Accounting for density reduction and structural loss in standing dead trees: Implications for forest biomass and carbon stock estimates in the United States

Practical Guide to Measuring Wetland Carbon Pools and Fluxes

Wetlands cover a small portion of the world, but have disproportionate influence on global carbon (C) sequestration, carbon dioxide and methane emissions, and aquatic C fluxes. However, the underlying biogeochemical processes that affect wetland C pools and fluxes are complex and dynamic, making measurements of wetland C challenging. Over decades of research, many observational, experimental, and analytical approaches have been developed to understand and quantify pools and fluxes of wetland C. Sampling approaches range in their representation of wetland C from short to long timeframes and local to landscape spatial scales. This review summarizes common and cutting-edge methodological approaches for quantifying wetland C pools and fluxes. We first define each of the major C pools and fluxes and provide rationale for their importance to wetland C dynamics. For each approach, we clarify what component of wetland C is measured and its spatial and temporal representativeness and constraints. We describe practical considerations for each approach, such as where and when an approach is typically used, who can conduct the measurements (expertise, training requirements), and how approaches are conducted, including considerations on equipment complexity and costs. Finally, we review key covariates and ancillary measurements that enhance the interpretation of findings and facilitate model development. The protocols that we describe to measure soil, water, vegetation, and gases are also relevant for related disciplines such as ecology. Improved quality and consistency of data collection and reporting across studies will help reduce global uncertainties and develop management strategies to use wetlands as nature-based climate solutions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13157-023-01722-2.

Comparison between Empirical Models and the CBM-CFS3 Carbon Budget Model to Predict Carbon Stocks and Yields in Nova Scotia Forests

In response to the global climate crisis, the Nova Scotia Department of Lands and Forestry is using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) and associated methodologies to assess the carbon dynamics of the provincial forestry sector. The CBM-CFS3 bases simulations on a range of studies and national forest inventory plots to predict carbon dynamics using merchantable volume yield curves. Nova Scotia has also maintained thousands of permanent forest sample plots (PSPs) for decades, offering the opportunity to develop empirical, province-specific carbon models. This study used PSP tree measurements and allometric equations to compute plot-level forest carbon models from the PSP dataset and compared their output to that of the CBM-CFS3 model. The PSP-based models were stratified into five forest types and predict the carbon for seven carbon pools as a function of the plot age. Predictions with the PSP- and CBM-CFS3 models were compared to observed PSP data at the plot level and compared against each other at the stand and landscape level. At the plot level, the PSP-derived models predicted carbon closer to the observed data than the CBM-CFS3 model, the extent of over- or under-estimation depending on the carbon pool and forest type. At the stand scale, the CBM-CFS3 model predicted forest carbon to within 3.1–17.6% of the PSP method on average. Differences in predictions between the CBM-CFS3 and PSP models decreased to within 2.4% of the PSP-based models at the landscape level. Thus, the implications of using one method over the other decrease as the prediction scale increases from stand to landscape level, and the implications fluctuate as a function of the forest type and age.

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 stock densities of semi-deciduous Atlantic forest and pine plantations in Argentina

Recent studies have shown the importance of subtropical forests as terrestrial carbon sinks and also their vulnerability to human disturbances and climate change. The Semi-deciduous Atlantic Forest presents large extensions replaced by productive uses, such as tree plantations, and forest remnants showing high levels of structural heterogeneity. No studies have performed carbon stock densities estimations in different pools in the region. We wonder how changes in forest structure and forest replacement by pine plantations affect ecosystem carbon stock densities in different pools and fluxes. We performed carbon estimates based on field data and compared closed (CF) and open (OF) canopy natural forest patches and Pinus taeda plantations at harvest age (PP). Structural changes in the natural forest had a profound effect on the ecosystem by halving the forest carbon stock while pulp-intended pine plantations reached the carbon stock of closed forest at harvest age. Main changes from CF to OF were a 55% decrease in the carbon of biomass and a 42% decrease in SOC. Instead, carbon stock density in biomass of PP was similar to CF but the carbon in fallen deadwood was 78% lower while in the litter layer was double; the SOC at 0-5 cm depth was 31% lower in PP than CF. Our study shows that structural changes in the natural forest halve the forest carbon stock while pulp-intended pine plantations can reach the closed forest carbon stock at harvest age. However, PP do not seem to be effective for carbon storage in the long term because of regular harvesting and clearing and their short-life products. Therefore, to effectively store the forest carbon, arresting deforestation, replacement and degradation of the original forest is crucial.

