松嫩平原榆树疏林生态系统退化机制的研究
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摘要
生态系统退化（ecosystem degradation）问题已经成为影响人类社会可持续发展和进步的重要因素。目前，人类社会赖以生存的生态系统，有60%正处于不断退化的状态中，地球上近2/3的自然资源已经消耗殆尽，生态系统的退化已经严重威胁到人类的生活。近几十年来，各国学者把研究的焦点集中在退化生态系统（degraded ecosystem）的上，试图从退化生态系统的种类组成、群落结构、生物多样性和生产力，土壤和微环境恶化，以及生物间相互关系等角度，揭示生态系统退化过程、机制及其原因。但由于生态系统结构和功能存在复杂性、多样性等特点，以致在生态系统退化机制和理论研究等方面依然存在争论。从结构相对简单的、对外界干扰响应敏感的生态系统入手，研究生态系统对外界干扰响应过程，分析生态系统退化的机制和原因，探讨退化生态系统恢复理论与方法，不仅在方法上更易于操作，而且更容易充分理解生态系统各组分之间对外界干扰的响应规律与机制。
榆树疏林（Ulmus woodland）生态系统的结构相对简单、功能多样。榆树疏林是相对脆弱的生态系统，为深入生态系统退化与恢复理论的研究提供了重要的研究对象。研究不同干扰下榆树疏林生态系统的生产力、群落结构和生物多样性，以及土壤理化特征的变化规律，不仅可以揭示榆树疏林退化的主要原因，明确榆树疏林退化的过程和机制，而且能够为退化榆树疏林的恢复和重建提供理论支持，也可以为丰富退化生态系统的理论提供科学依据。
本论文以不同人为干扰（农耕与放牧）下的榆树疏林生态系统为研究对象，分别从生态系统水平和优势种个体水平，研究榆树疏林生态系统对不同人为干扰的响应规律，并获得了以下重要成果：
（1）研究了近30年（1983-2011）间未受干扰的榆树疏林生态系统和人为干扰下榆树疏林生态系统的群落结构、物种组成、生物多样性等问题。结果表明：近30年的气候变化已经使榆树疏林的物种数量降低，但统计分析差异不显著；而放牧和农耕干扰，不仅使疏林植物物种数量降低50%以上，也降低了植物物种多样性、均匀度和地上生物量。通过Bray-Curtis排序分析所有榆树疏林取样类型和环境因子之间的关系，发现所有取样样方都可划分为3种类群，1983-2011年未受干扰榆树疏林划分为第I类，农耕干扰为第II类，放牧干扰为第III类。因此，榆树疏林植被特征变化的原因不是气候因子，而是放牧和农耕干扰，而放牧和农耕干扰对榆树疏林的具体影响有待进一步研究。如果对榆树疏林进行合理的农耕和放牧，或完全排除人类干扰进行围封，对榆树疏林生态系统的恢复和重建将具有重要作用。
（2）在控制条件下，研究了榆树幼苗对土壤营养、含水量和刈割的响应。实验结果表明：榆树幼苗株高的生长主要受到土壤营养条件的限制，而生物量的积累则与土壤含水量密切相关。榆树幼苗的光合能力和光合产物主要受到土壤含水量的制约，而影响榆树幼苗生长和存活的首要因素是土壤含水量，根据本文研究结果，榆树幼苗的水分生态阈限下限为3%的土壤含水量。刈割模拟放牧，直接作用于幼苗地上部分，影响幼苗的株高和生物量，但刈割处理没有显著影响幼苗的生长和存活率（P>0.05），但刈割和土壤营养之间存在显著的交互作用（P <0.05），尤其是土壤水分不受限制时，刈割对幼苗生长的负面影响可以通过提高土壤营养水平来削弱。本实验证明，决定榆树幼苗生长和存活的首要因素为土壤含水量，最低生长和存活的土壤含水量阈限为3%。而且土壤营养和土壤水分对榆树幼苗生长的作用不同，土壤营养水平决定榆树幼苗株高的生长，而土壤水分水平决定榆树生物量的积累。这样从理论上阐明了干旱贫瘠沙地环境是导致了榆树株高偏低的主要原因之一。因此建议加强土壤水分和营养管理是榆树疏林恢复和重建的必要措施。
（3）本研究从生长速率角度，探讨了榆树疏林建群种榆树幼苗生长模式对土壤水分和营养的响应规律。结果表明：榆树幼苗生物量生长速率和株高生长速率对土壤营养水平呈现出分级生长趋势，低营养条件下幼苗随土壤水分含量的升高而升高，生长速率低速上升，而当土壤营养升高后，幼苗生长速率随土壤水分升高而迅速上升；幼苗生物量和株高生长速率随土壤营养升高呈现单峰曲线的生长模式，并且受到土壤水分的限制，当土壤水分为3%时，幼苗的生长速率基本为0；进一步分析榆树幼苗光合能力，结果显示，提高土壤营养和水分能够使榆树幼苗光合速率迅速提高，最终提高幼苗生物量和株高生长速率。但当土壤营养过高时，榆树幼苗生物量、株高和茎粗的生长都受到了抑制，其具体原因有待进一步研究。该结果在一定程度上解释了长期生长在固定/半固定沙丘环境下的榆树，能够适应贫瘠的土壤环境的原因。而榆树株高生长与土壤营养密切相关，一定程度上解释了农耕条件下榆树普遍高大的原因。因此，培育榆树幼苗的最佳土壤营养和水分条件为N3W4处理组，但由于W4土壤含水量为10%，综合考虑在不同土壤含水量条件下榆树幼苗的生长效果，以及成本等问题，可以将榆树幼苗最适培育条件设置为土壤含水量8%，土壤营养沙壤土比例为1：1。
