Potentials for win-win alliances among animal agriculture and forest products industries: Application of the principles of industrial ecology and sustainable development

Commercial forests in many parts of the world are deficient in nitrogen and phosphorus. These nutrient-deficient forests often exist in close proximity to large animal feeding operations, meat processing and other food, textile, or other biomass-processing plants, and municipal waste treatment facilities. Many of these facilities produce large surpluses of nitrogen, phosphorus, and organic matter as gaseous ammonia, urea, uric acid, phosphorus compounds, bacterial sludges, and partially treated municipal wastewaters. These co-existing and substantial nutrient deficiencies and surpluses offer ready-made opportunities for discovery, demonstration, and commercial development of science-based, technology-facilitated, environmentally sound, economically viable, and socially acceptable "win-win alliances" among these major industries based on the principles of industrial ecology and sustainable development. The major challenge is to discover practical means to capture the surplus nutrients and put them to work in forest stands from which value-added products can be produced and sold at a profit.     

Potentials for win-win alliances among animal agriculture and forest products industries: Application of the principles of industrial ecology and sustainable development

Commercial forests in many parts of the world are deficient in nitrogen and phosphorus. These nutrient-deficient forests often exist in close proximity to large animal feeding operations, meat processing and other food, textile, or other biomass-processing plants, and municipal waste treatment facilities. Many of these facilities produce large surpluses of nitrogen, phosphorus, and organic matter as gaseous ammonia, urea, uric acid, phosphorus compounds, bacterial sludges, and partially treated municipal wastewaters. These co-existing and substantial nutrient deficiencies and surpluses offer ready-made opportunities for discovery, demonstration, and commercial development of science-based, technology-facilitated, environmentally sound, economically viable, and socially acceptable “win-win alliances” among these major industries based on the principles of industrial ecology and sustainable development. The major challenge is to discover practical means to capture the surplus nutrients and put them to work in forest stands from which value-added products can be produced and sold at a profit.     

Nutrient transport within and between habitats through seed dispersal processes by woolly monkeys in north‐western Amazonia

The contribution of vertebrate animals to nutrient cycling has proven to be important in various ecosystems. However, the role of large bodied primates in nutrient transport in neotropical forests is not well documented. Here, we assess the role of a population of woolly monkeys (Lagothrix lagothricha lugens) as vectors of nutrient movement through seed dispersal. We estimated total seed biomass transported by the population within and between two habitats (terra firme and flooded forests) at Tinigua Park, Colombia, and quantified potassium (K), phosphorus (P) and nitrogen (N) content in seeds of 20 plant species from both forests. Overall, the population transported an estimated minimum of 11.5 (±1.2 SD) g of potassium, 13.2 (±0.7) g of phosphorus and 34.3 (±0.1) g nitrogen, within 22.4 (±2.0) kg of seeds ha^?1 y^?1. Approximately 84% of all nutrients were deposited in the terra firme forest mostly through recycling processes, and also through translocation from the flooded forest. This type of translocation represents an important and high‐quality route of transport since abiotic mechanisms do not usually move nutrients upwards, and since chemical tests show that seeds from flooded forests have comparatively higher nutrient contents. The overall contribution to nutrient movement by the population of woolly monkeys is significant because of the large amount of biomass transported, and the high phosphorus content of seeds. As a result, the phosphorus input generated by these monkeys is of the same order of magnitude as other abiotic mechanisms of nutrient transport such as atmospheric deposition and some weathering processes. Our results suggest that via seed dispersal processes, woolly monkey populations can contribute to nutrient movement in tropical forests, and may act as important nutrient input vectors in terra firme forests. Am. J. Primatol. 72:992–1003, 2010. © 2010 Wiley‐Liss, Inc.     

