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Diversity of ecosystems through diversity in structure
Structural assemblage and diversity is a pivotal component of forest ecosystems. Variability in how vegetation is distributed three-dimensionally fundamentally influences habitat availability, species richness, productivity, and impacts ecosystem processes and functions.
Let’s use two contrasting forests as examples. First, a plantation, where the same tree species is planted in rows at predetermined distances, and the other, a natural mature forest. The plantation is fundamentally less diverse from a structural perspective because the locations of where trees grow has been systematically optimized to maximize growth. While this may be a viable management option for maximizing attributes like timber volume, the lack of diversity in structure is likely to negatively influence habitat availability and the range of ecosystem services that a forest may otherwise provide. While some generalist species may still find the plantation to be viable habitat, the low diversity in vegetation species and structure limit the overall ecosystem services, ecological processes, and function the stand can provide.
Now let’s switch perspectives and focus on our natural mature forest example. We would expect that a natural forest has greater diversity in vegetation species, as well as how they grow and are distributed. Opportunistic growth of trees and shrubs leads to a more varied distribution across a given area, which contrasts greatly with the uniform structure of the plantation. Likewise, we would expect to see a mix of species and age classes as a result of the forest's disturbance history. Oliver and Larson (1996) describe forests as dynamic ecosystems [2], which means that as they grow, disturbances occur that alter their vertical and horizontal structure of vegetation. Forest dynamics result in changes such as new vegetation growing where older trees have fallen over and opened up access to light. Likewise, structurally complex forests are complex in that vegetation is present in many layers. This could be a variety of shrub and understory species growing on the forest floor, shade tolerant species (species that don’t need direct sunlight to grow) growing in the low to mid canopy, while a dominant forest canopy makes up the principal component of the forest biomass. The variation of vegetation species, ages classes, and how they distribute themselves increases the potential for ecosystem processes such as habitat usage for birds, food production in shrubs, soil water filtration, and improved air quality to take place. In general, greater variability in forest structure helps to promote and support a wider range of species and leads to an increased rate of productivity.
The Power of 3D Remote Sensing in Ecosystem Analysis
While the understanding of the importance of vegetation structure to ecosystem processes and function are well researched and understood, traditional methods to objectively measure and characterize structure have been expensive and complex. Prior to the development of three-dimensional remote sensing datasets like Light Detection and Ranging (LiDAR), it was incredibly difficult and time consuming to map forest structure and the terrain underneath it. LiDAR datasets provide a means to characterize forest structure over large expanses, which allow us to not only provide high quality estimates of key forest attributes like tree height and density, but also about quantitative data like how variable forest structure is within its canopy. The data afforded to us by LiDAR can be analyzed to evaluate and map key criteria such as ecosystem services, function, and processes, and has become a pivotal tool for monitoring and managing our planet’s natural resources.
A paradigm shifting program for global ecosystem monitoring has been the Global Ecosystem Dynamics Investigation (GEDI), which is a satellite-based LiDAR system that was attached to the international space station. GEDI has provided unprecedented insights into the three-dimensional structure of ecosystems around the world. By measuring the height and density of forest canopies, GEDI has helped scientists better understand how different ecosystems function, provide estimates of their productivity, and their capacity to store carbon - a key factor in mitigating the impacts of climate change.
Importance of Field Measurements
While remote sensing offers a comprehensive overview, on-the-ground field measurements linked with these remotely sensed data provide the granular details essential for a thorough understanding. Field campaigns including forest inventory and wildlife surveys provide ground truthed measurements of species composition, age structures, carbon estimates, and other ecological interactions, and remain a critical component to mapping species habitat, and estimating carbon stocks. Field data help to validate and enrich the data obtained from remote sensing, ensuring more accurate and relevant outcomes.
Frameworks that combine remotely sensed data like LiDAR and field measurements provide a means to accurately quantify carbon sequestration potential of different forest types. Additionally, structural characterizations can help to better understand critical ecosystem components such as how resilient they may be to climate change and human impacts, or where the most critical areas for conservation may be. The findings from these structural assessments can be used to guide conservation strategies and policy decisions so that we can be more effective stewards for our forest resources.
Conclusion
Vegetation structure is a core component of ecosystems and the functions, processes and productivity they provide. Assessing structural diversity using LiDAR and field measurements is a powerful tool to better understand and manage our forested ecosystems. As we continue to face global environmental challenges, increasing assessments and integration of structural information should be prioritized.
References
Oliver, C. D., Larson, B. C., & Oliver, C. D. (1996). Forest stand dynamics.
