FWD 2 HerbalGram: The Effects of Climate Change on Medicinal and Aromatic Plants

Issue: 81 Page: 44-57

The Effects of Climate Change on Medicinal and Aromatic Plants

by Courtney Cavaliere

HerbalGram. 2009;81:44-57 American Botanical Council

By Courtney Cavaliere

Climate change* has become increasingly recognized as one of the greatest challenges to humankind and all other life on Earth. Worldwide changes in seasonal patterns, weather events, temperature ranges, and other related phenomena have all been reported and attributed to global climate change. Numerous experts in a wide range of scientific disciplines have warned that the negative impacts of climate change will become much more intense and frequent in the future—particularly if environmentally destructive human activities continue unabated.

The Ötztal Mountains of South Tyrol in the Central Alps. Glaciers have lost approximately 50% of their mass in the Alps, and 30% of glaciated area has disappeared.
Photo ©2009 GLORIA

Like all living members of the biosphere, medicinal and aromatic plants (MAPs) are not immune to the effects of climate change. Climate change is causing noticeable effects on the life cycles and distributions of the world’s vegetation, including wild MAPs. Some MAPs are endemic to geographic regions or ecosystems particularly vulnerable to climate change, which could put them at risk. Concerns regarding the survival and genetic integrity of some MAPs in the face of such challenges are increasingly being discussed within various forums.

Although scientists do not know whether climate change poses a more prominent or immediate threat to MAP species than other threats, it does have the potential to exert increasing pressures upon MAP species and populations in the coming years. The possible effects on MAPs may be particularly significant due to their value within traditional systems of medicine and as economically useful plants. The future effects of climate change are largely uncertain, but current evidence suggests that these phenomena are having an impact on MAPs and that there are some potential threats worthy of concern and discussion.

Medicinal and Aromatic Plants in the Arctic

Warming is occurring more rapidly in the Arctic than anywhere else in the world.1,2 Changes in snow patterns, ice cover, and temperatures are already affecting the distribution of some Arctic vegetation. Some experts postulate that climate change could affect the chemical composition and, ultimately, the survival of some MAPs in Arctic regions.

Alain Cuerrier, PhD, associate professor at the University of Montreal and a botanist at the Montreal Botanical Garden, has studied medicinal plants of the Canadian Arctic. According to Dr. Cuerrier, aerial photographs of the Canadian Arctic have indicated that tree and shrub lines have been changing over the past few decades (oral communication, April 9, 2008). The dry Arctic snow has traditionally stripped or otherwise harshly impacted tall trees or shrubs, in a manner Dr. Cuerrier compared to sandblasting. As a result, some species and populations have grown only in areas where they would be shielded from such harsh environmental conditions. The changing temperatures and wind patterns associated with climate change are affecting precipitation factors and giving some trees and shrubs the ability to grow taller and in more open areas. These taller plants then become barriers to snow, fencing it in and changing the surrounding biodiversity.

Dr. Cuerrier noted that many of the plants that thrive in the Arctic are able to do so because there is little competition from other species. With increased warming, more plants (such as those from the Boreal forest) will encroach into Arctic territories and compete for resources with preexisting flora. The preexisting flora, meanwhile, often do not have the option of migrating any farther north, as other plants typically do when threatened by warming temperatures and increased competition.

Charlène Lavallée and Philip Roy take measurements in open top chambers that mimic global warming and in control plots as part of a study being led by Alain Cuerrier. The study is to assess effects of climate change on berry producing shrubs of the Canadian Arctic.
©2009 Alain Cuerrier

Some studies have demonstrated that temperature stress can affect the secondary metabolites and other compounds that plants produce,3,4 which are usually the basis for their medicinal activity. But few studies have been conducted in situ (in natural settings) or ex situ (in a controlled non-natural setting) to mimic conditions of global warming. Dr. Cuerrier is currently involved in such a study, which should help analyze how increased global temperatures might affect berry-producing shrubs of the Canadian Arctic. Dr. Cuerrier and his team have developed experimental plots with open-top chambers and are simulating higher temperature levels to determine how warming might affect the plants’ production of berries, as well as the berries’ nutrient and antioxidant levels. These berry-producing shrubs, which include blueberry (Vaccinium spp., Ericaceae), cloudberry (Rubus chamaemorus, Rosaceae), and crowberry (Empetrum nigrum, Ericaceae), are important to the diets and traditional medicinal practices of Inuit communities.

Cloudberry Rubus chamaemorus ©2009 Alain Cuerrier

Crowberry Empetrum nigrum ©2009 Alain Cuerrier

Louise Bondo, director of the consulting company KULUK Consult and former section leader for Nordic Genetic Resource Center’s (Nord-Gen) agricultural department, likewise stated that the taste and medicinal effectiveness of some Arctic plants could possibly be affected by climate change (e-mail, April 22, 2008). She noted that such changes could either be positive or negative, although she said it seems more likely that the effects would be negative since secondary metabolites are produced in larger quantities under stressed conditions and—for Arctic plants— warmer temperatures would likely alleviate environmental stress. She added, however, that the production of plants’ secondary metabolites are influenced by multiple factors—including diseases, competition between plants, animal grazing, light exposure, soil moisture, etc—and that these other factors may mitigate the effects of climate change on plants’ secondary metabolites.