（4）连续3年（2006-2008）研究了松嫩平原榆树疏林生态系统的建群种榆树、植被群落和土壤对不同农耕和放牧干扰的响应规律，结果表明：农耕和放牧干扰对引起榆树疏林生态系统退化的过程和机制不同。农耕干扰破坏了榆树疏林的林下植被结构，促进了一、二年生草本植物的快速繁殖，加快系统内部物质周转率与营养循环，从而在一定程度上有利于建群种榆树的生长，表现为农耕样地中榆树高度（4.78±0.45m）、胸径（18.98±1.08cm）和单株生物量（278.5±13.28）较大。但由于农耕对林下植被结构的破坏，使系统结构逐渐简单化，物种多样性降低，加之对榆树的采伐，导致农耕干扰下疏林生态系统生产力下降，尤其在非耕作期，疏林地表植被覆盖度降低，土壤裸露，使疏林稳定性下降，最终提高榆树疏林生态系统发生退化的可能性。而放牧干扰能够直接作用于榆树疏林生态系统建群种榆树和其它植被进行：榆树高度下降（1.86±0.17m）、胸径减小（6.97±0.60cm）、单株生物量降低（51.9±5.9kg/stem），植被生产力降低，破坏疏林植被组成、以及生态系统的物质和营养循环，使系统结构简单化、物种多样性下降，最终降低榆树疏林抵御外界干扰的能力。放牧降低疏林内的土壤含水量较，土壤氮、磷和有机质含量，使得榆树疏林逐渐演变成为功能衰退、脆弱的退化生态系统。本研究揭示了放牧和农耕干扰是榆树疏林退化的原因，而且两者的作用机制不同：放牧通过对榆树和疏林植被的直接采食，破坏原有系统结构和营养物质循环，降低了土壤的营养状况，并形成恶性循环，而最终导致生态系统退化；农耕干扰则是通过机械作用破坏林下植被，降低建群种榆树的密度，但由于农耕促进了一、二年生植物的生长，而加速了系统的营养物质循环，从而在一定程度上降低了农耕的负面作用，因而一定程度上减缓了榆树疏林的退化过程。
总之，通过野外定位与受控实验，本研究发现，松嫩平原榆树疏林生态系统退化的原因主要为放牧和农耕干扰；而且，放牧和农耕的作用机制不同：农耕通过机械作用破坏榆树疏林的林下植被，促进了一、二生植物的生长，加速物质周转与营养循环，因此，在一定程度上有利于榆树个体的生长。榆树幼苗的实验结果也表明，土壤营养状态的改善有利于榆树株高的生长，这可能是农耕区榆树长势良好的原因，但因榆树密度过低，榆树疏林容易受到破坏，并最终成为退化生态系统；放牧动物对榆树疏林林下植被的采食，降低植被生产力和物质周转总量，减少土壤营养输入，使林下植被的生长进一步受到抑制，加速了系统的退化过程。而放牧动物对榆树和榆树幼苗的直接啃食，使建群种榆树的生长和自我更新受到抑制，并且较低的土壤含水量和营养水平都不利于榆树的生长。因此，放牧干扰是引起松嫩平原榆树疏林生态系统退化的最主要因素。
Ecosystem degradation had been a great issue concerning sustainabledevelopment of human society. According to "Millennium Ecosystem Assessment"report, more than60%ecosystem has been degraded or being degrading, and nearly2/3nature recourses almost had been exhausted. Ecosystem degradation is threateningnormal human life. In recent decades, ecologists focused on the restoration ofdegraded ecosystem, they tried to interpret the reasons caused ecosystem degradation,mechanisms and processes of ecosystem degradation. Ecosystem species composition,community structure, biodiversity and productivity of degraded ecosystems had beenall studied in recent decades. However, the complexity ecosystem structure anddiversity of ecosystem function increase the difficulty to clarify the ecosystemdegradation processes and mechanisms, and also increased the argument amongecologists. Therefore, if we could study a ecosystem which is simple in structure andsensitive to external disturbances, that will not only be very helpful in defining theecosystem degradation processes, and also could help us understood the relationshipbetween ecosystem units.
