Variations in the Volatile Organic Compound Emission Potential of Plant Functional Groups in the Temperate Grassland Vegetation of Inner Mongolia, China

Abstract: The biogenic volatile organic compounds (VOC) emitted by the vegetation of a terrestrial ecosystem play a key role in both regional air quality and tropospheric chemistry. To describe the general emission properties of VOC of different plant functional groups (PFG) in a typical temperate grassland in Inner Mongolia, China, we randomly selected 175 plant species and measured the quantities of isoprene and monoterpene in situ. Results showed that most plants had low VOC emission potential at the species level, especially for some dominant plants, such as Leymus chinensis Tzvel., Stipa grandis Smirn., and Agropyron cristatum Gaertn. At the PFG level, the lowest VOC emission potential was found for perennial rhizome grasses, a major PFG in a typical temperate grassland ecosystem. The effects of overgrazing and subsequent vegetation succession on the emission of VOC by different plant life form functional groups (PLFG) were also discussed.

Plant functional groups mediate effects of climate and soil factors on species richness and community biomass in grasslands of Mongolian Plateau

Aims Functional group composition of a plant community is mainly driven by environmental factors and is one of the main determinants of grassland biodiversity and productivity.Therefore,it is important to understand the role of plant functional groups(PFGs)in mediating the impact of environmental conditions on ecosystem functions and biodiversity.Methods We measured plant biomass and species richness(SR)of grasslands in 65 sites on the Mongolian Plateau and classified 157 perennial herbaceous plants into two main PFGs(namely grasses and forbs).Using the random forest model and ordinary least squares regression,we identified that environmental factors(i.e.aridity index,soil total nitrogen[STN]and pH)were significantly related to the SR and aboveground biomass(AGB)of PFGs.We then used structural equation modeling to explore the relationship between the identified environmental factors and community SR and biomass,and the role of PFGs in driving this relationship.Important Findings We found that aridity index had unimodal relationships with both AGB and SR of the PFGs and the whole community.All SR and biomass metrics were significantly related to STN and pH.The relationship between aridity index and community biomass was mediated by an increase in the AGB of grasses.The influence of STN and pH on community SR was mainly due to their regulation in the SR of forbs.Our results indicate that community composition and the identity of the PFGs play a key role in linking environmental factors to ecosystem functioning.

Species-area relationship within and across functional groups at alpine grasslands on the northern Tibetan Plateau,China

The species-area relationship （SAR） is one of the most fundamental concepts in community ecology and is helpful for biodiversity conservation. However, few studies have systematically addressed this topic for different alpine grassland types on the Tibetan Plateau, China. We explored whether the plant composition of different functional groups affects the manner in which species richness inereases with increasing area at scales ≤ 1.0 m^2. We also compared species richness （S） within and across forbs, legumes, sedges and grasses, with sampling subplot area （A） increasing from 0.0625 m^2 to 1.0 m^2 between alpine meadow and steppe communities. We applied a logarithmic function （S = b0 ＋ b1 ln A） to determine the slope and intercept of SAR curves within and across functional groups. The results showed that the logarithmic relationship holds true between species richness and sampling area at these small scales. Both the intercept and slope of the logarithmic forbs-area curves are significantly higher than those for the three other functional groups （P 〈 0.05）. Forb accounts for about 91.9 % of the variation in the intercept and 75.0% of the variation in the slope of the SAR curve when all functional groups＇ data were pooled together. Our results indicated that the different SAR patterns should be linked with species dispersal capabilities, environmental filtering, and life form composition within alpine grassland communities. Further studies on the relationship between species diversity and ecosystem functions should specify the differential responses of different functional groups to variations in climate and anthropogenic disturbances.