NordGen, an organization based in Alnarp, Sweden, that collects and conserves samples of genetic plant material from the Nordic countries, recently collected samples of 4 medicinal plant species from Greenland for preservation and evaluation: angelica (Angelica archangelica, Apiaceae), yarrow (Achillea millefolium, Asteraceae), Rhodiola rosea (aka golden root, Crassulaceae), and thyme (Thymus vulgaris, Lamiaceae).5 According to Bondo, these 4 MAPs are not currently endangered in Greenland, nor are they currently listed on the Convention in Trade in Endangered Species (CITES) appendices. However, collectors interested in preserving current plant genotypes from rapidly warming areas, such as Greenland, must do so before new genotypes arrive in response to climate change. Moreover, plant populations in Greenland are often isolated by the territory’s many huge ice sheets, and this can limit the populations’ available gene pools and subsequent abilities for genetically adapting to new climatic conditions. Capturing genetic diversity becomes increasingly important since it is possible that populations will lose genetic diversity in response to the changing environment.

Dr. Cuerrier pointed out that the traditional medicinal plant R. rosea is one plant species that could eventually face significant threats to its survival in the Canadian Arctic. Rhodiola rosea is circumboreal, growing primarily in Arctic areas of Europe, Asia, and North America. It has been used traditionally to treat fatigue, depression, and infections, strengthen the immune system, and protect the heart.6,7 The herb is becoming more popular in the mainstream herbal industry, and Dr. Cuerrier has suggested that the plant may one day serve as a promising business venture for the Canadian Inuit.6 Efforts to effectively and profitably cultivate R. rosea in the Canadian province of Alberta are already underway via the Rhodiola Rosea Commercialization Project.8 According to Dr. Cuerrier, Canadian populations of wild R. rosea may be significantly impacted by increased competition with invasive species due to climate change, and rising sea levels generated by global warming could pose further threats to the plant’s survival. Dr. Cuerrier pointed out that R. rosea naturally grows along the seashore in Canada, an area likely to be the first inundated through rising sea levels brought on by melting glaciers.

Medicinal and Aromatic Plants in Alpine Areas

Plants growing in alpine environments may also be particularly impacted by climate change. Advancing tree lines and extinctions of montane plant populations have become increasingly apparent and documented by researchers worldwide in recent years—and have been attributed as evidence of the impact of climate change on alpine ecosystems.9

“After polar regions, alpine areas are changing faster than any other areas on Earth,” said Jan Salick, PhD, senior curator of ethnobotany at the Missouri Botanical Garden, who has conducted research on alpine environments of the Eastern Himalayas (oral communication, January 11, 2008).

Primula glutinosa.
©2009 GLORIA

Researchers have found that some cold-adapted plant species in alpine environments have begun to gradually climb higher up mountain summits—a phenomenon correlated with warming temperatures.10 In some cases, these plants migrate upward until there are no higher areas to inhabit, at which point they may be faced with extinction.9,10 Additionally, the upward migration of plant species can lead to increased competition for space and resources, causing further stress among alpine plant populations.9

The Global Observation Research Initiative in Alpine Environments (GLORIA, www.gloria.ac.at), founded in 2001, has been focusing its efforts on documenting the changing biodiversity and landscapes of alpine environments throughout the world.11 GLORIA is an international network of researchers using a standard methodology to collect information on alpine environments and thereby monitor climate change and its effects.

Artemisia genipi.
©2009 GLORIA

Dr. Salick utilized GLORIA’s protocols in her own recent research in the Eastern Himalayas, and the information she collected has been added to the network’s data sets. Dr. Salick further supplemented this data by taking doctors of traditional Tibetan medicine on her trips into the Himalayas to obtain information on the uses of the observed alpine plants. She and a colleague also conducted interviews with Tibetan people to gauge their beliefs and understanding of climate change, its causes, and its relevance to their livelihoods, health, and cosmology.

“Alpine areas are very important for Tibetan doctors’ use,” explained Dr. Salick. “They traditionally spend a month each year going into the mountains to collect plants. A lot of their medicines come from the mountains.”

Dr. Salick and her team found that useful Tibetan plants (predominantly medicinal plants) accounted for 62% of all plant species in the alpine Himalayan sites that they examined.9 Further, although overall species richness was found to decline with elevation from the lowest summits to the highest, the proportion of useful plants stayed approximately constant. This high percentage of useful plants confirms the importance of the Himalayas for Tibetan medicine and reflects the dangers posed by potential plant losses from climate change.

In a paper based on their research, Dr. Salick and her co-authors noted the projection by the Intergovernmental Panel on Climate Change (IPCC) that the Himalayas are likely to experience some of the most drastic climate changes in the world outside of polar regions, with temperature increases of 5-6°C and precipitation increases of 2030%.9 Such figures indicate that climate change is likely to have equal or greater effects on Himalayan alpine vegetation than on vegetation found elsewhere in the world. According to Dr. Salick, several Tibetan medicinal plants are already threatened by over-harvest, and the additional challenges posed by climate change could push some species— which might otherwise have been sustainable—to extinction.

