Air-Pollution-Mediated Changes in Alpine Ecosystems and Ecotones.

Over the past decades, the Tibetan Plateau has experienced pronounced warming, yet the extent to which warming will affect alpine ecosystems depends on how warming interacts with other influential global change factors, such as nitrogen (N) deposition. A long-term warming and N manipulation experiment was established to investigate the interactive effects of warming and N deposition on alpine meadow. Open-top chambers were used to simulate warming. N addition, warming, N addition × warming, and a control were set up. In OTCs, daytime air and soil temperature were warmed by 2.0°C and 1.6°C above ambient conditions, but soil moisture was decreased by 4.95 m3 m−3. N addition enhanced ecosystem respiration (Reco); nevertheless, warming significantly decreased Reco. The decline of Reco resulting from warming was cancelled out by N addition in late growing season. Our results suggested that N addition enhanced Reco by increasing soil N availability and plant production, whereas warming decreased Reco through lowering soil moisture, soil N supply potential, and suppression of plant activity. Furthermore, season-specific responses of Reco indicated that warming and N deposition caused by future global change may have complicated influence on carbon cycles in alpine ecosystems.

<p><strong>Anthropogenic climate warming is a significant driver of global ecosystem change. High-altitude alpine habitats with low annual mean temperatures warm proportionally faster than the global average. These ecosystems contain cold-tolerant, alpine-specialist plants that are disproportionately sensitive to the impacts of climate warming. Climate warming is predicted to facilitate upward shifts in species ranges, increasing invasion pressure and disrupting species interactions. Plant functional trait responses can provide insights into the possible impacts of climate warming on species that may alter alpine communities, which are providers of valuable and unique ecosystem services. Research on plant functional trait responses to warming has focused on easily quantified morphological traits. Knowledge of plant chemical traits is less common, limiting understanding of coordinated shifts in plant function in response to drivers of global change. My thesis aimed to understand how alpine plants respond to global drivers of ecosystem change, such as warming and shifts in plant community composition, by altering their morphological traits and tissue chemistry and how this affects changes in ecosystem service provision.</strong></p><p>I measured the responses of 18 plant functional traits, including those related to leaf morphology, leaf and root tissue chemistry, aboveground biomass, and plant height, to warming and dominant species removal. Traits were measured across six species and two experiments based in Tongariro National Park, New Zealand: Warming and Removal in Mountains (WaRM) and Warming and Invasion (WIN). I hypothesised that trait responses would primarily indicate species benefit from warming and dominant species removal treatments. Plant functional trait responses were highly species-specific, with native species being the most responsive overall and often indicating a shift towards more acquisitive resource strategies, particularly at high-elevation sites. Under warming, high-alpine species Poa cita and Rytidosperma setifolium exhibited positive trait responses. The generative height of Poa cita increased under warming, and Rytidosperma setifolium demonstrated increases in leaf mass, length, and area, as well as vegetative height and aboveground biomass when the dominant species was also removed. Gaultheria colensoi had mixed responses, with decreased leaf size but increased tissue nutrients. Dracophyllum subulatum was measured in both WaRM and WIN experiments; leaf size increased under warming at the WIN site but decreased in the WaRM site, indicating higher stress levels. These trends were inversed at each site when warming and removal treatments were combined. Surprisingly, the traits of invasive species, Calluna vulgaris and the asters were largely unresponsive to treatments. Axes 1 and 2 from principal component analyses were also largely unresponsive to treatments despite individual trait responses, indicating a possible decoupling of trait correlations under warming and species removal.</p><p>Overall, my findings indicate that warming may alleviate temperature limitation to more acquisitive growth strategies in native alpine species, but that competition from invasive species may limit growth responses. These effects may be modulated by plant functional and mycorrhizal type, elevational range, site characteristics, and the length of the warming and invasion period. Alpine ecosystems will need careful management to navigate trade-offs in native plant survival and invasive species removal. An awareness of these trade-offs in future management and conservation plans may enhance outcomes for the culturally, scientifically, aesthetically, and recreationally valuable indigenous and endemic species found in Tongariro National Park.</p>

Plant production, and carbon and nitrogen source pools, are strongly intensified by experimental warming in alpine ecosystems in the Qinghai-Tibet Plateau

