Coarse Woody Debris At Huoditang Forest Region In The Qinling Mountains

The storage, tree species. decay classes, diameter classes of coarse woody debris (CWD), and the characteristics of density and water content in fallen woods of Pinus tabulaeformis and Toxicodendron vernicifluum were investigated in a natural secondary forest of P. tabulaeformis at Huoditang forest region in the Qinling Mountains. We compared the difference of the storage of CWD, discussed the effects on the decomposition of fallen woods, and revealed the reason for the formation of fallen woods. We also analyzed the change of the nutrients of fallen wood during the decomposition process, and discussed the effects of the decomposition of fallen woods on the soil nutrients. This study aimed to provide a theoretical basis for sustainable management of forest ecosystems in the Qinling Mountains, data on systematic assessment of fallen woods during the decomposition process, and a scientific basis to clarify the role of CWD in carbon cycle of the forest ecosystem. The main results are as follows:(1) The CWD storage was averagely12.56t-hm-2in a natural secondary forest of P. tabulaeformis at Huoditang forest region, of which logs, snags and stumps accounted for65.68%,33.13%and1.19%, respectively. The tree species of CWD on this land were Metasequoia glyptostroboides, Pinus armandi, Larix principis-rupprechtii, P. tabulaeformis, T. vernicifluum and Quercus aliena var. acuteserrata. However, the P. tabulaeformis was the main tree species of CWD, the storage of which was averagely7.21t·hm-2, occupying57.41%, followed T. vernicifluum, the storage of which was averagely3.53t·hm-2, occupying28.10%. Ⅰ and Ⅱ decay class of CWD distributed very widely at the region, accounted for72.93%of the average. Apart from I decay class of snags occupied the maximum proportion, the other four decay classes of logs all occupied dominantly.10～20cm diameter of CWD distributed very broad at the region, but the storage of30～40cm diameter of CWD was dominant, occupying46.26%of the total. The storage of30～40cm diameter of logs and10～20cm diameter of snags were dominant. While the storage of stumps only had10～20cm and30～40cm diameter, it had very few storage at the region.(2) There were significant differencs between the density of P. tabulaeformis and T. vernicifluum (P<0.0001), the same differences also existed among the density of the same tree at five different decay classes (P<0.0001). The model of density and decay class of logs of P. tabulaeformis is y=0.4571-0.04731x (R2=0.99264), while that of T. vernicifluum is y=0.40821-0.04485x (R2=0.99889). With the decomposition of logs, the density of P. tabulaeformis decreased faster than that of T. vernicifluum. In addition, the densities of logs at five decay classes of T. vernicifluum were all higher than those of P. tabulaeformis.(3) In terms of water content, there were also significant differences between P. tabulaeformis and T. vernicifluum (P=0.0047<0.01) and among the same tree at five different decay classes (P<0.0001). The model of water content and decay class of logs of T. vernicifluum is y=0.35328+0.09435x (R2=0.97164), while that of P. tabulaeformis is y=0.18911+0.15191x (R2=0.90611). With the decomposition of logs, the water content of P. tabulaeformis increased faster than that of T. vernicifluum.(4) The model of decomposition of logs of P. tabulaeformis is y=0.362e-0.03993a (R2=0.98178), while that of T. vernicifluum is y=0.37522e-0.04533a (R2=0.98268). The decomposition constant of P. tabulaeformis fallen wood was0.03993, while that of fallen wood of T. vernicifluum was0.04533. The monomial exponential attenuation model predicted that it took17and75years for P. tabulaeformis fallen wood to lose50%and95%of its mass. The monomial exponential attenuation model also estimated that it took15and66years for T. vernicifluum fallen wood to lose50%and95%of its mass.(5) There were no significant differences among C contents in the decay classes of P. tabulaeformis, but the C content of T. vernicifluum decreased significantly with decaying. Obviously N accumulated at the V decay class in P. tabulaeformis and T. vernicifluum. In regard to Mg there was no significant difference among the decay classes in P. tabulaeformis, while Mg increased significantly at the late stage of decomposition in T. vernicifluum. No differences could be found in P, K and Ca among the decay classes of P. tabulaeformis and T. vernicifluum, however with the decomposition of P. tabulaeformis and T. vernicifluum, the contents of P and Ca increased at linear trend, the content of K decreased at linear trend.(6) With the increase of soil depth, the content of C in the soil under P. tabulaeformis and T. vernicifluum fallen wood decreased significantly, yet no difference could be observed in those of K, Ca and Mg. The content of C increased significantly from decay I to V, while those of N, P, Ca and Mg were similar.