Climate change is having a dramatic effect not only on plant harvesting times, but also on the phytochemicals in plants. This article investigates how.
Observations from the field
Over the course of my field work, aiming to observe the harvesting of medicinal plants at the “right times” for production of pharmacopoeial quality herbs, done in the “right ways” for sustainable resource management and trade, I can state that, in recent decades, predictability of harvest periods has become uncertain in some regions of the world. In some locations there has been notable fluctuation from year-to-year.
Anecdotally, plant harvesters have shared their observations of the changing climate, uncertainties of harvest times and yields, as these factors directly impact livelihoods and rural economies. I have also observed some impacts of extreme weather on herb quality (e.g., stunted growth, lower yield, lower essential oil content) but also on the ability to even harvest herbs in some years due to, for example, torrential rains, flash flooding, and landslides (or conversely no monsoon at all). Changing or fluctuating growing seasons can also impact whether, or not, there will be sufficient labor available at harvest time. In rural communities, where some villagers make some – or all – of their household income harvesting medicinal plants for trade, different herbs or mushrooms are targeted for harvesting in different months based on traditional ecological knowledge (TEK). Labor was traditionally organized accordingly. Climatic unpredictability may be impacting such TEK.
But – these statements come only from my experiences and observations. The accumulated analyses of competent researchers worldwide are contributing to a growing body of scientific literature, that carries with it, I believe, urgency.
Phenological responses to climate change, endemism, extinction
While shifting flowering and fruiting phenology (1,2), and altered phenological synchrony between plants and pollinators (3), is observable, measurable, and documented, less is known about changes that may be occurring in the composition and content of phytochemicals, so-called secondary metabolites, and phytonutrients where much of the medicinal properties lie (4,5,6). Mismatches between plant and pollinator populations may lead to extinction of a plant species and/or its pollinator (7), especially in ecoregions of high endemism.
Plants are either adapting to changing weather patterns, and to the narrowing of suitable habitat, and/or are migrating to more suitable habitat areas (8). Or, if unable to adapt or migrate rapidly enough, some species may face a threat of extinction (9). Endemic plants with a limited geographic distribution may be the most vulnerable (7). However, in their recent paper on endemism hotspots, Harrison and Noss (2017) concluded “Current knowledge suggests that centres of endemism will remain relatively climatically buffered in the future, with the important caveat that absolute levels of climatic change and species losses in these regions may still be large.”(10)
To inform species conservation strategies as well as the development of suitable agro-techniques for the successful cultivation of threatened wild medicinal plants, of a reproducible therapeutic quality, research on habitat range and secondary metabolites production under different climate models is still in its infancy.” (7,11) Not enough is known, yet.
Impacts of abiotic and biotic stresses on phytochemical content
Abiotic plant stresses such as elevated atmospheric carbon dioxide (CO2), temperature and precipitation extremes, and changes in ultraviolet radiation exposure, among others, impact phenology, physiology, and phytochemistry, which, in turn, could also impact a plant’s chemical defences against biotic stresses such as attacks by pathogens and insect herbivores (12). Changes in phytochemical composition whether due to abiotic or biotic stressors, or both, could also have relevant implications for coevolutionary plant and pollinator interactions (13).
And it is not only insect herbivores. Arboreal folivores, such as the native Australian koala bear (up-listed from ‘vulnerable’ to an ‘endangered’ species in February 2022) (14), coevolved with eucalyptus trees, having a specialist diet, limited mainly to eucalyptus leaves (15). Due to increasingly frequent extreme heatwaves and droughts, the koala may not be able to rely on eucalyptus leaf moisture for hydration, and, in the future, may need supplemental sources of water for survival (16). The leaves and distilled essential oil of the leaves and terminal branchlets harvested from Eucalyptus globulus trees, endemic to Victoria and Tasmania, Australia (17), are used in traditional medicines (18).18 Changes in a medicinal plant’s content and composition, if significant, “could” also impact posology and pharmacological action(s) (19).
Ghazghazi et al (2022), in their recent study on the effect of drought stress on physio-biochemical traits and secondary metabolites production in Pinus halepensis, the leaves of which are used in traditional medicines of the Mediterranean region state: “The capability of plants to respond to abiotic stress is associated with their plasticity as well as the adaptableness of plant traits to fluctuating bioclimatic conditions.”(20) According to the VILLUM Research Center for Plant Plasticity:
“Plant plasticity refers to a plant’s ability to adapt to and cope with changes in its environment. In contrast to animals, which are able to actively move away to avoid challenges such as predators or a changing climate, plants have acquired the ability to biosynthesize an unprecedented array of structurally complex bioactive natural compounds with specialized roles in order to cope with environmental challenges.” (21)
Primarily the “secondary” metabolites
In a systematic review of papers published from 2015 to 2020, that analyzed climate change impacts on secondary metabolite production and accumulation in medicinal plants, Pant et al (2021) reported that impacts from environmental factors such as changes in temperature, elevated CO2, elevated ozone, UV light, and drought were not only species-specific but different variables were associated with adverse impacts on plant growth and yield, as well as significant increases or decreases in secondary metabolite content (22).
Changing habitat suitability, migration, adaptation, chemistry
Most of the following examples from the current literature concern medicinal plants used in China. This is not an intentional focus on traditional Chinese medicine (TCM), but rather an observation that much of the published climate change scenario research involving medicinal plants, that I have seen or have paid attention to, is focused on habitat suitability and quality of herbal drugs used in TCM. That is not to say that similar research is not being carried out involving medicinal plants of other regions – it is.
