Which of the remaining tropical forests are the highest priorities for conservation? That is the question addressed by our recent paper in Nature Ecology and Evolution. The question is more difficult to answer than one might suspect, yet vital to forest conservation.
Looking down from an airplane, patches of forest stand out distinctly from grasslands, fields, and cities as continuous green tracks. It is only when one walks within a forest that the wondrous variety becomes apparent. The types of trees and their sizes vary through a forest and influence the biodiversity and ecological functions of the forest. It is particularly the forest patches with very large trees and many canopy layers that best provide services for nature and people.
The main tool used to map forests, satellite sensors, however, has not been able until recently to look within the canopy to identify the forests with large trees and well developed “forest structure”.
Consequently, the international Convention on Biological Diversity (CBD), signed by 196 countries on Earth, set targets for forest extent but not for forest quality (Watson et al. 2018). Under the CBD all forest are treated equally and countries do not have good information to guide which of their forests should be maintained or restored to meet biodiversity goals.
This situation changed recently when a satellite used to measure the topography of glaciers was combined with sensors used to measure vegetation with the result being continuous maps of forest height (Hansen et al. 2016).
In this study, we integrated this new information on canopy height with measures of canopy cover and time since last disturbance to quantify “forest structural condition” across the humid tropics (Hansen et al. 2019). Places of high structural condition are the older forests with large trees and many canopy layers. These forests are known to have the potential to support provide habitat for the most species of plants and animals, best retain water, store the most carbon, and are most resilient to changing climate.
Human activities, however, can reduce the ecological benefits of these forests. Hunting can deplete wildlife populations. Selective logging can dry the forest and increase risk of devastating forest. Thus, we overlaid a global measure of human pressure, called the Human Footprint (Venter et al. 2016), over forest structural condition to derive an index of “Forest Structural Integrity”.
We initiated this study following extensive work in the United States using satellite data to provide information on how best to manage national parks and other federal lands under the pressures of land use and climate change (Hansen et al. 2016). We learned that the ecology of these places functioned over large areas, landscapes much larger than individual national parks, and consequently that collaborative management among federal agencies and with private land owners was needed to be effective. We worked with stakeholders particularly in the Greater Yellowstone Ecosystem to develop means for this large-landscape collaborative management. We found, however, that US federal land management agencies do not have legal mandate for collaborative conservation. Moreover, the federal initiative developed to champion such management, the Landscape Conservation Cooperative Program, was being partially defunded by the incoming federal administration in 2016.
We thus shifted our work to the global tropics where the 2010 Convention on Biodiversity was motivating countries to engage in coordinated national-scale conservation planning and reporting (CBD 2010). With the expertise of our United Nations Development Program partners, we worked effectively with eight tropical countries to pilot the methods and this provided a basis for scaling up to the full Moist Tropical and Subtropical Biome.
Forests of high structural integrity represent the best of the last remaining forests and should be considered high priorities for maintenance and restoration. We discovered that that tall forests with closed-canopies and low human pressure typical of natural conditions comprise less than half of the global moist tropical forest estate. These unique and valuable forests are largely limited to the Amazon and Congo basins. Most of the forests remaining in other parts of the tropics are either reduced in forest structure or relatively high in human pressure. Unfortunately, the majority of the forests with high structural integrity have no formal protection and, given recent rates of forest loss, are at significant risk.
With the rapid disappearance of these ‘best of the last’ forests at stake, we developed a policy-driven framework for their conservation and restoration, and recommend locations to maintain protections, add new protections, mitigate deleterious human impacts, and restore forest structure. With this framework, each country has an opportunity to contribute to conservation through policies to protect the forest or restore them by managing to modify how people use them or to encourage regrowth of forest structure.
This work is particularly timely in that the CBD is now developing post-2020 global biodiversity framework, which will set international priorities for nature for the next 30 years. As evidenced by our study, new satellite technology now allows for the mapping of not just forest extent but also forest quality. Many conservationists are advocating that forest quality be included among the CBD’s post-2020 targets. Our Forest Structural Condition and Forest Structural Integrity indices are among the best current measures of forest quality. We hope that they can contribute to international efforts to set targets to best maintain tropical forest biodiversity and associated ecosystem services.
The United States is one of the two countries that has not ratified in the CBD. While progress has been made globally in achieving some of the Aichi Biodiversity Targets and to formulate a global strategy for nature post-2020, places like the Greater Yellowstone Ecosystem have had increased pressure from crowding of visitors and from habitat fragmentation from private land development.
We hope to apply the methods developed in this study within the US to help promote national policy and implementation of large landscape cooperative conservation.
Our manuscript titled, "A policy-driven framework for conserving the best of Earth’s remaining moist tropical forests " can be found here for further details: https://www.nature.com/articles/s41559-020-1274-7.
Convention on Biological Diversity (CBD). COP 11 Decision X/2. Strategic Plan for Biodiversity 2011-2020. (2010).
Hansen, A. et al. Global humid tropics forest structural condition and forest structural integrity maps. Sci. Data 6, 232 (2019).
Hansen, A.J., W.B. Monahan, D.M. Theobald, T. Olliff. Climate Change in Wildlands: Pioneering Approaches to Science and Management. Island Press. (2016).
Hansen, M. C. et al. Mapping tree height distributions in Sub-Saharan Africa using Landsat 7 and 8 data. Remote Sens. Environ. 185, 221–232 (2016).
Venter, O. et al. Sixteen years of change in the global terrestrial human footprint and implications for biodiversity conservation. Nat. Commun. 7, 12558 (2016).
Watson, J. E. M. et al. The exceptional value of intact forest ecosystems. Nat. Ecol. Evol. 2, 599–610 (2018).