2010Effects of permafrost degradation on ecosystems
Effects of permafrost degradation on ecosystems
Yang Zhao-ping a,b,1,Ou Yang Hua c,*,Xu Xing-liang a ,Zhao Lin d ,Song Ming-hua a ,Zhou Cai-ping a
a
Institute of Geographic Sciences and Natural Resources Research,Chinese Academy of Sciences,Beijing 100101,China b
Graduate School of Chinese Academy of Sciences,Beijing 100049,China c
International Centre for Integrated Mountain Development,G.P.O.Box 3226,Khumaltar,Kathmandu,Nepal d
Observation and Research Station of Qinghai–Tibet Plateau,CAREERI,CAS,Lanzhou Gansu 730000,China
a r t i c l e i n f o Keywords:Permafrost Degradation
Ecosystem structure Ecosystem function Qinghai–Tibet Plateau
a b s t r a c t
Permafrost,covering approximately 25%of the land area in the Northern Hemisphere,is one of the key components of terrestrial ecosystem in cold regions.As a product of cold climate,permafrost is extremely sensitive to climate change.Climate warming over past decades has caused degradation in permafrost widely and quickly.Permafrost degradation has the potential to signi?cantly change soil moisture con-tent,alter soil nutrients availability and in?uence on species composition.In lowland ecosystems the loss of ice-rich permafrost has caused the conversion of terrestrial ecosystem to aquatic ecosystem or wet-land.In upland ecosystems permafrost thaw has resulted in replacement of hygrophilous community by xeromorphic community or shrub.Permafrost degradation resulting from climate warming may dra-matically change the productivity and carbon dynamics of alpine ecosystems.This paper reviewed the effects of permafrost degradation on ecosystem structure and function.At the same time,we put forward critical questions about the effects of permafrost degradation on ecosystems on Qinghai–Tibetan Plateau,included:(1)carry out research about the effects of permafrost degradation on grassland ecosystem and the response of alpine ecosystem to global change;(2)construct long-term and located ?eld observations and research system,based on which predict ecosystem dynamic in permafrost degradation;(3)pay extensive attention to the dynamic of greenhouse gas in permafrost region on Qinghai–Tibetan Plateau and the feedback of greenhouse gas to climate change;(4)quantitative study on the change of water-heat transport in permafrost degradation and the effects of soil moisture and heat change on vegetation growth.
ó2009Ecological Society of China.Published by Elsevier B.V.All rights reserved.
0.Introduction
Permafrost,de?ned as subsurface earth materials remaining be-low 0°C for two consecutive years [1,2].It is a geological entity that results from the exchange of material and energy between the earth and the atmosphere under the multiple impacts of regio-nal geographic condition,geological structure,lithologic charac-ters,hydrology and topographical features,and is more sensitive to the change of environments [3].Permafrost is widespread in the Arctic and boreal regions of the Northern Hemisphere,where permafrost regions occupy 25%of the exposed land surface area [4].Permafrost also occurs on the continental shelf of the Arctic Ocean and in mountainous regions [2].In the Southern Hemi-sphere,permafrost occurs in mountains,in subantarctic islands,and in the Antarctic continent [2].Russia [5],Canada [6],China [7,8],Alaska [9]and high mountains and subalpine [10]were coun-
tries and regions that underlain by permafrost widespread.Perma-frost in china is mainly distributed on the Qinghai–Tibetan Plateau,in northeastern China and in the mountains in northwestern and central China.Permafrost bodies on the Qinghai–Tibetan Plateau is the largest permafrost region in the mid and low-latitude,which is estimated at about 1.5?106km 2and account for 69.77%the to-tal permafrost area in china [11].Permafrost controlled by air tem-perature in the thickness,presence and geographic extent reacts sensitively to changes in atmospheric temperature and permafrost is identi?ed as one of the key cryospheric indicators of global cli-mate change [10,12,13].Global average surface temperatures have increased by about 0.74±0.18°C over the past hundred years [14].Global climate models project the strongest future warming in the high-latitudes,with some models predicting a 7–8°C warming over land in these regions by the end of the 21st century [14].Glo-bal circulation models project pronounced future warming in areas where discontinuous and continuous permafrost is currently pres-ent [15]and despite the many uncertainties associated with cli-mate predictions there is a general agreement that this climatic warming will have serious impacts on high-latitude permafrost [16].Increases in regional temperatures have led to widespread
1872-2032/$-see front matter ó2009Ecological Society of China.Published by Elsevier B.V.All rights reserved.doi:10.1016/j.chnaes.2009.12.006
*Corresponding author.Tel.:+97701064888767.
