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1. Biotic and anthropogenic forces rival climatic/abiotic factors in determining global plant population growth and fitness

   https://doi.org/10.1073/pnas.1918363117  

   Morris, William F. ; Ehrlén, Johan ; Dahlgren, Johan P. ; Loomis, Alexander K. ; Louthan, Allison M. ( January 2020 , Proceedings of the National Academy of Sciences)

   Multiple, simultaneous environmental changes, in climatic/abiotic factors, interacting species, and direct human influences, are impacting natural populations and thus biodiversity, ecosystem services, and evolutionary trajectories. Determining whether the magnitudes of the population impacts of abiotic, biotic, and anthropogenic drivers differ, accounting for their direct effects and effects mediated through other drivers, would allow us to better predict population fates and design mitigation strategies. We compiled 644 paired values of the population growth rate ( λ ) from high and low levels of an identified driver from demographic studies of terrestrial plants. Among abiotic drivers, natural disturbance (not climate), and among biotic drivers, interactions with neighboring plants had the strongest effects on λ . However, when drivers were combined into the 3 main types, their average effects on λ did not differ. For the subset of studies that measured both the average and variability of the driver, λ was marginally more sensitive to 1 SD of change in abiotic drivers relative to biotic drivers, but sensitivity to biotic drivers was still substantial. Similar impact magnitudes for abiotic/biotic/anthropogenic drivers hold for plants of different growth forms, for different latitudinal zones, and for biomes characterized by harsher or milder abiotic conditions, suggesting thatmore »all 3 drivers have equivalent impacts across a variety of contexts. Thus, the best available information about the integrated effects of drivers on all demographic rates provides no justification for ignoring drivers of any of these 3 types when projecting ecological and evolutionary responses of populations and of biodiversity to environmental changes.« less
2. Pan-Arctic seasonal cycles and long-term trends of aerosol properties from 10 observatories

   https://doi.org/10.5194/acp-22-3067-2022  

   Schmale, Julia ; Sharma, Sangeeta ; Decesari, Stefano ; Pernov, Jakob ; Massling, Andreas ; Hansson, Hans-Christen ; von Salzen, Knut ; Skov, Henrik ; Andrews, Elisabeth ; Quinn, Patricia K. ; et al ( January 2022 , Atmospheric Chemistry and Physics)

   Abstract. Even though the Arctic is remote, aerosol properties observed there arestrongly influenced by anthropogenic emissions from outside the Arctic. Thisis particularly true for the so-called Arctic haze season (January throughApril). In summer (June through September), when atmospheric transportpatterns change, and precipitation is more frequent, local Arctic sources,i.e., natural sources of aerosols and precursors, play an important role.Over the last few decades, significant reductions in anthropogenic emissionshave taken place. At the same time a large body of literature shows evidencethat the Arctic is undergoing fundamental environmental changes due toclimate forcing, leading to enhanced emissions by natural processes that mayimpact aerosol properties. In this study, we analyze 9 aerosol chemical species and 4 particleoptical properties from 10 Arctic observatories (Alert, Kevo, Pallas,Summit, Thule, Tiksi, Barrow/Utqiaġvik, Villum, and Gruvebadet and ZeppelinObservatory – both at Ny-Ålesund Research Station) to understand changesin anthropogenic and natural aerosol contributions. Variables includeequivalent black carbon, particulate sulfate, nitrate, ammonium,methanesulfonic acid, sodium, iron, calcium and potassium, as well asscattering and absorption coefficients, single scattering albedo andscattering Ångström exponent. First, annual cycles are investigated, which despite anthropogenic emissionreductions still show the Arctic haze phenomenon. Second, long-term trendsare studied using the Mann–Kendall Theil–Sen slope method. We find in total41 significant trendsmore »over full station records, i.e., spanning more than adecade, compared to 26 significant decadal trends. The majority ofsignificantly declining trends is from anthropogenic tracers and occurredduring the haze period, driven by emission changes between 1990 and 2000.For the summer period, no uniform picture of trends has emerged. Twenty-sixpercent of trends, i.e., 19 out of 73, are significant, and of those 5 arepositive and 14 are negative. Negative trends include not only anthropogenictracers such as equivalent black carbon at Kevo, but also natural indicatorssuch as methanesulfonic acid and non-sea-salt calcium at Alert. Positivetrends are observed for sulfate at Gruvebadet. No clear evidence of a significant change in the natural aerosolcontribution can be observed yet. However, testing the sensitivity of theMann–Kendall Theil–Sen method, we find that monotonic changes of around 5 % yr−1 in an aerosol property are needed to detect a significanttrend within one decade. This highlights that long-term efforts well beyonda decade are needed to capture smaller changes. It is particularly importantto understand the ongoing natural changes in the Arctic, where interannualvariability can be high, such as with forest fire emissions and theirinfluence on the aerosol population. To investigate the climate-change-induced influence on the aerosolpopulation and the resulting climate feedback, long-term observations oftracers more specific to natural sources are needed, as well as of particlemicrophysical properties such as size distributions, which can be used toidentify changes in particle populations which are not well captured bymass-oriented methods such as bulk chemical composition.« less
3. Over half of western United States' most abundant tree species in decline

   https://doi.org/10.1038/s41467-020-20678-z  

   Stanke, Hunter ; Finley, Andrew O. ; Domke, Grant M. ; Weed, Aaron S. ; MacFarlane, David W. ( January 2021 , Nature Communications)