One of the medicinal plant species that Dr. Salick and her former graduate student Wayne Law, PhD, have studied specifically is snow lotus (Saussurea laniceps, Asteraceae), a plant that has been traditionally used in Tibetan medicine to treat high blood pressure, heart conditions, and women’s conditions (i.e., childbirth, dysmenorrhea). Snow lotus, which is endemic to the Eastern Himalayas, is currently considered by local experts to be in danger from both over-harvest12 and negative effects of climate change, even though this herb has not yet been officially listed as “threatened” by any government body or reputable nongovernmental organization (NGO) like the International Union for Conservation of Nature (IUCN). In light of these perceived threats, repeated attempts have been made to cultivate snow lotus, largely without success.

Dr. Salick explained that many threatened alpine plant species have similarly proven difficult or impossible to cultivate. “The conditions [that they need to grow] are just so unique and somewhat unfathomable,” she said. She added that one species of snow lotus in India has been shown to germinate in cultivation, yet even a success such as this is diminished by the fact that cultivated varieties of alpine plants can take years to mature.

Dr. Georg Grabherr, professor of vegetation ecology and conservation biology at the University of Vienna and chair of GLORIA, recently explored the effects of climate change on alpine medicinal plants of a different mountain range—the Central Alps in Europe.13 Dr. Grabherr identified 25 native alpine plants considered medicinally useful by local traditional healers and determined that many of them could potentially invade (or escape to) higher elevations as a result of global warming and that the plants’ risk of extinction under predicted scenarios appears to be relatively low. However, he added that a few medicinal alpine species are restricted to the upper alpine zone, such as Artemisia genipi (Asteraceae) and Primula glutinosa (Primulaceae). These species may experience greater impacts from warming temperatures, possibly leading to local endangerment.

Compared to the Himalayas, the percentage of alpine plants used medicinally in the Alps is much lower, at approximately 10%. Dr. Grabherr noted that imported or cultivated medicinal plants have replaced many native ones for medicinal use, traditional knowledge has largely disappeared, and conventional medical treatment options have long been practiced in the Alps. For these reasons, the fate of wild medicinal alpine plant species does not appear to be a pressing concern for local inhabitants, as is the case among Tibetan societies of the Himalayas and as may possibly be the case for other mountain civilizations.

According to Dr. Salick, the Andes of South America is likely to also provide evidence of the effects of climate change on medicinal alpine plant populations. She stated that traditional healers in the Andes have begun to comment on species loss and encourage tracking of these resources. Dr. Salick plans to move westward along the Himalayas to conduct further research in the alpine environments of India and Nepal. She intends to once again use GLORIA’s methodology to collect further data, which may vary from her earlier findings as climate change and vegetation effects differ according to rainfall levels, elevation, and biogeography.

Medicinal and Aromatic Plants in Other Threatened Regions

Although Arctic and alpine areas are experiencing some of the most rapid changes from global warming, other ecosystems are also considered particularly threatened by the ongoing effects of climate change. Among these ecosystems are islands and rainforests.

Islands are considered especially at risk from rising ocean levels, in addition to changing temperatures and weather patterns.1 According to the 2007 IPCC report, global average sea levels rose at an average rate of about 3.1 mm per year from 1993 to 2003.1 Climate change is expected to accelerate this process through the melting of glaciers and polar ice caps, which adds water into the oceans.2,14 The world’s oceans also absorb excess heat from the atmosphere, and as water warms it expands in volume (a process known as thermal expansion), which will similarly contribute to global sea level rise. In 2006, a team of scientists, using computer models and climate change scenarios proffered by IPCC, calculated that up to 65% of some islands in the Northwestern Hawaiian Islands could be submerged by the year 2100.14

Despite these threats, experts have indicated that island MAPs may not be significantly affected by conditions related to climate change. Many of the plants used by island communities are common species that are widespread and highly adaptable.

“Very few plants that are used as medicines are actually rare or endangered and most are in fact so common that they are often taken as weeds,” said Will McClatchey, PhD, professor of botany at the University of Hawaii at Manoa, regarding medicinal plants of the semi-tropical and tropical Pacific islands (e-mail, April 15, 2008). “In fact, one odd aspect is that the most commonly used medicinal plants of the Pacific are coastal strand plants, or plants that live on the edge of the beach and ocean. These plants will actually do just fine as sea level rises. Individuals will be lost, but they are very tough and well adapted to storms and damage much like climate change will throw at them. Probably the greatest threat that they will face is complete loss of some of the smallest islands, called atolls.”

Dr. McClatchey noted that common medicinal plants of the Pacific islands include noni (Morinda citrifolia, Rubiaceae), naupaka (Scaevola spp., Goodeniaceae), kukui (Aleurites moluccana, Euphorbiaceae), and milo (Thespesia populnea, Malvaceae). These and other medicinal plant species of the area grow relatively fast, have high reproduction rates, and are typically resistant to salt water and wind, making them more resilient to some of the predicted effects of global climate change.

Bertrand de Montmollin, chair of the Mediterranean Islands Plant Specialist Group of the IUCN, similarly stated that medicinal plants of the Mediterranean islands do not appear to be under any considerable threat from conditions of climate change (e-mail, April 17, 2008). According to de Montmollin, most wild collected MAPs, such as thyme (Thymus spp., Lamiaceae) and rosemary (Rosmarinus spp., Lamiaceae), are rather widespread and located at lower altitudes, making them less vulnerable to climate change than plants with narrower ecological requirements.

“The mountain climate in the Mediterranean islands is not so extreme,” he added. “Most of the plants can easily be cultivated ex situ.