E-mail addresses:yangzp04@https://www.sodocs.net/doc/3215166730.html, (Z.-p.Yang),ohua@https://www.sodocs.net/doc/3215166730.html, (Y.H.Ou).1
14.10.1980,Zaozhuang Shandong province,China,Ph.D.Candidate,Mainly engaged in ecosystem ecology and landscape ecology.
Acta Ecologica Sinica 30(2010)
33–39
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degradation of permafrost,increase of thaw depths and disappear-ance of permafrost in local area,especially in the discontinuous and sporadic permafrost zones[17,18].Widespread increases in active layer thickness can cause great changes in hydrological pro-cesses,distribution of vegetation,soil organic carbon and can also affect the infrastructure and economy of permafrost region[19–22].This paper reviewed the effects of permafrost degradation on ecosystem structure and function and put forward critical ques-tions about the effects of permafrost degradation on alpine ecosys-tems on Qinghai–Tibetan Plateau to promote research on permafrost ecosystem developing further.
1.Importance of permafrost for alpine ecosystem
Under the assemblage of permafrost molded by freeze–thaw ac-tion,cryogenic ecosystems and related processes of water and heat variations in the high-cold environment(alpine cold area and high-latitude cold area)are called permafrost ecosystem[23–25].The permafrost plays an active role on vegetation growth and produc-tion in the alpine grassland ecosystem[26].Permafrost degrada-tion alter the water–heat process of soil,which lead to changes in species composition[27]and give rise to community succession [28]and ecosystem degradation[29]further.Therefore,develop-ment and protection well of permafrost is the material foundation that permafrost ecosystem keep ecological balance.
There is strong interdependence between vegetation and per-mafrost in permafrost region.The relationship of vegetation and permafrost is complex,which mainly embodies the interaction between vegetation and permafrost.Low ground temperature of permafrost restrict roots grow deeply and is bene?cial to accumu-lation of organic materials.Permafrost is regarded as widely dis-tributing semi-permeable which could prevent surface water and soil moisture in?ltrating and migrating downward.In the mean-while,soil nutrients leached from surface soil are apt to accumu-late at the bottom of active layer[31].Permafrost plays an important role of supplying soil moisture and nutrient to vegeta-tion.Permafrost environment is essential to maintain stable cli-mate and keep the balance of alpine ecosystem.On the other hand,the vegetation which can reduce sunlight and shed soil sur-face,lowered surface temperature and transpiration rate[30]is vi-tal for thermal insulating and cooling permafrost[29].Moreover, vegetation play an important role in soil water conservation,cli-mate regulation,erosion prevention,sand?xation and keeping water balance between vegetation and soil.Permafrost and vegeta-tion mutual restricting,interacting and complementing each other are key complements to maintain the balance of alpine ecosystem in permafrost region.
2.Effects of permafrost degradation on ecosystem structure 2.1.Effects of permafrost degradation on species composition
Environmental condition such as high elevation,low tempera-ture,large t emperature difference between day and night,low air pressure,strong solar radiation in permafrost region on QTP is harsh for plant growth[32,33].Frost,snow,hail and gale are pre-vailing in growth season[32,33].Plant has narrow ecological niche as a result of living in harsh habitat for a long time and extremely sensitively to environment change.Widespread degradation of permafrost and the concomitant expansion of thermokarst-related landforms have the potential to signi?cantly affect the composi-tion,distribution and extent of plant communities in the arctic and subarctic[34].The collapsed areas caused by rich-ice perma-frost degradation in central Alaska are rapidly colonized by aquatic herbaceous plants,leading to the development of a thick,?oating organic mat(Table1)[35].The studies about effects of permafrost degradation on woody vegetation at Arctic treeline on the Seward Peninsula,Alaska show that improved soil drainage was the most important factor controlling the growth and distribution of vegeta-tion and the second factor was the depth of active layer.Plant com-munities in steep pond banks were distinct from those found on surrounding level tundra.Thaw pond banks supported a signi?-cantly higher density of trees and large shrub species and shrubs were signi?cantly taller on banks than elsewhere(Table1)[34]. Permafrost degradation resulted in improving soil drainage might in?uence species composition considerably.