Uptake and Recovery of Soil Nitrogen by Bryophytes and Vascular Plants in an Alpine Meadow

Due to their particular physiology and life history traits, bryophytes are critical in regulating biogeochemical cycles and functions in alpine ecosystem. Hence, it is crucial to investigate their nutrient utilization strategies in comparison with vascular plants and understand their responses to the variation of growing season caused by climate change. Firstly, this study testified whether or not bryophytes can absorb nitrogen(N) directly from soil through spiking three chemical forms of 15N stable isotope tracer. Secondly, with stronger ability of carbohydrates assimilation and photosynthesis, it is supposed that N utilization efficiency of vascular plants is significantly higher than that of bryophytes. However, the recovery of soil N by bryophytes can still compete with vascular plants due to their greater phytomass. Thirdly, resource acquisition may be varied from the change of growing season, during which N pulse can be manipulated with 15N tracer addition at different time. Both of bryophytes and vascular plants contain more N in a longer growing season, and prefer inorganic over organic N. Bryophytes assimilate more NH4+ than NO3– and amino acid, which can be indicated from the greater shoot excess 15N of bryophytes. However, vascular plants prefer to absorb NO3– for their developed root systems and vascular tissue. Concerning the uptake of three forms N by bryophytes, there is significant difference between two manipulated lengths of growing season. Furthermore, the capacity of bryophytes to tolerate N-pollution may be lower than currently appreciated, which indicates the effect of climate change on asynchronous variation of soil N pools with plant requirements.

Mechanism of plant-soil feedback in a degraded alpine grassland on the Tibetan Plateau

Although biotic and abiotic factors have been confirmed to be critical factors that affect community dynamics,their interactive effects have yet to be fully considered in grassland degradation.Herein,we tested how soil nutrients and microbes regulated plant-soil feedback(PSF)in a degraded alpine grassland.Our results indicated that soil total carbon(STC;from 17.66 to 12.55 g/kg)and total nitrogen(STN;from 3.16 to 2.74 g/kg)exhibited significant(P<0.05)decrease from non-degraded(ND)to severely degraded(SD).Despite higher nutrients in ND soil generating significantly(P<0.05)positive PSF(0.52)on monocots growth when the soil was sterilized,a high proportion of pathogens(36%)in ND non-sterilized soil resulted in a strong negative PSF on monocots.In contrast,the higher phenotypic plasticity of dicots coupled with a higher abundance of mutualists and saprophytes(70%)strongly promoted their survival and growth in SD with infertile soil.Our findings identified a novel mechanism that there was a functional group shift from monocots with higher vulnerability to soil pathogens in the ND fertile soil to dicots with higher dependence on nutritional mutualists in the degraded infertile soil.The emerging irreversible eco-evolutionary in PSF after degradation might cause a predicament for the restoration of degraded grassland.

Soil microbial community composition and its driving factors in alpine grasslands along a mountain elevational gradient

Understanding the vertical distribution patterns of soil microbial community and its driving factors in alpine grasslands in the humid regions of the Tibet Plateau might be of great significance for predicting the soil microbial community of this type of vegetation in response to environmental change. Using phospholipid fatty acids （PLFA）, we investigated soil microbial community composition along an elevational gradient （3094-4131 m above sea level） on Mount Yajiageng, and we explored the impact of plant functional groups and soil chemistry on the soil microbial community. Except for Arbuscular Mycorrhizal fungi （AM fungi） biomarker 18：2ω6,9 increasing significantly, other biomarkers did not show a consistent trend with the elevational gradient. Microbial biomass quantified by total PLFAs did not show the elevational trend and had mean values ranging from 1.64 to 4.09 ktmol per g organic carbon （OC）, which had the maximum value at the highest site. Bacterial PLFAs exhibited a similar trend with total PLFAs, and its mean values ranged from 0.82 to 1.81 μmol （g OC）-1. The bacterial to fungal biomass ratios had the minimum value at the highest site, which might be related to temperature and soil total nitrogen （TN）. The ratios of Gram-negative to Gram-positive bacteria had a significantly negative correlation with soil TN and had the maximum value at the highest site. Leguminous plant coverage and soil TN explained 58% of the total variation in the soil microbial community and could achieve the same interpretation as the whole model. Other factors may influence the soil microbial community through interaction with leguminous plant coverage and soil TN. Soil chemistry and plant functional group composition in substantial amounts explained different parts of the variation within the soil microbial community, and the interaction between them had no impact on the soil microbial community maybe beeause long-term grazing greatly reduces litter. In sum, although there were obvious differences in soil microbial communities along the elevation gradient, there were no clear elevational trends found in general. Plant functional groups and soil chemistry respectively affect the different aspects of soil microbial community. Leguminous plant coverage and soil TN had important effects in shaping soil microbial community.