Elm woodland（Ulmus pumla woodland）, which is simple in structure anddiverse in function, is kind of fragile ecosystem. It is an exact target approaching theneeds above to the study of mechanism and process of ecosystem degradation. In thispaper, we carried out a study about the changes of constructive species, communitystructure, biodiversity, productivity and soil characters of elm woodland underdifferent disturbances. This study could explain the reason caused degradation of elmwoodland, the processes and mechanisms of elm woodland degradation, and also giveinstructions to further restoration work of elm woodland and restoration theory ofdegraded ecosystem.
We compared the difference in community structure, species composition andbiodiversity between undisturbed elm woodlands and the human disturbed ones in thelast nearly30years (1983-2011). The results showed that climate change in the30years reduced the number of species, but there was no significant difference. Speciescomposition decreased more than50%in the grazing and agricultural cultivation elmwoodlands, and there was decrease in the species diversity, evenness andaboveground biomass. Results of Bray-Curtis ordination analysis with the interactionbetween sampling types and environmental factors of elm woodlands showed that allthe sampling quadrats can be divided into3groups. The first group is the undisturbedelm woodlands in the period of1983-2011. The woodland disturbed by agriculturalcultivation is the second group and the third group is that disturbed by grazing. So, climate change, rather than grazing or agricultural cultivation, was the reason for thechange in vegetation feature of elm woodlands. There is still need for further study onthe detailed influence of grazing and agricultural cultivation on elm woodlands.Rational utilization of elm woodlands for agricultural cultivation or grazing, orenclosing without human disturbance, may play an important role in restoration andreconstruction of elm woodlands.