In Crete, medicinal plants are mostly cultivated in the mountains, in a climate very close to their original one.”

Rainforest ecosystems are also considered to be particularly threatened by climate change. Climate modeling studies have indicated that these regions are likely to become warmer and drier, with a substantial decrease in precipitation over much of the Amazon.15 Such changes could ultimately convert areas of the Amazon’s tropical forest into dry savannah and result in significant loss in biodiversity, according to the latest IPCC report.1

There is not much, if any, published evidence on MAPs that could be at risk in the rainforest from climate change, and regional experts that were contacted for this article were unable to comment on specific MAPs that may be vulnerable to climate change in rainforests. However, the expected loss of general biodiversity in the Amazon, as noted in the IPCC report, indicates the potential to lose both known and undiscovered MAP species.

“There are plenty of data to show that 70% of our drugs are based on natural plant medicines, at least in precursor forms,” said John Janovec, PhD, research botanist of the Botanical Research Institute of Texas and director of the Andes to Amazon Biodiversity Program (e-mail, May 8, 2008). “Yet only about 1% of the 400,000 flowering plant species on the planet have ever been screened for medicinal compounds. Compare this to the distribution of the diversity of plants on the planet. A large percentage of plants occur in tropical rainforests, and a large portion of those are found in the neotropical region, especially the Andes-Amazon region of South America.”

According to Dr. Janovec, impending climate change is just one factor that could contribute to the loss of MAPs in the Amazon. The region is also being negatively affected by deforestation and burning, unwise exploitation of the land, and rapid population growth. Loss of MAPs, coupled with loss of traditional knowledge by the indigenous peoples of these areas, could prevent important health discoveries and options in the future.

Thus, experts interviewed for this article expressed limited concern over the fate of MAPs within islands and rainforests, despite the fact that these ecosystems are also occasionally cited as being particularly threatened by future climate change.

Widespread Effects of Climate Change on Medicinal and Aromatic Plants

Some effects of climate change appear to be impacting plants worldwide. For instance, evidence has shown that climate change has been affecting vegetation patterns such as phenology (the timing of life cycle events in plants and animals, especially in relation to climate) and distribution.1 Some wild plants, including MAPs, have begun to flower earlier and shift their ranges in response to changing temperatures and weather patterns. Shifting phenologies and ranges may seem of little importance at first glance, but they have the potential to cause great challenges to species’ survival. They further serve as harbingers of future environmental conditions from climate change. Increased weather extremes are also predicted to accompany climate change,1 and plant species’ resilience in the face of these weather events may also factor into their abilities to adapt and survive.

Shifts in Phenology

The life cycles of plants correspond to seasonal cues, so shifts in the timing of such cycles provide some of the most compelling evidence that global climate change is affecting species and ecosystems.16,17 Available evidence indicates that spring emergence has generally been occurring progressively earlier since the 1960s.17 Such accelerated spring onset has generated noticeable changes in the phenolgical events of many plant species, such as the timing of plants’ bud bursts, first leafings, first flowerings, first seed or fruit dispersal, etc. Records indicate that many plants—including MAPs—have started blooming earlier in response to the earlier occurrences of spring temperatures and weather.

A 2003 meta-analysis of 9 phenological studies from various countries, involving 172 plant and animal species, found a mean shift toward earlier spring timing of 2.3 days per decade.18 Many studies have shown that plant species that normally flower in early spring are experiencing some of the greatest acceleration from warming, whereas species active later in the growing season can be unresponsive or experience delayed phenological events.16 Plants in the wild appear to be more disposed toward phenological shifts than cultivated plants.

Researchers from Boston University, led by Biology Professor Richard Primack, PhD, have spent the past several years collecting phenological data of plants and birds in the Concord area of New England in the United States.19 They have used the notes of renowned American naturalist Henry David Thoreau (1827-1862), in addition to herbarium records and local historical accounts, to create a baseline of spring events dating back to the 1850s. Their data indicate that many of the region’s plants are now flowering more than a week earlier than when Thoreau recorded such events, just a little more than 150 years ago.

Of the hundreds of plant species tracked by the Boston University researchers, several are medicinal plants. According to Abraham Miller-Rushing, PhD, who compiled data with Dr. Primack and now serves as a postdoctoral researcher for the Rocky Mountain Biological Laboratory, wild medicinal plants in the Concord area that have experienced accelerated flowering times include wormwood (Artemisia absinthium, Asteraceae), feverfew (Tanacetum parthenium, Asteraceae), and cranberry (Vaccinium macrocarpon, Ericaceae). St. John’s wort (Hypericum perforatum, Clusiaceae) now flowers 6 days earlier than in Thoreau’s day, and peppermint (Mentha x piperita, Lamiaceae) now blooms 10 days earlier (A. Miller-Rushing, oral communication, March 20, 2008).

The phenological data gathered by the Boston University researchers may offer a good prediction of how plants’ life-cycles may be changing across similarly developed areas of the United States. According to Dr. Miller-Rushing, heavily developed urban areas such as Concord are typically experiencing greater temperature rises than rural areas. Much of this greater temperature rise is attributable to urban heat island effect, a phenomenon related to increased urbanization in which parking lots, streets, and buildings absorb heat while vegetation loss lessens the natural cooling effects of shade and evaporation of water from soil and leaves. Long-term weather data show that spring temperatures in Concord have risen by approximately 4.5° F, which is above the global average of about 1.5° F. Dr. Miller-Rushing noted that many rural areas, on the other hand, are experiencing temperature increases close to the global average.

