Estimating uncertainty in the volume and carbon storage of downed coarse woody debris

Impacts of a severe storm on carbon accumulation in coarse woody debris within a secondary Atlantic Forest fragment in Brazil

The alarming increase in extreme weather events, such as severe storms with torrential rain and strong winds, is a direct result of climate change. These events have led to discernible shifts in forest structure and the carbon cycle, primarily driven by a surge in tree mortality. However, the impacts caused by these severe storms on the production and carbon increment from coarse woody debris (CWD) are still poorly understood, especially in the Brazilian Atlantic Forest. Thus, the goal proposed by the study was to quantify the CWD volume, necromass, and carbon stock before and after the occurrence of a severe storm and to determine the importance of spatial, structural, and qualitative variables of trees in the CWD carbon increment. The increase in carbon by the storm was 2.01 MgC ha-1, with a higher concentration in the CWD less decomposed and smaller diameter class. The forest fragment plots showed distinct increments (0.05-0.35 MgC), being influenced by spatial (elevation, declivity, and slope angle) structural (basal area) and qualitative factors (trunk quality and tree health), intrinsic to the forest. Thus, it is concluded that severe storms cause a large increase in carbon in CWD, making it essential to understand the susceptibility of forests to the action of intense rains and strong winds to model and monitor the future impacts of these extreme weather events on Atlantic Forest and other tropical forests in the world.

Physical and chemical properties of Coarse Woody Debris submitted to the natural process of decomposition in a Secondary Atlantic Forest Fragment in Brazil

Coarse Woody Debris (CWDs) are constantly exposed to the natural decomposition process of wood, which can lead to a change in its physical-chemical properties. However, these changes have not yet been fully elucidated, requiring further studies to help to understand the effect of this process on CWDs degradation. Thus, the objectives of this study were: (i) verify if the decomposition affects the physical-chemical properties of the CWDs; (ii) verify if the structural chemical composition of the CWDs is altered as a function of decomposition, using immediate chemical and thermogravimetric analysis. Wood samples were collected from the CWDs to carry out these analyses, considering pieces with diameters ≥ 5 cm separated into 4 decay classes. The results indicated that the average apparent density decreased as a function of the increase of CWDs decomposition (0.62-0.37 g cm^-3). The averages contents of Carbon and Nitrogen suffered less impact with the increase of CWDs decompositions, ranging from 49.66 to 48.80% and 0.52 to 0.58%, respectively. Immediate chemical and thermogravimetric analysis indicated a loss of holocelluloses and extractives and an increase in the concentration of lignin and ash throughout the decomposition process. The weight loss analyzed by thermogravimetric analysis was greater for less decomposed CWDs and with larger diameters. The use of these analyzes removes the subjectivity of CWDs decay classes, reducing the number of tests to determine CWDs physical-chemical properties and increasing the studies accuracy focused on the carbon cycle of these materials.

Predicting downed woody material carbon stocks in forests of the conterminous United States

Downed woody material (DWM) is a unique part of the forest carbon cycle serving as a pool between living biomass and subsequent atmospheric emission or transference to other forest pools. Thus, DWM is an individually defined pool in national greenhouse gas inventories. The diversity of DWM carbon drivers (e.g., decay, tree mortality, or wildfire) and associated high spatial variability make this a difficult-to-predict component of forest ecosystems. Using the now fully established nationwide inventory of DWM across the United States (US), we developed models, which substantially improved predictions of stand-level DWM carbon density relative to the current national-reporting model ('previous' model, here). The previous model was developed from published DWM carbon densities prior to the NFI DWM inventory. Those predictions were tested using NFI DWM carbon densities resulting in a poor fit to the data (coefficient of determination, or R² = 0.03). We present new random forest (RF) and stochastic gradient boosted (SGB) regression models to prediction DWM carbon density on all NFI plots and spatially on all forest land pixels. We evaluated various biotic and abiotic regression predictors, and the most important were standing dead trees, long-term annual precipitation, and long-term maximum summer temperature. A RF model scored best for expanding predictions to NFI plots (R² = 0.31), while an SGB model was identified for DWM carbon predictions based on purely spatial data (i.e., NFI-plot-independent, with R² = 0.23). The new RF model predicts conterminous US DWM carbon stocks to be 15% lower than the previous model and 2% higher than NFI data expanded according to inventory design-based inference. The new NFI data-driven models not only improve the predictions of DWM carbon density on all plots, they also provide flexibility in extending these predictions beyond the NFI to make spatially explicit and spatially continuous estimates of DWM carbon on all forest land in the US.

Carbon fractions in the world's dead wood

A key uncertainty in quantifying dead wood carbon (C) stocks—which comprise ~8% of total forest C pools globally—is a lack of accurate dead wood C fractions (CFs) that are employed to convert dead woody biomass into C. Most C estimation protocols utilize a default dead wood CF of 50%, but live tree studies suggest this value is an over-estimate. Here, we compile and analyze a global database of dead wood CFs in trees, showing that dead wood CFs average 48.5% across forests, deviating significantly from 50%, and varying systematically among biomes, taxonomic divisions, tissue types, and decay classes. Utilizing data-driven dead wood CFs in tropical forests alone may correct systematic overestimates in dead wood C stocks of ~3.0 Pg C: an estimate approaching nearly the entire dead wood C pool in the temperate forest biome. We provide for the first time, robust empirical dead wood CFs to inform global forest C estimation.

Tree mortality is increasing with climate change, which suggests that the biomass of dead wood is likely becoming more and more important to the global carbon cycle. Here, the authors perform a meta-analysis of the carbon content of dead wood and find that past estimates of total forest carbon were overestimated.

Carbon Storage Dynamics of Secondary Forest Succession in the Central Loess Plateau of China

Research Highlights: This study comprehensively revealed the carbon sequestration characteristics of secondary forests in the central Loess Plateau during vegetation succession. Background and Objectives: The secondary succession of Loess Plateau forests is of great significance in global climate change, but their carbon storage dynamics are poorly understood. The study objectives were to clarify the pattern of changes and contribution level of carbon stocks in various components of ecosystem during succession. Materials and Methods: We selected 18 plots for Pinus tabuliformis Carr. forest at the early stage of succession, 19 for pine-broadleaved mixed forest at the middle stage, and 12 for Quercus-broadleaved mixed forest at the climax stage to determine the tree, shrub, herb, fine root, litter, coarse wood debris (CWD), and soil carbon stocks. Results: Ecosystem carbon stocks increased from 160.73 to 231.14 Mg·ha−1 with the succession stages. Vegetation (including tree, shrub and herb) and soil were the two largest carbon pools, and carbon was mainly sequestrated in tree biomass and shallow soil (0–50 cm). In the early stage, soil contributed more carbon stocks to the ecosystem than vegetation, but with succession, the soil contribution decreased while vegetation contribution increased, finally reaching a balance (46.78% each) at the climax stage. Fine root, litter, and CWD contributed little (average 6.59%) to ecosystem carbon stocks and were mainly involved in the turnover of vegetation biomass to soil carbon. Conclusions: Our results provide direct evidence for carbon sequestration of secondary forests on the Loess Plateau. The dynamic results of carbon storage provide an important basis for forest restoration management under climate change.