ABSTRACTAimDeep soils (> 30 cm) store considerable amounts of carbon and are often assumed to be less responsive to warming than topsoil. However, recent evidence indicates that deep soils are more sensitive to climate change in alpine grasslands, yet their influence on ecosystem productivity is not well understood. Here, we tested the key environmental drivers, particularly the roles of deep soil moisture and temperature, of alpine ecosystem productivity across different vegetation types and plant growth constraints.LocationThe Qinghai‐Tibetan Plateau (QTP), the Earth's Third Pole.Time Period2003–2021.Major Taxa StudiedSedge, grass and forbs.MethodsWe introduced the plant growth limitation index (GLI) to classify alpine sites as either energy‐ or water‐limited sites, using solar radiation and soil moisture as proxies for energy and water availability. We employed the random forest models to quantify dominant environmental drivers of gross primary productivity (GPP) and net ecosystem productivity (NEP) at 14 alpine sites across four vegetation types (alpine meadow, steppe, wetland and shrub) on the QTP.ResultsWe identified divergent drivers of ecosystem productivity varying with soil depth and GLI classifications. In water‐limited sites, productivity was more responsive to soil variables than to radiation, with deep soil temperature and moisture exerting greater influence than their surface counterparts. In contrast, energy‐limited sites were primarily driven by topsoil temperature and radiation, with deep soil moisture remaining more influential than topsoil moisture. In alpine wetlands, deep soil temperature emerged as the dominant driver.Main ConclusionsWe advance the concept of plant growth constraints by introducing deep soil moisture as a key regulator, demonstrating its direct controls on alpine vegetation productivity. Our findings challenge previous studies that focused solely on topsoil, offering new insights into the interactions between productivity and environmental drivers in alpine ecosystems. This improved understanding supports more accurate projections of carbon sequestration under global change.

Aims Variations in vegetation spring phenology are widely attributed to temperature in temperate and cold regions. However, temperature effect on phenology remains elusive in cold and arid/semiarid ecosystems because soil water condition also plays an important role in mediating phenology. Methods We used growing degree day (GDD) model and growing season index (GSI) model, coupling minimum temperature (Tmin) with soil moisture (SM) to explore the influence of heat requirement and hydroclimatic interaction on the start of carbon uptake period (SCUP) and net ecosystem productivity (NEP) in two alpine meadows with different precipitation regimes on the Qinghai-Tibet Plateau (QTP). One is the water-limited alpine steppe-meadow, and the other is the temperature-limited alpine shrub-meadow. Important Findings We observed two clear patterns linking GDD and GSI to SCUP: SCUP was similarly sensitive to variations in preseason GDD and GSI in the humid alpine shrub-meadow, while SCUP was more sensitive to the variability in preseason GSI than GDD in the semiarid alpine steppe-meadow. The divergent patterns indicated a balance of the limiting climatic factors between temperature and water availability. In the humid meadow, higher temperature sensitivity of SCUP could maximize thermal benefit without drought stress, as evidenced by the stronger linear correlation coefficient (R2) and Akaike’s information criterion (AIC) between observed SCUPs and those of simulated by GDD model. However, greater water sensitivity of SCUP could maximize the benefit of water in semiarid steppe-meadow, which is indicated by the stronger R2 and AIC between observed SCUPs and those of simulated by GSI model. Additionally, although SCUPs were determined by GDD in the alpine shrub-meadow ecosystem, NEP was both controlled by accumulative GSI in two alpine meadows. Our study highlights the impacts of hydroclimatic interaction on spring carbon flux phenology and vegetation productivity in the humid and semiarid alpine ecosystems. The results also suggest that water, together with temperature should be included in the models of phenology and carbon budget for alpine ecosystems in semiarid regions. These findings have important implications for improving vegetation phenology models, thus advancing our understanding of the interplay between vegetation phenology, productivity and climate change in future.

