Some impacts of Climate Change on Economically Important Plants

 Evidence is accumulating that recent climate change particularly overall warming,and rising night temperatures is affecting growth and yield of important crops. Changes in the timing and intensity of precipitation also can cause additive effects. Here are some crops that either have already, or, are projected to respond to climatic changes based on field observations, crop yields or computer simulation models. No attempt was made to be comprehensive thus other crops not discussed may also be responding to climate change. Also, in many species, programs to develop crop varieties better adapted to climate change are underway and may improve future yield prospects.  See Larson, C., Science 348: 953-4 (2015) for general discussion of the topic covered in this post. 

In  tea (Camellia sinensis)  the  distinctive flavors  of the many varieties are traceable to a mix of organic compounds in leaves that vary depending on growing conditions. A change in these conditions can have economic significance tied to alterations in the balance of the leaf organic compounds that determine tea flavor. Even if changing rainfall and temperatures barely influence plant growth rates, tea connoisseurs can detect subtle changes in tea quality. For example, average temperatures in the southern Chinese province of Yunnan have risen 1.5 C in the last half-century. The monsoon rains have also changed- coming 22 days later in 2011 than the three previous decades (Larson 2015).  Both these changes have economic consequences for the local black teas (particularly the esteemed pu’er variety) because of altered drinking  experience.

Coffee (Coffea arabica and C. canephora)- Coffee plants are descendents of wild populations native to relatively cool upland areas in tropical Africa thus the concern about warming impacts on coffee production.  The Arabica variety which provides 70 % of global supply, will be severely affected by a temperature rise of 2-2.5 C that could occur by mid-century. The loss of area suitable for Arabica cultivation in Brazil may reach 25% as cooler uplands formerly ideal for coffee growing become too warm.

 Chocolate- Production of cocoa which is harvested from the pods of the cacao tree (Theobroma cacao) grown in West Africa, especially Ghana and Ivory Coast, likely will be reduced by expected climate change. Increased droughts and higher temperatures will increase evapotranspiration (loss of water by evaporation from surfaces and by release from foliage), reducing soil water content and leaving less water for pod development.

 Maple Syrup- Sugar maples (Acer saccharum) yield sugary xylem solution that flows from roots to shoots in early spring when temperatures range from just above to just below freezing throughout the 24 hour day. Warming in North America has already shortened the sap flow season and reduced syrup yield. Projections of future temperatures in maple syrup areas suggest continued decline in sap flow season will occur. Fortunately some mitigation may result from northward shifting of maple syrup production areas assuming local conditions support flourishing sugar maple growth.

 Cherries (Prunus cerasus and others) are sensitive to wide temperature fluctuations, which may increase in coming decades according to some climatic modeling. In 2012, a frigid winter was followed an exceptional warm spring in the Great Lakes states (north- central USA). Effects on cherry production were severe in some areas. In Michigan, 90% of the tart cherry crop was lost that year.

 Wheat- extrapolating from agronomic models that simulate wheat growth responses to temperature indicates that warming is already reducing yields in most wheat-growing regions. For each rise in 1 degree Celsius above the present average temperature, wheat yield is estimated to decline by 6% with increased variability in yield both temporally and spatially (Asseng et al 2015).

Hops – an essential ingredient of beer, hops  (Humulus lupulus of  the Moraceae, the fig family) are widely grown in the cooler regions of Europe and North America, especially in the Pacific Northwest. Hops prices have risen 250% in the past decade in response to both rising demand and to lowered yields. Warmer temperatures  and extreme weather events are implicated in  the production decline according to the Brewer’s Climate Declaration, an industry position paper signed by 42 breweries (ceres. org). In the Czech Republic, declines in the quality of the Saaz hops which imparts delicate flavor to pilsner beers, was already reported in 2009. The Czech Hydrometerological Institute analyzed detailed data on local climate, hops quality (based on alpha acid content)  and crop yield for a 52 year period, concluding that hops quality declined at an average annual rate of 0.06% from 1954 to 2006 (

