Climatic change controls productivity variation in global grasslands.

Detection and identification of the impacts of climate change on ecosystems have been core issues in climate change research in recent years. In this study, we compared average annual values of the normalized difference vegetation index (NDVI) with theoretical net primary productivity (NPP) values based on temperature and precipitation to determine the effect of historic climate change on global grassland productivity from 1982 to 2011. Comparison of trends in actual productivity (NDVI) with climate-induced potential productivity showed that the trends in average productivity in nearly 40% of global grassland areas have been significantly affected by climate change. The contribution of climate change to variability in grassland productivity was 15.2-71.2% during 1982-2011. Climate change contributed significantly to long-term trends in grassland productivity mainly in North America, central Eurasia, central Africa, and Oceania; these regions will be more sensitive to future climate change impacts. The impacts of climate change on variability in grassland productivity were greater in the Western Hemisphere than the Eastern Hemisphere. Confirmation of the observed trends requires long-term controlled experiments and multi-model ensembles to reduce uncertainties and explain mechanisms

Complex responses of spring vegetation growth to climate in a moisture-limited alpine meadow.

Since 2000, the phenology has advanced in some years and at some locations on the Qinghai-Tibetan Plateau, whereas it has been delayed in others. To understand the variations in spring vegetation growth in response to climate, we conducted both regional and experimental studies on the central Qinghai-Tibetan Plateau. We used the normalized difference vegetation index to identify correlations between climate and phenological greening, and found that greening correlated negatively with winter-spring time precipitation, but not with temperature. We used open top chambers to induce warming in an alpine meadow ecosystem from 2012 to 2014. Our results showed that in the early growing season, plant growth (represented by the net ecosystem CO2 exchange, NEE) was lower in the warmed plots than in the control plots. Late-season plant growth increased with warming relative to that under control conditions. These data suggest that the response of plant growth to warming is complex and non-intuitive in this system. Our results are consistent with the hypothesis that moisture limitation increases in early spring as temperature increases. The effects of moisture limitation on plant growth with increasing temperatures will have important ramifications for grazers in this system

Excessive plant compensatory growth: a potential endogenous driver of meadow degradation on the Qinghai-Tibetan Plateau

Degradation of meadow ecosystems in the largest alpine region of the world, i.e., the Qinghai-Tibetan Plateau (QTP), is a crucial ecological issue that has ardently discussed in recent years. Many factors, such as livestock overgrazing, climate change and overpopulation of small mammals are treated as important factors that cause the degradation of meadow ecosystems in the QTP. However, there are few hypotheses focus on the potential role of plant compensatory growth on meadow degradation. We proposed a compensatory growth-related hypothesis to understand the potential degradation process of meadow ecosystems in the QTP. We discussed that there are two stages of meadow degradation, i.e. the beginning stage of meadow degradation that is triggered by high-strength overcompensation; and the intensification stage of meadow degradation, which are driven by external factors such as climate warming, small mammals and thawing of permafrost.The mechanism of meadow degradation driven by plant compensatory growth is the asynchronism of plant consumption and the availability of soil nutrients. Our hypothesis that plant compensatory growth drives meadow degradation under the overgrazing condition requires re-examination and modification by testing the balance between soil nutrient cycling rates and the strength of plant compensatory growth in alpine regions

Sensitivity of Grassland Coverage to Climate across Environmental Gradients on the Qinghai-Tibet Plateau

Grassland cover is strongly influenced by climate change. The response of grassland cover to climate change becomes complex with background climate. There have been some advances in research on the sensitivity of grassland vegetation to climate change around the world, but the differences in climate sensitivity among grassland types are still unclear in alpine grassland. Therefore, we applied MODIS NDVI data and trend analysis methods to quantify the spatial and temporal variation of grassland vegetation cover on the Qinghai-Tibet Plateau. Then, we used multiple regression models to analyze the sensitivity of fractional vegetation cover (FVC) to climatic factors (Temperature, Precipitation, Solar radiation, Palmer drought severity index) and summarized the potential mechanisms of vegetation sensitivity to different climatic gradients. Our results showed (1) a significant increasing trend in alpine desert FVC from 2000–2018 (1.12 × 10−3/a, R2 = 0.56, p < 0.001) but no significant trend in other grassland types. (2) FVC sensitivity to climatic factors varied among grassland types, especially in the alpine desert, which had over 60% of the area with positive sensitivity to temperature, precipitation and PDSI. (3) The sensitivity of grassland FVC to heat factors decreases with rising ambient temperature while the sensitivity to moisture increases. Similarly, the sensitivity to moisture decreases while the sensitivity to thermal factors increases along the moisture gradient. Furthermore, the results suggest that future climate warming will promote grassland in cold and wet areas of the Qinghai-Tibet Plateau and may suppress vegetation in warmer areas. In contrast, the response of the alpine desert to future climate is more stable. Studying the impact of climate variation at a regional scale could enhance the adaptability of vegetation in future global climates