Changes in leaf stomatal traits of different aged temperate forest stands

Stomata control carbon and water vapor exchange between the leaves and the atmosphere,thus infl uencing photosynthesis and transpiration.Combinations of forest patches with different stand ages are common in nature,however,information of which stomatal traits vary among these stands and how,remains limited.Here,seven different aged forest stands(6,14,25,36,45,55,and 100 years)were selected in typical temperate,mixed broadleaf-conifer forests of northeast China.Stomatal density,size and relative area of 624 species,including the same species in stands of different ages were selected.Stomatal density,size and relative area were distributed log-normally,differing across all species and plant functional groups.Stomatal density ranged from 4.2 to 1276.7 stomata mm^(–2),stomatal size ranged from 66.6 to 8315.7μm^(2),and stomatal relative area 0.1–93.3%.There was a significant negative relationship between density and size at the species and functional group levels,while the relative stomatal area was positively correlated with density and size.Stomatal traits of dominant species were relatively stable across different stand ages but were significantly different for herbs.The results suggest that stomatal traits remain relatively stable for dominant species in natural forests and therefore,spatial variation in stomatal traits across forest patches does not need to be incorporated in future ecological models.

Self-stabilizing maintenance process in plant communities of alpine meadows under different grazing intensities

Backgrounds:Grazing prohibition and reduced grazing intensity,as two important“vegetation close-to-nature recovery”methods,have been suggested as economical and effective technologies for enhancing forage production.However,numerous studies have found that the yield of forage could be increased by removing or reducing grazing in a short time in some steady stage of alpine Kobresia meadows,but not in others.To reveal the mechanism behind this phenomenon,we proposed a series of experiments.Methods:We monitored the plant and soil characteristics in the key steady stages of Kobresia meadows under reduced and prohibited grazing conditions in the same geographic and climatic environments in the northeastern Qinghai-Tibet Plateau for 6 years.We estimated the relationships between the plant community and soil nutrients and obtained the following results.Results:All measured variables were positively correlated with each other.The plant community structure had higher path coefficients to aboveground biomass,soil organic matter,total nitrogen,and nitrate nitrogen than to other factors.The plant community structure played an important role in response to grazing intensity.Different plant functional groups(PFGs)had different responses to grazing intensity,which led to plant community re-establishment or re-organization under different grazing intensities.Poaceae and Kobresia were more sensitive to grazing intensity than other PFGs,and the ratio of Kobresia biomass(including Kobresia humilis and Kobresia pygmaea)to the total biomass of Poaceae and Kobresia could be used as an indicator of regime shifts within plant communities.With Kobresia pygmaea as the dominant species,the prohibition of grazing was not an efficient approach to increase the yield in the steady stages because this treatment needed more time to recover aboveground biomass.If Poaceae is the dominant PFG,grazing should only be prohibited for 3 years in the steady stages because the aboveground biomass will decrease if grazing is prohibited for more than 3 years.Conclusions:Therefore,the different steady stages of alpine meadows require different recovery methods to increase recovery efficiency and speed.

Resource enrichment combined with biomass removal maintains plant diversity and community stability in a long-term grazed grassland

Aims Long-term heavy grazing reduces plant diversity and ecosystem function by intensifying nitrogen(N)and water limitation.In contrast,the absence of biomass removal can cause species loss by elevating light competition and weakening community stability,which is exacerbated by N and water enrichment.Hence,how to maintain species diversity and community stability is still a huge challenge for sustainable management of worldwide grasslands.Methods We conducted a 4-year manipulated experiment in six long-term grazing blocks to explore combination of resource additions and biomass removal(increased water,N and light availability)on species richness and community stability in semiarid grasslands of Inner Mongolia,China.Important Findings In all blocks treated with the combination of resource additions and biomass removal,primary productivity increased and species richness and community stability were maintained over 4 years of experiment.At both species and plant functional group(PFG)levels,the aboveground biomass of treated plants remained temporally stable in treatments with the combination of N and/or water addition and biomass removal.The maintenance of species richness was primarily caused by the biomass removal,which could increase the amount of light exposure for grasses under resource enrichment.Both species asynchrony and stability of PFGs contributed to the high temporal stability observed in these communities.Our results indicate that management practices of combined resource enrichment with biomass removal,such as grazing or mowing,could not only enhance primary productivity but also maintain plant species diversity,species asynchrony and community stability.Furthermore,as overgrazing-induced degradation and resource enrichment-induced biodiversity loss continue to be major problems worldwide,our findings have important implications for adaptive management in semiarid grasslands and beyond.