Increases in soil and woody biomass carbon stocks as a result of rangeland riparian restoration

BACKGROUND

Globally, vegetation in riparian zones is frequently the target of restoration efforts because of its importance in reducing the input of eroded sediment and agricultural nutrient runoff to surface waters. Here we examine the potential of riparian zone restoration to enhance carbon sequestration. We measured soil and woody biomass carbon stocks, as well as soil carbon properties, in a long-term chronosequence of 42 streambank revegetation projects in northern California rangelands, varying in restoration age from 1 to 45 years old.

RESULTS

Where revegetation was successful, we found that soil carbon measured to 50 cm depth increased at a rate of 0.87 Mg C ha^-1 year^-1 on the floodplain and 1.12 Mg C ha^-1 year^-1 on the upper bank landform. Restored sites also exhibited trends toward increased soil carbon permanence, including an increased C:N ratio and lower fulvic acid: humic acid ratio. Tree and shrub carbon in restored sites was modeled to achieve a 50-year maximum of 187.5 Mg C ha^-1 in the channel, 279.3 Mg ha^-1 in the floodplain, and 238.66 Mg ha^-1 on the upper bank. After 20 years of restoration, the value of this carbon at current per-ton C prices would amount to $US 15,000 per km of restored stream.

CONCLUSION

We conclude that revegetating rangeland streambanks for erosion control has a substantial additional benefit of mitigating global climate change, and should be considered in carbon accounting and any associated financial compensation mechanisms.

Airborne and Terrestrial Laser Scanning Data for the Assessment of Standing and Lying Deadwood: Current Situation and New Perspectives

LiDAR technology is finding uses in the forest sector, not only for surveys in producing forests but also as a tool to gain a deeper understanding of the importance of the three-dimensional component of forest environments. Developments of platforms and sensors in the last decades have highlighted the capacity of this technology to catch relevant details, even at finer scales. This drives its usage towards more ecological topics and applications for forest management. In recent years, nature protection policies have been focusing on deadwood as a key element for the health of forest ecosystems and wide-scale assessments are necessary for the planning process on a landscape scale. Initial studies showed promising results in the identification of bigger deadwood components (e.g., snags, logs, stumps), employing data not specifically collected for the purpose. Nevertheless, many efforts should still be made to transfer the available methodologies to an operational level. Newly available platforms (e.g., Mobile Laser Scanner) and sensors (e.g., Multispectral Laser Scanner) might provide new opportunities for this field of study in the near future.

From models to measurements: comparing downed dead wood carbon stock estimates in the U.S. forest inventory

The inventory and monitoring of coarse woody debris (CWD) carbon (C) stocks is an essential component of any comprehensive National Greenhouse Gas Inventory (NGHGI). Due to the expense and difficulty associated with conducting field inventories of CWD pools, CWD C stocks are often modeled as a function of more commonly measured stand attributes such as live tree C density. In order to assess potential benefits of adopting a field-based inventory of CWD C stocks in lieu of the current model-based approach, a national inventory of downed dead wood C across the U.S. was compared to estimates calculated from models associated with the U.S.’s NGHGI and used in the USDA Forest Service, Forest Inventory and Analysis program. The model-based population estimate of C stocks for CWD (i.e., pieces and slash piles) in the conterminous U.S. was 9 percent (145.1 Tg) greater than the field-based estimate. The relatively small absolute difference was driven by contrasting results for each CWD component. The model-based population estimate of C stocks from CWD pieces was 17 percent (230.3 Tg) greater than the field-based estimate, while the model-based estimate of C stocks from CWD slash piles was 27 percent (85.2 Tg) smaller than the field-based estimate. In general, models overestimated the C density per-unit-area from slash piles early in stand development and underestimated the C density from CWD pieces in young stands. This resulted in significant differences in CWD C stocks by region and ownership. The disparity in estimates across spatial scales illustrates the complexity in estimating CWD C in a NGHGI. Based on the results of this study, it is suggested that the U.S. adopt field-based estimates of CWD C stocks as a component of its NGHGI to both reduce the uncertainty within the inventory and improve the sensitivity to potential management and climate change events.

