Old Forest Ecosystems

Old forests are delicate and complex ecosystems.  Commonly referred to as “old-growth” both scientifically and colloquially, these forests usually contain very old trees and conditions that require a long time to develop in the absence of intensive human impact.  Old-growth forest conditions were commonplace in North America prior to European settlers’ arrival, as Indigenous communities engaged in forest stewardship activities (e.g. cultural burning) that helped develop and maintain characteristics of forest structure and composition that Western scientists today refer to as ‘old-growth’.  However, European and early-American settler-colonialist practices of intensive logging and forest removal for pasture and agriculture decimated the forested landscape. 

As a result, today, old-growth forests in the United States are rare. This is particularly true of the deciduous broadleaf and mixed deciduous broadleaf-evergreen needleleaf systems in the eastern United States, where almost all forested landscapes were entirely cleared or selectively harvested at one time or another in the past 300 years, many times repeatedly.  Old-growth conditions are more common in western needleleaf dominated systems, but still comprise a minority of the forest estate.

Importance of Old Forests

Old forests provide a myriad of ecosystem services, not limited to:

1. increased biodiversity relative to younger or anthropogenically disturbed forests,
2. large and long term carbon sequestration potential
3. improved air and water quality
4. societal benefits such as a venue for recreational and spiritual activities

Thus, in recognition of both the scarcity and importance of old forests, there has been recently renewed motivation to better understand, identify, and protect these cardinal ecosystems.  This motivation is shared by various state and federal agencies, indigenous tribal nations, NGO’s, and academic institutions alike, and collaborative partnerships between these groups are growing.

Mapping old forests with GEDI

Due to the many ways in which forests age and change over time, the range of structural and compositional conditions within old forests is vast.  In the absence of widespread and spatially explicit information about forest history and land use that spans a centuries-long timeframe relevant for old forest development, present day structural conditions can be used as a proxy for longevity.  In this capacity, GEDI data are proving extremely useful in a) characterizing structural conditions associated with old forests, and 2) identifying where on the landscape the structural conditions associated with old forests exist.

Researchers at the University of Maryland are using GEDI forest structure observations in combination with other remote sensing forest structure and reflectance data to map old-growth probability throughout the United States.  This project, funded by the United States Forest Service (USFS), is one of several efforts to better understand, identify, and map old forests in response to the 2022 Biden Administration Executive Order (EO #14072) “Strengthening the Nation’s Forests, Communities, and Local Economies”.

GEDI data are fundamental to all aspects of this project.  In the context of wall-to-wall mapping of potential old-growth conditions, GEDI data are first used to train algorithms to characterize the “structural signature” of forest stands classified as old-growth by the USFS.  Other GEDI data are then used in applying those algorithms to identify areas of forest with a high probability of old-growth conditions everywhere throughout the continental United States.

Example old-growth probability map in the Willamette National Forest, OR. Recently logged areas with a homogeneous canopy structure and smaller trees correspond with the darker brown regions in the map, while forest areas with larger trees and more complex and heterogeneous canopy structure correspond with the darker green regions.

Collaborations

GEDI data are also being used to direct a large field campaign intended to sample forest age structure, composition, and various structural attributes within old-growth forests.  In the spring and summer of 2024, our collaborators at the Harvard Forest are conducting ground-based forest sampling within GEDI footprints in areas of dense GEDI coverage within the Cherokee, Gila, White River, and Superior national forests.  The data collected include individual tree ages determined through dendrochronological sampling and analysis, as well as extensive species composition and other, inferred qualitative information about forest and land use history. These data will be linked to the GEDI structure observations, enabling an independent assessment of the old-growth probability maps using dendroecology-based age structure information, as well as new insights into the complex and nuanced relationships between physical- and age-based measures of forest structure.

Image credit: Neil Pederson.

Additionally, GEDI observations within the areas sampled for age structure and composition are being used by project collaborators at NASA Goddard Space Flight Center to initialize individual-based forest gap models in an experimental simulation environment designed to assess and interrogate temporal trends in forest dynamics and the development of old-growth characteristics.

Related Publications

Bruening, J. M., Dubayah, R. O., Pederson, N., Poulter, B., & Calle, L. (2024). Definition criteria determine the success of old-growth mapping. Ecological Indicators, 159, 111709. https://doi.org/10.1016/j.ecolind.2024.111709

May, P. B., Dubayah, R. O., Bruening, J. M., & Gaines III, G. C. (2024). Connecting spaceborne lidar with NFI networks: A method for improved estimation of forest structure and biomass. International Journal of Applied Earth Observation and Geoinformation, 129, 103797. https://doi.org/10.1016/j.jag.2024.103797