In controlled condition, we studied response of elm seedlings to soil nutrient,water content and clipping. Results showed that height of seedlings was mainlylimited by soil nutrient conditions, while accumulation of biomass was stronglycorrelated with soil water content. The photosynthesis ability and the product of elmseedlings were mainly constrained by soil water content, which was the primaryfactor to influence growth and survival of elm seedlings. Results of this paper showedthat3%of soil water content was the lower limit of water of seedlings. Clipping,simulating grazing, had direct impacts on aboveground parts of seedlings, influencingtheir height and biomass, but had no significant effects on growth and survival ofseedlings. However, clipping and soil nutrient had significant interactive effects,especially when soil water was not limited. The negative effect of clipping onseedling growth could be by improving soil nutrient conditions. This study showedthat soil water content was the determinant factor for growth and survival of elmseedlings, and3%of water was the lowest water limit. Soil nutrient and water haddifferent effects on elm seedlings. Soil nutrient availability determined height of elmseedlings while soil water controlled accumulation of biomass. This demonstrated themain reason for the low height of elm woodlands in dry and infertile sand. So, weproposed that there was necessary to strengthen management of soil water andnutrient for restoration and reconstruction of elm woodlands.
Our experiments mainly studied how the growth rate of elm seedlings respondedto soil humidity and nutrient availability in the community assembly of elm woodland.The results showed that the growth rate of seedling biomass and seedling heightgradually increased with the increase of soil humidity, when these seedlings grew inlow nutrient environments, while sharply increased with the increased soil humidity,when these seedlings grew in high nutrient environments. The response pattern ofbiomass and height to soil nutrient was unimodal curve, and the growth rate waslimited by soil humidity. When soil humidity was3%, the growth rate of seedling wasalmost0. The increase of soil humidity and soil nutrient improved the rapid increaseof the photosynthesis rate of seedlings, and resulted in the increase of growth rate ofseedling biomass and seedling height. However, too high nutrient availability was alimitation to the accumulation of seedling biomass and the growth of plant height andstem. To some extent, seedling growing in fixed/semi-fixed sand dunes has adapted toinfertile soil environments. The severe response of plant height to soil nutrient might be the explanation to the common hugeness of plant individuals under the cultivationconditions. Thus, the optimum soil nutrient and humidity for the growth of elmseedlings was N3W4in our experiments. In natural conditions,8%soil humidity andthe mixture of1:1sand and loam could be treated as the appropriate environments forelm growth.
We studied the response pattern of vegetation structure and communitycomposition to soil conditions in the consecutive three years (2006-2008). The resultsindicated the different degraded mechanisms of elm woodland ecosystems resultedfrom cultivation and grazing disturbance were different. Cultivation damaged theunderstory vegetation structure, improved the rapid propagation of annual/perennialgrasses, and increased the rate of matter turnover and nutrient cycles inside theecosystems. So this is beneficial for the growth of constructive species, elm, withincreased height (4.78±0.45m), DBH (18.98±1.08cm), and individual biomass(278.5±13.28kg/stem). However, the damage to understory vegetation by cultivationgradually resulted in the singleness of ecosystem structure and the decrease ofbiodiversity. The further harvest cutting for elms significantly reduced theproductivity of elm woodland ecosystem, especially in the non-cultivation term. Thedecreased vegetation cover and bared soil reduced the stability of elm woodlandecosystem, and increased the possibility of the degradation. Compared withcultivation, the direct grazing damage to constructive species and other vegetation inelm woodland led to the opposite effects on height (1.86±0.17m), DBH (6.97±0.60cm), and individual biomass (51.9±5.9kg/stem). In addition, low soil humidity andlow concentration of nitrogen and phosphorus in grazing treatments gradually reducedthe ecosystem function.