Plant nitrogen concentration is more sensitive in response to degradation than phosphorus concentration in alpine meadow

An assessment of soil microbial community environmental stress in alpine marginal ecosystems using microbial indicators

Nitrogen (N) deposition can affect the global ecosystem carbon balance. However, how plant community assembly regulates the ecosystem carbon exchange in response to the N deposition remains largely unclear, especially in alpine meadows. In this study, we conducted a manipulative experiment to examine the impacts of N (ammonium nitrate) addition on ecosystem carbon dioxide (CO2) exchange by changing the plant community assembly and soil properties at an alpine meadow site on the Qinghai–Tibetan Plateau from 2014 to 2018. The N-addition treatments were N0, N7, N20, and N40 (0, 7, 20, and 40 kg N ha–1year–1) during the plant growing season. The net ecosystem CO2 exchange (NEE), gross ecosystem productivity (GEP), and ecosystem respiration (ER) were measured by a static chamber method. Our results showed that the growing-season NEE, ER and GEP increased gradually over time with increasing N-addition rates. On average, the NEE increased significantly by 55.6 and 65.2% in N20 and N40, respectively (p < 0.05). Nitrogen addition also increased forage grass biomass (GB, including sedge and Gramineae) by 74.3 and 122.9% and forb biomass (FB) by 73.4 and 51.4% in N20 and N40, respectively (p < 0.05). There were positive correlations between CO2 fluxes (NEE and GEP) and GB (p < 0.01), and the ER was positively correlated with functional group biomass (GB and FB) and soil available N content (NO3––N and NH4+–N) (p < 0.01). The N-induced shift in the plant community assembly was primarily responsible for the increase in NEE. The increase in GB mainly contributed to the N stimulation of NEE, and FB and the soil available N content had positive effects on ER in response to N addition. Our results highlight that the plant community assembly is critical in regulating the ecosystem carbon exchange response to the N deposition in alpine ecosystems.

Although soils are often the largest sink for N in terrestrial ecosystems, litter can immobilize N during the first year of decay and could affect N retention in seasonally snow-covered ecosystems that have high N losses during snowmelt, a time when soil N retention and plant uptake can be low, and litter N retention could be high. In alpine ecosystems, the annual loss of 0.5 g N m−2 is dominated by a yearly peak in N loss during snowmelt when frozen soils and declining winter microbial populations decrease soil N retention. Here, we aimed to quantify litter N immobilization over winter and identify the potential for high phenolic accumulation in an alpine plant to increase the capacity of litter to immobilize N. A one-year litter decomposition experiment was carried out using litter of an alpine species high in phenolics, Acomastylis rossii (R. Br.) Greene, and an alpine species with similar litter chemistry but low in phenolics, Deschampsia caespitosa (L.) Beauv. We also doubled litter production to characterize the capacity of litter to immobilize N when substrate is not limiting. D. caespitosa and A. rossii litter immobilized 0.1 ± 0.1 and 0.3 ± 0.1 g N m−2, respectively, over winter. Doubling litter production led to the immobilization of 1.2 ± 0.2 g N m−2 in A. rossii litter, but did not affect immobilization in D. caespitosa litter. Higher N immobilization in A. rossii litter than in D. caespitosa litter was consistent with our expectation of phenolic control; but the lack of an order of magnitude difference in litter N immobilization at the natural rate of litter production was not consistent with the 15-fold greater concentration of phenolics in A. rossii litter. Yet, A.␣rossii litter also lost significantly more mass over winter than D. caespitosa litter. By decomposing pre-senescent litter, the difference between the species in litter decay rates was eliminated, and A. rossii tissues retained an order of magnitude more N than D. caespitosa tissues. Litter mass loss explained over 50% of the variation in litter N retention, and this relationship implies that A. rossii litter needs to lose two times more mass than D. caespitosa litter prior to the initiation of litter N loss. These results indicate that litter N immobilization under the snow contributes to ecosystem N retention in the alpine tundra and strongly suggest that high phenolic accumulation in A. rossii increases the capacity of litter to immobilize N.

Snowpatch plant community distribution and composition are strongly tied to the duration of long-lasting snow cover in alpine areas; they are vulnerable to global climatic changes that result in warmer temperatures and longer growing seasons. We used a revisitation study to quantify early-melting snowpatch floristic and functional diversity change in southern Australia, and document shrub size-class distributions over time to detect evidence for their encroachment into snowpatches, a key prediction with climatic change. Early-melting snowpatch vegetation has declined in areal extent, changed in species composition, and shrub and tussock grass cover has increased, but changes in functional trait diversity were less clear. Species gains, particularly by non-clonal species, accounted for most of the floristic change observed. Shrub recruitment was ongoing by most shrub species. Biotic differentiation is occurring; many early-melting snowpatches are transitioning to a novel state with changed composition and taller vegetation structure, but there is little evidence for species loss having occurred. Given enough time, however, the long-term loss of species is likely (i.e. biotic homogenisation) if taller shrubs outcompete short-statured snowpatch species. Our results provide evidence that this alpine ecosystem is forming a novel community with an uncertain future.