Using MaxEnt (maximum entropy) modelling, Zhan et al (2022) evaluated and predicted the distribution area of Panax notoginseng (san qi) under future climate scenarios, as well as the relationship between total saponins content (about 90% are ginsenosides) and habitat suitability. They predict a gradual decrease in suitable habitat area with migration towards high-altitude areas of central-eastern Yunnan province, and decreased ginsenoside content in future highly suitable habitat areas for this species (8).
Also using MaxEnt ecological niche modeling, Shi et al (2022) predict a sharp decline in suitable habitat for Meconopsis punicea (hong hua lü rong hao) by 2050, with a likely upward migration to inhabit a narrower elevational range in the Qinghai-Tibet Plateau. Other studies found the luteolin content of a related species Meconopsis quintuplinervia (wu mai lü rong hao) to be significantly impacted by changes in altitude. While these researchers predict that the secondary metabolites content will change as the distribution area shrinks and changes in altitude in the coming decade, no specific predictions in content levels or impacts on therapeutic use are made (23).
In a similar study on Gentiana rigescens (dian long dan), Shen et al (2021) predict that current highly suitable habitats in southwest China will turn into lowly suitable habitats or unsuitable habitat for this species. In their 2050 and 2070 models, migration to high elevations is predicted with an increased accumulation of bioactive constituents, i.e., iridoids including loganic acid, swertiamarin, and sweroside, showing a correlation between altitude and iridoid concentration (24). Using ArcGIS (geographic information system mapping software) with MaxEnt, Yan et al (2020) predict that, by the 2050s, the unsuitable habitat areas for Gentiana macrophylla (qin jiao) in China would increase by 11.92% under the moderate greenhouse gas emissions/climate scenario, concurrently with significant losses of suitable habitat. Furthermore, an accelerated speed of fragmentation of most of the suitable habitat area for this species is predicted, threatening reproduction and long-term survival (25).
Using fuzzy theory and a MaxEnt model, Guo et al (2016) predicted the future distribution range of Schisandra sphenanthera (nan wu wei zi) in the Qinling mountains, under three different climate change scenarios, for the periods 2020s, 2050s and 2080s, as well as determining any associations with synthesis and accumulation of schisantherin A in the fruits with various climatic variables. The Pharmacopoeia of the People’s Republic of China requires that dried S. sphenanthera fruits contain not less than 0.20% schisantherin A, while the Hong Kong Chinese Materia Medica Standards require that the dried fruits contain not less than 0.67% of the total content of schisandrin A and schisantherin A. This study predicted that under the three future climate scenarios, the habitats of pharmacopoeial quality S. sphenanthera fruits will continue to decrease towards near extinction; suggesting that, at least in this specific study area in the Qinling mountains, highly suitable habitat areas for S. sphenanthera may disappear
if the annual mean temperature begins to exceed 20 ºC and/or if the annual precipitation level should exceed 1,200 mm (26).
For some species, the suitable habitat area may expand with climate change. That is the case, according to Wang et al (2022) for Artemisia annua (qing hao), used in TCM but also the only plant source of the antimalarial drug substance artemisinin. In this study, the future suitable habitat for A. annua is projected to expand inland, perhaps significantly, although the effects of increasing temperatures differ in different time periods. However, the authors raise uncertainty as to whether the expansion of A. annua habitat area will be beneficial, or not, to its medicinal value in terms of the content and yield of artemisinin secondary metabolites. For this reason, these researchers suggest implementing a system of protecting genetic resource reserves and experimental cultivation bases within the geographic origin areas of the species, particularly areas where the wild populations possess higher artemisinin content such as Sichuan, Guangxi, Guizhou, and Yunnan (27).
What if herbs adapt and change in content and composition?
In traditional herbal medicine, safe and effective formulation and dosage can be based, in part, on accumulated knowledge, stemming from the clinical observations of many generations of practitioners, often monographed in the pharmacopoeias and formularies of the codified systems of traditional medicine. Traditional pharmacopoeias generally describe the identification, composition, properties, and quality of each herbal drug based on an assessment of representative materials that have been observed to be effective for their intended traditional uses. In this way, traditional medicine is informed by local or traditional ecological knowledge (TEK) and traditional medical knowledge (TMK).
If, by the year 2050, or the year 2070, or the year 2100 (young herbalists reading this should still be around!), certain medicinal herbs have adapted or migrated and survived the predicted changes – but, in doing so, their composition and content have significantly changed, by comparison to their ancestors described in the traditional medicine literature, TEK and TMK will also necessarily evolve.
Traditional medicine is not stagnant, not stuck in time. It is reasonable to predict that medical herbalists will continue to be keen observers of nature and adjust formulations and dosage, if need be, in response to the changing environment, changes in herbal drug content, and patient response to herbal drug preparations.
No conclusion
Scientists have developed elaborate climate models, indeed useful to help predict changes in suitable habitat areas, and whether the plants and pollinators can or will migrate together, in time to survive, and whether different plants will respond differently in terms of secondary metabolite production. Increasing uncertainty of continued access to certain medicinal plants, of traditional origins and qualities, may, however, cause the practice of herbal medicine to also adapt and change.
The full extent of impacts of the climate crisis on availability and quality of medicinal plants in general cannot be known at this point. Assumptions and variables, from any region, that could exhibit a global impact on climate predictions, are adjusted each time new data is published. For example, while a study published in January 2019, in the journal Proceedings of the National Academy of Sciences, showed that Greenland’s ice was melting far more rapidly than scientists previously thought, (28) in February 2022, University of Cambridge scientists subsequently reported an unprecedented melt rate of the Greenland Ice Sheet that would change sea level rise projections, AGAIN (29). What happens in Greenland, doesn’t stay in Greenland.
References
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