Identifying restoration opportunities beneath native mesquite canopies

Effective restoration strategies are needed to address habitat degradation that accompanies worldwide environmental change. One method used to enhance restoration outcomes is the leveraging of beneficial relationships (facilitation) among plants. In the southwestern United States, native mesquite trees (Prosopis spp.) are commonly planted to stabilize soil, but the value of using mesquite canopies for enhancing restoration success is unknown. We explored this possibility in an attempt to understand how common species, that both are and are not typically used for restoration, might differentially respond to mesquite canopies. We used a Bayesian multivariate generalized mixed model structure to analyze a dataset describing natural vegetation density in the Santa Rita Experimental Range, Arizona, United States. We found that more dominant species were not more likely to be distributed under mesquite. We also found that, while all of the focal species were more likely to be under mesquite with increased mesquite cover, they varied in the strength of their responses and the degree of saturation. Finally, we found that the aggressive invasive grass Eragrostis lehmanniana was found at lower incidences with increasing mesquite canopy cover, compared to the total species average as well as several of the natives investigated in this study. This work highlights the importance of being conscious of canopy size and continuity when considering understory species for restoration. This work also suggests that mesquite canopies can be used to provide a “safe site” for restoration species because competitive pressure from invasives is slightly reduced. © 2020 Society for Ecological Restoration12 month embargo; first published: 05 December 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Phenological changes offset the warming effects on biomass production in an alpine meadow on the Qinghai–Tibetan Plateau

Phenology is an important indicator of plant responses to environmental changes and is closely correlated with biomass production. However, how changes in phenological events affect plant biomass production when exposed to changing temperature and precipitation remain unclear. We conducted a 4-year manipulative experiment of warming and precipitation addition to explore phenology-biomass interactions under climate change in a dry alpine meadow on the central Qinghai-Tibetan Plateau from 2015 to 2018. In dry and warm years, warming delayed phenology and precipitation addition advanced them. Warming decreased the biomass of Kobresia pygmaea in 2018 and the biomass of Poa pratensis in 2015, 2017 and 2018. However, precipitation addition significantly increased the biomass of Poa pratensis and Potentilla multifida in most of the experimental years. Phenological changes regulated the responses of biomass to treatments. Specifically, delay of green up of P. pratensis and delay of withering of K. pygmaea induced by warming can increase biomass production, but it can be offset by the direct negative effects of warming on biomass. Synthesis. Here we show how warming-induced drought tend to decrease the biomass production of graminoids and the negative effects of warming on the biomass of P. pratensis and K. pygmaea were partially offset by green up postponement and withering postponement respectively. Our results highlights phenology is a crucial regulator for biomass production under climate change. Hence, both direct and indirect effects of warming and precipitation addition on phenology and biomass cannot be ignored when predicting biomass responses to climate change.National Natural Science Foundation of China12 month embargo; published 5 November 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Spatiotemporal Variations of Water Stable Isotope Compositions in Nujiang Headwaters, Qinghai-Tibetan Plateau

The variations of the stable isotope compositions in water provide critical information on hydroclimatic mechanisms. The climatological and hydrological processes in the Nujiang headwaters in the central Qinghai&ndash;Tibetan Plateau are extremely complex and are controlled by alternating continental/local recycled and maritime moisture. However, previous studies have only derived limited data from different types of water in the Nujiang headwaters. Therefore, aiming to understand the sources of stable oxygen (&delta;18O) and hydrogen (&delta;2H) isotopes&rsquo; compositional variability and how these are related to hydroclimatic processes, we measured &delta;18O and &delta;2H values from surface waters, snow and precipitation across the Nujiang headwaters from April to September 2018. We found higher &delta;18O (&minus;13.7&permil;), &delta;2H (&minus;101.8&permil;) and deuterium excess (d-excess; 7.6&permil;) values in the non-monsoon season and lower values in the summer monsoon season. Our findings indicated that the &delta;18O and &delta;2H compositions were significantly affected by different moisture sources in this region. The slope (6.66) and intercept (&minus;14.90) of the surface water line (SWL: &delta;2H = 6.66 &delta;18O &minus; 14.90, R2 = 0.98) were lower than those of the local meteoric water line (LMWL: &delta;2H = 9.50 &delta;18O + 41.80, R2 = 0.99) and global meteoric water line (GMWL), indicating that precipitation was the primary water vapor source for surface water, and evaporation was the dominant hydrological process for the Nujiang headwaters. In general, &delta;18O and &delta;2H tended to be negatively correlated with precipitation and air temperature. In addition, &delta;18O and &delta;2H values in the Nagqu River were inversely correlated with the intensity of discharge, highlighting a precipitation-driven isotope-discharge pattern. Our findings provide a theoretical basis for the hydroclimatic mechanisms occurring in the Nujiang headwaters and further augment our understanding of the southern&ndash;middle&ndash;northern hydroclimate in the Qinghai&ndash;Tibetan Plateau

