Changes Of Species Diversity Of Understory Vegetation In Pinus Massoniana Forest After Disturbance By Pine Wilt Disease And Its Relation To Soil Nutrients

Pine wilt disease is one of the most harmful plant inspection and quarantine pest.Once infected by Bursaphelenchus xylophilus, the pine tree will die off within two tothree momths. Thus the pine wilt disease has a destructive harm to pine forestecosystem. This disease was firstly found in Shushan Forest Park in1999. The damagehas been constantly enlarged. Several hectars of masson pine stands have beenseriously damaged with the pine wilt disease since2005. A large number of massonpine trees were withered and gradually died off.Changes in the forest structure will cause a series of changes in soil propertiesespecially after Bursaphelenchus xylophilus invasion. This change can cause greatchange of species diversity in the undergrowth. In order to understand the changes ofunderstory vegetation in Pinus massoniana forest after disturbance by pine wilt disease,an investigation of variation in understory species diversity and its relations to thedominant tree and soil nutrients was conducted at Shushan forest park of suburbanHefei.Results showed that the diversities of both shrub layer and herb layer are slightlyhigher in summer than in spring and autumn. Among the shrub species with theimportant value over10%, more than60%was from the overstory broadleaf trees. Thepeak value of the Margalef index for the understory shrub layer appeared in summer,the peak values of both Shannon-Winner index and Pielous index emerged in spring,which indicated that the proportion of individual plant species from understory shrublayer reached the most even balance in spring. As for the herb layer, three diversityindices showed the peak values in summer. Among the five sample forest stands, thespecies diversity index was greater for the shrub layer than for the herb layer. Thereexisted significant correlations among the three indices used in this study.In addition, a negative and significant relationship was found between theMargalef indices of understory vegetation and the importance value of Pinusmassoniana. In the stands with the importance value of Pinus massoniana over80%,the Margalef indices of understory plants was relatively low. On the contrary, theMargalef indices of understory plants was great in the stands with the importance valueof Pinus massoniana less than80%. Moreover, the negative and significant relationship was found between Margalef indices of understory shrub layer andimportance value of Pinus massoniana in summer. The relationship between speciesdiversity of the herb layer and importance value of Pinus massoniana was moreobvious.In spring, soil moisture content, pH and total Mg reached the peak value. Insummer, NH+4-N, total N, soil organic matter, total K, total Ca achieved the peak value.In autumn, NO-3-N, available P, TOC, total P came to the peak value. In the samplingseasons, the concentrations of total Ca, total K, total Mg, total N, and total P werehigher in the stands with the importance value of Pinus massoniana less than80%,implying that stands which had higher species diversity indices of understoryvegetation possessed higher soil nutrients. Similarly, stands with lower speciesdiversity indices of understory vegetation were usually lower in soil nutrients.Soil organic matter, total P, total N, Ca and Mg were positively correlated with thespecies diversity of understory vegetation while a negative relationship was foundbetween soil moisture content and the species diversity of understory vegetation.Besides, correlations between soil nutrients and the species diversity of understoryherb layer were more significant than those between soil nutrients and the speciesdiversity of understory shrub layer.