Assessing carbon sequestration and wildfire risk across open-to-closed forest gradients to guide restoration and foster resilience in eastern U.S. forests

Background

Intentional fire exclusion across the U.S. is contributing to widespread shifts in forest structure and composition, and consequently, alterations to forest ecosystem function and services. In the absence of fire and other canopy-opening disturbances in the eastern U.S., open forest landscapes characterized by pyrophytic oaks (Quercus spp.), pines (Pinus spp.), and an herbaceous understory maintained by frequent, low-intensity fires are transitioning to dense, closed forest systems dominated by shade-tolerant, fire-sensitive species with a leaf litter understory of low flammability. This has cascading effects on ecosystem services, including decreasing understory plant biodiversity and habitat provision for wildlife and pollinator species that largely depend on fire- maintained, open forests. Preventing these shifts through application of low-intensity, prescribed fire is a common management approach throughout regions, yet there may be unquantified trade-offs between these known ecosystem services and the potential for an open-to-closed forest transition to enhance carbon storage through stand densification and decrease risk of catastrophic wildfires through shading and the proliferation of species with fire-suppressing leaf litter and woody debris traits. While these traits may lead to fire suppression in some scenarios, open-forest structure may in fact decrease catastrophic risk more successfully given increased fuels associated with closed forests and their potential to ignite regardless of species traits under an increasingly warm and dry climate regime. Ultimately, which ecosystem services are more desirable may reflect changing perceptions of forest value, especially with continued increases in atmospheric warming and detrimental fire activity. 

Nonetheless, assessing the carbon sequestration potential and wildfire risk of shifting forest ecosystem states is critical for future forest management, restoration, and fostering forest resilience. We will investigate how oak and pine forest structure and composition across management gradients of open-to-closed-forest structure and pyrophyte-to-mesophyte composition affect ecosystem services including carbon storage and reduction in forest flammability. We will utilize a network of newly established sites across the region, while also leveraging existing datasets, field sites, and collaborative partnerships. This effort will include cross-station collaborations of USDA Forest Service researchers  and academic institutions. 

Collaborators will use pre-existing connections to extension agents and land managers to generate a network committed to affecting landscape-scale change through forest management and restoration of open forest systems. They will apply the tools developed here to reduce wildland fire risk and restore resilient open forest systems on public and private lands. Additionally, they will communicate available tools to land managers to assess climate and fire risks and management trade-offs, and to manage land accordingly. They will use the tools to assess ecosystem integrity; improve current fire behavior models; evaluate assumptions used to formulate indicators and/or incorporate management actions; evaluate existing plans for effective approaches and potential opportunities to enhance restoration and support pollinator populations and habitats; develop estimates of restoration needs; and assess the role of wildfires as opportunities for restoration.

Approach

Objective 1

Study sites will be distributed across a latitudinal gradient from Alabama to Ohio and include fire-dependent forest types dominated by longleaf pine, pine-hardwood mixtures, and hardwoods. Sites will include mature stands representing distinct forest ecosystem states indicative of the transition between open and closed forest systems based on size and composition of overstory trees, groundlayer cover, and midstory development of shade-tolerant trees. Based on initial conversations, we anticipate using sites in South Carolina, North Carolina, and Ohio which are associated with the Fire and Fire Surrogates Study, as well as several sites in western Kentucky, Mississippi, and Alabama frequently utilized by PI Alexander. In each site, we will sample carbon pools, fuel loading, and fuel traits associated with flammability in at least three stands across an open-closed forest gradient. Carbon sampling in these sites is distinct from that in long-term Forest Inventory and Analysis plots because each site will contain multiple stands representing an open-to-closed forest gradient. 

We will estimate carbon pools and fuel loading of each stand within fixed-radius plots by estimating biomass of live trees, snags, downed woody debris, understory vegetation, leaf litter, and organic and surface mineral soils using current or slightly modified Forest Inventory and Analysis protocols, in combination with published and/or laboratory-determined bulk density and carbon content values for soils and understory vegetation. We will extend our current assessment of leaf litter and woody debris flammability traits to additional sites and species, including understory vegetation, as well as combinations of species and their potential for additive, synergistic, or buffering flammability effects. We anticipate sampling 10-15 sites, each with at least three stands, between spring and fall 2024. Absolute and relative carbon pools, fuels, and flammability traits will be analyzed as a function of canopy cover, vertical forest structure, species composition of each stratum, and fire and other management history, controlling for site-specific factors such as climate and soil type. 