Differential resistance and resilience of functional groups to livestock grazing maintain ecosystem stability in an alpine steppe on the Qinghai-Tibetan Plateau

Ecosystem stability is one of the main factors maintaining ecosystem functioning and is closely related to temporal variability in productivity. Resistance and resilience reflect tolerance and recovering ability, respectively, of a plant community under perturbation, which are important for maintaining the stability of ecosystems. Generally, heavy grazing reduces the stability of grassland ecosystems, causing grassland degradation. However, how livestock grazing affects ecosystem stability is unclear in alpine steppe ecosystems. We conducted a five-year grazing experiment with Tibetan sheep in a semi-arid alpine steppe on the Qinghai-Tibetan Plateau, China. The experimental treatments included no grazing (NG), light grazing (LG, 2.4 sheep per ha), moderate grazing (MG, 3.6 sheep per ha) and heavy grazing (HG, 6.0 sheep ha). We calculated resistance and resilience of three plant functional groups and ecosystem stability under the three grazing intensities using aboveground primary productivity. The results showed that with increasing grazing intensity, aboveground biomass of each functional group significantly decreased. As grazing intensity increased, the resistance of forbs first increased then decreased. The resilience of graminoids in HG was significantly lower than in LG plots, but the resilience of legumes in HG was higher than in LG and MG plots. The resilience of graminoids was significantly higher than legume and forbs under LG and MG treatments. In HG treatments, resilience of legumes was higher than graminoids and forbs. Ecosystem stability did not change under different grazing intensities, because of dissimilar performance of the resilience and resistance of functional groups. Our results highlight how the differential resistance and resilience of different function groups facilitate the tolerance of alpine steppe to grazing under even a heavy intensity. However, the degradation risk of alpine steppe under heavy grazing still needs to be considered in grassland management due to sharp decreases of productivity.National Key R&D Program of China [2016YFC0502003]; Central Public-interest Scientific Institution Basal Research Fund [BSRF201713]24 month embargo; published online: 26 September 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Warming and precipitation addition interact to affect plant spring phenology in alpine meadows on the central Qinghai-Tibetan Plateau

Temperature and precipitation are primary regulators of plant phenology. However, our knowledge of how these factors might interact to affect plant phenology is incomplete. The Qinghai-Tibetan Plateau, a cold and high region, has experienced no consistent changes in spring phenology, despite a significant warming trend. We conducted a manipulative experiment of warming and precipitation addition in an alpine meadow on the Qinghai-Tibetan Plateau in 2015 (cold and wet), 2016 (warm and dry) and 2017 (mild and very wet). We found that warming increased annual variability of plant spring phenology. Warming delayed green up of all monitored species in 2016, advanced green up of early flowering species in 2015, and did not alter green up in 2017. For example, green up of the shallow rooted Kobresia pygmaea advanced 8 (+/- 2) days in 2015 and was delayed by 23 (+/- 3) days in a dry year (2016) under warming compared with control. Early spring precipitation addition can offset the delaying effects of warming in a dry year on the Qinghai-Tibetan Plateau. Under warming plus precipitation addition, community average green up advanced compared to control plots in 2015 and 2016, and community average flowering advanced for all three years. In 2016, flowering of K. pygmaea (an early flowering species) advanced under warming plus precipitation addition compared to control while flowering of other species did not change. Our results highlight that annual variation of soil moisture condition plays a critical role in determining the magnitude and direction of spring phenology response to warming. We provide insights in how plant spring phenology might change in a warmer future in the presence or absence of precipitation increase.National Natural Science Foundation of China24 month embargo; published online: 21 February 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]