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Implications of Climate Change for Medicinal Plant Distribution and Composition

In 1992, the Union of Concerned Scientists published a “World Scientists’ Warning to Humanity,” with 1,575 signatories that alerted the public that human damage to the Earth, its resources, and its lifeforms must be curbed to avoid catastrophe. In 2017, a second notice, with 15,364 signatories from 184 nations, noted that few concerns raised in the 1992 statement had been adequately addressed, and most had worsened. They named climate change (CC) as one of the major threats. After the publication of the second notice, the Alliance of World Scientists called for additional, discipline-specific “Scientists’ Warnings” that highlight effects of CC on environmental and human well-being. Early responses addressed impacts on wetlands, microbial communities, and wildfire regimes. In January 2020, the journal Planta Medica published a new Scientists’ Warning that assessed threats to medicinal plants due to CC, habitat loss, and overharvesting.

Medicinal plants are considered a vital component of health care for an estimated 70% to 95% of people living in developing countries, and medicinal plants increasingly are sought by those in wealthier developed nations. The use of herbal remedies in veterinary medicine can affect the health of humans consuming livestock products and the economic security of animal owners. Traditional medicinal plant collection and sales offer livelihoods to millions of people, and the global trade in medicinal plants has been estimated at more than $32.6 billion annually.

CC is predicted to decrease the availability and abundance of many medicinal plants, some to the point of extinction. CC may also affect the chemical composition of plants, potentially altering their biological effects. Each of these threats is discussed.

Nearly 600 plant species have become extinct since the publication of Carl Linnaeus’ Species Plantarum in 1753. In general, wild plants are endangered by CC, habitat loss and fragmentation, invasive species and new pathogens, and increased grazing due to low predator populations. Medicinal plants also are threatened by unsustainable harvests and harvesting methods. American ginseng (Panax quinquefolius, Araliaceae), for example, has declined in abundance and average size due to the high demand in China and illegal harvesting in its native ranges in North America. Other slow-growing, popular herbs show similar declines.

CC may further reduce or alter suitable ranges for the continued existence of many medicinal plants. Many are moving to higher elevations or latitudes, increasing species competition there. For some species, relationships with commensal organisms or pollinators will be harmed by these moves. (Commensalism is a type of relationship in which one organism benefits and the other is not affected.) With an overall reduction in beneficial insects, harmful ones, like the bark beetles (Dendroctonus spp.) wreaking havoc on North America’s coniferous forests, have proliferated and increased their range. Many fungal pathogens thrive in warmer winters and are an increasing threat to forest species, as are fires, which are occurring with greater frequency and intensity. In central Canadian black spruce (Picea mariana, Pinaceae) populations, these factors reportedly have changed net productivity, carbon stocks, and soil nitrogen levels. CC may not only increase the damage of plant pests and pathogens, fire, and drought, but these factors may contribute to further CC due to feedback loops that are “inadequately understood but perhaps catastrophic.” Farmland expansion and development, intensive grazing, and overharvesting may interact additively and synergistically with other factors driving and resulting from CC.

Phenological changes (i.e., changes in an organism’s life cycle in relation to seasonal variations in climate) due to CC have been noted, especially in nival (perpetually snow-covered regions) or subnival species in montane ecosystems. Alpine meadows are among the most threatened ecosystems, with species at the highest elevations at risk from other species seeking higher ground.

Arid zones are also likely to see rapid changes from CC. Chinese licorice (Glycyrrhiza uralensis, Fabaceae), which grows wild in desert steppes, has degraded substantially due to CC and human disturbance. So far, cultivated licorice plants have been shown to produce roots with “considerably lower” active constituents than wild-harvested ones. China, once a major exporter of this herb, has become a major importer, potentially threatening sustainability in other producer nations.

Secondary metabolites, many of which are bioactive compounds that provide medicinal benefits, may increase or decrease with different stressors. For some plants, temperature, elevation, and rainfall are important determinants for secondary metabolite concentrations. The interplay among these factors affects each species differently, making predictions about the effects on phytoconstituents difficult. Decreased or increased potency of plant medicines would most affect those who lack resources for complex testing and quality control. Higher atmospheric carbon dioxide, a driver of CC, may partly counter effects of drought on metabolism for some species. Effects of CC on plants used as both food and medicine are particularly important to determine.

Various methods to mitigate the harmful effects of CC on medicinal plants, such as ex situ conservation and assisted migration, have been proposed and are underway. However, they will not completely alleviate harm to indigenous peoples from decreased availability or access to economically and culturally important plants. While advocating strong efforts to slow CC, the authors of the Planta Medica paper suggest that local and national communities should act quickly to assure preservation of valuable medicinal plants. Examples include community gardens, preservation of traditional knowledge, conservation programs including habitat protection, training wildcrafters in sustainability, use of sustainability certification programs for commercial material, as well as regional quality monitoring programs.