Permafrost degradation on QTP decrease species number of gramineae and sedge suitable to live in the environment of low temperature and high soil moisture and increase the weeds(e.g. Potentilla nivia,Oxytropis)in the plant communities which caused the Economic value of plants decreased[27,28].Accompanying by permafrost degradation,wet plant species like Kobresia tibetica and Kobresia humilis in communities decreased gradually and drought plants such as Stipe purpurea,Poa annua and Agropyron cristatum expanded rapidly.With the permafrost degradation fur-ther,soil moisture decreased and soil environment extremely drought which caused mesophyte in alpine meadow disappear, drought enduring plant expanded fully and alpine meadow change into alpine desert steppe gradually[37].The response of species composition to permafrost degradation is signi?cantly correlated with permafrost environment.In rich-ice permafrost region with thin active layer,permafrost degradation increased soil moisture, and aquatic plants or hygrophytes expanded gradually[35].Per-mafrost degradation in regions with thick active layer and well drainage soil lowered soil moisture which increased the number of xerophytes or mesophytes and decreased the number of aquatic plants and hygrophytes(Table1)[38].
Alpine ecosystem is extremely sensitive to permafrost degrada-tion and species composition responds permafrost degradation in two different ways.For plant community in lowland on?at with rich-ice soil(e.g.Tanana Flats in central Alaska),permafrost degra-dation cause drought habitat moistening gradually.Accompanying by permafrost degradation,soil moisture increased and the xero-phytes were substituted by hygrophytes.Permafrost on QTP and upland in arctic degraded resulted in improved soil drainage and drought soil environment which caused the xerophytes expand gradually.Permafrost degradation affected species composition through altering soil water conditions that increased or decreased soil moisture.
2.2.Effects of permafrost degradation on succession
Permafrost degradation increased permafrost table,thickened active layer,and altered water conditions of vegetation growth environment,which caused the changes of?oristic composition and community structure,and arouse the community succession [36,39–41].For forests on ice-rich permafrost,the loss of ice-rich permafrost can caused the wholesale conversion of ecosystems from terrestrial to aquatic or wetland systems[42–45].Studies on the thermokarst in the Mentasta Pass area in Alaska showed that wet sedge meadows,bogs,thermokarst ponds,and lakes are replacing forests[45].Permafrost on the Tanana Flats in central Alaska degradation is widespread and rapid,causing large shifts in ecosystems from birch forests to fens and bogs which birch for-est have decreased35%and fens have increased29%from1949to 1995(Table1)[35].In lowland ecosystems and hillside of Alaska, permafrost degradation improved soil drainage which provided a microsite that favours tall woody vegetation and caused the shift of communities from tussock-tundra communities to shrub-tundra communities(Table1)[34]and shrub expanded widespread in arctic tundra was a typical example[39].
34Z.-p.Yang et al./Acta Ecologica Sinica30(2010)33–39
Wang Genxu et al.studied the permafrost ecosystems on QTP and presented the primary succession mode of alpine vegetation during the permafrost degradation:alpine marsh meadow changed into alpine meadow with the permafrost degradation,then meso-phyte community,and then drought community(Table1)[37]. The study on the relation between permafrost degradation and Kobresia meadow change on the southern piedmont of the Tangula Range showed accompanying by permafrost degradation,species occurred in cold–wet habitat disappear gradually and the species suitable to live in cold–drought habitat invaded community,and K.tibetica meadow changed into Kobresia pygmaea meadow gradu-ally(Table1)[27].
The response of alpine ecosystem to permafrost degradation showed two different trends which was complex due to the vari-ability of vegetation,content of ground ice and thaw settlement. In general,the degradation of permafrost with ice-rich soil would result in the conversion of terrestrial ecosystem to aquatic or wet-land systems.However,drought community or shrub community might become dominant community of alpine ecosystem in re-gions that permafrost degradation caused improved soil drainage
(e.g.QTP or upland in Arctic).
