Abstract:The responses of diameter at breast height(DBH)in tree layer to nitrogen(N)additions were studied in three representative forest types in Dinghushan Biosphere Reserve：a pine(Pinus massoniana)forest，a pine— broadleaved mixed forest，and a broadleaved forest．Dissolved NH4NO3 was sprayed monthly with equal applications dose onto the forest floor of 10 m×20 m plots within a year to simulated elevated N deposition，Four treatments were established in the broadleaved forest：control，low N(50 kg N hm-2a-1 )，moderate N (100 kg N hm-2a-1 ) and high N(1 50 kg N hm-2a-1 )，and three treatments(control，low N and moderate N)，in mixed and pine forests．All the treatments were in three replicates. Annual increment rates of DBH in control plots were estimated to be 4.84％ , 4.09％ and 2.99％ for the pine, mixed and broadleaved forests， respectively. After treatment with N additions for one year， in the pine and mixed forest, the DBH increment rate in low N plots was similar to that in the control， while in moderate N plots the increment rates in pine forest and mixed forest were by 77．8％ and 105．6％， respectively， higher than those of the contro1． In contrast， in the broadleaved forest， all N addition treatments decreased the tree growth，though the treatment efect was not significant，and the increment rates in the low，moderate and high N plots were by 36．8％，28．5％ and 41.0％，respectively，lower as compared to the contro1．The efects of N addition on tree growth diferent in species. N additions stimulated the growth of Pinus massoniana both in pine forest and mixed forest， but decreased that of most broadleaved species in all the three forests.
