Relationship Between Litter Production And Stand Structure Under Nitrogen And Phosphorus Addition In Subtropical Evergreen Broad-leaved Forests

Nitrogen?N?deposition is one of the important factors driving global changes.Continuous high N deposition has a serious impact on plant diversity and functioning of forest ecosystems,becoming the focus of global researches.Litterfall is an important part of forest productivity and is the main way of material cycling and energy flow.This is the basis of forest self-sustaining growth and nutrient balance.The forest structure is an important control of litterfall production.In this study,three subtropical evergreen broad-leaved forests across an age sequence?46-a middle-aged Castanopsis sclerophylla forest,78-a mature Castanopsis eyrei forest,and about 200-a overmature Castanopsis eyrei forest?were selected for a simulated N deposition experiment in order to reveal the effects of N deposition and stand structure on litterfall production.The experiment was designed with three treatments including High nitrogen(HN,100 kg N·(hm²·a^–1),high nitrogen plus phosphorus(HN+P,100 kg N·hm^–2·a^–1+50 kg P·hm^–2·a^–1,and control?CK,No nitrogen and phosphorus added?.The litterfall amount,C,N,P content and their stoichiometric ratios were analyzed.The findings are as follows.?1?The concentrations of C,N,P in the different litter components were as follows:overmature forest>middle-aged forest>mature forest.The C,N,and P concentrations for different forest were significantly different under N and P treatments.The order of nutrient return among the components were as follows:C>N>P.Compared with CK,the addition of N and P increased N and P returns,but the effect on C return was not significant?p>0.05?.The N return was significantly high under HN treatment?p<0.05?,and the P return was high under HN+P treatment?p<0.05?.?2?In this study,the nutrient stoichiometric ratio analysis of three forests was conducted under the same treatment.The results showed that average C:N ratio was lower than the global and national averages,and the utilization efficiency of N was lower than the global and national levels.The addition of N+P alleviated the scarcity of P to a certain extent,and the forests in the region had a great P use efficiency.The ratio of N:P was also higher than the global and national averages,further proving that the study area maybe P limitation.?3?Changes in the monthly litterfall were affected by factors such as terrain,climate,and forest type.Under natural conditions,the litterfall showed two peaks for the foliar litter with the autumn peak occurred from October to November and the spring peak from April to May.Due to the influence of the other external factors,there were inconspicuous fluctuations among various forests.The annual litterfall in this study was 7.68 t·hm^-2for middle-aged forest,6.62 t·hm^-2in mature forest,and 6.24t·hm^-2in the overmature forest.?4?The addition of N and P increased the total litterfall.In overmature forest,there was HN+P(6.17 t·hm^-2)>CK(5.99 t·hm^-2)>HN(5.78 t·hm^-2),mature forest showed HN+P(6.70 t·hm^-2)>HN(6.44 t·hm^-2)>CK(6.30 t·hm^-2),and middle-aged forest was HN+P(6.70 t·hm^-2)>HN(5.70 t·hm^-2)>CK(5.40 t·hm^-2).?5?The total litterfall,branch,foliar,and fruit litters were negatively significantly correlated the mean DBHs of the forests?p<0.01?.The total litter litterfall and fruit litter had a significant positive correlation with the average tree height?p<0.01?.The foliar litter had a significant positive correlation with the average tree height?p<0.05?.This result indicates that stand structure has a certain impact on litter productivity.?6?Our finging showed that stand density was significantly negatively correlated to the litterfall C content and their N:P ratio.It indicates that the forest structure are major control of litterfall production and its nutrient stoichiometric trait.