 Both barley and oats are cool season grasses with optimum growth temperatures  between 68 and 70 degrees F. and substantial reduction of growth above 82-86 F. This is a concern in Minnesota where temperatures have already risen in the past few decades and are expected to continue. Mean maximum  temperatures in the 1980-2007 period increased 1.4 F in Minnesota while the mean minimum increased  2.2 F.  Minnesota is the leading oats producer in the US and 6th in barley production. Spring and summer temperatures are “highly likely” to increase in Minnesota and the Upper Midwest and growing season precipitation is expected to decrease in the region.  If drought accompanies warming, adverse effects on yield of barley and oats will increase. Fortunately to some degree, development of adapted cultivars is likely to compensate at some level for the warming effects.

 Interaction with other stresses also is likely.  Fungal pathogens, for example, will develop faster as temperatures rise. Fusarium graminearum, the cause of head blight fungus (a harmful pathogen of all grains) is favored by temperatures of 65-86 F.  Ambient temperatures in the higher end of this range can support Fusarium growth sufficient to cause large yield decreases in grain crops. If temperature increases continue  in spring and summer, further declines in the yield of cool-season crops would be  expected especially  if growing season precipitation decreased as predicted. (Klink, et al. Climate Variability and the Production of Barley and Oats in Minnesota. {})

 Potato- Crop simulation models of the effects of climate change on yield suggests potato yields in Ireland will decline beginning in 2055 due to limiting water availability  resulting from rising temperature and increased rainfall variability. (Holden, N.M., et al 2003  Agricultural and Forest Meteorology 116:181.

  Barley yield however, in Ireland, was projected to increase in response to warming temperature without being adversely affected by  the projected increase in rainfall variability

 Rice-  The  IFPRI (International,Food Policy Research Institute) forecasts that by 2050  rice prices will rise 32-37% and yield losses will decline by 10 to 15%. For every 1 C rise in night-time temperature, rice yield is projected to decrease by 10%. (these considerations ignore other climate-related interactions such as sea level rise effects on wetland rice in Bangladesh, coastal Myanmar and other coastal areas). Rice is a unique crop that it thrives in lowland, swampy soils that are flooded  for long periods. Under these conditions, paddies release methane (CH4) a potent greenhouse gas. Both higher temperatures and higher atmospheric CO2 levels stimulate rice growth but also accelerate methane releases to the atmosphere from flooded padi soils (van Groenigen, K.J. 

 Apples and grapes- flowering in these crops is triggered by a degree-days threshold that varies with variety. In some cases, warming effects may be mitigated by switching of varieties but because apples are orchard crops that grow for many decades, (often lasting more than a century) adaptation may require the expensive task of replacing mature trees with more heat-tolerant varieties.

 Forests – Leafing out in the northern hemisphere spring is triggered by rising daytime temperatures. Plants require a threshold amount of accumulated heat to trigger bud swelling and subsequent leafing out. Recent warming has already advanced the timing of leaf expansion in the Northern Hemisphere. A further increase of a single degree Celsius in Tmax (maximum daily temperature) would advance leafing out in deciduous forests by 4.7 days in Europe and 4.3 days in the US.  The effect is due more to Tmax (in spring)  than the rising minimum night temperatures (Tmin) based on models incorporating data from satellite greenness indices, field observations, and CO2 observations.

 Traditional Medicines-  Some evidence suggests that warming may affect the quality of traditional medicines ( Ziska, L., Agric. Research Serv., US Department of Agriculture). Of the 7.2 billion people on Earth, about 5 billion rely on traditional medicines to cure ills. Where herbs are gathered from wild areas, changing distributions, especially shifting northward and declining or disappearing more locally may affect availability to gatherers of  wild herbs.. Presently, an important phytochemical of concern is artemisinin, (derived from Artemisia annua)- the most effective anti-malarial drug presently available. A. annua is naïve to Asia but has established wild populations in portions of North America. Although higher atmospheric CO2 may increase the availability of artemisinin (through increased plant growth), too little is known about carbon and nitrogen relationships in medicinal plants in response to climate change to draw any firm conclusions.

Written January 2016