Effects Of Thinning On The Undergrowth Of Platycladus Orientalis And Pinus Tabulaeformis Plantation In Beijing Mountainous Areas

The Platycladus orientalis plantation and Pinus tabulaeformis plantation, which cover an area of 60,106 hm² and 59,816.8 hm², all count for 20% of the total non-commercial forests, are major non-commercial plantation in Beijing mountainous areas and improving quality of the stand is the key stone of ecological construction in Beijing. In order to investigate the dynamic variance of the undergrowth and to provide reference for reasonable tending and the appropriate stand density, the Platycladus orientalis plantation and Pinus tabulaeformis plantation were selected in Beijing mountainous areas and conducting light, medium and heavy tending on each sample plots respectively and the changes of the growth of forest, the environment and the undergrowth diversity were investigated and analyzed. The results showed:Tending adjusted the stand structure, decreased the canopy density, improved the forest environment, and hence made it a more favorable condition for stand growing and undergrowth renewing. After the tending process, the bulk density was decreased, and the heavier tending the more density that was decreased. Contrasted with the controlled group, the bulk density could reduce as much as to 8.6% ～17.5% for the heavy tending group. Similarly, with the strengthening of the tending, the saturated water contend, capillary moisture content and the field water holding capacity all increased. After tending, the light transmission conditions for each month were all improved; and as the strengthening of the tending, the photosynthetically active radiation (PAR) was increased too; compared with the controlled group, the PAR at the post-tending stand could increase up to 186.3%～343.6%. Tending changed the light transmission conditions, the correlative air temperature, relative humidity both had changed; compared with the controlled group, the air temperatures in different seasons all increased after tending, the average increasing in temperatures were 0.67～1.14 ℃ for Summer. As the tending became heavier, the relative humidity were decreased, the average drop in the relative humidity for Summer were 3.33%～7.71%. Each environmental factor all gained a most significant change after the heavy tending.In the 2^nd year after tending of the Platycladus orientalis plantation on sunny slope with thin soil, sunny slope with thick soil and half sunny slope with thick soil, the DBH, height, crown length and IV of the tending group were higher than that of the control group, and the growth the 3 groups were ranked in such order: heavy tending > medium tending > light tending. In the 1 st and 2nd tending year, the species and quantity of undergrowth, Simpson index, Shannon-Wiener bio-diversity index and the total amount of the undergrowth biomass all increased along with enhancing tending intensity; and the distributing of vertical structure of the community became greater significantly as the tending intensity was enhanced; After the heavy tending, the distributing height for undergrowth increased to 150 cm; After 2 years of heavy tending, the species increased in the sunny slope with thin soil, sunny slope with thick soil and half sunny slope with thick soil are reached 13～17, and the undergrowth indicator plants (above level 2) increased 4～5 species, besides, the heavy tending has an effect of improving the undergrowth turning to meso-xerophytes; After the medium and heavy tending, the undergrowth biomass can reach to 6 150.59 ～ 7 989.98 kg/hm². For the Platycladus orientalis plantations on 34 a sunny slope with thin soil, 20 a sunny slope with thick soil and 22 a half sunny slope with thick soil, are 3 400～4 700 truncks/hm², 3 300～3 900 truncks/hm² and 2 500～3 400 truncks/hm² respectively.In the 2^nd year after tending of Pinus tabulaeformis plantation on shade slope with thick soil, the DBH, height, crown length, IV and stocking of the tending group were significantly higher than that of the control group, while no clear evidence was observed regarding the variance on growth of the Pinus tabulaeformis among the light tending, medium tending and heavy tending groups. 6 years after tending, the growth of the Pinus tabulaeformis of the 3 groups were ranked in such order: heavy tending > medium tending > light tending; and the heavy tending group obtained the best growth. The DBH, height, crown length, IV and stocking of the heavy tending group increased 70. 2%, 40. 4%, 52. 3%, 236. 4% and 41. 2% respectively. In the first 2 years of tending, the species and quantity of the undergrowth, Simpson index, Shannon-Wiener index, Evenness index and the total amount of the undergrowth biomass all increased along with the enhancing of the tending; After 6 years of the tending, The species, Simpson index, Shannon-Wiener index and Evenness index all depressed and for the Simpson index and Shannon-Wiener index, the 3 groups were ranked under the order: heavy tending > medium tending > light tending. Compared with the first 2 years after tending, the quantity of the undergrowth was decreased for every group, but the undergrowth coverage was increased as the tending was enhancing; the community vertical structure became much clearer and undergrowth plants that higher than 150 cm appeared for the heavy tending group. Besides, the medium and heavy tending increased the ratio of the highly water demanded plants and had an effect of turning the undergrowth from xero-mesophytes to meso-xerophytes and mesophytes. After the tending, the total amounts of the undergrowth biomass were increased as the tending was enhancing. After 6 years' tending, the undergrowth biomass are increased by 70.9% respectively for the heavy tending group. For increasing the growth and diversity of undergrowth, the heavy tending has the best effect; the appropriate stand density for the Pinus tabulaeformis plantations, on 35～37 a sunny slope with thick soil and an original density of 3 2003～ 467 trunks/hm² is 1240～1500 truncks/hm². In conclusion, tending has significant effects on the forest environment, stand growth and undergrowth diversity of the Platycladus orientalis plantation and Pinus tabulaeformis plantation in Beijing Mountainous area. Reasonable tending methods should be applied to control the stand density, so as to make the plantation growing in a healthy and sustainable way.