| Quantifying the effects of forest structure on its different functions and then determiningthe rational stand structure index which can meet the needs of multiple functions, are the keytechniques to realize the multifunctional forest management. After realizing the problems in themanagement of mountainous water-retention forests of northern China, such as the unclear ofdominant function, focusing only on the timber production function, neglecting the water yieldrequirement from head-water catchments and biodiversity protection, this study was aimed tosearch the multifunctional forest management technique which can ensure the water supply asthe dominant function of mountainous forest area and possibly more meet the requirement forother functions. In this study, the plantation of Larix principis-rupprechetii in the Xiangshuihesmall catchment of Liupan Mountain, Ningxia in northwestern China, was selected as it is themain tree of plantation in this region. Many sample slots were established to investigate thestand structure (mainly of tree density) and its relation with the tree growth and timberproduction, diversity of undergrowth plant species, plant biomass and carbon sequestration,water cycle and water yielding capacity; also the relationship between stand structure and thesnow-load bearing capacity of trees was analyzed. Finally, an integrated analysis was made todetermine the rational tree density of stands, in which the ability to lower the snow-loaddamage can be ensured for maintaining the stand structure stability, to reduce theevapotranspiration for keeping certain water yielding ability, to promote the growth of largetrees for producing high-value timber, to maintain undergrowth for protecting the biodiversity,to sustain a high vegetation biomass for using function of carbon sequestration. The resultsmay provide a theoretical base and practical guideline for the multifunctional management ofplantation in the study area and other places. Our mainly results were as following:1. The tree growth was influenced jointly by tree density and tree age. The growthprocesses of Larix principis-rupprechetii can be divided into3stages: initial stage, rapid growth stage, and gentle growth stage. The age thresholds for dividing these3stages are10thand30thyear for the growth of DBH and tree height; while the14th. The growth of tree heightwas less influenced by the stand density. Increase of tree density decreased the DBH andtimber volume of single trees when the tree age is above16years. The ratio of tree height toDBH (H/DBH) increased with increasing tree density, meaning a decreasing stability againstthe snow damage. However, it was suggested to keep the tree density in the range of1000-1500trees/hm2, for producing big sized and high-value timber and for lowering the waterconsumption used for tree growth.2. The growth of undergrowth plants was significantly affected by tree density. Thespecies quantity of undergrowth plants reached to the largest when the tree density is1300trees/hm2, while the coverage and biomass of undergrowth plants decreased with increasingtree density. The species diversity, dominance index and Evenness Index of understory shrub-and herb-layers was maximums at the tree density of1300trees/hm2density. From theviewpoint of increasing undergrowth plant diversity and promoting natural regeneration oftrees, it is recommended to take the tree density of1300trees/hm2as the reference for theclose-to-nature conversion of the pure Larix principis-rupprechtii plantation.3. The vegetation biomass and carbon sequestration were influenced by tree density. Thetree layer composed the majority of vegetation biomass and carbon sequestration of stands. Thebiomass of tree layer increased slowly with rising tree density when it is less than500trees/hm2, and then increased more faster when the density is in the range of500-1500trees/hm2, and then slowly again when it is above1500trees/hm2. This illustrated that theincrease of tree density could not proportionally increase the forest biomass production. Inparticular, the high tree density is not favorable for producing high-value and big-sized timber,and for maintaining of stand stability. Therefore, the tree density should be controlled in therange of less than1500trees/hm2. The biomass and carbon sequestered of understory shrublayer decreased drastically with increasing tree density in the range of650-1500trees/hm2, andthen maintained in a very low level and gradually grew to zero when the tree density increases in the range of more than1500trees/hm2. The biomass and carbon sequestered in undergrowthgrass layer decreased slowly with increasing tree density in the range of below1125trees/hm2,and decreased rapidly in the range above1125trees/hm2, and then gradually grew to zerowhen the tree density reached2500trees/hm2. These statistics showed that in order to maintaina certain herb layer, the tree density should be controlled around1125trees/hm2. By composingthe impact of tree density on the vegetation biomass and carbon sequestered in the arbor, shruband grass layers, the tree density should be controlled1500trees/hm2from the perspective ofthe maintenance of higher vegetation biomass and carbon sequestration function.4. Forest snow damage highly relates to the forest structure. There is a close relationshipbetween the snow damage degree and the site character, stand structure and the weatherconditions. The snow damage rate was significantly higher in barren site at high altitude andfacing wind. The stand with higher tree density suffered from significantly higher snowdamage compared with the stands with lower density. This is because the increasing treedensity resulted in a higher ratio of tree height to tree DBH. The snow damage appeared whenthe H/DBH ratio was above0.7, but the damage rate increased not fast with increasing H/DBHratio. Otherwise, when the H/DBH ratio was higher than0.9, the snow damage rate increasedrapidly with increasing H/DBH ratio; and when the H/DBH ratio was over1.0, the snowdamage increased sharply with the H/DBH ratio. Therefore, the H/DBH ratio should bemaintained around0.7, and always below0.9, to enhance the resistance ability of forest againstsnow disaster. According to the relationship between tree density and the H/DBH ratio, the treedensity should be controlled in the range below1200trees/hm2.5. The water yielding function from stand decreased with increasing tree density. Thecanopy interception and tree transpiration measured in growing season increased with risingtree density; but the understory evapotranspiration decreased with increasing tree density; Thewater yield from stand increased non-linearly with decreasing tree density. When the treedensity in the range above1500trees/hm2, the water yield was lower and had little change(109.7-106.9mm) with varying tree density; In the range of1300-1500trees/hm2, the water yield increased slowly with decreasing density, with an increase rate of6.6mm per decrease of100trees/hm2; when the tree density was in the range below1300trees/hm2, the water yieldincreased rapidly with decreasing tree density, with an increasing rate of11.7mm per treedensity decrease of100trees/hm2. To the age range studied, the thinning of high density standsshould be implemented within the range of below1300trees/hm2, in order to increase thewater yield effectively.6. As an suggestion based on the integrated analysis of the findings above, the tree densityshould be controlled in the range below1300trees/hm2if the purpose is to ensure the dominantfunction of water yielding for the mountainous water-retention forests. But when otherfunctional requirements are considered, it was proposed to control the tree density in the rangeof1000-1200trees/hm2.
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