   Changing forest disturbance regimes and climate are driving accelerated tree mortality across temperate forests. However, it remains unknown if elevated mortality has induced decline of tree populations and the ecological, economic, and social benefits they provide. Here, we develop a standardized forest demographic index and use it to quantify trends in tree population dynamics over the last two decades in the western United States. The rate and pattern of change we observe across species and tree size-distributions is alarming and often undesirable. We observe significant population decline in a majority of species examined, show decline was particularly severe, albeit size-dependent, among subalpine tree species, and provide evidence of widespread shifts in the size-structure of montane forests. Our findings offer a stark warning of changing forest composition and structure across the western US, and suggest that sustained anthropogenic and natural stress will likely result in broad-scale transformation of temperate forests globally.
4. Estimating the timescale of fluvial response to anthropogenic disturbance using two generations of dams on the South River, Massachusetts, USA

   https://doi.org/10.1002/esp.4886  

   Dow, S. ( June 2020 , Earth surface processes and landforms)

   Centuries-long intensive land-use change in the north-eastern United States provides the opportunity to study the timescale of geomorphic response to anthropogenic disturbances. In this region, forest-clearing and agricultural practices following EuroAmerican settlement led to deposition of legacy sediment along valley bottoms, including behind mill dams. The South River in western Massachusetts experienced two generations of damming, beginning with mill dams up to 6-m high in the eighteenth–nineteenth century, and followed by construction of the Conway Electric Dam (CED), a 17-m-tall hydroelectric dam near the watershed outlet in 1906. We use the mercury (Hg) concentration in upstream deposits along the South River to constrain the magnitude, source, and timing of inputs to the CED impoundment. Based on cesium-137 (137Cs) chronology and results from a sediment mixing model, remobilized legacy sediment comprised urn:x-wiley:01979337:media:esp4886:esp4886-math-0001 % of the sediment load in the South River prior to 1954; thereafter, from 1954 to 1980s, erosion from glacial deposits likely dominated (63 ± 14%), but with legacy sediments still a substantial source (37 ± 14%). We also use the CED reservoir deposits to estimate sediment yield through time, and find it decreased after 1952. These results are consistent with high rates of mobilization of legacy sediment asmore »historic dams breached in the early twentieth century, and suggest rapid initial response to channel incision, followed by a long decay in the second half of the century, that is likely dependent on large flood events to access legacy sediment stored in banks. Identifying sources of sediment in a watershed and quantifying erosion rates can help to guide river restoration practices. Our findings suggest a short fluvial recovery time from the eighteenth–nineteenth century to perturbation during the first half of the twentieth century, with subsequent return to a dominant long-term signal from erosion of glacial deposits, with anthropogenic sediment persisting as a secondary source.« less
5. Understanding recent tropospheric ozone trends in the context of large internal variability: a new perspective from chemistry-climate model ensembles

   https://doi.org/10.1088/2752-5295/ac9cc2  

   Fiore, Arlene M. ; Hancock, Sarah E. ; Lamarque, Jean-François ; Correa, Gustavo P. ; Chang, Kai-Lan ; Ru, Muye ; Cooper, Owen ; Gaudel, Audrey ; Polvani, Lorenzo M. ; Sauvage, Bastien ; et al ( November 2022 , Environmental Research: Climate)

   Observational records of meteorological and chemical variables are imprinted by an unknown combination of anthropogenic activity, natural forcings, and internal variability. With a 15-member initial-condition ensemble generated from the CESM2-WACCM6 chemistry-climate model for 1950–2014, we extract signals of anthropogenic (‘forced’) change from the noise of internally arising climate variability on observed tropospheric ozone trends. Positive trends in free tropospheric ozone measured at long-term surface observatories, by commercial aircraft, and retrieved from satellite instruments generally fall within the ensemble range. CESM2-WACCM6 tropospheric ozone trends are also bracketed by those in a larger ensemble constructed from five additional chemistry-climate models. Comparison of the multi-model ensemble with observed tropospheric column ozone trends in the northern tropics implies an underestimate in regional precursor emission growth over recent decades. Positive tropospheric ozone trends clearly emerge from 1950 to 2014, exceeding 0.2 DU yr⁻¹at 20–40 N in all CESM2-WACCM6 ensemble members. Tropospheric ozone observations are often only available for recent decades, and we show that even a two-decade record length is insufficient to eliminate the role of internal variability, which can produce regional tropospheric ozone trends oppositely signed from ensemble mean (forced) changes. By identifying regions and seasons with strong anthropogenic change signals relative tomore »internal variability, initial-condition ensembles can guide future observing systems seeking to detect anthropogenic change. For example, analysis of the CESM2-WACCM6 ensemble reveals year-round upper tropospheric ozone increases from 1995 to 2014, largest at 30 S–40 N during boreal summer. Lower tropospheric ozone increases most strongly in the winter hemisphere, and internal variability leads to trends of opposite sign (ensemble overlaps zero) north of 40 N during boreal summer. This decoupling of ozone trends in the upper and lower troposphere suggests a growing prominence for tropospheric ozone as a greenhouse gas despite regional efforts to abate warm season ground-level ozone.

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