Effects Of Forest Type Conversion On Rainfall Partitioning Processes In The Subtropical Forest

Atmospheric precipitation not only serves as the primary water source for vegetation growth in forest ecosystems,but also drives the migration of carbon and various elements or substances in the canopy,trunk,and soil.Due to differences in vegetation composition,canopy and leaf structure,and their responses to different rainfall events,different types of forests often exhibit distinct ecohydrological processes.The subtropical region of China is abundant in water and heat,and rapid forest growth has made it the most important production base for commercial forests in the country,providing a large amount of timber and other forest products for regional economic and social development.At the same time,more than half of the area of native evergreen broad-leaved forests in China has been replaced by secondary forests and plantations.The change in forest structure resulting from the conversion of forest types affects forest ecohydrological processes,further impacting forest water conservation and soil conservation functions,and may pose a risk of declining comprehensive forest service value,but this is not well recognized.Therefore,relying on the forest hydrological research platform of the Fujian Sanming Forest Ecosystem National Field Science Observation and Research Station,a study was conducted from April 2021to March 2022 on the throughfall,stemflow,canopy interception,surface runoff,and net input of natural forests dominated by Castanopsis carlesii that have been converted into secondary forests,Castanopsis carlesii plantations,and Cunninghamia lanceolata plantations.As well as examined the migration processes of carbon,nitrogen,phosphorus,and sulfur elements carried by these hydrological processes.（Due to the influence of topography,no significant subsurface infiltration was collected during the study period）.The aim was to fully understand the impact of subtropical forest type conversion on hydrological processes.The main research results are as follows:（1）During the study period,a total of 36 effective rainfall events were monitored,with a total precipitation amount of 1242.67 mm,a total precipitation duration of 1152.72 h,and mean precipitation intensity of 1.57 mm h^-1.The rainfall intensity during the rainy season and dry season was 1.66 and 1.34 mm h^-1,respectively.The rainy season was characterized by short-duration rainfall events with high rainfall intensity,while the dry season was dominated by continuous rainfall events with low rainfall intensity but long duration.The throughfall rates for natural forest,secondary forest,C.carlesii plantation,and C.lanceolata plantation were 65.83%,69.78%,75.69%,and 75.96%,respectively.The stemflow rates for natural forest,secondary forest,C.carlesii plantation,and C.lanceolata plantation were 6.00%,4.28%,1.43%,and 1.29%,respectively;Canopy interception rates were 30.92%,27.93%,26.25%,and 24.51%respectively;surface runoff amounts were9.53,9.05,7.26,and 6.28 mm respectively;and net precipitation amounts were 833.08,911.28,951.11,and 953.69 mm respectively.Overall,after the conversion of natural forest to secondary forest,C.carlesii plantation,and C.lanceolata plantation,the throughfall rate and net precipitation showed an increasing trend.Among them,the increase was most significant in the C.lanceolata plantation,with a 15.39%increase in throughfall rate and a14.48%increase in net precipitation.In addition,the stemflow rate,canopy interception capacity,and surface runoff showed a decreasing trend,with the greatest decrease observed in the C.lanceolata plantation,with reductions of 78.50%,20.73%,and 16.03%,respectively.There was significant correlation between gross rainfall and throughfall,stemflow,and surface runoff for all four forest types.In addition,there was a significant negative correlation between temperature and both throughfall and stemflow.The canopy interception capacity increased with the increase in rainfall intensity and gross rainfall but decreased with the increase in temperature.forest stand characteristics such as canopy closure,tree height,and basal area had significant effects on the throughfall,stemflow,canopy interception,and surface runoff of the four forest types.Stand characteristics such as canopy closure,tree height,and basal area had significant effects on passage,stem flow,canopy interception,and surface runoff for the four forest types.（2）The average concentrations of dissolved organic carbon（DOC）,total nitrogen（TN）,dissolved organic nitrogen（DON）,nitrate nitrogen（NO₃^-）,ammoniacal nitrogen（NH₄⁺）,total phosphorus（TP）,and sulfate(SO₄^2-)in atmospheric precipitation were 28.66,1.12,0.55,0.21,0.36,0.05,and 2.57 mg L^-1,respectively.The enrichment ratios of DOC,TN,DON,NO₃^-,and SO₄^2-in throughfall of natural forest,secondary forest,C.carlesii plantation,and C.lanceolata plantation were all greater than 1.The enrichment ratios of DOC,TN,DON,and NO₃^-were higher in C.lanceolata plantation,ranging from 1.07 to1.67 times that of natural forest.The enrichment ratios of SO₄^2-in throughfall was highest in natural forest,indicating that this element is easily leached and washed from the canopy.The enrichment ratios of DOC and SO₄^2-in stemflow of the four forest types were all greater than 1,and the enrichment ratios in C.lanceolata plantation were higher than those in other forest types.The enrichment ratios of TN,NH₄⁺,and TP in stemflow of the four forest types were all less than 1,indicating that these elements were adsorbed when passing through the tree trunks.In addition,the surface runoff in the four forest types showed enrichment of DOC,TN,DON,NO₃^-,NH₄⁺,TP,and SO₄^2-,with the enrichment degree of DOC and NO₃^-being higher in secondary forest,and that of TN,DON,NH₄⁺,TP,and SO₄^2-being higher in natural forest.