Known as the “Third Pole” and “Early-warning region” of the world, the Qinghai-Tibetan Plateau (QTP) had an intense increase in surface temperature and marked change in the amplitude of diurnal temperature (ADT), which could have significant influences on the ecosystem carbon cycle. The increasing rate of the mean daily minimum temperature (MinTa) was about two times higher than that of maximum temperature (MaxTa) in the last five decades, and this asymmetric pattern has resulted in a smaller ADT, which substantially affected the plant phenology and vegetation productivity. The reduction in the ADT caused by global climate change would have a profound impact on the carbon balance of alpine ecosystems. However, the response of carbon budgets to the ADT over the QTP remained unclear. Here, we analyzed the 15-a growing seasonal (June−September) carbon fluxes (measured by the eddy covariance [EC] technique) in alpine meadow at the southern foot of Qilian Mountains, which is one of the most extensive vegetation types on the QTP. This study aimed to clarify how carbon fluxes respond to ADT at different temporal (daily, monthly and annual) scales in alpine meadow and to understand their potential response to future climate change. The results indicated that both the MaxTa and MinTa showed bell-shaped seasonal patterns, whereas the ADT failed to show an obvious trend during the growing season from 2002 to 2016. Besides, there was a non-significant increase in annual MaxTa, MinTa and ADT ( P >0.05). Meanwhile, daily gross primary productivity (GPP) and ecosystem respiration (Re) exhibited a single-peaked trend that increased and then decreased, whereas daily net ecosystem CO2 exchange (NEE) showed a v-shaped trend. The alpine meadow ecosystem is a carbon sink during the growing season, and the annual NEE, GPP, and Re were −230.4±17.3, 668.8±25.5, 438.3±27.5 g C m–2, respectively. Moreover, the annual GPP and Re of alpine meadow in the growing season showed a significant increase trend ( P P >0.05). Annual CO2 fluxes were not related annual ADT. Only annual MaxTa exerted significant influence on variations in annual GPP and Re. Interestingly, the slopes of GPP and Re with respect to MaxTa were similar, also indicating the little impact of MaxTa on annual NEE. On a monthly scale, ADT exerted a negligible influence on CO2 fluxes ( P >0.05), but there were significant correlations between MinTa and MaxTa with CO2 fluxes. On a daily scale, there was a significant quadratic relationship between daily NEE and ADT during the whole growing season ( P P

Differences in respiration components and their dominant regulating factors across three alpine grasslands on the Qinghai−Tibet Plateau

It is the purpose of this paper to clarify relationship between forest distribution and its controlling factors such as cumulative air-temperature (degree-day), landform characteristics and snow-cover duration using remote sensing images, DEM (Digital Elevation Model) and climatological data. The outline of procedure of investigation will be described below.The spring, summer and autumn LANDSAT TM/MSS images were used to analyze seasonal change of vegetation and snow-cover. At first all images were converted into images resampled on a 56/58 m grid of DEM by using the nearest neighborhood method. Then the image data were processed in order to eliminate shadows caused only by azimuth and dip of each grid of DEM and direction of sunbeam. This processing is useful to improve the accuracy of landcover classification for a high relief mountainous region like this study area, the northernmost part of the Hida Range, Central Japan. Finally five categories ; dwarf pine, evergreen coniferous forests, deciduous broad-leaved forests, alpine meadow, bare grounds were interpreted, and a digital map of the estimated date of snow disappearance was obtained from the multi-temporal images.Air-temperature for each pixel of the DEM was calculated using daily surface air-temperature at a local AMeDAS station (Automated Meteorological Data Acquisition System) and daily lapse rate in free air at the nearest aeorological observatory. The estimated values were tested comparing the values observed at temporal observatories of mountain huts during summer season. The cumulative air-temperature is defined as an integrated excess of 5°C of daily air temperature, and it is widely admitted that it controls primarily the distribution of forest plants in rainy Japanese Islands. The other controlling factors such as snow-cover duration and slope azimuth and dip were discussed statistically.In conclusion ;(1) Dwarf pine (Pinus pumila) is distributed in gentle and westward slopes of ridge zones with the maximum occupation ratio at ca. 2, 500 m. This is replaced by alpine meadows or bare grounds in and around snow patches durable to late spring or summer.(2) Evergreen coniferous (Abies mariestii, A. veitchii, Tsuga diversifolia and Picea jesoensis var. hondoensis) forests are distributed with the maximum occupation ratio at ca. 2, 000 m. The coniferous forests will prefer a habitat which is West- to North-facing slope with inclination of less than 25. In the higher zone than 2, 000 m, these forests compete with dwarf pine or birch (Betula ermanii) forsts for occupation according to controlling factors. The coniferous plants are seemed to be snow-intolerant and easily damaged by a loss of the cumulative air-temperature due to snow-cover duration, and by mechanical and physiological injury by snow-cover itself.(3) Deciduous broad-leaved (Fagus crenata, Quercus mongolica var. crispula, Betula ermanii etc.) forests are widely distributed in this area. The Occupation ratio curve has bi-modal peaks at 2, 300 m and 1, 500 m. The lower one indicates the normal altitudinal distribution of forest zone according to thermal condition. The higher one is introduced by occupation of Betula ermanii in alpine-subalpine transitional zone. Betula ermanii is snow-tolerant and distributed in mountain slopes of all directions with the maximum frequency in South to East. The east-facing slope is leeward of the winter snow-bearing winds, but receives much snow accumulation by the blowing effect. Betula ermanii will occupay a habitat unfavorable for the coniferous mainly because of much snow accumulation.