Our studied discovered the different degradation cause of elm woodland underthe disturbance of grazing and cultivation. In general, cultivation and grazing are themain driving forces for the degradation of elm woodland ecosystem, depending ontheir different affecting mechanisms. Cultivation improved the growth ofannual/perennial plants and increased matter turnover and nutrient cycle. Therefore,cultivation is beneficial for the individual growth of elms to some extent. Andincrease soil nutrient content is favorable for elm seedlings’ growth. Grazingdisturbance resulted in damaged understory vegetation in elm woodlands, reducedecosystem productivity matter turnover, decreased soil nutrient inputs and limitedplant growth. The vicious spiral accelerated the processes of ecosystem degradation inelm woodlands. Moreover, grazing obviously limits the growth of constructivespecies and seedling regeneration, reduced soil moisture and nutrient, and greatlyaffected the growth of U. pumila. Therefore, grazing disturbance is the main factorresulting in the degradation of elm woodlands in the Songnen Plains.
榆树疏林（Ulmus woodland）生态系统的结构相对简单、功能多样。榆树疏林是相对脆弱的生态系统，为深入生态系统退化与恢复理论的研究提供了重要的研究对象。研究不同干扰下榆树疏林生态系统的生产力、群落结构和生物多样性，以及土壤理化特征的变化规律，不仅可以揭示榆树疏林退化的主要原因，明确榆树疏林退化的过程和机制，而且能够为退化榆树疏林的恢复和重建提供理论支持，也可以为丰富退化生态系统的理论提供科学依据。
本论文以不同人为干扰（农耕与放牧）下的榆树疏林生态系统为研究对象，分别从生态系统水平和优势种个体水平，研究榆树疏林生态系统对不同人为干扰的响应规律，并获得了以下重要成果：
（1）研究了近30年（1983-2011）间未受干扰的榆树疏林生态系统和人为干扰下榆树疏林生态系统的群落结构、物种组成、生物多样性等问题。结果表明：近30年的气候变化已经使榆树疏林的物种数量降低，但统计分析差异不显著；而放牧和农耕干扰，不仅使疏林植物物种数量降低50%以上，也降低了植物物种多样性、均匀度和地上生物量。通过Bray-Curtis排序分析所有榆树疏林取样类型和环境因子之间的关系，发现所有取样样方都可划分为3种类群，1983-2011年未受干扰榆树疏林划分为第I类，农耕干扰为第II类，放牧干扰为第III类。因此，榆树疏林植被特征变化的原因不是气候因子，而是放牧和农耕干扰，而放牧和农耕干扰对榆树疏林的具体影响有待进一步研究。如果对榆树疏林进行合理的农耕和放牧，或完全排除人类干扰进行围封，对榆树疏林生态系统的恢复和重建将具有重要作用。
（2）在控制条件下，研究了榆树幼苗对土壤营养、含水量和刈割的响应。实验结果表明：榆树幼苗株高的生长主要受到土壤营养条件的限制，而生物量的积累则与土壤含水量密切相关。榆树幼苗的光合能力和光合产物主要受到土壤含水量的制约，而影响榆树幼苗生长和存活的首要因素是土壤含水量，根据本文研究结果，榆树幼苗的水分生态阈限下限为3%的土壤含水量。刈割模拟放牧，直接作用于幼苗地上部分，影响幼苗的株高和生物量，但刈割处理没有显著影响幼苗的生长和存活率（P>0.05），但刈割和土壤营养之间存在显著的交互作用（P <0.05），尤其是土壤水分不受限制时，刈割对幼苗生长的负面影响可以通过提高土壤营养水平来削弱。本实验证明，决定榆树幼苗生长和存活的首要因素为土壤含水量，最低生长和存活的土壤含水量阈限为3%。而且土壤营养和土壤水分对榆树幼苗生长的作用不同，土壤营养水平决定榆树幼苗株高的生长，而土壤水分水平决定榆树生物量的积累。这样从理论上阐明了干旱贫瘠沙地环境是导致了榆树株高偏低的主要原因之一。因此建议加强土壤水分和营养管理是榆树疏林恢复和重建的必要措施。
（3）本研究从生长速率角度，探讨了榆树疏林建群种榆树幼苗生长模式对土壤水分和营养的响应规律。结果表明：榆树幼苗生物量生长速率和株高生长速率对土壤营养水平呈现出分级生长趋势，低营养条件下幼苗随土壤水分含量的升高而升高，生长速率低速上升，而当土壤营养升高后，幼苗生长速率随土壤水分升高而迅速上升；幼苗生物量和株高生长速率随土壤营养升高呈现单峰曲线的生长模式，并且受到土壤水分的限制，当土壤水分为3%时，幼苗的生长速率基本为0；进一步分析榆树幼苗光合能力，结果显示，提高土壤营养和水分能够使榆树幼苗光合速率迅速提高，最终提高幼苗生物量和株高生长速率。但当土壤营养过高时，榆树幼苗生物量、株高和茎粗的生长都受到了抑制，其具体原因有待进一步研究。该结果在一定程度上解释了长期生长在固定/半固定沙丘环境下的榆树，能够适应贫瘠的土壤环境的原因。而榆树株高生长与土壤营养密切相关，一定程度上解释了农耕条件下榆树普遍高大的原因。因此，培育榆树幼苗的最佳土壤营养和水分条件为N3W4处理组，但由于W4土壤含水量为10%，综合考虑在不同土壤含水量条件下榆树幼苗的生长效果，以及成本等问题，可以将榆树幼苗最适培育条件设置为土壤含水量8%，土壤营养沙壤土比例为1：1。
