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1. Restoration and resilience to sea level rise of a salt marsh affected by dieback events

   https://doi.org/10.1002/ecs2.4467  

   Rolando, J. L. ; Hodges, M. ; Garcia, K. D. ; Krueger, G. ; Williams, N. ; Carr, Jr., J. ; Robinson, J. ; George, A. ; Morris, J. ; Kostka, J. E. ( April 2023 , Ecosphere)

   The frequency of salt marsh dieback events has increased over the last 25 years with unknown consequences to the resilience of the ecosystem to accelerated sea level rise (SLR). Salt marsh ecosystems impacted by sudden vegetation dieback events were previously thought to recover naturally within a few months to years. In this study, we used a 13‐year collection of remotely sensed imagery to provide evidence that approximately 14% of total marsh area has not revegetated 10 years after a dieback event in Charleston, SC. Dieback onset coincided with a severe drought in 2012, as indicated by the Palmer drought stress index. A second dieback event occurred in 2016 after a historic flood influenced by Hurricane Joaquin in 2015. Unvegetated zones reached nearly 30% of the total marsh area in 2017. We used a light detection and ranging‐derived digital elevation model to determine that most affected areas were associated with lower elevation zones in the interior of the marsh. Further, restoration by grass planting was effective, with pilot‐scale restored plots having greater aboveground biomass than reference sites after two years of transplanting. A positive outcome indicated that the stressors that caused the dieback are no longer present. Despite that, many affected areas have not recovered naturally, even though they are located within the typical elevation range of healthy marshes. A mechanistic modeling approach was used to assess the effects of vegetation dieback on salt marsh resilience to SLR. Predictions indicate that a highly productive restored marsh (2000 g m⁻² year⁻¹) would persist at a moderate SLR rate of 60 cm in 100 years, whereas a nonrestored mudflat would lose all its elevation capital after 100 years. Thus, rapid restoration of marsh dieback is critical to avoid further degradation. Also, failure to incorporate the increasing frequency and intensity of extreme climatic events that trigger irreversible marsh diebacks underestimates salt marsh vulnerability to climate change. Finally, at an elevated SLR rate of 122 cm in 100 years, which is most likely an extreme climate change scenario, even highly productive ecosystems augmented by sediment placement would not keep pace with SLR. Thus, climate change mitigation actions are also urgently needed to preserve present‐day marsh ecosystems.

    

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2. Arctic greening associated with lengthening growing seasons in Northern Alaska

   https://doi.org/10.1088/1748-9326/ab5e26  

   Arndt, Kyle A. ; Santos, Maria J. ; Ustin, Susan ; Davidson, Scott J. ; Stow, Doug ; Oechel, Walter C. ; Tran, Thao T. P. ; Graybill, Brian ; Zona, Donatella ( December 2019 , Environmental Research Letters)

   Many studies have reported that the Arctic is greening; however, we lack an understanding of the detailed patterns and processes that are leading to this observed greening. The normalized difference vegetation index (NDVI) is used to quantify greening, which has had largely positive trends over the last few decades using low spatial resolution satellite imagery such as AVHRR or MODIS over the pan-Arctic region. However, substantial fine scale spatial heterogeneity in the Arctic makes this large-scale investigation hard to interpret in terms of vegetation and other environmental changes. Here we focus on one area of the northern Alaskan Arctic using high spatial resolution (4 m) multispectral satellite imagery from DigitalGlobe^™to analyze the greening trend near Utqiaġvik (formerly known as Barrow) over 14 years from 2002 to 2016. We found that tundra vegetation has been greening (τ = 0.65,p = 0.01, NDVI increase of 0.01 yr⁻¹) despite no overall change in vegetation community composition. The greening is most closely correlated to the number of thawing degree days (R² = 0.77,F = 21.5,p < 0.001) which increased in a similar linear trend over the 14 year study period (1.79 ± 0.50 days per year,p < 0.01,τ = −0.56). This suggests that in this Arctic ecosystem, greening is occurring due to a lengthening growing season that appears to stimulate plant productivity without any significant change in vegetation communities. We found that vegetation communities in wetter locations greened about twice as fast as those found in drier conditions supporting the hypothesis that these communities respond more strongly to warming. We suggest that in Arctic environments, vegetation productivity may continue to rise, particularly in wet areas.

    

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3. Long-Term Community Dynamics Reveal Different Trajectories for Two Mid-Atlantic Maritime Forests

   https://doi.org/10.3390/f12081063  

   Woods, Natasha N. ; Tuley, Philip A. ; Zinnert, Julie C. ( August 2021 , Forests)

   null (Ed.)