Nitrogen stable isotopic composition of leaves and soil: Tropical versus temperate forests

Several lines of evidence suggest that nitrogen in most tropical forests is relatively more available than N in most temperate forests, and even that it may function as an excess nutrient in many tropical forests. If this is correct, tropical forests should have more open N cycles than temperate forests, with both inputs and outputs of N large relative to N cycling within systems. Consequent differences in both the magnitude and the pathways of N loss imply that tropical forests should in general be more¹⁵N enriched than are most temperate forests. In order to test this hypothesis, we compared the nitrogen stable isotopic composition of tree leaves and soils from a variety of tropical and temperate forests. Foliar ¹⁵N values from tropical forests averaged 6.5 higher than from temperate forests. Within the tropics, ecosystems with relatively low N availability (montane forests, forests on sandy soils) were significantly more depleted in¹⁵N than other tropical forests. The average ¹⁵N values for tropical forest soils, either for surface or for depth samples, were almost 8 higher than temperate forest soils. These results provide another line of evidence that N is relatively abundant in many tropical forest ecosystems.     

Aspects of the nitrogen cycle in peatland and plantation forest ecosystems in western Ireland

Extensive plantation forests cover large areas of blanket peatland in western Ireland. Sites are characterised by the ombrotrophic nature of the peat and the often extreme maritime conditions prevailing. The study area is located close to two coastlines and in consequence, ions of marine origin are dominant in the bulk precipitation. Mean annual nitrogen deposition is 2.26 kg ha^-1. Forestry development in the region dates from the early 1950s. Deficiency of phosphorus is universally encountered, sometimes accompanied by a shortage of nitrogen. A fertilizer experiment in the study area was maintained for 16 years. The principal response was to applied phosphorus and although nitrogen had a positive influence on growth in the early years, it was of little consequence in the longer term.Over 900 kg N ha^-1 was accumulated in the forest floor. In a mineralisation study of peat collected from plots fertilized 14 years previously, differences in total mineral nitrogen production between treatments were small, but in nitrogen-treated plots a higher proportion of the mineral nitrogen was as nitrate than in those which had not received fertilizer nitrogen. Throughfall measurements in pole-stage crops of Sitka spruce and lodgepole pine which had received no fertilizer nitrogen, showed significantly greater quantities of nitrogen than bulk precipitation.     

Herbivory makes major contributions to ecosystem carbon and nutrient cycling in tropical forests

The functional role of herbivores in tropical rainforests remains poorly understood. We quantified the magnitude of, and underlying controls on, carbon, nitrogen and phosphorus cycled by invertebrate herbivory along a 2800 m elevational gradient in the tropical Andes spanning 12°C mean annual temperature. We find, firstly, that leaf area loss is greater at warmer sites with lower foliar phosphorus, and secondly, that the estimated herbivore‐mediated flux of foliar nitrogen and phosphorus from plants to soil via leaf area loss is similar to, or greater than, other major sources of these nutrients in tropical forests. Finally, we estimate that herbivores consume a significant portion of plant carbon, potentially causing major shifts in the pattern of plant and soil carbon cycling. We conclude that future shifts in herbivore abundance and activity as a result of environmental change could have major impacts on soil fertility and ecosystem carbon sequestration in tropical forests.     

中亚热带森林转换对土壤磷积累的影响

磷是植物生长的必需元素之一,是维持亚热带森林生态系统生产力的关键因子。研究森林转换后土壤因素对磷素的影响,对生态系统的稳定和森林经营具有重要意义。选取由亚热带常绿阔叶林转换而成的米槠次生林(SF)、米槠人促林(AR)和杉木人工林(CF)为研究对象,测定了土壤理化性质、铁铝氧化物、各形态磷含量以及酸性磷酸酶活性,旨在探究土壤磷对森林转换的响应和驱动土壤磷变化的影响因子。结果显示:米槠人促林土壤的全磷、有机磷和微生物生物量磷显著高于米槠次生林和杉木人工林;冗余分析(RDA)发现,土壤含水量、总氮和无定型铁是影响淋溶层土壤磷的主要因子,而在淀积层,则是酸性磷酸酶、游离型铁和总氮起主要作用;土壤生物化学属性和微生物特性都会影响着不同形态土壤P的积累,其中土壤中的水分和酸性磷酸酶活性是调控土壤磷的关键因子。研究表明,中亚热带地区天然林转换为人促更新林更有利于森林土壤磷的储存和供应,有助于维持本区域森林生态系统的稳定。     

Nutrient limitations to plant growth during primary succession in Hawaii Volcanoes National Park