（4）连续3年（2006-2008）研究了松嫩平原榆树疏林生态系统的建群种榆树、植被群落和土壤对不同农耕和放牧干扰的响应规律，结果表明：农耕和放牧干扰对引起榆树疏林生态系统退化的过程和机制不同。农耕干扰破坏了榆树疏林的林下植被结构，促进了一、二年生草本植物的快速繁殖，加快系统内部物质周转率与营养循环，从而在一定程度上有利于建群种榆树的生长，表现为农耕样地中榆树高度（4.78±0.45m）、胸径（18.98±1.08cm）和单株生物量（278.5±13.28）较大。但由于农耕对林下植被结构的破坏，使系统结构逐渐简单化，物种多样性降低，加之对榆树的采伐，导致农耕干扰下疏林生态系统生产力下降，尤其在非耕作期，疏林地表植被覆盖度降低，土壤裸露，使疏林稳定性下降，最终提高榆树疏林生态系统发生退化的可能性。而放牧干扰能够直接作用于榆树疏林生态系统建群种榆树和其它植被进行：榆树高度下降（1.86±0.17m）、胸径减小（6.97±0.60cm）、单株生物量降低（51.9±5.9kg/stem），植被生产力降低，破坏疏林植被组成、以及生态系统的物质和营养循环，使系统结构简单化、物种多样性下降，最终降低榆树疏林抵御外界干扰的能力。放牧降低疏林内的土壤含水量较，土壤氮、磷和有机质含量，使得榆树疏林逐渐演变成为功能衰退、脆弱的退化生态系统。本研究揭示了放牧和农耕干扰是榆树疏林退化的原因，而且两者的作用机制不同：放牧通过对榆树和疏林植被的直接采食，破坏原有系统结构和营养物质循环，降低了土壤的营养状况，并形成恶性循环，而最终导致生态系统退化；农耕干扰则是通过机械作用破坏林下植被，降低建群种榆树的密度，但由于农耕促进了一、二年生植物的生长，而加速了系统的营养物质循环，从而在一定程度上降低了农耕的负面作用，因而一定程度上减缓了榆树疏林的退化过程。
总之，通过野外定位与受控实验，本研究发现，松嫩平原榆树疏林生态系统退化的原因主要为放牧和农耕干扰；而且，放牧和农耕的作用机制不同：农耕通过机械作用破坏榆树疏林的林下植被，促进了一、二生植物的生长，加速物质周转与营养循环，因此，在一定程度上有利于榆树个体的生长。榆树幼苗的实验结果也表明，土壤营养状态的改善有利于榆树株高的生长，这可能是农耕区榆树长势良好的原因，但因榆树密度过低，榆树疏林容易受到破坏，并最终成为退化生态系统；放牧动物对榆树疏林林下植被的采食，降低植被生产力和物质周转总量，减少土壤营养输入，使林下植被的生长进一步受到抑制，加速了系统的退化过程。而放牧动物对榆树和榆树幼苗的直接啃食，使建群种榆树的生长和自我更新受到抑制，并且较低的土壤含水量和营养水平都不利于榆树的生长。因此，放牧干扰是引起松嫩平原榆树疏林生态系统退化的最主要因素。
Ecosystem degradation had been a great issue concerning sustainabledevelopment of human society. According to "Millennium Ecosystem Assessment"report, more than60%ecosystem has been degraded or being degrading, and nearly2/3nature recourses almost had been exhausted. Ecosystem degradation is threateningnormal human life. In recent decades, ecologists focused on the restoration ofdegraded ecosystem, they tried to interpret the reasons caused ecosystem degradation,mechanisms and processes of ecosystem degradation. Ecosystem species composition,community structure, biodiversity and productivity of degraded ecosystems had beenall studied in recent decades. However, the complexity ecosystem structure anddiversity of ecosystem function increase the difficulty to clarify the ecosystemdegradation processes and mechanisms, and also increased the argument amongecologists. Therefore, if we could study a ecosystem which is simple in structure andsensitive to external disturbances, that will not only be very helpful in defining theecosystem degradation processes, and also could help us understood the relationshipbetween ecosystem units.
Elm woodland（Ulmus pumla woodland）, which is simple in structure anddiverse in function, is kind of fragile ecosystem. It is an exact target approaching theneeds above to the study of mechanism and process of ecosystem degradation. In thispaper, we carried out a study about the changes of constructive species, communitystructure, biodiversity, productivity and soil characters of elm woodland underdifferent disturbances. This study could explain the reason caused degradation of elmwoodland, the processes and mechanisms of elm woodland degradation, and also giveinstructions to further restoration work of elm woodland and restoration theory ofdegraded ecosystem.