2.3.Effects of permafrost degradation on vegetation coverage
Permafrost degradation lead to changes in soil hydrological properties[34]which in?uenced vegetation coverage eventually since soil moisture is the key ecological factor limiting vegetation distribution,plant growth and community structure and function. In the headwater area of the Yellow River,alpine marsh meadow commonly with exuberant Kobresia(K.tibetica,K.humilis,Kobresia prattii,etc.)occurred in the area where suprapermafrost water bur-ied depth was between0.1m and0.8m and the coverage was about85–95%.When the suprapermafrost water buried depth was between0.8m and2m the dominant vegetation type was cold–arid grassland with the coverage between30–50%in which the dominant species were drought-tolerant plant like S.purpurea, A.cristatum,Elymus nutans and so on.Alpine desert steppe was the outstanding landscape type and the vegetation was composed of drought-tolerant weed(e.g.Artemisia arenaria,Leontopodium na-num,Thermopsis lanceolata)with the coverage below30%when the suprapermafrost water buried depth was larger than2m.In general,community type and vegetation zoning have the signi?-cant relation with the buried depth of suprapermafrost water in the permafrost region on QTP.In turn,the vegetation coverage is the important factor affecting the buried depth of suprapermafrost. In regions where alpine marsh meadow was with high vegetation coverage,the buried depth of suprapermafrost water was totally less than1m[46].Permafrost table has a closely relation with veg-etation coverage;low permafrost table always accompanied by high vegetation coverage.At the time of permafrost table being less than2m,soil moisture content and vegetation coverage are high,and permafrost table is negative relation with vegetation cov-erage.When permafrost table is larger than2m,most plant growth is limited and only few drought-tolerant species with developed root system are survival with low coverage being less than35%,as a result of which the growth environment is drought dramatically due to low level of suprapermafrost water,little water recharge,few quantity of suprapermafrost water and rising height of capillary water being not reach the surface soil layer in which the root mainly distributed[36].
2.4.Effects of permafrost degradation on biodiversity
Permafrost degradation changed habits and species composi-tion which affected species richness.The result based on the quad-rat investigating indicated that little difference existed between the different communities in successional series.However,the richness index was signi?cantly different within the100m2sam-pling zones in the degraded permafrost region on QTP,which spe-cies richness in alpine meadow and steppe meadow was signi?cantly larger than that in marsh meadow and desert steppe. The difference of results obtained from two different sampling ap-proaches indicated though the species richness in single quart had little difference,the role and importance of species in communities changed because the distribution and occurrence frequency of spe-cies was different.Species evenness indices were signi?cantly dif-ferent between communities in permafrost region on QTP that increased from marsh meadow to steppe meadow,and decreased
Table1
Model of community succession and change of species composition in permafrost degradation.
Model of community succession Succession
stage
Dominant species Associate species Area
Swamp meadow–true
meadow–steppe meadow–sandy meadow Swamp
meadow
Kobresia tibetica Blysmus sinocom pressus,Polygonum macrophyllum,Pedicularis
longi?ora var tubifrmis,Caltha scaposa,Carex atrofusca,Hippuris
vulgaris
Xidatan-the southern
hillside of tanggula
mountain[38]
Ture
meadow
Kobresia humilis K.capillifolia,Poa pratensis,Stipa sp.,Leuntopodium nanum,
Oxytrapis ansuensis
Steppe
meadow
Kobresia.pygmaea Stipa aliena,Saussurea superba,Festuca rubra,Elymus nutans,Aster
?accidus,Gentiana sp.
Sandy
meadow
Stipa
purpurea+Carex sp.
Stipa subsessili?ora,Festuca spp.,Poa pratensis,Astragalus spp.,
Potentilla bifurca,Saussurea sp,M yricariap rostrata
Kobresia tibetica meadow–Kobresia pygmaea meadow Kobresia
tibetica
meadow
Kobresia tibetica,
Kobresia pygmaea
Anemone sp.,Gentiana leucom elaena,Trollius pum ilus,Cham
aesium parad oxum,Saussurea sp.,Poa sp.,Crem anthodium sp.
The southern hillside of
tanggula mountain[27] Kobresia
pygmaea
meadow
Kobresia pygmaea Blysmus sinocom pressus,Potentilla nivia,Androsace sp.,
Leuntopodium nanum,Oxytrapis kansuensis,Saussurea sp.,Poa
pratensis
Tussock-tundra–shrub-tundra Tussock-
tundra Eriophorum
vaginatum
C.bigelowii,Empetrum niigrum,Arctostaphylos spp.,Rubus
chamaemoris,Salix arctica,Vaccinium spp.