We determined the effects of nutrient amendments on plant growth in three tropical montane rainforest sites representing a sequence of soil ages (< 30, 200, and 2000 y). Factorial fertilization with nitrogen, phosphorus, and all other essential nutrients (combined) was applied to the two younger sites; only nitrogen was applied to the oldest one. Nitrogen supply represented the most important limitation to plant growth in the two younger sites; additions of nitrogen caused significant increases in tree diameter increment, height growth, litterfall, and most other growth-related parameters. In contrast, nitrogen additions had no significant effect on plant growth in the oldest site. Phosphorus additions increased extractable soil phosphorus and plant tissue phosphorus, but did not increase plant growth at the young sites. The results are consistent with Walker & Syers' (1976) model for the control of nutrient limitation during soil development.     

中亚热带次生林和人工林凋落枝水溶性碳氮磷动态特征

凋落枝是森林地上部分凋落物的重要组分,揭示其水溶性碳氮磷的动态规律对于认识森林物质循环过程具有重要意义,但目前研究集中于凋落叶,而对凋落枝缺乏必要关注。因此,以中亚热带典型马尾松（Pinus massoniana）人工林、杉木（Cunninghamia lanceolata）人工林和米槠（Castanopsis carlesii）次生林为研究对象,在一个自然年内调查了凋落枝水溶性碳、氮、磷含量及其芳香化指数以及化学计量比的动态变化过程。结果显示：（1）米槠次生林凋落枝水溶性碳、氮、磷含量及芳香化指数明显大于马尾松和杉木人工林;（2）水溶性碳和磷、水溶性碳比磷、水溶性氮比磷和芳香化指数有明显的季节变化;（3）水溶性碳、水溶性磷、水溶性氮比磷和芳香化指数在不同林分和季节间有交互作用。（4）马尾松和杉木人工林、米槠次生林凋落枝水溶性物质含量的季节变化多数与气温和降水呈显著负相关。这些结果表明亚热带次生林可能相对于人工林具有更为高效的以凋落枝为载体的物质循环过程,在未来气候变暖背景下亚热带森林由凋落枝归还给土壤的养分浓度可能降低。     

Topography and soil content contribute to plant community composition and structure in subtropical evergreen-deciduous broadleaved mixed forests

Topography and soil factors are known to play crucial roles in the species composition of plant communities in subtropical evergreen-deciduous broadleaved mixed forests. In this study, we used a systematic quantitative approach to classify plant community types in the subtropical forests of Hubei Province (central China), and then quantified the relative contribution of drivers responsible for variation in species composition and diversity. We classified the subtropical forests in the study area into 12 community types. Of these, species diversity indices of three communities were significantly higher than those of others. In each community type, species richness, abundance, basal area and importance values of evergreen and deciduous species were different. In most community types, deciduous species richness was higher than that of evergreen species. Linear regression analysis showed that the dominant factors that affect species composition in each community type are elevation, slope, aspect, soil nitrogen content, and soil phosphorus content. Furthermore, structural equation modeling analysis showed that the majority of variance in species composition of plant communities can be explained by elevation, aspect, soil water content, litterfall, total nitrogen, and total phosphorus. Thus, the major factors that affect evergreen and deciduous species distribution across the 12 community types in subtropical evergreen-deciduous broadleaved mixed forests include elevation, slope and aspect, soil total nitrogen content, soil total phosphorus content, soil available nitrogen content and soil available phosphorus content.     

Phosphatase activity and nitrogen fixation reflect species differences,not nutrient trading or nutrient balance,across tropical rainforest trees

A fundamental biogeochemical paradox is that nitrogen‐rich tropical forests contain abundant nitrogen‐fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen‐fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen‐for‐phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen‐for‐phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen‐fixing trees cannot provide a positive nitrogen‐phosphorus‐carbon feedback to alleviate nutrient limitation of the tropical carbon sink.     