In addition to collection and analysis of novel regional datasets, we will leverage preexisting data and knowledge on mechanisms and outcomes of forest shifts. The first step of this process will be conducting a “state of the knowledge” review, including a workshop where collaborators meet to discuss the existing data, knowledge, and projects associated with the effects of forest shifts and identify current knowledge gaps. Leaf litter, woody debris, and other trait data will be compiled into a species flammability traits database, with the goal of characterizing relevant tree species along a mesophyte-pyrophyte gradient. The integrated database will additionally hold data on estimated absolute and relative carbon pools across stand types. We will use these data, in combination with stand characteristics such as canopy cover and vertical structure, to create a metric of forest ecosystem state change, allowing for a standardized characterization of forests along the open-to- closed forest gradient and their associated ecosystem services.

Objective 2

The suite of results and tools generated through Objective 1 research will be disseminated and implemented in several ways to promote large-scale open forest restoration and revegetation efforts. The flammability traits and forest carbon databases will be compiled and curated by the postdoctoral
position funded through this grant and will be made publicly available. Such information can inform the ongoing development of high-resolution fire behavior models. The process of integrating trait and carbon database information with stand characteristics to quantify forest ecosystem state and
ecosystem service tradeoffs will be presented to Forest Service employees through the mountain module training, and landowners and managers through outlets including workshops, seminars, podcasts, and webinars. In addition to dissemination and education, collaborators will work directly with stakeholders to utilize these tools to assess stand-specific ecosystem service trade-offs and select appropriate fire management strategies and approaches accordingly.

Objectives 

The primary objectives of this research are to:

1. Evaluate trade-offs between known ecosystem services associated with open forest woodlands and savannas and those of increased carbon sequestration and/or wildfire buffering capacity that may occur with ongoing open-to-closed forest structural and compositional shifts across the eastern U.S.
2. Incorporate information about ecosystem services provided by open forest woodlands and savannas, and their potential trade- offs, to promote large-scale forest restoration and revegetation efforts through collaborations between Forest Service researchers, university experts, extension agents, and land managers.

Expected Project Results 

A. Outputs:

1. Review paper on the a “State of the Knowledge” on carbon sequestration and flammability consequences of shifting forest states
2. Databases: 
   1. Flammability trait database documenting leaf litter and woody debris traits of tree species along a mesophyte-pyrophyte gradient and their relationship to forest flammability
   2. Carbon database documenting absolute and relative forest carbon pools by location and forest ecosystem state
3. Two scientific articles reporting the findings of this study
4. Conference presentations at the Ecological Society of America and Association of Fire Ecology annual meetings in 2024/2025
5. Educational and network activities across study sites including webinars and stakeholder workshops focused on sharing findings and product outcomes of research with land managers and research station scientists
6. Training materials on developed tools and research outcomes provided to Forest Service employees through integration with the mountain module continuing education platform
7. Training of post-doctoral researchers

B. Expected Outcomes:

The proposed project supports priority research areas of the Forest Service, including: 

• Improving Fire Management and Wildland Fire Risk Reduction: Provide a better understanding of how forests of varying structure and composition resulting from different fire and other management histories impact future forest flammability in the central and eastern U.S. This will allow for assessments of forest stands best suited for fire management and help slow compositional and structural shifts from pyrophytic to mesophytic species.
• Fostering Forest Resilience and Restoration: This work will specifically assess open stand restoration treatments, as well as regeneration dynamics of longleaf pine, shortleaf pine, and upland oaks, which are declining after decades of fire suppression. Research will also assess problematic species that are encroaching into eastern forests and pose competitive threats to the desired pyrophytic species mentioned above as well as to native understory herbaceous plants.

C. Metrics of Success:

Project success will be determined by:

1. Completion of sampling forest sites across the regional and forest ecosystem state gradients
2. Generation of flammability trait and forest above and belowground carbon databases
3. Presentation of findings via conferences, workshops, training modules, and extension articles
4. Creation of a large network of stakeholders collaborating on open forest restoration
5. Publication of review paper and research findings as scientific articles

Geographies and High-Risk Landscapes to be Addressed

Research will span a broad latitudinal gradient across the eastern U.S. from southern Alabama pine savannas to mixed hardwood forests in Ohio.