The Effect Of Plant Debris On Elemental Export From Headwater Streams In An Alpine Forest

Headwater streams are a major component of the initial end of the river network structure and are also the main material exporters of the forest ecosystem,maintaining natural discharge,regulating sediment export,retaining and removing nutrients,and playing an irreplaceable role in water security in the watershed.Forest plant debris provide significant amounts material as a resource recharge to headwater streams and play a vital role in the transport.Woody debris includes fallen wood,large dead branches,large twigs,and root stumps,usually larger than 1 cm in diameter;non-woody debris includes fallen leaves,flowers,fruits,seeds,bark and twigs smaller than 1 cm in diameter.They play a vital role in the transport of material and energy between the forest-stream continuum.However,how about the dynamics of elemental contents and exports from forest streams in alpine forests,and how non-woody and woody debris affect the exports of elements from streams still lack studies.This study was conducted in headwater streams in an alpine forest in the upper reaches of the Minjiang River(an important tributary of the upper reaches of the Yangtze River),and the effects of different types of elements export dynamics(carbon,nutrient,salt-based ions,aluminium and iron inos,and heavy metals)in non-woody debris,woody debris,and mixed debris in streams were investigated in three growing seasons from2018 to 2020(streams were frozen in winter and could not be sampled)using a field in situ controlled experiment approach with a control no-plant debris treatment.The main findings are as follows,(1)Non-woody debris in alpine forests reduced stream velocity and discharge and has the potential to reduce the export of most elements from streams.In comparison with streams without plant debris,the magnitude of the reduction in total stream elemental export from non-woody debris was found to be as follows: Cu(17.83%)> DOC(14.4%)> Fe(12.89%)>TDP(8.27%)> Al(6.69%)> Na(6.36%)> Mg(5.60%)> TDN(5.11%)> Pb(4.49%)>K(4.33%)> Ca(1.04%)> Cr(-1.51%)> Mn(-6.15%).(2)The alpine forest woody debris significantly altered stream flow characteristics,increasing the depth of water downstream as well as the discharge,thus significantly increasing the export of elements from the stream.Overall,woody debris increased the export of heavy metals by 71.85%,96.79%,66.74% and 61.15% of the stream export of Cu,Mn,Cr and Pb,respectively;it also significantly increased the export of other metals in the stream by 62.26% and 59.29% of the stream export of Al and Fe,respectively,and increased the export of salt-based ions K,Na,Ca,and Mg by 50.29%,37.62%,59.50%,and 41.26%,respectively;although the increase in stream C and N from woody debris was relatively small compared to the other elements,29.50% and 33.91%,respectively,the increase in stream P export was as high as 95.98%.(3)The effect of mixed non-woody and woody debris in alpine forests on stream elemental export was similar to that of woody debris,but the increase was smaller than that of woody debris.The effect of mixed debris on stream C and N export was smaller,at 23.80%and 21.52%,respectively;the increase in stream P was 43% lower than that of woody debris,but still increased P export by 53.25%;the increase in stream salt-based ion K,Na,Ca and Mg export was 29.87%,19.64%,40.05% and 23.80%,respectively.Significantly higher increases were observed for stream other metallic and heavy elements,increasing the export of Al and Fe by 55.73% and 34.37%,respectively;and for Cu,Mn,Cr and Pb by 49.81%,94.36%,42.53% and 43.19%,respectively.(4)Litter input was the main influence on stream elemental export,with a relative importance of 30.7% ～ 52.7%.In addition,we also found that litter input was the main influencing factor for DOC,TDP,Ca,Al,Fe,Cu,Mn and Cr content.Overall,the influence of litter input(22.63%)on stream elemental content was comparable to that of rainfall(22.48%).(5)Stream DOC has the same trend as TDN,Na,Mg,Al,Mn and Cr.Plant debris enhance the synergistic variation between Fe and the above elements,and the export of all these elements is regulated by the input of litter matter and the content is influenced by moisture conditions at the same time.The nutrient elements TDN and TDP behaved in opposite ways,with stream TDN export following the same trend as DOC,Na,Mg,Al,Fe,Mn and Cr,and TDP export being synergistic with Ca and Pb.Na and Mg export behaved similarly and were synergistic with DOC,TDN,Al,Fe,Mn and Cr,which were more influenced by hydrothermal conditions;K was more influenced by soil moisture and K is more influenced by soil moisture and rainfall and has a better fit with Na and Mg,but the links between the other elements associated with them are not significant;Ca shows similar variations with TDP,Fe and Cr.Al and Fe are mainly influenced by rainfall and litter inputs,their exports behave in the same way and have synergism with DOC,TDN,Na,Mg,Mn and Cr.The heavy metals Mn,Cr and Cu are mainly regulated by the input of litter,Mn and Cr behave similarly and synergistically with DOC,TDN,Na,Mg,Al and Fe;Cu shows the opposite trend to them;Pb has the same trend as TDP and the opposite trend to Fe,Mn and Cr.Other common heavy elements such as Cd,As and Hg did not reach the detection limit.In summary,woody debris increased the elements export in headwater streams,and the increase in element export from a mixture of non-woody and woody debris was less than the increase in woody debris.This suggested that non-woody debris have a retention effect on stream elements that increases with accumulation in the vicinity of woody debris.Woody debris can alter water flow conditions and thus influence stream element export patterns,contributing approximately 31% to source stream element export.For most elements,the effect of plant debris on stream element contents was comparable to that of precipitation,with plant debris enhancing the sensitivity of stream elements to change in hydrothermal conditions.In addition,we suggest that management and conservation measures for alpine forest water-harvesting sites need to pay attention to the distribution of plant debris in headwater streams to avoid significant elemental export downstream.These findings are useful for further insight into the processes of material transport along the forest-stream continuum,and provide some scientific basis for the management and protection of the upper Yangtze River water containment area.