防护林学研究现状与展望

防护林学是研究防护林构建及经营的理论与技术的科学, 其核心内容包括防护林构建理论与技术、防护林经营理论与技术和防护林效益评价。防护林学发展的终极目的是构建与经营防护林, 使其防护功能或生态服务功能高效、稳定并可持续。防护林学是偏重实用的应用基础学科, 其发展始终依托防护林工程建设需求, 特别是以国家运作方式开展的大型防护林工程建设, 对推动防护林学发展做出了巨大贡献。国外著名的防护林工程有美国大平原各州林业工程(罗斯福工程)、前苏联斯大林改造大自然计划、日本的治山治水防护林工程和北非五国“绿色坝”跨国防护林工程等。围绕这些工程, 在防护林规划设计、树种选择、空间配置、造林方法, 结构、抚育、间伐、衰退机制与更新, 以及效益评价等各个方面开展了相关研究, 其中, 以效益评价及效益与结构的关系研究最为广泛与深入。中国幅员辽阔、自然条件复杂、森林资源相对匮乏且分布不均、自然环境恶劣, 对防护林的需求极大, 自“三北”防护林体系建设工程启动以来, 中国防护林建设规模已居世界首位, 防护林学在中国得到了长足发展, 尤其在防护林经营理论与技术研究方面取得突破性进展。防护林学以效益评价为桥梁将防护林构建和经营组合在一起, 效益与结构的关系为防护林构建及现有防护林经营提供了科学依据。未来防护林学研究将以更广泛的生态公益林或防护性森林为对象, 在研究方法上将由以林分尺度为主向更微观和更宏观两个方向拓展; 在防护林构建方面, 仍以林学理论与技术为主体, 并重点与生态系统稳定性原理、景观生态学原理相结合, 开展防护林(体系)区域分异规划设计、营建理论与技术研究; 在防护林经营方面, 将以防护林衰退与恢复机制、带状防护林更新和非带状防护林近自然经营理论与技术为重点开展研究; 在效益评价方面, 将采用遥感等技术, 以防护林(体系)、大规模防护林建设的生态环境效应评价等为主要内容开展研究。     

Productive phenology of tropical montane forests: Fertilization experiments along a moisture gradient

Phenological responses of leaves and roots were studied in the tropical montane forests of Mount Kinabalu, Borneo. Soil nutrient supply, in addition to the supply of light and water, is a potential abiotic factor influencing plant phenology in the tropics. The main objective of this study was to evaluate the contribution of soil nutrient supply to plant productive phenology. Fertilization experiments, including controls, nitrogen fertilized and nitrogen and phosphorus fertilized treatments, were conducted on three vegetation types in different moisture environments. Responses of leaves and roots were compared among treatments and among vegetation types. Leaf flushing was induced by nitrogen fertilization in the upper montane forest, where extremely wet moisture conditions are associated with cloud cover. This induction of leaf flushing by fertilization was not observed in the other forests. Root growth was suppressed by fertilization when leaf flushing was not induced by fertilization. These results indicate that soil pulsed nutrient release could be a cue for leaf flushing in a tropical wet environment, and that leaf phenology could be regulated by external abiotic factors and root phenology could be regulated by internal plant demands.     

Phosphorus Fertilization Increases the Abundance and Nitrogenase Activity of the Cyanolichen Pseudocyphellaria crocata in Hawaiian Montane Forests

Nitrogen-fixing epiphytes (especially lichens with a cyanobacterial symbiont—cyanolichens) have the potential to contribute significant amounts of nitrogen (N) to montane tropical forests, which are typically low in N—but the factors controlling the abundance and distribution of epiphytic cyanolichens are poorly understood. In long-term fertilization experiments in montane forests on a 4.l million-yr-old Oxisol on the island of Kauà'i and on a 152-yr-old lava flow on the island of Hawai'i, the epiphytic cyanolichen Pseudocyphellaria crocata increased significantly in abundance in canopies of host trees fertilized with phosphorus (P). There was no response to fertilization with N or other essential elements. Nitrogen-fixation rates were also elevated in lichens in P-fertilized plots at both sites. Phosphorus supply to host trees may be an important factor controlling N inputs to montane tropical forests by N-fixing epiphytes.     