Naturalist organizations and collaborations around the world have been tracking phenological shifts of plants and animals by encouraging citizens to enter personal observations and historical records of phenological events into online databases. Such a national initiative, called Project Budburst, was launched in February 2008 in the United States.20 Nature’s Calendar, a similar organization based in the United Kingdom, receives data entries from thousands of recorders each year. Nature’s Calendar was launched as a national initiative in 2000, and its database contains phenological information on many species from previous centuries. Data from Nature’s Calendar provides evidence that many UK plants have begun to bloom earlier, including such widely used medicinal plants as hawthorn (Crataegus monogyna and C. laevigata, Rosaceae) and horse chestnut (Aesculus hippocastanum, Hippocastanaceae).

“There has been an advance in the flowering dates of most British plants,” explained Kate Lewthwaite, phenology manager of Nature’s Calendar (e-mail, April 4, 2008). “Species like hawthorn and horse chestnut are good [to track] because they are widely distributed and recognized and are very responsive to temperature. Hawthorn in particular may be up to 10 days earlier for each 1° C rise in temperature. In 2007, the warmest spring since records began in 1659, it flowered on 18 March (UK average). A typical date for the 30 year average would be 3 April. Horse chestnut flowered on 4 April; the 30 year average is 15 April.” (The 30 year average refers to data from 1961-1990, which the UK Meteorological Office identifies as the latest complete set of data that meets the definition of a long term temperature period.)

Shifts in plant phenology could lead to any of several disruptive ecological effects. The timing of a plant’s life cycle can affect whether it reaches optimal seed set before the end of the growing season. Phenological variation between plant species in the same ecosystem can reduce competition for pollinators and other resources, and, conversely, phenological similarities can benefit plants that rely on other species to attract pollinators. The timing of growth stages can also determine the length of the growing season. Furthermore, it is possible that interdependent species—such as particular plants and their pollinators—may not shift their phenologies in harmony with one another, and such mistimed relationships could pose a danger not only to the plant species’ survival but also the pollinators, having a concomitant cascade effect on the rest of the ecosystem.2,17,19

Dr. Miller-Rushing posits that phenological shifts will likely result in “winners” and “losers.” Some species may benefit from phenological shifts; others may become threatened by these shifts. He added that most medicinal plant species are relatively common, so they are less likely to be under significant risk of threat from these phenological changes. Rare MAP species occurring in small populations, however, would be more endangered by any form of environmental stress, including climate change.

“Plants have a lot of time-sensitive relationships, and many will be disrupted in the future from climate change,” said Dr. Miller-Rushing. “Our best guess is that things are going to get shaken up, but we don’t know exactly how.”

Dr. Miller-Rushing noted that there is a lot of variability between species, and it can be difficult to predict how climate change will affect the phenologies of different plants. Highbush blueberry (V. corymbosum), for instance, is flowering 2 to 3 weeks earlier in the Concord area than it did in the mid-19th century, whereas many other plants have not demonstrated any change at all.

According to Chuck Wanzer of Botanics Trading LLC, a company based in Blowing Rock, North Carolina, phenological shifts of medicinal plants are not significantly affecting wild harvesting practices. He noted that there have always been variations in the timing of the seasons, and collectors of wild medicinals are accustomed to adjusting their harvesting schedules accordingly. “There’s always been earlier springs and later springs, and you can adapt to that,” he explained (oral communication, April 14, 2008). “But if you keep having early springs, it can change whether a species can survive in that area.”

In particular, Wanzer noted that early blooming can become detrimental if an area is prone to cold spells late in the spring season. If a cold spell occurs a few days or weeks after early blooming has commenced, then those early buds or fruits could freeze, potentially killing or affecting the production of some economically useful plants. Apple orchards of North Carolina suffered severely from this type of scenario in 2007,21 and Wanzer noted that the medicinal plant bloodroot (Sanguinaria canadensis, Papaveraceae) has also been susceptible to frost following early blooming.

In some areas of the world, plants are experiencing not only earlier springs but also warmer spring temperatures that are more typical of summer. Dr. Hans-Jürgen Hannig, head of the department of cultivation and plant breeding for the German-based company Martin Bauer, arguably the world’s largest botanical raw materials supplier, commented that this phenomenon has been affecting many areas of Europe over the past few years (e-mail from V. Wypyszyk, July 20, 2008). Several European countries have experienced severe heat waves lately; researchers have found that the frequency of hot days has nearly tripled and that the duration of heat waves has doubled in Western Europe since 1880.22

“We are observing that for the last 5 to 10 years, a ‘normal’ spring period is absent,” said Dr. Hannig. “There is an abrupt transition from winter to summer with temperatures in April and May that are more typical for summer temperatures. This has caused, for example, that throughout Europe spring planting of chamomile [Matricaria recutita, Asteraceae] has been disastrous with an average loss of yield of 80%.”

