Understanding the impact that climate change has on tea crops (derived from the plant Camellia sinensis (L) Kuntze
) is critical given the significance of the product to human societies and cultures globally. Tea is the most consumed beverage in the world after water and is an important economic engine for rural communities in many countries [1
]. Camellia sinensis
is an evergreen perennial shrub that produces leaves that are used to make a variety of tea types including green, black, white, and oolong. In 2012, 4.6 million tons of it were produced in 50 countries worldwide [2
]. Tea is grown predominately in Southeast Asia, Eastern Africa, and in parts of Latin America and can be grown in a variety of ecosystems from 49°N to 30°S and from sea level up to 2700 m in altitude [3
]. In China and India, the top two global tea producers, 80 million and three million rural laborers, respectively, are involved in tea production or processing [4
(hereafter referred to as tea) has been found to have health benefits when consumed regularly. For example, some epidemiological studies have found tea consumption to be associated with a lower risk of developing Type 2 Diabetes, cardiovascular disease, cognitive impairment, depressive symptoms, and reduced incidence of cold and flu symptoms [6
]. One recent meta-analysis found a statistically significant decrease in lung cancer risk in people consuming green tea [8
]. A literature review of the association between tea consumption and cardiovascular disease found a dose response association on the incidence of mortality from stroke [9
]. These health benefits are attributed to the chemicals contained in tea. Flavonoids, L-theananine, caffeine, and nonproteinic amino acid have known health benefits and make up between 25% and 35% of tea’s fresh weight [10
The quantity of these chemicals in tea is highly dependent on the prevailing climate conditions where tea production occurs [12
]. These chemicals, in turn, drive both harvesting and consumption decisions. For example, teas harvested in Southern Yunnan Province, China before and after the onset of the East Asian Monsoon exhibited dramatic changes in secondary metabolite chemistry [14
]. According to research on tea production in Australia, tea shoot growth favors warm and rainy conditions that prevail during the season of heaviest rainfall. However, it is during slow growth periods when there is less rainfall that shoots are plucked because leaves accumulate greater secondary metabolites that drive quality [17
]. Farmers interviewed in Yunnan China confirm this finding; they believe tea quality varies by season and is especially affected by the arrival date of the East Asian Monsoon [18
]. Another study in a controlled greenhouse environment found that tea grown with higher water availability had significantly lower concentrations of epigallocatechin 3-gallate, a flavonoid found in many teas that helps to drive quality of the consumable product [19
]. Clearly, the monsoon rainfall plays a key role in plant growth and quality and harvesting decisions. This serves as a key motivating factor for the analysis in this paper.
Our analysis is also motivated by the fact that precipitation, temperature, and surface radiation patterns have already significantly changed across China’s tea growing regions over the last half century. One study reports that rainfall has increased 20 to 60 mm per decade since 1950 in southern and southwestern China [20
]. This increase has come in the form of fewer rain days but more intense rain events, which can be detrimental to agricultural crops [21
]. The number of days per year with a daily maximum temperature of 35 °C or greater has increased during the past 50 years as well [21
]. Wild (2005) also found that solar radiation has been increasing across China since 1990 [22
These climatic changes have also already had an impact on agricultural production, including tea, in China according to recent research. Using simple pairwise correlations and trend analysis one study found that changes in temperature and precipitation between 1981 and 2000 resulted in declining yields for rice, wheat, and maize across China [23
]. For tea, rising temperatures in tea growing regions of China have been found to result in later spring freezes which can severely impact tea growth and development [24
]. Changes in prevailing weather patterns are expected to continue across China in the future according to various climate change projections. The 2014 Intergovernmental Panel on Climate Change (IPCC) found that mean surface temperatures in China are expected to increase between 2 °C and 5 °C over the next century [25
]. The number of extreme warm temperature events across China are expected to increase significantly over the same time period even in the most conservative global warming scenarios [26
]. The IPCC also concludes that future climatic changes will increase East Asian Monsoon precipitation via an earlier monsoon onset and a delayed retreat across much of southwestern China [27
]. All of this suggests the need for continued research quantifying the impact of weather factors on agricultural yields. These types of analyses provide critical information for predicting the impact that climate change will have on agricultural production, farmer livelihoods, and food security outcomes for the global population.
