Mary Farina

Permafrost thaw is transforming boreal forests into mosaics of wetlands and drier uplands. Topographic controls on hydrological and ecological conditions impact methane (CH₄) fluxes, contributing to uncertainty in local and regional CH₄ budgets and underlying drivers. The objective of this study was to explore CH₄ fluxes and their drivers in a transitioning boreal forest-fen ecosystem (Goldstream Valley, Alaska, USA). This landscape is characterized by thawing discontinuous permafrost and heterogeneous mosaics of fens, collapse-scar channels, and small mounds of permafrost soils. From a survey in July 2021, observed chamber CH₄ fluxes included fen areas with intermediate to very high emissions (29.8–635.3 mg CH₄ m⁻² d⁻¹), clustered locations with CH₄ uptake (−2.11 to −0.7 mg CH₄ m⁻² d⁻¹), and three anomalous emission hotspots (342.4–772.4 mg CH₄ m⁻² d⁻¹) that were located near samples with lower emissions. Some surface and near-surface variables partially explained the spatial variation in CH₄ flux. Log-transformed CH₄ flux had a positive linear relationship with soil moisture at 20 cm depth (R ² = 0.31, p-value < 1e-5) and negative linear relationships with microtopography (R ² = 0.13, p-value < 0.006) and slope (R ² = 0.28, p-value < 2e-5). Methane emissions generally occurred in flat, wet, graminoid-dominated fens, whereas CH₄ uptake occurred on permafrost mounds dominated by feather mosses and woody vegetation. However, the CH₄ hotspots occurred on drier, slightly sloped locations with low or undetectable near-surface methanogen abundance, suggesting that CH₄ was produced in deeper soils. When the hotspot samples were omitted, log-transformed CH₄ flux had a positive linear relationship with near-surface methanogen abundance (R ² = 0.29, p-value = 0.0023), and stronger linear relationships with soil moisture, slope, and soil macronutrient concentrations. Our findings suggest that some CH₄ emission hotspots could arise from CH₄ in deep taliks. The inference that methanogenesis occurs in deep taliks was strengthened by the identification of intrapermafrost taliks across the study area using low-frequency geophysical induction. This study assesses surface spatial heterogeneity in the context of subsurface permafrost conditions and highlights the complexity of CH₄ flux patterns in transitioning forest-wetland ecosystems. To better inform regional CH₄ budgets, further research is needed to understand the spatial distribution of terrestrial CH₄ hotspots and to resolve their surface, near-surface, and subsurface drivers.

This work is focused on characterizing and understanding the aboveground biomass of Caatinga in a semiarid region in northeastern Brazil. The quantification of Caatinga biomass is limited by the small number of field plots, which are inadequate for addressing the biome's extreme heterogeneity. Satellite-derived biomass products can address spatial and temporal changes but they have not been validated for seasonally dry tropical forests. Here we combine a compilation of published field phytosociological observations with a new 30m spatial resolution satellite biomass product. Both data were significantly correlated, satellite estimates consistently captured the wide variability of the biomass across the different physiognomies (2-272 Mg/ha). Based on the satellite product we show that in year 2000 about 50 percent of the region had very low biomass (<2 Mg/ha) and that the majority of the biomass (86%) is concentrated in only 27% of the area. Our work confirm other estimates of biomass 39 Mg/ha (9-61 Mg/ha) and carbon 0.79 PgC. The satellite products together with ground based estimates has the potential to improve forest management in Caatinga and other seasonally dry tropical forests through improved approximation of spatial variability, how they relate to climate, and support numerical modeling experiments in semiarid regions.

The Hansen et al . critique centers on the lack of spatial agreement between two very different datasets. Nonetheless, properly constructed comparisons designed to reconcile the two datasets yield up to 90% agreement (e.g., in South America).

Forests out of balance: Are tropical forests a net source or net sink of atmospheric carbon dioxide? As fundamental a question as that is, there still is no agreement about the answer, with different studies suggesting that it is anything from a sizable sink to a modest source. Baccini et al. used 12 years of MODIS satellite data to determine how the aboveground carbon density of woody, live vegetation has changed throughout the entire tropics on an annual basis. They find that the tropics are a net carbon source, with losses owing to deforestation and reductions in carbon density within standing forests being double that of gains resulting from forest growth. Science , this issue p. 230