Shifting Ranges

Changes in climate are also causing plants to migrate into new ranges. Studies and computer modeling programs have found that plants’ ranges have begun to shift towards the poles and/or to higher elevations in an effort to “reclaim” appropriate growing zones.17,18

A 2003 meta-analysis of 3 studies, including data on climate-related range shifts of 99 plants and animals of different Northern-hemisphere countries, found that range limits of species have advanced an average of 6.1 km per decade northward (or meters per decade upward).18 The Mapped Atmosphere-Plant-Soil System Study (MAPSS), a global scale vegetation distribution model that simulates the potential natural vegetation that could exist in areas under present and future climate scenarios, also indicates a global poleward migration of plants from conditions of climate change, including northward migration of vegetation in the United States.23 The Arbor Day Foundation released an updated map of US hardiness zones for plants and crops in 2006 based upon climate statistics, showing significant northward shifts of warm zones within the country.24

Few, if any, studies have focused on range shifts of medicinal plant species (although some medicinal plants may be included in studies of larger plant populations). However, Wanzer of Botanics Trading mentioned that, from his own observations, the range of economically useful saw palmetto (Serenoa repens, Arecaceae) plants appears to be slowly shifting northward due to warming temperatures and changing rain patterns. Saw palmetto grows only in the Southeastern United States—primarily in Florida but also in southern areas of Georgia, Alabama, and South Carolina. In some of the northern areas of the plant’s range, saw palmetto is able to grow but does not produce a significant amount of berries. Over the last 2 years, Wanzer has noticed that wild saw palmetto berries seem to be more dominant in central Florida than southern Florida, although this has not been confirmed by any published reports of saw palmetto populations. He predicts that more berry-producing plants may eventually spread farther into Georgia (where the plants currently grow but do not typically bear fruit).

Computer models suggest that range shifts may accelerate over time, as a result of continued climate change. A 2005 article in the journal BioScience noted that the preferred ranges of many species could shift tens to hundreds of kilometers over only 50 to 100 years according to some simulations—“nearly an order of magnitude faster than may have occurred since the last glaciation.”25

A team of researchers from Germany and France recently used computer modeling to project how future climate and land use changes could affect the distribution of plant species in Germany.26 This was the first study to estimate expected range changes of plants at a national scale under climate and land use conditions.

The researchers used different models and 3 different climate scenarios—forecasting moderate, intermediate, and severe climate changes—to estimate the ranges of 845 European plant species within Germany up to the year 2080. Only 550 of the selected plant species are currently recorded in Germany. As might be expected, the authors found that range loss and gain were generally most dramatic under conditions of severe climate change, with less extensive range changes associated with lower levels of warming. However, even the results of the moderate climate change scenario suggest that German flora would be affected negatively by future climate change and land use patterns. Species that are currently not recorded in Germany could migrate into the country as the climate warms, potentially disrupting existing species pools.

One medicinal plant included in the researchers’ projections was aconite (Aconitum napellus, Ranunculaceae), which demonstrated a loss of over 70% of its current range within Germany under all 3 climate scenarios. A medicinal species with large expected range gains under all 3 climate scenarios, on the other hand, is peony (Paeonia officinalis, Paeoniaceae).

The researchers further found that certain areas of Germany may be particularly susceptible to species loss and turnover (variation in species composition), such as the eastern and southwestern regions of Germany.

“In our analysis, we identified differences of local vulnerability of species distribution under scenarios,” said lead author Sven Pompe of the Helmholtz Centre for Environmental Research in Germany (e-mail, June 25, 2008). “For instance, Stellaria media [chickweed (Caryophyllaceae)] or Salix alba [white willow (Salicaceae)] have great range loss in the east of Germany: We hypothesize that drier conditions and higher temperatures (in combination) lead to local extinctions of species.”

Many uncertainties exist regarding how the ranges of plant species might be affected by future climate change, including the extent to which cultivated crops may be affected. According to Ronald Neilson, PhD, bioclimatologist for the US Forest Service, models such as MAPSS do not typically include simulations of agricultural crops (oral communication, June 4, 2008). Although it is expected that range shifts of cultivated plants would occur due to climate change, he said it is not entirely clear what type or degree of range shifts might take place. He added that there is some indication that agricultural crops will be more adaptable to climate change than natural ecosystems, but some regions may become significantly less productive. A recent report published by Botanic Gardens Conservation International noted that changes in patterns of crop distribution will have to occur with altered temperature ranges and rainfall availability. Some areas of the world are expected to gain increased suitable cropland, whereas others—particularly developing regions of Africa and South Asia—are predicted to experience marked declines in crop yield.27

The range shifts of wild plants from climate change could ultimately jeopardize the survival of some species. Dr. Neilson explained that all ecosystems contain a spectrum of species with varying degrees of migratory potential. At one end of this spectrum are those species that can migrate rapidly and aggressively, and at the opposite end are the specialist species with limited mobility. According to Dr. Neilson, vegetation will migrate in bands, with the fastest, most invasive species at the front, the slowest at the rear, and all others in between.25 This could cause situations where early successive species invade areas with slow-moving endangered species, and the 2 species may have to compete with one another. Moreover, some slow-moving species may not be able to migrate quickly enough to keep pace with range shifts generated by climate change. Natural and human-made barriers to migration could also affect the survival of some species undergoing climate-induced range shifts.