Phosphorus and Nitrogen Regulate Arbuscular Mycorrhizal Symbiosis in Petunia hybrida

Phosphorus and nitrogen are essential nutrient elements that are needed by plants in large amounts. The arbuscular mycorrhizal symbiosis between plants and soil fungi improves phosphorus and nitrogen acquisition under limiting conditions. On the other hand, these nutrients influence root colonization by mycorrhizal fungi and symbiotic functioning. This represents a feedback mechanism that allows plants to control the fungal symbiont depending on nutrient requirements and supply. Elevated phosphorus supply has previously been shown to exert strong inhibition of arbuscular mycorrhizal development. Here, we address to what extent inhibition by phosphorus is influenced by other nutritional pathways in the interaction between Petunia hybrida and R. irregularis. We show that phosphorus and nitrogen are the major nutritional determinants of the interaction. Interestingly, the symbiosis-promoting effect of nitrogen starvation dominantly overruled the suppressive effect of high phosphorus nutrition onto arbuscular mycorrhiza, suggesting that plants promote the symbiosis as long as they are limited by one of the two major nutrients. Our results also show that in a given pair of symbiotic partners (Petunia hybrida and R. irregularis), the entire range from mutually symbiotic to parasitic can be observed depending on the nutritional conditions. Taken together, these results reveal complex nutritional feedback mechanisms in the control of root colonization by arbuscular mycorrhizal fungi.     

樟子松固沙林生长对土壤磷素变化的影响

磷是森林生态系统重要养分元素之一,是干旱半干旱地区植物生长的限制性因子。然而沙质草地转化为人工林生态系统后,林分的生长对沙地土壤磷素的变化及其影响机理还不清晰,为沙地合理经营和管理人工林带来了不确定性。以辽宁省章古台地区各生长阶段(幼龄林、中龄林、成熟林和过熟林阶段)的20块樟子松固沙林样地(在各生长阶段林分附近寻找1块天然草地作为对照样地)为研究对象,取样并测定样地各土层(0—10、10—20、20—40、40—60、60—80、80—100 cm)的土壤磷(全磷和速效磷)、土壤氮(全氮和速效氮)、土壤钾(全钾及速效钾)、土壤有机碳等含量以及土壤含水率、土壤pH值、土壤质地、土壤容重等因子值,并进行统计分析。结果表明:沙质草地营造樟子松人工林后,土壤全磷含量随林龄的增加而逐渐递增,成熟林时期达到最高,过熟林地全磷降低,且土壤全磷对土层深度不敏感。成熟林地的速效磷含量略高于幼林和中龄林;虽然与幼林、中龄林没有显著差异,但过熟林的土壤速效磷含量是所有林分中最低的。除了林分生长影响外,草地营造樟子松林后,土壤全磷的变化还受土壤容重和土壤速效氮含量的影响,而土壤速效磷的变化受土壤有机碳和pH...     

滇中退化山地不同植被恢复下土壤碳氮磷储量与生态化学计量特征

植被恢复对土壤营养元素的存赋及其生态化学计量特征的影响广受关注,为了深入了解不同植被恢复类型下土壤碳、氮、磷储量与生态化学计量特征,选择滇中地区退化山地飒马场流域具有代表性的4种不同修复阶段的典型植被(荒坡灌草丛、云南松林、针阔混交林和次生常绿阔叶林)为研究对象,分析了不同植被类型下不同深度土壤中有机碳(SOC)、全氮(TN)、全磷(TP)储量和化学计量变化特征。结果表明,退化山地的植被恢复显著改变土壤碳氮磷储存能力和化学计量比,这种改变作用整体上随土壤深度增加而降低。其中,在0—60 cm土层上,SOC储量在次生常绿阔叶林最高,达123.41 t/hm²,其次是针阔混交林(115.69 t/hm²)和云南松林(93.08 t/hm²),荒坡灌草丛(89.56 t/hm²)最低;TN储量针阔混交林(4.91 t/hm²)>次生常绿阔叶林(4.58 t/hm²)>云南松林(4.43 t/hm²)>荒坡灌草丛(3.98 ...     