Seward Peninsua in
western Alaska[34]
Shrub-tundra Salix glauca Betula nana,Salix arctica,Picea glauca,Vaccinium uliginosum,
Ledum groenlandicum
Forest–fen Lowland
birch forests Betula papyrifera Picea glauca,Picea mariana,Rosa acicularis,Equisetum arvense,
Calamagrostis anadensis,Lemna minor,Calla palustris
Tanana Flats[35]
Lowland fen meadows Menyanthes
trifoliata,Equisetum
?uviatale
Cutriculata,C.aquatilis,Potentilla palustris,Typha latifolia,Cicuta
mackenzieana,Galium tri?dum,Myrica gale
Z.-p.Yang et al./Acta Ecologica Sinica30(2010)33–3935
from steppe meadow to steppe grassland[38,47].a Diversity was signi?cantly different among the successional series and its change trend was in accord with the evenness index that increased from marsh meadow to steppe meadow,and then decreased from steppe meadow to steppe grassland.With the degradation of per-mafrost,marsh meadow was drained and changed into alpine meadow which brought about invasion and development of meso-phytes.Species richness index in quadrat was almost no change between marsh meadow and alpine meadow because even distri-bution of species in communities,however,a diversity in alpine meadow increased compared with in marsh meadow.At the suc-cession stage of steppe meadow,xerophytes replaced mesophytes and hygrophytes gradually and the proportion of meadow plant decreased.Although species composition in steppe meadow chan-ged compared with in alpine meadow,a diversity was not signi?-cant different.Xerophytes had high ecological dominance with disappearance of mesophytes because of extremely degraded per-mafrost environment and a diversity index decreased[38,47].Yan et al.[27]carried about the study on the relation between perma-frost degradation and Kobresia meadow change on the southern piedmont of the Tangula Range and the results showed that when the permafrost environment changed from permafrost island to seasonally frozen-ground area,ecological dominances and species diversities were increasing in a certain degree.The discrepancy be-tween the results obtained by Yan et al.[27]and Guo et al.[38,47] demonstrated that it is not a universal law about the response of biodiversity and ecological dominance to permafrost degradation. The effect of permafrost degradation on biodiversity was complex which should be contingent on permafrost type,speci?c region and succession mode.
b Diversity is often used to analyze the difference of species composition between communities;it re?ects species replacement along an environmental gradient.b Diversity is high when different communities had large number of common species[48].At the stage of marsh meadow during permafrost degradation on QTP, the heterogeneity of habitat was low due to high soil moisture con-tent and the rate of species replacement was small.Along with per-mafrost degradation on QTP,soil moisture decreased gradually and mesophytes invaded community that led to increase of the b diver-sity.The rate of species replacement increased further when the soil moisture decreased further resulting in the disappearance of hygrophytes and the invasion of xerophytes.Soil environment was greatly drought due to permafrost degraded extremely low-ered the heterogeneity of habitat and steppe plant had high ecolog-ical dominance which decreased the rate of species replacement. The change trend of b diversity re?ected the heterogeneity of microhabitat increased from marsh meadow to steppe meadow, and decreased from steppe meadow to steppe grassland;also re-?ected the change rate of species diversity of alpine grassland com-munity during permafrost degradation on QTP[38,47].
3.Effects of permafrost degradation on ecosystem function 3.1.Effects of permafrost degradation on soil moisture
Under unique plateau climate environment,especially in per-mafrost region,variations in the process of soil moisture were the signi?cant change of eco-environment on QTP.Soil moisture was important factor in?uencing vegetation growth which de-pended on vegetation type,soil characteristics and permafrost environment.Changes in freezing and thawing process caused by altered permafrost environment in?uenced soil moisture drasti-cally.Soil moisture was the key factor linking the changes of per-mafrost environment and vegetation dynamics.The melting of permafrost impacted alpine ecosystem mainly through changes in soil moisture conditions.Thickened active layer and lowered le-vel of suprapermafrost water caused by permafrost degradation on QTP were one of important reasons responsible for a dropping groundwater table.With the increase in the permafrost table,the thickness of active layer thickens.With the development of active layer during permafrost degradation,soil moisture converge at the permafrost table,in the meanwhile,vaporous water migrate to-ward and condense at permafrost table under the vapor pressure leading to decrease of moisture in surface soil which cause the loss of soil moisture in upper active layer and drought gradually[46].
