SIAM News Blog

Optimizing the Sustainable Harvest of Timber and Non-Timber Forest Products

By Lina Sorg

Non-timber forest products (NTFPs), also known as non-wood forest products, are biological materials, commodities, or services that one can obtain from forests without harvesting and chopping down the trees themselves. Such products include fruits and vegetables; nuts and seeds; game and fish; oils, resins, and syrups; medicinal plants; and plant materials like palms, grasses, rubber, and fibers. Up to 300 million people earn part or all of their annual livelihood and food supply from wild plants and other NTFPs; that amounts to a yearly NTFP harvest of approximately 90 billion dollars. Additionally, 80 percent of the world’s population relies on medicinal plants for primary healthcare. Thus, the harvest of NTFPs can be both economically and ecologically sustainable by providing products and livelihoods without decimating forests for timber or severely disrupting the surrounding ecosystem. This is particularly true of poor communities in tropical systems.

In recent years, governments, conservation agencies, and other organizations have encouraged the harvest and marketing of NTFPs to promote forest conservation, sustainability, and continued biodiversity. However, exclusive harvest of NTFPs without supplemental logging efforts is likely not economically sustainable, and may fail to provide sufficient revenue for local people in the long term. Prior NTFP studies of ecological factors and socio-economic demands have failed to reconcile the two. At the 2018 SIAM Conference on Mathematics of Planet Earth, currently taking place in Philadelphia, Pa., Orou Gaoue of the University of Tennessee presented a simple model that optimizes a combination of nonlethal NTFP harvest and lethal timber harvest in tropical forests. 

Forest harvesting traditionally refers to the removal of timber for profit and consequential economic stability. Yet in the last two decades, the definition of forest harvest has evolved. A published 1989 exhibition to Peru and exploration of the Amazon rainforest confirmed the importance of tropical forest products and suggested that timber’s actual market value is much smaller than that of NTFPs. The authors also affirmed that sustainable exploitation of so-called “minor” forest products is two to three times higher than traditional forest harvesting. Although researchers later debated the validity of these statistics, the idea of NTFPs as a lucrative and environmentally-friendly harvest alternative has persisted.

The harvest of a unique NTFP plays a significant social-economic role on the African savannas. Local Fulani herdsmen routinely and perilously climb to the tops of trees in the genus Khaya—which yield African mahogany timber—and cut off the upper branch plumes to feed their cows. The cows produce milk that locals drink, mix with porridge and other foods, and use for cheese-making — a lucrative business. Because this harvest affects only the uppermost branches and leaves the trees intact, it is a type of NTFP exploitation. 

A particular social prestige is affiliated with this branch cutting as well. Members of the Fulani communities consider the harvest of Khaya treetops a courageous and worthy act due to the danger of the task. Society correlates a successful harvest with the ability to marry and provide for a family. “The interactions between humans, trees, and the environment define a social status,” Gaoue said. However, this nonlethal NTFP harvest presents a conflict of interest with companies that lethally raze the trees for their mahogany, as the notches that men use for climbing actually devalue the timber. 

Beginning in the 1990s, scientists began investigating the coexistence of nonlethal NTFP harvest and traditional lethal timber harvest via matrix progression models. Instead, Gaoue created an implicit model of synergistic harvest effects that accounts for both lethal and nonlethal harvest through plant population growth rate and removal of whole plants respectively. The corresponding system of equations yields a globally- and asymptotically-stable nontrivial positive equilibrium system that estimates the intensity of the contrasting harvests. Gaoue then compared his model with existing matrix projection models. When analyzing multiple systems in various locations, he observed some overestimation in the matrix projection model regarding realistic levels of nonlethal harvest. 

In response, Gaoue employed optimal control theory to account for economic value, test the impact of species’ longevity and growth rate on optimal harvest strategies, and minimize the cost of harvest while maximizing revenue and conservation efforts. He noted decoupling between growth rate and life span, and concluded that beginning with NTFP harvest while delaying the harvest of slow-growth species (lethal timber harvest) would maintain ecosystem sustainability. Gaoue also observed that one must be prepared to add additional ecosystem stressors to a harvest model, particularly in the case of forests that are burned, suffering, or otherwise compromised. 

In concluding discussion, Gaoue indicated that the way in which people define “harvest” can extend well beyond the traditional ecological and economical value of lethal and nonlethal products. This open-mindedness ties into the notion of “ecosystem services,” which broadly refer to the various benefits that humans naturally gain from the environment and working ecosystems. For example, one could consider trees’ absorption of carbon dioxide and release of oxygen—as well as water quality and aesthetic tourist appeal—when making harvest-based decisions. “If you start adding those components, then I would be fine with using ‘ecosystem services’ to describe NTFPs,” Gaoue said. 

Another factor worth considering is forest size, as smaller forest fragments tend to limit ecological interactions. “The smaller the fragment, the faster you would expect the plant population to decline,” Gaoue said. The presence of other species that simultaneously interact with the harvested plant/species is also noteworthy, because harvest intensity might have a limited effect on the targeted species but a significant ecological effect on related species. “Nonlethal harvesting of a targeted species can weaken overall species’ coexistence,” Gaoue continued. “We probably need to redefine sustainable harvest, because other side species are sometimes affected.” 

Gaoue admitted that people do not often like thinking about this type of interconnectivity, and instead prefer to focus on immediate acquisition of their product. “They don’t want to spare trees for next year, because this is an open-access system,” he said. “If you do not harvest it, somebody else will harvest it. If you maximize harvesting today, it might not affect the future of that tree because it might be cut tomorrow.”

Ultimately, Gaoue’s model predictably demonstrates that the optimal harvesting rates of slow-growth species (timber) are lower than those of fast-growth species (NTFPs). However, he was surprised to learn that species’ lifespan only loosely affects optimal harvest, meaning that life history—not lifespan—serves as a more valuable indicator of species’ resilience to and recovery from both lethal and nonlethal harvest.

Lina Sorg is the associate editor of SIAM News.
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