Seasonality regulates the effects of resource addition on plant diversity and ecosystem functioning in semi-arid grassland

Aims Seasonal variations in species richness,aboveground net primary productivity(ANPP)and stability under resource enrichment are frequently ignored.This study explores how the impacts of resource enrichment on species richness,ANPP and stability vary among seasons in semi-arid grasslands.Methods We conducted a 3-year experiment in an Inner Mongolia grassland to determine the effects of resource input(water[W],nitrogen[N])on species richness,community ANPP and stability using seasonal sampling during the growing season(2013–2015).Structural equation modeling(SEM)was used to examine the relative importance of resource input on community stability via mechanistic pathways in each month and the whole growing season.Important Findings Resource inputs did not affect community ANPP in May and June,while N and/or NW enhanced ANPP in July and August.Resource inputs generally did not affect species richness,asynchrony or community stability in most of the time.Positive responses of perennial bunchgrasses(PB)to N and/or NW treatments contributed to the increased community ANPP in July and August.Species asynchrony may be the major mechanism contributing to community stability in May and June and the entire growing season,and PB stability is potentially the primary factor controlling community stability in July and August under resource enrichment.Our results indicate that season and resource availability could interact to regulate species richness,community ANPP and stability in semi-arid grasslands.These findings have important implications for management practices in semi-arid grasslands in order to mitigate the impact of land use and global change.

Climate change-driven shifts in plant–soil feedbacks:a meta-analysis

Background:Climate change is expected to affect plant–soil feedbacks(PSFs,i.e.,the effects of a plant on the growth of another plant or community grown in the same soil via changes in soil abiotic and biotic properties),influencing plant community dynamics and,through this,ecosystem functioning.However,our knowledge of the effects of climate changes on the magnitude and direction of PSFs remains limited,with considerable variability between studies.We quantified PSFs associated with common climate change factors,specifically drought and warming,and their corresponding ambient(control)conditions using a meta-analytical approach.We investigated whether drought and warming effects on PSFs were consistent across functional groups,life histories(annual versus perennial)and species origin(native versus non-native),planting(monoculture,mixed culture)and experimental(field,greenhouse/laboratory)conditions.Results:PSFs were negative(a mechanism that encourage species co-existence)under drought and neutral under corresponding ambient conditions,whereas PSFs were negative under both ambient and elevated temperatures,with no apparent difference in effect size.The response to drought was largely driven by stronger negative PSFs in grasses,indicating that grasses are more likely to show stronger negative PSFs than other functional groups under drought.Moreover,non-native species showed negative drought-induced PSFs while native species showed neutral PSFs under drought.By contrast,we found the opposite in pattern in response to warming for native and non-native species.Perennial herbs displayed stronger drought-induced negative PSFs than annual herbs.Mixed species communities displayed more negative PSFs than monocultures,independent of climate treatment.Finally,warming and drought treatment PSF effect sizes were more negative in experiments performed in the field than under controlled conditions.Conclusions:We provide evidence that drought and warming can induce context-specific shifts in PSFs,which are dependent on plant functional groups,life history traits and experimental conditions.These shifts would be expected to have implications for plant community dynamics under projected climate change scenarios.

The interactive effects of nitrogen addition and increased precipitation on gross ecosystem productivity in an alpine meadow

The ecological consequences of precipitation change and increased atmospheric nitrogen(N)deposition have profound impacts on ecosystem CO2 exchange in grassland ecosystems.Water and N can largely influence grassland productivity,community composition and ecosystem functions.However,the influences of water and N addition on the ecosystem CO2 exchange of alpine grassland ecosystems remain unclear.A field manipulative experiment with water and N additions was conducted in an alpine meadow on the Tibetan Plateau over 4 years with contrasting precipitation patterns.There were four treatments:control(Ctrl),N addition(N),water addition(W)and N and water addition(NW),each replicated three times.N addition,but not water addition,increased gross ecosystem productivity(GEP),plant biomass,community cover and community-weighted mean height.The responses of ecosystem CO2 exchange to water and N addition varied between the wet and dry years.Water addition had a positive effect on net ecosystem carbon exchange(NEE)due to a larger increase in GEP than in ecosystem respiration(ER)only in the dry year.On the contrary,N addition significantly enhanced ecosystem CO2 exchange only in the wet year.The increased GEP in N addition was attributed to the larger increase in NEE than ER.Moreover,N addition stimulated NEE mainly through increasing the cover of dominant species.Our observations highlight the important roles of precipitation and dominant species in regulating ecosystem CO2 exchange response to global environmental change in alpine grasslands.