Nutrient Addition Differentially Affects Soil Carbon Sequestration in Secondary Tropical Dry Forests: Early‐ versus Late‐Succession Stages

There is considerable interest in the potential use of soils to sequester carbon for climate change mitigation. As such, there is a need to evaluate the potential for carbon accumulation in tropical regions. We compared the effects of three annual additions of nitrogen and/or phosphorus on soil carbon and nitrogen contents and pools (bulk soil, macro‐, meso‐, and microaggregates) of two regenerating secondary tropical dry forest differing in nutrient status and succession stage (10‐year‐old early‐succession stage and approximately 60‐year‐old late‐succession stage). The selected forest sites were located on a shallow calcareous soil in the Yucatán Peninsula (Mexico). The primary production is limited by nitrogen and phosphorus in early‐succession stage and by phosphorus in late‐succession stage. In each forest site, four independent plots (12 × 12 m²) were established, the treatments being: controls and plots fertilized during three consecutive years with nitrogen, phosphorus, or nitrogen plus phosphorus. In both forests, soil carbon and nitrogen contents were consistently high, with soil carbon:nitrogen ratios generally greater than 10. Results indicate that usually there are no significant increases of soil carbon stock associated to late succession but can be increased to 3.7 Mg·ha^?1·yr^?1 with adoption of fertilizer practices. The potential soil carbon sequestration in early‐succession forest was estimated to be 2.7 Mg·ha^?1·yr^?1, and there is no indication that fertilization improves carbon sequestration. In short, results suggest that the soil potential for carbon sequestration in these ecosystems is high and depends on the specific nutrient status of the site.     

中国南亚热带不同造林模式碳汇林土壤碳、氮、磷的积累及化学计量特征

碳汇林的固碳效益十分显著,是实现“碳中和”的主要碳增汇途径之一,在研究南亚热带地区不同造林模式碳汇林土壤碳汇的同时,研究碳汇林土壤氮、磷储量及土壤碳、氮、磷的生态化学计量特征,能够为不同造林模式碳汇林的土壤碳、氮、磷储量的评估提供科学依据。采取单因素随机区组设计,选择立地条件基本一致的研究区和4种造林模式(新造林、封山育林、补植套种、更新改造)的碳汇林,分析了不同造林模式土壤有机碳(SOC)、全氮(TN)、全磷(TP)的含量、储量、生态化学计量比,以及微生物生物量碳(MBC)的变化特征。结果表明,不同造林模式碳汇林的土壤理化性质,碳氮磷储量、化学计量比及微生物生物量碳含量差异显著,且土壤碳氮磷储量随土壤深度增加而减少,表现出明显的垂直变化特征。4种造林模式下碳氮比(C/N)介于11.11—17.86,碳磷比(C/P)介于17.00—242.59,氮磷比(N/P)介于1.18—15.99之间,在0—60 cm土层上均以更新改造模式林最低,封山育林模式下C/P和N/P均显著高于其他造林模式。土壤SOC与TN、N/P呈极显著正相关关系,土壤TN与C/P呈极显著正相关关系。综合来看,封山育林模...     

川西亚高山次生林恢复过程中乔木层优势种变化及其影响因子

川西亚高山森林在20世纪下半叶遭受长时期连续砍伐,形成了采伐迹地上天然次生林恢复演替序列。该研究采用空间代替时间方法,以立地条件相似的暗针叶林作为对照,分析川西亚高山暗针叶林采伐迹地上次生林自然恢复过程中乔木层优势种组成及其影响因子。结果表明:(1)研究区亚高山天然次生林恢复演替过程中,不同演替阶段土壤各养分差异显著,其中土壤总氮、土壤总磷、土壤有效磷、土壤有效氮均在针阔混交林阶段最高,土壤pH值的变化在5.42～7.39范围波动,土壤有机碳随恢复的进行而增加,土壤C/N主要集中在15左右,只有针叶林达到18.62。(2)研究区次生林以16个乔木树种为主,针叶树种主要有粗枝云杉、岷江冷杉,阔叶树种主要有红桦、青榨槭等,且不同恢复阶段的群落乔木层优势种差异显著(R_(ANOSINM)=0.439,P0.001)。(3)多元回归树分析发现,次生林乔木层优势种组成主要受林龄、土壤有机碳和总氮的影响,其中在次生林恢复演替后期(80a后),乔木层组成可能主要由于种间竞争导致,在次生林恢复演替前、中期(80a前),环境选择可能是影响乔木层组成差异的主导因素。(4)反向选择筛选结果显示,土壤总氮、海拔高度、土壤有效磷和土壤有机碳是影响乔木层优势种差异的主要因子。