We compared the difference in community structure, species composition andbiodiversity between undisturbed elm woodlands and the human disturbed ones in thelast nearly30years (1983-2011). The results showed that climate change in the30years reduced the number of species, but there was no significant difference. Speciescomposition decreased more than50%in the grazing and agricultural cultivation elmwoodlands, and there was decrease in the species diversity, evenness andaboveground biomass. Results of Bray-Curtis ordination analysis with the interactionbetween sampling types and environmental factors of elm woodlands showed that allthe sampling quadrats can be divided into3groups. The first group is the undisturbedelm woodlands in the period of1983-2011. The woodland disturbed by agriculturalcultivation is the second group and the third group is that disturbed by grazing. So, climate change, rather than grazing or agricultural cultivation, was the reason for thechange in vegetation feature of elm woodlands. There is still need for further study onthe detailed influence of grazing and agricultural cultivation on elm woodlands.Rational utilization of elm woodlands for agricultural cultivation or grazing, orenclosing without human disturbance, may play an important role in restoration andreconstruction of elm woodlands.
In controlled condition, we studied response of elm seedlings to soil nutrient,water content and clipping. Results showed that height of seedlings was mainlylimited by soil nutrient conditions, while accumulation of biomass was stronglycorrelated with soil water content. The photosynthesis ability and the product of elmseedlings were mainly constrained by soil water content, which was the primaryfactor to influence growth and survival of elm seedlings. Results of this paper showedthat3%of soil water content was the lower limit of water of seedlings. Clipping,simulating grazing, had direct impacts on aboveground parts of seedlings, influencingtheir height and biomass, but had no significant effects on growth and survival ofseedlings. However, clipping and soil nutrient had significant interactive effects,especially when soil water was not limited. The negative effect of clipping onseedling growth could be by improving soil nutrient conditions. This study showedthat soil water content was the determinant factor for growth and survival of elmseedlings, and3%of water was the lowest water limit. Soil nutrient and water haddifferent effects on elm seedlings. Soil nutrient availability determined height of elmseedlings while soil water controlled accumulation of biomass. This demonstrated themain reason for the low height of elm woodlands in dry and infertile sand. So, weproposed that there was necessary to strengthen management of soil water andnutrient for restoration and reconstruction of elm woodlands.