（3）During the study period,the input of DOC in atmospheric precipitation was 305.23 kg ha^-1,and the input of DOC during the rainy season was 1.06 times that of the dry season.The interception capacity（interception amount）of canopy for DOC in precipitation shows that natural forest(43.30 kg ha^-1)>secondary forest(36.42 kg ha^-1)>C.lanceolata plantation(22.62 kg ha^-1),but precipitation leached DOC from the canopy of C.lanceolata plantation,with a leaching amount of 21.45 kg ha^-1.The stand characteristics and meteorological factors have no significant impact on the interception of DOC in the canopy of natural forest and C.carlesii plantations,but the increase in stemflow significantly reduces the interception capacity of DOC in the canopy of secondary forests and promotes the leaching of DOC in the canopy of C.lanceolata plantations.In addition,the net input of DOC in natural forests,secondary forests,C.carlesii plantations,and C.lanceolata plantation is 312.79,319.37,334.23,and 378.62 kg ha^-1,respectively.（4）During the study period,the total annual input amounts of TN,DON,NO₃^-,and NH₄⁺in atmospheric precipitation were 11.33,5.06,2.53,and 3.74 kg ha^-1,respectively.In addition,the input amounts of these elements in the rainy season were 2.40-4.67 times higher than those in the dry season.The range of interception amounts of TN,DON,and NH₄⁺by the canopy from precipitation were 0.88-3.22 kg ha^-1,0.26-1.49 kg ha^-1,and1.17-1.62 kg ha^-1,respectively.The interception capacity of the canopy to TN and DON in the atmospheric precipitation is shown as secondary forest>C.carlesii plantation>natural forest>C.lanceolata plantations,and the interception capacity of NH₄⁺was natural forest>secondary forest>C.carlesii plantation>C.lanceolata plantations.In addition,the interception capacity（interception）of NO₃^-in the canopy is shown as secondary forest(0.37 kg ha^-1)>C.carlesii plantation(0.33 kg ha^-1)>natural forest(0.03 kg ha^-1).The forest characteristics and meteorological factors explained the changes of TN,DON,NO₃^-and NH₄⁺interception/leaching in the canopy of natural forests,secondary forests,C.carlesii plantation and C.lanceolata plantations of 12%-30%,14-18%,36%-55%,31%-34%,respectively.In addition,the net inputs of TN,DON,NO₃^-and NH₄⁺in C.lanceolata plantations were 10.27,4.75,2.99 and 2.55 kg ha^-1,respectively.（5）The total annual input of TP in atmospheric precipitation is 0.47 kg ha^-1,and the input of TP in atmospheric precipitation in rainy season is 14.67 times that in dry season.The interception effect of canopy on TP in atmospheric precipitation is shown as secondary forest=C.carlesii plantation>natural forest=C.lanceolata plantation.The increase of basal area significantly promotes the interception of TP by natural forest canopy,but the stand characteristics and meteorological factors have no significant impact on the interception of TP by secondary forest,C.carlesii plantation and C.lanceolata plantations canopy.In addition,the net input range of TP in natural forest,secondary forest,C.carlesii plantation,and C.lanceolata was 0.019-0.031 kg ha^-1.The annual input amount of SO₄^2-in atmospheric precipitation was 25.38 kg ha^-1,and the input amount during the rainy season was 1.40 times higher than during the dry season.The SO₄^2-leaching from the crowns of natural forests,secondary forests,spruce plantations,and C.lanceolata plantation was17.30,16.70,11.75,and 14.57 kg ha^-1,respectively.The increase of basal area promoted the leaching of SO₄^2-from the crowns of C.carlesii plantation and C.lanceolata plantations,while the forest characteristics and meteorological factors had no significant effect on SO₄^2-leaching from the crowns of natural forests and secondary forests.In addition,the net input amounts of SO₄^2-in natural forests,secondary forests,C.carlesii plantation and C.lanceolata plantations were 41.98,41.51,36.76,and 39.56 kg ha^-1,respectively.In summary,the conversion of natural forests to secondary forests,C.carlesii plantation and C.lanceolata plantations led to an increase in throughfall and net precipitation input,and a decrease in stemflow,interception capacity,and surface runoff,with C.lanceolata plantations showing the most significant changes.As is well known,inorganic nitrogen is a major air pollutant,and the interception capacity of NH₄⁺and NO₃^-in precipitation by the crowns of the C.carlesii plantation and C.lanceolata plantations is lower than that of natural forests and secondary forests,indicating a decrease in the purifying effect of the crowns.In addition,although the presence of C.lanceolata plantations leads to the maximum net input of DOC,TN,and DON,which further increases the potential for nutrient input to the community under the canopy through the hydrological pathway,the net input of NH₄⁺and NO₃^-is higher in C.lanceolata plantations than in other forest types,which may increase the risk of forest soil acidification.The above results,from the perspective of forest ecohydrological processes,indicate that the risk of a decline in ecological service functions of subtropical evergreen broad-leaved forests after conversion to plantation forests.Therefore,in the management and operation of the subtropical forest ecology,it is necessary to vigorously protect the natural subtropical evergreen broad-leaved forests,while promoting the harmonious and orderly development of secondary forest resources,to achieve sustainable forest management and restoration of forest ecological functions,and further enhance the ecological service capacity of existing forests.