| Pinus elliottii is an excellent dual-use species for producing rosin and lumber with more than80years history since introduced to China, and has become one of the most important afforestation species in hilly area of southern China as its strong adaptability, wide distribution, quick growth in early period, straight trunk and good timber quality. In this paper, different densities of slash pine forest planted in MiLuo city, Hunan province were studied on its growth, biomass, root development and nutrient cycling, and the influence of fertilization and pruning on the growth of middle-aged slash pine forest was studied and analyzed so as to provide the theoretical basis for developing silvicultural measures of slash pine plantations in northern Hunan. The results showed that:1. Densities had significantly or extremely significant influence on each growth index except height. The DBH, height and individual volume all decreased since8years old, the stand volume increased since4years old. The higher density, the higher H/D and the lower C/H with better developed trunk. For11years old plantations, the proportion of middle class wood which diameter was≥14cm was higher in low density while the proportion of small class wood which diameter was8-12cm was higher in high density. As analyzing the cost and benefits comprehensively, we should select two densities of1667and2000tree·hm-2to cultivate middle-large diameter timber and dual-use species for producing rosin and lumber, and select2400tree-hm-2to cultivate middle-small diameter timber of slash pine.2. Stand density had extremely significant effect on biomass and its distribution of individuals and plantations. With the increase of stand density, the biomass of individual tree in arbor layer were significantly decreased, and the stand biomass of arbor layer, litter and the total biomass of stand increased, and the ground cover biomass generally decreased in9and11-year-old slash pine plantations. The biomass of arbor layer accounts for the largest proportion in different components of stand, which accounted for more than81.6%-88.1%and83.0%-87.6%of the total biomass of stand respectively. The stem biomass accounted for the largest proportion in different organs of arbor layer, which accounted for more than37.7%-43.7%and41.4%-45.8%of the stand biomass of arbor layer respectively. The biomass mathematical models of all organs had high accuracy, and various organs biomass and the total biomass can be estimated by the models. The average productivity of stands in different densities were8.9t-hm-2a-1and9.1t-hm-2a-1, and the stand structure with density of2400tree-hm-2was more reasonable which was suitable for slash pine pulpwood in the northern Hunan.3. Stand density had significantly or extremely significant effect on root biomass of individual tree and stand. With the increase of stand density, individual root biomass decreased, and stand root biomass generally increased in9and11-year-old slash pine plantations, among which the tap root and large-root accounted for more than60.7%-70.9%and63.9%-75.2%of the total root biomass respectively. The root biomass decreased significantly with increased soil depth in all stand densities, and more than63.1%the root biomass distributed in0-40cm depth soil and more than81.4%of the root biomass distributed around0-50cm from the trunk in the horizontal direction in11-year-old slash pine plantations. Therefore, to ensure that the fertilizer maximum absorbed by roots of slash pine, fertilization should be applied from the trunk50cm, depth20cm. The mathematical models of root biomasses of individual tree with different stem diameter had high accuracy, and root biomass of each class and the total root biomass can be estimated for the basis of the utilization and development of the root system.4. Stand density had significantly effect on nutrient accumulations of both individual tree and stand of slash pine. With the increase of the stand density, the nutrient accumulation of individual tree in arbor layer decreased, and that in stand of arbor layer, litter and the total quality of the stand increased in9and11-year-old slash pine plantations. The majority of all parts was arbor layer which accounted for more than67.7%-79.2%and69.3%-76.5%of the total nutrient accumulation respectively. The most nutrients accumulated in branch, leaves and timber in arbor layer, which was more than44.1%-50.1%and39.1%-44.9%in11years old forest. With the increase of the stand density, the nutrient uptake and retention of the stand increased significantly, and the return slightly decreased, the turnover became longer, the nutrient cycling rate decreased a little, and the nutrient use efficiency increased. Therefore,1667and2000tree-hm"2were the suitable densities, and the cultivation measures including selectively removing other species, reserving the litter and residue of cutting, applying P-N-K fertilizer should be conducted to maintain the land capability of slash pine plantations.5. Fertilization had obvious effect on the growth and soil et al of slash pine. The DBH and the individual volume growth increased significantly compared with the control3years after fertilization, and the growth of each index was the highest treated by fertilization of700g-tree-1. The2years after fertilization, pH value and available P content in each soil layer decreased, and organic matter, available K content increased, and available N content increased in a large scale. The7months after fertilization, the number of Pisolithus fruiting body generally increased with the increase of the fertilizer, which was2.0-24.0times of the control, and excessive fertilizer maybe suppress the growth of Pisolithus tinctorius. By analyzing the effects of the fertilization on the growth and the cost and benefits, the rational measure was N-P-K fertilization of700g-tree-1for middle aged slash pine plantations.6. Pruning had adversely effected on the growth of each growth index except crown width in the middle aged slash pine. Two years after high intensity pruning, the growth of DBH, individual volume and stand volume in different density of slash pine were significantly lower than those of the control, while crown growth was significantly higher. The middle to light and stubble reserve method pruning showed less adverse effects on the growth. Pruning had bigger effect on stem form of the high density. Middle to heavy pruning and stubble method increased the H/D and decreased the C/H with well developed stem form. The number of rosin knot increased with the increase of the pruning intensity in the3rd-4th month. Lighter pruning and stubble method might decrease the loose knot resinosis. Comprehensively analyzing the cost and benefits of2years pruning, no pruning was suitable for6years old (2500tree-hm-2) and7years old (833tree-hm-2) slash pine plantations.In a conclusion, the invest-output of the forest which density was1667and2000tree-hm-2were better as their shorter nutrient turnover and higher cycling rate, which could be the rational afforestation density to cultivate middle-high diameter timber. While the forest with density of2400tree-hm-2had better stand structure and higher biomass, which could be the primary density to cultivate middle-low diameter timber. In the process of cultivating of slash pine, it were conducive to maintain the land capability and improve the productivity of middle-aged slash pine plantations through selectively removing weeds and bushes of the woodland, applying formula P-N-K fertilizer (700g-tree-1), reserving the litter and residue of cutting in the forest land. |
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