At community scale,surface soil moisture with decreasing trend changed dramatically and the difference between succession stages reached signi?cant level.The difference of soil moisture during permafrost degradation on QTP was mainly because that permafrost could prevent soil moisture in?ltrating and maintain high soil moisture content in root zone;however,increased active layer during permafrost degradation weakened the support action of permafrost to groundwater,decreased the level of groundwater, and brought about the drought trend of surface soil gradually[38]. Soil moisture distributed in vertical pro?le in different vegetation types(e.g.shrub grassland,Kobresia meadow,and black soil land) with the same trend that the content of soil moisture decreased with the increase of soil depth.However,there were apparent dif-ferences in the extent of soil moisture change in pro?le,which soil moisture distributed vertically in black soil land and Kobresia mea-dow changed much more dramatically than that in shrub grass-land.The content of soil moisture in shrub grassland was highest and it had the lowest value in Kobresia meadow.Black soil land with more degraded permafrost environments had higher soil moisture content than Kobresia meadow due to compact soil struc-ture of black soil land restricted in?ltration maintaining high soil moisture[49].At region scale,the melting of permafrost and the thickening of active layer were the crucial factors that caused a dropping groundwater table at the source areas of the Yangtze Rive and Yellow River,which in turn results in lowing lake water levels, drying swamps and shrinking grasslands[50].
3.2.Effects of permafrost degradation on soil nutrient
The main driving factor for soil organic material(SOM)storage is the thickness of the active layer[51].The degradation of perma-frost and the related deepening of the active layer make large amounts of previously frozen SOM may become bioavailable [54,55]and increase SOM mineralization[53,56]dramatically by improving soil drainage and oxygen availability[52,53].The recent studies on permafrost on QTP showed that accompanying by the process of permafrost degradation,SOM reduced signi?cantly. The content of SOM in alpine marsh meadow was?ve and seven times higher than that of in alpine steppe meadow and alpine des-ert steppe,respectively[11,38].Additionally,the melting of perma-frost decreased the content of the hydrolysable nitrogen,available potassium,and available phosphorous[49].
Permafrost degradation led to thermokarst formation that in?u-enced the content of soil organic carbon and total nitrogen.Soil or-ganic and total nitrogen in thermokarst caused by permafrost degradation was much less than in intact bogs[51].Some study showed contradictory?ndings that total canopy N was signi?-cantly higher at thermokarst compared to Tussock[57]suggesting permafrost degradation increased soil temperature resulting in im-proved nitrogen mineralization and nitrogen availability,more soil nitrogen to be utilized in thermokarst which caused the increase of community productivity limited by soil nutrient currently[57,59–61].Therefore,the effects of permafrost degradation and long-term development of thermokarst pond on carbon and nitrogen cycling are not insightful and it is a great challenge for the study about the
36Z.-p.Yang et al./Acta Ecologica Sinica30(2010)33–39
effect of permafrost degradation on the nutrient cycling in the fu-ture[58,61].
3.3.Effects of permafrost degradation on ecosystem productivity
Permafrost play a signi?cant role on vegetation production in the alpine grassland ecosystem in permafrost region due to that permafrost as semi-permeable could prevent soil moisture in?l-trating and supply an ample water resource for plant growth,in the meanwhile,permafrost is bene?cial for plant survivor and organism remaining in soil and played an important role on nutri-ents cycling[26].Therefore,the degradation of permafrost would alter vegetation pattern and change ecosystem productivity[62–64].Guo et al.[38]revealed that primary productivity of grassland decreased gradually during permafrost degradation;plant biomass lowed60%when alpine marsh meadow changed into alpine mea-dow and alpine desert steppe had the lowest plant biomass,only about500kg/hm2.However,there were not signi?cantly different in plant biomass between alpine meadow and alpine steppe mea-dow.Although the difference in plant biomass between some suc-cessional series was not signi?cant,carrying capacity,quality of grassland decreased dramatically because the proportion of avail-able forage decreased,and toxic and hazardous weeds in plant communities increased in the successional series,which was in-stanced in a fact that sedge decreased and the species richness of weeds such as P.nivia,Stellera chamaejasme,etc.increased as a re-sult of shift in vegetation type from alpine steppe meadow to al-pine desert steppe during permafrost degradation on QTP[38]. The main cause being responsible for permafrost degradation led to the decrease of ecosystem productivity was that ecological water level and groundwater level lowered due to increased depth of seasonal thawing layer,and soil moisture was not restricted in the surface soil reducing the available water to plant growth,caus-ing the root withered and resulting in the decrease of primary pro-ductivity of alpine ecosystem[65,66].In upland tundra in Alaskan, the development of thermokarst decreased the aboveground plant biomass which changed from being dominated by sedges,to becoming increasingly dominated by deciduous and evergreen shrubs.Tussock had the lowest shrub biomass and the highest graminoid biomass indicated permafrost degradation changed cold,dry microsites suitable for graminoids into warm,moist microsites associated with shrubs closely[57].
