Effects Of Burning And Nitrogen Addition On Leaf Function Traits Of Coniferous-Broadleaf Forest In The Climatic Transitional Zone

It has been predicted that fire frequency and atmospheric nitrogen(N)deposition will increase resulting from intensifying human activities.Both fire and N deposition,the dominant disturbance sources in forest ecosystems,can substantially influence forest ecosystem structure and functioning.Plant functional traits are important indicators for forest plant growth.After fire disturbance,forest plants evolved and many physiological forms of adaptation countermeasures are produced.In addition,N deposition also affects plant functional traits.However,responses of plant functional traits to fire disturbance and N deposition are still unclear.An experiment with burning and N addition was conducted in a Quercus acutissima-Pinus massoniana mixed forest in the Mountain Xian,Xinyang,Henan Province,Central China,after an artificial fire occurred in 2014.Field sampling experiments of mature leaves and senescent leaves of plants were carried out in the growing season(July and November)of 2016 and 2017.Six species were four shrub species(Vitex negundo Linn,Symplocos chinensis(Lour.)Druce,Lindera glauca Sieb.et Zucc,and Rubus corchorifolius L.f.)and two tree species(Quercus acutissima Carruth.,and Pinus massoniana Lamb.).Leaf functional traits including leaf area,carbon(C),N,and phosphorus(P)and soil physical and chemical properties(e.g.soil temperature and moisture)were measured to investigate their response to burning and N addition as well as their relationships.Burning enhanced photosynthetically active radiation,resulting in the decreases in specific leaf area of shrubs and trees.Burning increased photosynthetically active radiation and photosynthesis,leading to C accumulation in leaves and hence increasing leaf C content of shrubs and trees.Burning decreased soil inorganic N because of leaching,mineralization and volatilization,which reduced the plant absorption for soil inorganic N and then reduced leaf N content.Nitrogen addition had no effect on specific leaf area,leaf C,N,and P content.The response of leaf P content to burning varied with species.Burning decreased soil total P because of the reduction of available P and depressing the uptake of plants for soil available P,which subsequently decreased plant leaf P content.Nitrogen addition did not affect specific leaf area,leaf C,N,or P content at the leaf level.The changes of the ratio of leaf C: N,C: P and N: P also showed different responses to burning.Burning decreased C but increased N content at the leaf level,which resulting the increases of C: N ratio and C: P ratio as well as reduction of N: P ratio.Burning had no effects on leaf N reabsorption efficiency of shrubs and trees.Nitrogen addition had no effect on leaf N reabsorption efficiency of shrubs,but reduced leaf N reabsorption efficiency of trees.The negative relationships between leaf N reabsorption efficiency of trees and soil moisture in the result.Nitrogen addition reduced soil moisture,which depressed leaf N reabsorption efficiency of trees.Burning decreased leaf P reabsorption efficiency of shrubs,but had no effect on leaf P reabsorption efficiency of trees.Nitrogen addition did not affect on leaf P reabsorption efficiency of shrubs,but increased leaf P reabsorption efficiency of trees.The positive relationships between leaf P reabsorption efficiency of trees and soil moisture in the result.Nitrogen addition reduced soil moisture,which leading to the increase of P reabsorption efficiency of trees.In summary,the insensitivity of plant functional traits to N addition proves that the climatic transitional zone is not N-limited area,and it can be inferred from the range of leaf N: P ratio could determine belongs to P-limited area.The results suggest that the effects of fire and N addition on foliar stoichiometry are independent and have different mechanisms.In addition,this study demonstrated that it is inaccurate to assess the effects of natural and human disturbances on litter quality by the effects of burning and N addition on senescent leaves.These findings will help us better understand the effects of different management strategies on nutrient cycling and the response of senescent leaves to burning and N addition,thus providing a basis for forest management in the future.