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1. Response of Natural Vegetation to Climate in Dryland Ecosystems: A Comparative Study between Xinjiang and Arizona

   https://doi.org/10.3390/rs12213567  

   Zhang, Fang ; Wang, Chenghao ; Wang, Zhi-Hua ( November 2020 , Remote Sensing)

   As one of the most sensitive areas to climate change, drylands cover ~40% of the Earth’s terrestrial land surface and host more than 38% of the global population. Meanwhile, their response to climate change and variability carries large uncertainties as induced by background climate, topography, and land cover composition; but there is a lack of intercomparison of different dryland ecosystems. In this study, we compare the changing climate and corresponding responses of major natural vegetation cover types in Xinjiang and Arizona, two typical drylands with similar landscapes in Asia and North America. Long-term (2002–2019) quasi-8-day datasets of daily precipitation, daily mean temperature, and Normalized Difference Vegetation Index (NDVI) were constructed based on station observations and remote sensing products. We found that much of Xinjiang experienced warming and wetting trends (although not co-located) over the past 18 years. In contrast, Arizona was dominated by warming with insignificant wetting or drying trends. Significant greening trends were observed in most parts of both study areas, while the increasing rate of NDVI anomalies was relatively higher in Xinjiang, jointly contributed by its colder and drier conditions. Significant degradation of vegetation growth (especially for shrubland) was observed over 18.8% of Arizona due to warming. Ourmore »results suggest that responses of similar natural vegetation types under changing climate can be diversified, as controlled by temperature and moisture in areas with different aridity.« less
2. China’s terrestrial ecosystem carbon balance during the 20th century: an analysis with a process-based biogeochemistry model

   https://doi.org/10.1186/s13021-022-00215-9  

   Lu, Yanyu ; Huang, Yao ; Zhuang, Qianlai ; Sun, Wei ; Chen, Shutao ; Lu, Jun ( October 2022 , Carbon Balance and Management)

   China’s terrestrial ecosystems play a pronounced role in the global carbon cycle. Here we combine spatially-explicit information on vegetation, soil, topography, climate and land use change with a process-based biogeochemistry model to quantify the responses of terrestrial carbon cycle in China during the 20th century.

   At a century scale, China’s terrestrial ecosystems have acted as a carbon sink averaging at 96 Tg C yr^− 1, with large inter-annual and decadal variabilities. The regional sink has been enhanced due to the rising temperature and CO₂concentration, with a slight increase trend in carbon sink strength along with the enhanced net primary production in the century. The areas characterized by C source are simulated to extend in the west and north of the Hu Huanyong line, while the eastern and southern regions increase their area and intensity of C sink, particularly in the late 20th century. Forest ecosystems dominate the C sink in China and are responsible for about 64% of the total sink. On the century scale, the increase in carbon sinks in China’s terrestrial ecosystems is mainly contributed by rising CO₂. Afforestation and reforestation promote an increase in terrestrial carbon uptake in China from 1950s. Although climate change has generally contributedmore »to the increase of carbon sinks in terrestrial ecosystems in China, the positive effect of climate change has been diminishing in the last decades of the 20th century.

   This study focuses on the impacts of climate, CO₂and land use change on the carbon cycle, and presents the potential trends of terrestrial ecosystem carbon balance in China at a century scale. While a slight increase in carbon sink strength benefits from the enhanced vegetation carbon uptake in China’s terrestrial ecosystems during the 20th century, the increase trend may diminish or even change to a decrease trend under future climate change.

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3. Rapid increases in shrubland and forest intrinsic water-use efficiency during an ongoing megadrought

   https://doi.org/10.1073/pnas.2118052118  

   Kannenberg, Steven A. ; Driscoll, Avery W. ; Szejner, Paul ; Anderegg, William R. ; Ehleringer, James R. ( December 2021 , Proceedings of the National Academy of Sciences)

   Globally, intrinsic water-use efficiency (iWUE) has risen dramatically over the past century in concert with increases in atmospheric CO 2 concentration. This increase could be further accelerated by long-term drought events, such as the ongoing multidecadal “megadrought” in the American Southwest. However, direct measurements of iWUE in this region are rare and largely constrained to trees, which may bias estimates of iWUE trends toward more mesic, high elevation areas and neglect the responses of other key plant functional types such as shrubs that are dominant across much of the region. Here, we found evidence that iWUE is increasing in the Southwest at one of the fastest rates documented due to the recent drying trend. These increases were particularly large across three common shrub species, which had a greater iWUE sensitivity to aridity than Pinus ponderosa , a common tree species in the western United States. The sensitivity of both shrub and tree iWUE to variability in atmospheric aridity exceeded their sensitivity to increasing atmospheric [CO 2 ]. The shift to more water-efficient vegetation would be, all else being equal, a net positive for plant health. However, ongoing trends toward lower plant density, diminished growth, and increasing vegetation mortality across themore »Southwest indicate that this increase in iWUE is unlikely to offset the negative impacts of aridification.« less
4. Potential shifts in the aboveground biomass and physiognomy of a seasonally dry tropical forest in a changing climate