3.4.Effects of permafrost degradation on greenhouse gas emission
Large quantities of carbon are sequestered in the permafrost of boreal peatlands and tundra regions[67]and the near-surface layer of permafrost is a substantial proportion of the carbon pool in terrestrial ecosystems[68],which approximately970Gt of car-bon are contained in permafrost[69].Therefore,distribution of permafrost and composition and quantity of organic materials in permafrost are essential for understanding the response of ecosys-tems in high-latitude region to global warming[53,70].That change permafrost C from the frozen to the thawed state rapidly increase the C pool size available for decomposition was crucial for determining C emission from permafrost to atmosphere.Under a warming climate,release rate and type of C from permafrost to the atmosphere were closely associated with thaw mechanisms of permafrost at landscape scale and the thaw mechanisms mainly included active layer thickening,talik formation,river and coastal erosion and thermokarst development[2].However,the main causes controlled thaw rate and in?uence extent of permafrost were the content of ground ice and the characteristics of landscape such as terrain,slope,etc.[45].The melting of permafrost im-proved release of C from permafrost to the atmosphere mainly through accelerated microbial decomposition of organic matter.Accompanying with the melting of permafrost,large quantities of CO2were emitted from frozen C pool to atmosphere due to in-creased soil temperature and improved soil drainage and oxygen availability[55].The melting of permafrost in peatlands across western Canada led to1.6-and30-fold increased in CO2and CH4 emissions,respectively and the function of high-latitude terrestrial ecosystems as an atmospheric CO2sink was weakened gradually or even reverse because of permafrost degradation[52,55].The func-tion of wetlands in permafrost regions on the QTP contribute sig-ni?cantly to the global budgets of greenhouse gases.However, the functions of these wetlands as an atmospheric CO2sink may change substantially because permafrost under these wetlands has been degrading[8].Studies on the effects of the degradation of swamp and alpine meadows on CO2emission on the QTP showed that CO2emission?ux of the swamp meadow gradually decreased with increasing degradation degree,while that of the al-pine meadow gradually increased with increasing degradation de-gree[80].Fire disturbance is an important abiotic mechanism for transferring C thawed from permafrost to the atmosphere that oxi-dizes organic C primarily to CO2,but also releases smaller quanti-ties of CH4,carbon monoxide,and other volatile C compounds[2]. The size of C pool of permafrost,biological decomposition and?re disturbance were the important in?uencing factors that control the release of C from permafrost to atmosphere.Under a warming cli-mate,increased biological decomposition and frequency of?re dis-turbance would cause large quantities of organic C stored in permafrost release to atmosphere in the form of greenhouse [81,82].It is about100PgC in permafrost predicted to be lost by 2100[83].
Permafrost degradation in?uenced climate change through complex mechanisms,both positive and negative feedbacks relat-ing to CO2?uxes.Emission of greenhouse caused by permafrost degradation accelerated global warming through positive feedback mechanisms.However,increased photosynthetic rate and pro-longed growth season could sequester C from atmosphere that re-strained the climate warming to some extent through negative mechanism.One important offset of release C from permafrost was that increased soil temperature and improved nutrient avail-ability during permafrost degradation promote plant growth lim-ited by soil nutrients,cold temperature and extreme seasonality with a short growing season and sequestered more gaseous C in al-pine ecosystems[84].Studies have revealed that net ecosystem C uptake in growing season increased1–6g/m2caused by the in-crease of temperature[85,86].Vegetation C in bogs caused by per-mafrost degradation increased[71]due to that accelerated mineralization of organic matter released ample available nutri-ents led to increase in net primary productivity of ecosystem [72].Carbon sequestration in ecosystem at long-term scale could not offset the release of C from permafrost to atmosphere resulted from the melting of permafrost and the development of thermok-arst[2].Therefore,the response of ecosystem dynamics in perma-frost region to global warming was not only carbon emission and it is actually great complex[73,74].Long-term observation about the ?ux of CO2,nutrient status,net primary productivity and thaw depth of permafrost was crucial for estimating the balance of C in permafrost region accurately in the future.