Mowing and fertilizer effects on seedling establishment in a successional old field

Aims We used a 10-year field experiment that consisted of mowing and fertilizer treatments to evaluate the role of niche limitation in seedling establishment of species from different functional groups and of varying local abundance in an old field undergoing succession.Methods Seedlings of nine different species were planted into a successional field subjected to mowing and fertilizer treatments for 10 years that resulted in different plant communities and resource availability.Species representative of the factorial combination of three functional groups(C4 grasses,C3 grasses and legumes)and three abundance categories(abundant,present,or absent in the old field)were planted in four treatments resulting from the factorial combination of annual spring mowing(mowed and unmowed)and fertilizer application(annually fertilized and unfertilized).Survivorship,relative growth rate(RGR)and biomass were measured to determine the role of niche limitation on recruitment and growth.Important Findings Mowing increased the establishment success of seedlings.Fertilization had little influence on seedling performance and survivorship.C3 grasses had the highest survivorship,while C4 grasses and legumes had equivalent RGRs,but higher than C3 grasses.By contrast,survivorship of legumes was unrelated to mowing or fertilizer,suggesting that establishment of this functional group was dependent on other,unmeasured conditions or processes.Species already present,but at low abundance,performed better than locally abundant or absent species.Propagule limitation may restrict the arrival of a species.However,recruitment and establishment was subject to niche limitation,which varied among species,functional groups and whether a species is already resident at the site and its abundance.Thus,species interactions restrict establishment during old-field succession,supporting the niche limitation hypothesis.

Unexpected ecotone dynamics of a sand dune vegetation complex following water table decline

Aims Central Hungarian inland dune ranges harbor heterogeneous grassland vegetation with an extensive network of ecotones,arranged perpendicular to topography-driven hydrologic gradients.The area suffers from severe aridification due to climate change and local anthropogenic factors,which have led to a dramatic decline of the water table.As a result,groundwater is no longer reachable for lowlying plant communities;thus,we expect they are bound to undergo profound changes.This study investigates how the plant communities respond to this changing environment over time by monitoring ecotones,since they are frequently the hotspots of ecosystem change.We monitored five ecotones along permanent belt transects for 15 years to characterize their dynamic response,and to identify the internal structural changes of the plant communities the ecotones delimit.Methods Ecotones were delineated with the split moving window technique.The dynamics of two ecotone parameters,location and contrast,were analyzed with linear regression models incorporating two independent variables:study year as a measure of time since the loss of groundwater,and precipitation as a possible driver of interannual variations.The internal changes of the patches separated by the ecotones were analyzed using plant functional groups.Important Findings Precipitation had no detectable effect on the ecotone descriptors,but study year influenced ecotones in an unusual fashion.The position of the ecotones appeared to be very stable in time;their dynamics are stationary,not directional as we predicted.The contrasts had clear tendencies;two ecotones disappeared,one new one was formed and two ecotones showed no trend.The internal changes of the patches over time were dramatic,showing a shift toward more xeric and more open plant assemblages in most stretches of the transects.Thus,the dynamic response of the vegetation was not patch expansion vs.shrinking,but fusion vs.division,which profoundly restructured the vegetation pattern.Analysis of plant functional groups revealed that the trends of the ecotone contrasts could be traced back to internal changes of the patches and not to processes within ecotones.Hence,in situations where stationary ecotone dynamics prevail,ecotone position may be a poor indicator of the effects of strong directional environmental changes.However,in this study we show that ecotone contrast can serve as a sensitive tool for monitoring landscape pattern transformations in these cases.Also,this highlights the long-term nature of ecotone responses,which can have implications in landscape planning and restoration measures.