers is different.The major enrichment elements in gap,edge and understory are K,Mg and Na respectively.The largest enrichment element in stemflow is K,which is 5.32 times as much as that in precipitation.(3)The total input of heavy metals in precipitation was 1.41 kg/hm2.The input of heavy metals in different canopy layers was as follows: forest gap > forest edge > closed canopy and the input was 1.00 kg/hm2,0.99 kg/hm2 and 0.82 kg/hm2,respectively.The input of heavy metals in the throughfall of forest gaps and forest edge was mainly Al and Fe element under the forest.The input of heavy metals during rainfall is 1.78-3.40 times as much as that during snowfall.The input of heavy metals in stemflow was 0.26 kg/hm2 and the main input was Al.The concentrations of heavy metals in different precipitation patterns were as follows: stemflow > precipitation > throughfall.Mn elements was enriched in forest gaps,forest edges and closed canopy,especially under closed canopy,which 4.58 times higher than in precipitation.The concentration of four kinds of heavy metal elements is enriched in the stemflow,among which Al element has the greatest enrichment,which is 13.16 times times that of precipitation.(4)The interception rates of heavy metals and major metallic elements in forest ranged from 18.09% to 35.18% and from-48.52% to 15.21%,respectively.The interception effects were as follows: forest gap >forest edge > closed canopy.For different metal elements,the interception effect of canopy is inconsistent.The canopy showed a positive interception effect on heavy metal of Fe,and the best interception effect was under the forest edge,which intercepted 54.12% of the Fe element from precipitation.Al element has the highest interception in closed canopy,the interception rate is 16.89%,and the maximum interception rate of Zn is 65.31% in forest edge.Mn shows negative interception in the forest edge and closed canopy.It shows that precipitation leaches Mn under different canopy layers,and the leaching effect in forest edge was the largest,up to 206.75%.Canopy interception has a significant positive interception effect on Ca and Mg,indicating that canopy interception filters Ca and Mg in the precipitation.The leaching effect of K element in forest edge and closed canopy is obvious,and the leaching effect of precipitation on K element in closed canopy is greater.The leaching effect of K element in understory canopy is 70.36 kg/hm2,which is 4.06 times higher than that under forest edge.In summary,the forest gap,forest edge and closed canopy of Picea asperata plantation has significantly changed the migration process of metal elements in atmospheric precipitation,and the canopy has a certain interception and filtering effect on heavy metal elements,and the filtering effect is higher than that of major metallic metal.These results are helpful to understand the distribution process of metal elements in atmospheric precipitation and its impact on the canopy,which is the "atmospherevegetation layer".The cyclic relationships between elements provide basic data and provide new basis and ideas for the management of sub-alpine forest ecosystems.