The melting of permafrost may not only affect the CO2ef?ux but also the exchange of the greenhouse gases CH4[75].The direction and the size of the net CH4exchange were mainly controlled by the degree of soil water saturation,the temperature,and the amount of bioavailable organic matter[76,77].Little knowledge about N2O release from permafrost soil was known due to low nitrogen turn-over and availability and emission was considered to be low[70]. Large quantities of organic nitrogen accumulated in boreal peat-lands[78]and permafrost degradation induced a lowering water tables or drainage of boreal peatlands might increase emission of
Z.-p.Yang et al./Acta Ecologica Sinica30(2010)33–3937
N2O.High N2O emissions were found in permafrost soils rich in or-ganic nitrogen where appeared to be a strong potential for denitri-?cation[70,79].
4.Problems and perspectives on the effects of permafrost degradation on ecosystem on QTP
Compared with geocryology in other countries,permafrost study in china is closely related with economic construction and development of cold region and has obtained great achievements in engineering geocryology concerned exploitation of frozen-ground regions and geocryology physics(e.g.classi?cation of per-mafrost,type of permafrost,temperature and thickness of perma-frost).Unique high altitudinal permafrost on QTP attracted much attention in the context of global warming since the QTP is the sen-sitive and promoter region of climate change in china.However, investigation about the effects of permafrost degradation on eco-system on QTP was not systematical.
At present,there is lack of long-term observations in studying succession dynamic of alpine ecosystem during permafrost degra-dation and space sequence instead of chronosequence approach or static analogy approach is mostly adopted.A hypothesis was based on by utilizing space sequence instead of chronosequence ap-proach that swamp meadow degrade into true meadow,and then steppe meadow,desert steppe eventually during permafrost degra-dation.However,the hypothesis is not scienti?c suf?ciently due to the fact that each vegetation type exited previously and changes of vegetation type is unnecessarily resulted from permafrost degrada-tion.In the meanwhile,the true meadow changed into‘‘black soil land”instead of steppe meadow under the in?uence of climate change and human activity.So,the hypothesis maybe weakens the actual effects of permafrost degradation on grassland ecosys-tem.To investigate the mechanism of alpine grassland degradation in permafrost region on QTP,the long-term and located?eld obser-vations and research system should be constructed,based on which predict ecosystem dynamic in permafrost degradation.
Permafrost on QTP is a huge C pools since alpine grassland soils of the Qinghai–Tibetan Plateau’s permafrost region bear a total or-ganic carbon content of9.3–10.7kg C/https://www.sodocs.net/doc/3215166730.html,anic carbon stored in permafrost is vulnerability to change under an increasingly war-mer climate.Permafrost degradation has resulted in decreasing the content of soil organic carbon in alpine meadow and alpine steppe.Climate warming could increase the active layer depth in permafrost soils and subsequently release the carbon sequestered in the upper permafrost layers.This would drastically increase soil organic matter mineralization by improving soil drainage and oxy-gen availability which cause positive and negative feedback to cli-mate change.In future,study should pay extensive attention to the dynamic of greenhouse gas in permafrost region on Qinghai–Tibe-tan Plateau and the feedback of greenhouse gas to climate change.
Grassland ecosystem in cold region has close relation with per-mafrost and its water–heat process.Changes in freezing and thaw-ing process resulted from permafrost degradation in?uence vegetation growth environment directly through altering soil moisture and indirectly through changing nutrient availability. Quantitative study on the mechanism of water–heat transport in permafrost degradation is the theoretical premises that discuss the response of grassland ecosystem in permafrost region on QTP to global climate change.Therefore how to quantitative analyze changes in water–heat process during permafrost degradation and its effects on plant growth;reveal the relationship between al-pine grassland ecosystem degradation and changes of soil moisture and temperature during permafrost degradation and predict fur-ther the trend of dynamics change of alpine grassland ecosystem under future climate change is a great challenge in the study about the effects of permafrost degradation on alpine grassland ecosys-tem on QTP.
Acknowledgments
This study was funded by NKBRSF,PR China(No. 2005CB422005)and National Basic Task Project(No. 2006FY110200).Special thanks owe to editors and anonymous re-viewer whose constructive suggestions and detailed comments helped to clarify and improve the paper.
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