Imputing forest carbon stock estimates from inventory plots to a nationally continuous coverage

The U.S. has been providing national-scale estimates of forest carbon (C) stocks and stock change to meet United Nations Framework Convention on Climate Change (UNFCCC) reporting requirements for years. Although these currently are provided as national estimates by pool and year to meet greenhouse gas monitoring requirements, there is growing need to disaggregate these estimates to finer scales to enable strategic forest management and monitoring activities focused on various ecosystem services such as C storage enhancement. Through application of a nearest-neighbor imputation approach, spatially extant estimates of forest C density were developed for the conterminous U.S. using the U.S.'s annual forest inventory. Results suggest that an existing forest inventory plot imputation approach can be readily modified to provide raster maps of C density across a range of pools (e.g., live tree to soil organic carbon) and spatial scales (e.g., sub-county to biome). Comparisons among imputed maps indicate strong regional differences across C pools. The C density of pools closely related to detrital input (e.g., dead wood) is often highest in forests suffering from recent mortality events such as those in the northern Rocky Mountains (e.g., beetle infestations). In contrast, live tree carbon density is often highest on the highest quality forest sites such as those found in the Pacific Northwest. Validation results suggest strong agreement between the estimates produced from the forest inventory plots and those from the imputed maps, particularly when the C pool is closely associated with the imputation model (e.g., aboveground live biomass and live tree basal area), with weaker agreement for detrital pools (e.g., standing dead trees). Forest inventory imputed plot maps provide an efficient and flexible approach to monitoring diverse C pools at national (e.g., UNFCCC) and regional scales (e.g., Reducing Emissions from Deforestation and Forest Degradation projects) while allowing timely incorporation of empirical data (e.g., annual forest inventory).

Estimation of snag carbon transfer rates by ecozone and lead species for forests in Canada

Standing dead trees (snags) and downed woody debris contribute substantially to the carbon (C) budget of Canada's forest. Accurate parameterization of the C transfer rates (CTRs) from snags to downed woody debris is important for forest C dynamics models such as the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3), but CTRs are rarely measured or reported in the literature. Therefore, forest C models generally use snag fall rates (FRs) available in the literature, as a proxy for CTRs. However, FRs are based on stem counts while CTRs refer to mass transfers. Stem mass and stem number are not linearly related, with small diameter trees representing disproportionately lower C mass transfers. Therefore this proxy, while convenient, may bias C transfer from standing dead to downed woody material. Here, we combined tree data from 10802 sample plots and previously published species-specific individual-tree relationships between tree diameter (diameter at breast height, dbh) and fall rate to derive stand-level estimates of CTRs for the CBM-CFS3. We estimated CTRs and FRs and used the FR values to validate this approach by comparing them with standardized FR values compiled from the literature. FRs generally differed from CTRs. The overall CTR (4.78% +/- 0.02% per year, mean +/- SE) was significantly smaller than the overall FR (5.40% +/- 0.02% per year; mean +/- SE). Both the difference between FR and CTR (FR - CTR) and the CTR itself varied by ecozone, with ecozone means for CTR ranging from 3.94% per year to 10.02% per year. This variation was explained, in part, by heterogeneity in species composition, size (dbh distribution), structure, and age of the stands. The overall mean CTR estimated for the Snag_Stemwood (4.78% per year) and the Snag_Branches (11.95% per year) pools of the CBM-CFS3 were approximately 50% and 20% higher than the current default rates used in the CBM-CFS3 of 3.2% and 10.0%, respectively. Our results demonstrate that using CTRs to estimate the annual C transfer from standing dead trees to downed woody biomass will yield more accurate estimates of C fluxes than using a FR proxy, and this accuracy could be further improved by accounting for differences in ecozone, stand component (hardwood or softwood), or lead species.