   https://doi.org/10.1088/1748-9326/ab7394  

   Castanho, Andrea D. A. ; Coe, Michael T. ; Brando, Paulo ; Macedo, Marcia ; Baccini, Alessandro ; Walker, Wayne ; Andrade, Eunice M. ( March 2020 , Environmental Research Letters)

   Seasonally dry tropical forests (SDTFs) account for one-third of the interannual variability of global net primary productive (NPP). Large-scale shifts in dry tropical forest structure may thus significantly affect global CO₂fluxes in ways that are not fully accounted for in current projections. This study quantifies how changing climate might reshape one of the largest SDTFs in the world, the Caatinga region of northeast Brazil. We combine historical data and future climate projections under different representative concentration pathways (RCPs), together with spatially explicit aboveground biomass estimates to establish relationships between climate and vegetation distribution. We find that physiognomies, aboveground biomass, and climate are closely related in the Caatinga—and that the region’s bioclimatic envelope is shifting rapidly. From 2008–2017, more than 90% of the region has shifted to a dryer climate space compared to the reference period 1950–1979. An ensemble of global climate models (based on IPCC AR5) indicates that by the end of the 21st century the driest Caatinga physiognomies (thorn woodlands to non-vegetated areas) could expand from 55% to 78% (RCP 2.6) or as much as 87% (RCP8.5) of the region. Those changes would correspond to a decrease of 30%–50% of the equilibrium aboveground biomass by the end ofmore »the century (RCP 2.6 and RCP8.5, respectively). Our results are consistent with historic vegetation shifts reported for other SDTFs. Projected changes for the Caatinga would have large-scale impacts on the region’s biomass and biodiversity, underscoring the importance of SDTFs for the global carbon budget. Understanding such changes as presented in this study will be useful for regional planning and could help mitigate their negative social impacts.

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5. Expanded lacustrine sedimentation in the Qaidam Basin on the northern Tibetan Plateau: Manifestation of climatic wetting during the Oligocene icehouse

   https://doi.org/https://doi.org/10.1016/j.epsl.2021.116935  

   Wu, M. ; Zhuang, G. S. ; Hou, M. ; and Liu, Z. ( January 2021 , Earth and planetary science letters)

   Bendick, R. (Ed.)

   The Qaidam Basin in the core area of arid Inner Asia has been considered undergoing continuous aridification over the Cenozoic. However, the Qaidam Basin is marked with expanded lacustrine sedimentation during the Oligocene, which contrasts with the fluvial or deltaic facies stratigraphically below (Eocene) and above (Miocene-present). The Oligocene lacustrine expansion challenges the idea of persistent aridification. To solve the conundrum, we reconstruct a long-term compound-specific hydrogen isotope (δ2H) record from sedimentary leaf wax n-alkanes to evaluate the paleoclimatic context before, during, and after the Oligocene lacustrine expansion. The δ2H results reveal three shifts at ca. 40 Ma, 34 Ma, and 24 Ma. The leaf wax δ2H values range from −176.8to −166.7from 51 to 40 Ma, followed by an abrupt increase of 23.9at 40 Ma. We interpret this rapid increase as enhanced aridification due to the coeval retreat of the Paratethys Sea from the region. At 34 Ma, the δ2H plunges across the Eocene-Oligocene transition (EOT). Post-EOT δ2H values are the lowest, vary with high amplitude from −187.1to −153.2, and are associated with the lacustrine facies expansion, indicating a wetter climate. By compiling the regional isotopic proxy studies, we observe the contrasting patterns in paleohydrology conditions since the EOT: themore »relaxation of aridity in the westerlies region versus the enhanced aridification in the East Asian summer monsoon region. We interpret that the west-east contrasting patterns represent the different climatic responses to global cooling: wetting in the west as a result of the enhanced moisture transport via westerlies replacing the subtropical high, and drying in the east due to the reduction in moisture content associated with weakening East Asian summer monsoon. Wetting in Inner Asia is synchronous with cooling in the ocean (North Atlantic) and on land (Xining Basin). Since 24 Ma, δ2H increases in response to warming during the latest Oligocene to the early Miocene when the subtropical high re-occupied Inner Asia, causing the aridity. This study reveals a dynamic climate in Inner Asia with different mechanisms responding to global change.« less