The Degradation Mechanisms Of Ulmus Pumila Woodlands In The Songnen Plains

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.