   Maritime forests are threatened by sea-level rise, storm surge and encroachment of salt-tolerant species. On barrier islands, these forested communities must withstand the full force of tropical storms, hurricanes and nor’easters while the impact is reduced for mainland forests protected by barrier islands. Geographic position may account for differences in maritime forest resilience to disturbance. In this study, we quantify two geographically distinct maritime forests protected by dunes on Virginia’s Eastern Shore (i.e., mainland and barrier island) at two time points (15 and 21 years apart, respectively) to determine whether the trajectory is successional or presenting evidence of disassembly with sea-level rise and storm exposure. We hypothesize that due to position on the landscape, forest disassembly will be higher on the barrier island than mainland as evidenced by reduction in tree basal area and decreased species richness. Rate of relative sea-level rise in the region was 5.9 ± 0.7 mm yr−1 based on monthly mean sea-level data from 1975 to 2017. Savage Neck Dunes Natural Area Preserve maritime forest was surveyed using the point quarter method in 2003 and 2018. Parramore Island maritime forest was surveyed in 1997 using 32 m diameter circular plots. As the island has been eroding over the past two decades, 2016 Landsat imagery was used to identify remaining forested plots prior to resurveying. In 2018, only plots that remained forested were resurveyed. Lidar was used to quantify elevation of each point/plot surveyed in 2018. Plot elevation at Savage Neck was 1.93 ± 0.02 m above sea level, whereas at Parramore Island, elevation was lower at 1.04 ± 0.08 m. Mainland dominant species, Acer rubrum, Pinus taeda, and Liquidambar styraciflua, remained dominant over the study period, with a 14% reduction in the total number of individuals recorded. Basal area increased by 11%. Conversely, on Parramore Island, 33% of the former forested plots converted to grassland and 33% were lost to erosion and occur as ghost forest on the shore or were lost to the ocean. Of the remaining forested plots surveyed in 2018, dominance switched from Persea palustris and Juniperus virginiana to the shrub Morella cerifera. Only 46% of trees/shrubs remained and basal area was reduced by 84%. Shrub basal area accounted for 66% of the total recorded in 2018. There are alternative paths to maritime forest trajectory which differ for barrier island and mainland. Geographic position relative to disturbance and elevation likely explain the changes in forest community composition over the timeframes studied. Protected mainland forest at Savage Neck occurs at higher mean elevation and indicates natural succession to larger and fewer individuals, with little change in mixed hardwood-pine dominance. The fronting barrier island maritime forest on Parramore Island has undergone rapid change in 21 years, with complete loss of forested communities to ocean or conversion to mesic grassland. Of the forests remaining, dominant evergreen trees are now being replaced with the expanding evergreen shrub, Morella cerifera. Loss of biomass and basal area has been documented in other low elevation coastal forests. Our results indicate that an intermediate shrub state may precede complete loss of woody communities in some coastal communities, providing an alternative mechanism of resilience. 

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4. Climate Change Driving Widespread Loss of Coastal Forested Wetlands Throughout the North American Coastal Plain

   https://doi.org/10.1007/s10021-021-00686-w  

   White, Elliott E. ; Ury, Emily A. ; Bernhardt, Emily S. ; Yang, Xi ( August 2021 , Ecosystems)

   Coastal forested wetlands support many endemic species, sequester substantial carbon stocks, and have been reduced in extent due to historic drainage and agricultural expansion. Many of these unique coastal ecosystems have been drained, while those that remain are now threatened by saltwater intrusion and sea level rise in hydrologically modified coastal landscapes. Several recent studies have documented rapid and accelerating losses of coastal forested wetlands in small areas of the Atlantic and Gulf coasts of North America, but the full extent of loss across North America’s Coastal Plain (NACP) has not been quantified. We used classified satellite imagery to document a net loss of  13,682 km2 (8%) of forested coastal wetlands across the NACP between 1996 and 2016. Most forests transitioned to scrub-shrub (53%) and marsh habitats (24%). Even within protected areas, we measured substantial rates of wetland deforestation and significant fragmentation of forested wetland habitats. Variation in the rate of sea level rise, the number of tropical storm landings, and the average elevation of coastal watersheds explained about 78% of the variation in coastal wetland deforestation extent along the south Atlantic and Gulf Coasts. The rate of coastal forest loss within the NACP (684 km2/y) exceeds the recent estimate of global losses of coastal mangroves (210 km2/y). At the current rate of deforestation, in the absence of widespread protection or restoration efforts, coastal forested wetlands may not persist into the next century. 

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5. Siberian taiga and tundra fire regimes from 2001–2020

   https://doi.org/10.1088/1748-9326/ac3f07  

   Talucci, Anna C. ; Loranty, Michael M. ; Alexander, Heather D. ( January 2022 , Environmental Research Letters)

   Circum-boreal and -tundra systems are crucial carbon pools that are experiencing amplified warming and are at risk of increasing wildfire activity. Changes in wildfire activity have broad implications for vegetation dynamics, underlying permafrost soils, and ultimately, carbon cycling. However, understanding wildfire effects on biophysical processes across eastern Siberian taiga and tundra remains challenging because of the lack of an easily accessible annual fire perimeter database and underestimation of area burned by MODIS satellite imagery. To better understand wildfire dynamics over the last 20 years in this region, we mapped area burned, generated a fire perimeter database, and characterized fire regimes across eight ecozones spanning 7.8 million km²of eastern Siberian taiga and tundra from ∼61–72.5° N and 100° E–176° W using long-term satellite data from Landsat, processed via Google Earth Engine. We generated composite images for the annual growing season (May–September), which allowed mitigation of missing data from snow-cover, cloud-cover, and the Landsat 7 scan line error. We used annual composites to calculate the difference Normalized Burn Ratio (dNBR) for each year. The annual dNBR images were converted to binary burned or unburned imagery that was used to vectorize fire perimeters. We mapped 22 091 fires burning 152 million hectares (Mha) over 20 years. Although 2003 was the largest fire year on record, 2020 was an exceptional fire year for four of the northeastern ecozones resulting in substantial increases in fire activity above the Arctic Circle. Increases in fire extent, severity, and frequency with continued climate warming will impact vegetation and permafrost dynamics with increased likelihood of irreversible permafrost thaw that leads to increased carbon release and/or conversion of forest to shrublands.

    

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