Abstract. Litter decomposition and N release are the key processes that strongly determine the nutrient cycling at the soil–plant interface; however, how these processes are affected by grazing or grazing exclusion in the alpine grassland ecosystems on the Qinghai-Tibetan Plateau (QTP) is poorly understood. So far few studies have simultaneously investigated the influence of both litter quality and incubation site on litter decomposition and N release. Moreover, previous studies on the QTP investigating how grazing exclusion influences plant abundance and biodiversity usually lasted for many years, and the short-term effects have rarely been reported. This work studied the short-term (6 months) effects of grazing and grazing exclusion on plant community composition (i.e., plant species presented) and litter quality and long-term (27–33 months) effects on soil chemical characteristics and mixed litter decomposition and N release on the QTP. Our results demonstrate that (1) shorter-term grazing exclusion had no effect on plant community composition but increased plant palatability and total litter biomass; (2) grazing resulted in higher N and C content in litter; and (3) grazing accelerated litter decomposition, while grazing exclusion promoted N release from litter and increased soil organic carbon. In addition, incubation site had significantly more impact than litter quality on litter decomposition and N release, while litter quality affected decomposition in the early stages. This study provides insights into the mechanisms behind the nutrient cycling in alpine ecosystems. We suggest that periodic grazing and grazing exclusion is beneficial in grassland management on the QTP.

Climate variations and human activities, as two major driving forces, have profound impacts on alpine ecosystems. The Three-River Headwaters Region (TRHR) is located in the alpine region and is the source of three major rivers flowing to eastern China and Southeast Asia. Grassland is the dominant vegetation type in the TRHR and is fragile and sensitive to climate variations and human activities due to the alpine environment. Different types of grassland may have varying coping mechanisms with disturbances due to their unique environments and physiological functions. However, there is limited quantitative research on the response of different grassland types to climate variations and human activities in the TRHR. Therefore, the Carnegie–Ames–Stanford approach (CASA) was selected to simulate the net primary productivity (NPP) affected by climate (NPPC) and the actual NPP (NPPA) of steppes and meadows in the TRHR from 2001 to 2022, and the NPP affected by human activities (NPPH) was calculated by subtracting the NPPA from the NPPC. Results showed that the NPPA increased by 0.53 gC/m2/a during the study period, with the NPPA of steppes and meadows increasing by 0.55 gC/m2/a and 0.51 gC/m2/a, respectively. The regions dominated by climate variations, human activities, and the combined impact of the two accounted for 22.01%, 29.42%, and 48.57% of the NPPA changes. In terms of climate change, the impact of temperature and soil moisture on the NPP is equally important. It is worth noting that the alpine meadows (67.60%) contributed more to the increases in the NPPA than the steppes (32.40%). In addition, climate variations and human activities contributed more to the increased total NPPA of the meadows (20.54 GgC and 36.41 GgC) than that of the steppes (14.35 GgC and 10.20 GgC). The results clarify the quantitative evaluation system for the impact of human activities and climate change on different types of grasslands in the TRHR, providing guidance for the protection and management of these grasslands.