Some modeling programs do indicate that habitat loss and migratory challenges related to climate change could result in extinctions of many endemic species throughout the world.28,29 Because of the migratory challenges that some species will face as they respond to climate change, some government and wildlife organizations are discussing the possibility of actively assisting species migration. This could be done by opening broad swaths of wild land as migratory pathways, establishing new protected areas, restoring degraded habitat, and reducing the intensity of management in some areas to help connect fragmented habitats and encourage successful species dispersion.27,30

The Impact of Extreme Weather Events

Mounting evidence indicates that extreme weather events such as storms, droughts, and floods have become more prevalent and intense across the globe in recent years.1,2 The frequency and severity of these events are expected to increase in the future as a result of continued warming, having negative effects on human health, infrastructure, and ecosystems. (It is important to note, however, that although trends in extreme weather events have been observed and projected, it is still difficult to attribute individual weather events directly to global warming.) Extreme weather events have been known to affect harvesters’ and cultivators’ abilities to grow and/or collect medicinal plant species, and such difficulties have certainly been reported in recent years.

Europe has experienced particularly devastating droughts and floods in recent decades. An intense heat wave and drought struck Europe in 2003 that caused up to 35,000 deaths, while heavy precipitation and flooding occurred throughout areas of Europe in 2002 and 2005.31,32 Climate model simulations indicate that Europe could experience a pronounced increase in year-to-year climate variability and that both droughts and floods could become even more frequent and intense in the future.1,33,34

Extreme weather conditions throughout Europe are impacting medicinal plant production from seeding to harvesting. According to Dr. Hannig of the Martin Bauer company, the extremely dry soil conditions resulting from recent abnormally hot summers has prevented successful fall reseeding of some medicinal plants, such as chamomile in Germany and Poland. Dr. Hannig added that 2007 marked the first year that fennel (Foeniculum vulgare, Apiaceae) was recorded as having no yield at all in Bulgaria, due to drought conditions during the spring in that country. Serbia’s long and dry summers, which have been accompanied by other extreme weather conditions such as strong rains and winds, have sometimes made it impossible for harvesters to perform second cuttings of the aerial parts of cultivated herbs such as peppermint. Hungary, meanwhile, has been experiencing increasingly severe flooding for the past 3 to 4 years, which has led to yield deficits for both fennel and anise (Pimpinella anisum, Apiaceae).

Medicinal plants on other continents have also been impacted by severe weather conditions. Africa’s Sahel region experienced one of the most severe droughts of the 20th century. Although there has been debate as to the primary cause of this drought, some climate models indicate that human-induced aerosol loading and increased greenhouse gases are at least partly responsible for the region’s drying trend.35 Climatic conditions in the Sahel have improved slightly in recent years, but the future is uncertain. Some models suggest that rainfall in the Sahel may increase in future years; others suggest that drought conditions could become even more severe than that already experienced in the 20th century.35

According to Denzil Phillips, business advisor for the Association for African Medicinal Plant Standards (AAMPS), an organization that has been developing and promoting quality control standards for African medicinal plants, medicinal plants of the Sahel include hibiscus (Hibiscus sabdariffa, Malvaceae), myrrh (Commiphora africana, Burseraceae), frankincense (Boswellia spp., Burseraceae), baobab (Adansonia digitata, Malvaceae), moringa (Moringa oleifera, Moringaceae), and various aloes (Aloe spp., Liliaceae). Phillips stated that most of these medicinal plants are primarily grown or collected for local markets. He stressed that environmental degradation associated with the areas’ civil wars and mass migrations have, to date, had a far greater impact on medicinal plant production and collection than problems attributable to climate change. However, future drought from climate change could have devastating effects on the region’s already suffering ecosystems and harvesting capabilities.

“Since only a very small minority of cultivated medicinal plants in Africa are grown using irrigation, the impact of global warming and low rainfall is extremely serious,” said Phillips (e-mail, June 27, 2008). He explained that prices of medicinal plants have already risen substantially in many areas due to increased collection costs and declining yields. This often leads healers to switch to cheaper alternatives that are typically of inferior quality or to use less ingredients than are traditionally employed. He added that Africans may ultimately begin to move away from wild harvesting practices and reliance upon rain-fed crops in favor of irrigation, where possible, and there may also be a shift of production toward the more humid tropics.

India, whose climate is largely controlled by an annual monsoon, appears to be experiencing increasingly severe and erratic precipitation. A recent study found that the overall amount of monsoon rainfall across central India has remained relatively stable over the past century; however, moderate rainfall events during monsoon seasons have significantly decreased while extreme rainfall events have greatly increased since the early 1980s.36 This increase in extreme rainfall events could indicate greater potential for future natural disasters. Experts have claimed that the frequency and intensity of flooding has likewise been increasing in India in recent years,37 and hailstorms have caused huge agricultural losses across areas of India lately.38

CE Roeper, a German-based supplier of natural raw materials such as resins, gums, and herbal ingredients for the food and pharmaceutical industry, obtains some of its raw materials from Northern India. Jörn Herrmann, a sales manager for CE Roeper, visited the states of Rajasthan and Gujarat in April 2008. According to Herrmann, these 2 states experienced hailstorms and rains in 2006, 2007, and 2008, at times when such events traditionally have not occurred within the past 50 years (e-mail, July 30, 2008). During his trip there, Herrmann spoke with “some very old farmers” who indicated that they had never seen these phenomena before. Hail and rainstorms have also damaged psyllium (Plantago ovata, Plantaginaceae), wheat (Triticum aestivum, Poaceae), and cumin (Cuminum cyminum, Apiaceae) crops in the area. The destruction of Indian psyllium crops from hail and rainstorms resulted in a smaller than usual annual yield for 2008. Similarly, Herrmann noted that the availability of menthol crystals was affected by heavy monsoon rainfall, which occurred earlier than usual in Northern India and reportedly damaged wild mint (Mentha arvensis, Lamiaceae) crops in 2008.