Our experiments mainly studied how the growth rate of elm seedlings respondedto soil humidity and nutrient availability in the community assembly of elm woodland.The results showed that the growth rate of seedling biomass and seedling heightgradually increased with the increase of soil humidity, when these seedlings grew inlow nutrient environments, while sharply increased with the increased soil humidity,when these seedlings grew in high nutrient environments. The response pattern ofbiomass and height to soil nutrient was unimodal curve, and the growth rate waslimited by soil humidity. When soil humidity was3%, the growth rate of seedling wasalmost0. The increase of soil humidity and soil nutrient improved the rapid increaseof the photosynthesis rate of seedlings, and resulted in the increase of growth rate ofseedling biomass and seedling height. However, too high nutrient availability was alimitation to the accumulation of seedling biomass and the growth of plant height andstem. To some extent, seedling growing in fixed/semi-fixed sand dunes has adapted toinfertile soil environments. The severe response of plant height to soil nutrient might be the explanation to the common hugeness of plant individuals under the cultivationconditions. Thus, the optimum soil nutrient and humidity for the growth of elmseedlings was N3W4in our experiments. In natural conditions,8%soil humidity andthe mixture of1:1sand and loam could be treated as the appropriate environments forelm growth.
We studied the response pattern of vegetation structure and communitycomposition to soil conditions in the consecutive three years (2006-2008). The resultsindicated the different degraded mechanisms of elm woodland ecosystems resultedfrom cultivation and grazing disturbance were different. Cultivation damaged theunderstory vegetation structure, improved the rapid propagation of annual/perennialgrasses, and increased the rate of matter turnover and nutrient cycles inside theecosystems. So this is beneficial for the growth of constructive species, elm, withincreased height (4.78±0.45m), DBH (18.98±1.08cm), and individual biomass(278.5±13.28kg/stem). However, the damage to understory vegetation by cultivationgradually resulted in the singleness of ecosystem structure and the decrease ofbiodiversity. The further harvest cutting for elms significantly reduced theproductivity of elm woodland ecosystem, especially in the non-cultivation term. Thedecreased vegetation cover and bared soil reduced the stability of elm woodlandecosystem, and increased the possibility of the degradation. Compared withcultivation, the direct grazing damage to constructive species and other vegetation inelm woodland led to the opposite effects on height (1.86±0.17m), DBH (6.97±0.60cm), and individual biomass (51.9±5.9kg/stem). In addition, low soil humidity andlow concentration of nitrogen and phosphorus in grazing treatments gradually reducedthe ecosystem function.
Our studied discovered the different degradation cause of elm woodland underthe disturbance of grazing and cultivation. In general, cultivation and grazing are themain driving forces for the degradation of elm woodland ecosystem, depending ontheir different affecting mechanisms. Cultivation improved the growth ofannual/perennial plants and increased matter turnover and nutrient cycle. Therefore,cultivation is beneficial for the individual growth of elms to some extent. Andincrease soil nutrient content is favorable for elm seedlings’ growth. Grazingdisturbance resulted in damaged understory vegetation in elm woodlands, reducedecosystem productivity matter turnover, decreased soil nutrient inputs and limitedplant growth. The vicious spiral accelerated the processes of ecosystem degradation inelm woodlands. Moreover, grazing obviously limits the growth of constructivespecies and seedling regeneration, reduced soil moisture and nutrient, and greatlyaffected the growth of U. pumila. Therefore, grazing disturbance is the main factorresulting in the degradation of elm woodlands in the Songnen Plains.
引文
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