Hurricane seasons could also be affected by climate change, although experts do not agree on the possible effects. Some experts believe that hurricanes will increase in frequency, duration, and intensity; others predict that hurricanes will either not be significantly affected or might even be inhibited by factors related to warming.39 Regardless, shifts (whether increasing or decreasing) in hurricane activity have the potential to affect the availability of medicinal plants. Several hurricanes that hit the Southeastern United States in 2004, for instance, decimated saw palmetto crops that year, contributing to a temporary shortfall of saw palmetto available on the market.40

Increasing evidence and studies have thus shown that at least some types of extreme weather events have been striking more frequently and with greater force throughout the world. Although particular weather events cannot be definitively blamed on climate change, the negative effects of some recent droughts, storms, and floods on herbal crops demonstrate the threat that increased extreme weather could pose to the availability and supply of MAPs.


The effects of climate change are apparent within ecosystems around the world, including medicinal and aromatic plant populations. MAPs in Arctic and alpine areas face challenges associated with their rapidly changing environments, and some researchers have raised concerns regarding the possible losses of local plant populations and genetic diversity in those areas. Shifting phenologies and distributions of plants have been recorded worldwide, and these factors could ultimately endanger wild MAP species by disrupting synchronized phenologies of interdependent species, exposing some early-blooming MAP species to the dangers of late cold spells, allowing invasives to enter MAP species’ habitats and compete for resources, and initiating migratory challenges, among other threats. Extreme weather events already impact the availability and supply of MAPs on the global market, and projected future increases in extreme weather are likely to negatively affect MAP yields even further.

“As research is emerging on the effects of climate on vegetation in general, there continues to be a dearth of information on medicinal (economic) plants,” said Patricia DeAngelis, PhD, botanist at the US Fish and Wildlife Service and chair of the Plant Conservation Alliance—Medicinal Plant Working Group (e-mail, September 17, 2008). “Although some information can be gleaned from general floral research, what makes medicinal plants unique from other flora is the fact that they, along with other economically useful plants, are collected for human use.” Dr. DeAngelis continued that there is a need for more research into the effects of climate fluctuations on plants in general, but there is especially a need for research that incorporates ethnobotanical information (such as the perceived availability of species, changes in collection practices, etc) as it pertains to effects of climate fluctuations on MAP species.

Climate change may not currently represent the biggest threat to MAPs, but it has the potential to become a much greater threat in future decades. Many of the world’s poorest people rely on medicinal plants not only as their primary healthcare option, but also as a significant source of income. The potential loss of MAP species from effects of climate change is likely to have major ramifications on the livelihoods of large numbers of vulnerable populations across the world.27 Further, the problems associated with climate change are likely to be much more difficult to combat than other threats to MAPs. Dr. Salick of the Missouri Botanical Garden noted that laws can be passed to stop deforestation and over-collection, and in some cases such laws have achieved immediate results. The problems posed by warming temperatures, disrupted seasonal events, extreme weather, and other effects of climate change, on the other hand, cannot be so quickly and easily resolved.

Climate change and its effects will certainly increase in the near future, although the extent to which they do so cannot presently be determined. The effects of climate change on medicinal plants, in particular, has not been well-studied and is not fully understood. As the situation unfolds, climate change may become a more pressing issue for the herbal community, potentially affecting users, harvesters, and manufacturers of MAP species.

* Although the terms “global warming” and “climate change” are often used interchangeably, “climate change” is often the preferred term of many environmental organizations and government agencies. Climate change refers to any significant change in measures of climate (such as temperature, precipitation, or wind) over an extended period of time (decades or longer). Global warming refers to an increase in the temperature of the atmosphere that can contribute to change in global climate patterns. The Intergovernmental Panel on Climate Change considers “climate change” to mean any change in climate over time, whether due to natural variability or as a result of human activity. The United Nations Framework Convention on Climate Change defines “climate change” as a change in climate that is attributable directly or indirectly to human activity that alters atmospheric composition.

† Although the primary focus of this article concerns medicinal plants, much of the threat to these plants includes aromatic plants harvested for their essential oils, which could be used for medicinal, fragrance, culinary, and/or other purposes. This article therefore refers to all such plants under the widely-used acronym MAPs.

‡ Arctic plants typically produce phenolic compounds that protect plant cells from free radicals resulting from photoinhibition (a mechanism that can be caused by a high light regimen and low temperatures). Also, anthocyanins are known to pigment Arctic plants in reddish color and have been shown to attenuate the amount of light reaching photosynthetic cells and therefore reduce the risk of photoinhibition. Flavonoids are also common in Arctic plants, and they protect against UV damage such as cell apoptosis related to DNA breakage. If Arctic plants begin to produce less of these compounds as a result of higher temperatures, they may partly lose their ability to serve as antioxidants for human health benefit.



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