The link between climate and tea production has been studied in detail in some parts of the world and in field-scale experiments [17
]. However, no study to date has typified this association on a large scale or specific to the Chinese context. To fill this knowledge gap the present study assesses how historical climate changes in China are associated with tea yields using a yield response model. A yield response model uses Ordinary Least Squares (OLS) linear regression to model yield as a function of various exogenous weather factors. Our analysis focuses on quantifying the association between the East Asian Monsoon onset, duration, and retreat and tea yields. We also examine how other weather factors are associated with tea yields, including minimum temperatures, precipitation before, during and after the estimated monsoon period, and solar radiation. Previous large to medium scale studies employing yield response or Ricardian analysis techniques have focused on staple crops like corn and rice, as well as on fruits and vegetables, but none have examined tea [29
]. There are a variety of other statistical methods not used in this paper to identify the contribution of climate change to crop yield variation and these are discussed in some detail in [33
Our analysis is novel because unlike previous studies examining the relationship between monsoon dynamics and agricultural yields, which have assumed the monsoon is stationary over time, we estimate the actual monsoon onset and retreat empirically [29
]. As noted before, the timing of rains, at least anecdotally, is important to Chinese tea productivity and management decisions. Studies of other agricultural crops also confirm this association between timing of rainfall and yields and that the onset of seasonal rains may not be stationary because of a changing climate conditions [34
]. As such we hypothesize the monsoon period to be non-stationary over the study period and so the onset and retreat should be estimated empirically. To do this we find the break-points in the linear daily year-by-province cumulative precipitation function to arrive at approximate East Asian Monsoon onset and retreat dates for the study period in the tea growing provinces of China [36
]. This method provides new insights into how climate change affects tea yields through changes in the timing of seasonal rains. We hope the application of this technique more accurately captures how monsoon and seasonal dynamics affect crop productivity in tropical and subtropical regions globally.
The primary unit of analysis in the present study is the Chinese province. Chinese provinces are relatively large geographical regions, which does present some inferential limitations to our study. Nevertheless, this study is an important contribution to the examination of the relationship between Chinese tea yields and weather patterns, especially with respect to the timing of the monsoon season. Additionally, understanding at a large-scale the association between weather and yields is, in its own right, important for policymaking purposes and for informing farm management practices in a broad sense. We also hope the methodology in this study will spur future analyses of Chinese tea production at the sub-province level since data limitations currently preclude that level of detail. We will discuss this limitation further in later sections of this paper.
We use general bounds of acceptable temperature and precipitation for tea to develop our analytical model. Camellia sinensis
varieties achieve optimal growth with maximum daily temperatures between 20 °C and 30 °C and can continue to grow without risk of serious heat stress up to 35 °C [38
]. Minimum temperatures should remain above −10 °C for optimal growth and can reach -20 °C without causing irreversible damage [28
]. In the spring, bud bursting, leafing, and flowering occur when the mean maximum daily temperature is above 10 °C [38
]. In general, extreme minimum temperatures are detrimental to tea growth [40
]. Temperatures in the spring are particularly important for tea quality and yields and in recent years late spring frosts have caused significant financial loss for Chinese tea farmers [24
The annual rainfall needed for optimal growth is 1500–2000 mm [5
]. However, as noted in Carr (1972), the distribution of rainfall over the course of a growing season is as important to tea growth as total rain accumulation [28
]. A relatively old study showed that rainfall during the early part of the growing season had a greater positive impact on yield than precipitation falling during the rainy season [43
]. There are also reports that rains in excess of 5000 mm per year could affect tea growth success [28
]. More generally, additional research has found that extreme rainfall can negatively affect tropical plant growth and development [19
]. However, the upper threshold of rainfall (daily or at other temporal scales) has not been definitively ascertained according to our search of the literature. In this regard, our study helps to address this particular knowledge gap, albeit at a very aggregated geographic scale.
Finally, we also consider the relationship between solar radiation and tea yields. Solar radiation provides energy for plants to photosynthesize and grow [45
]. Further, as noted earlier, recent evidence suggests that solar radiation is changing over China and this change could have an impact on tea yields [22
]. As such, solar radiation its own right an important factor to consider in the assessment of weather factors and tea yields.
Our results show that tea yields in China are impacted by daily precipitation and temperatures, as well as by seasonality as determined by the empirically estimated onset and retreat of the East Asian Monsoon. Notably, we find a consistent negative association between tea yields and the monsoon retreat date in all of our model specifications. We find that a 1% increase in the monsoon retreat date is associated with a 0.481% and 0.535% reduction in tea yield. We can consider this association in practical terms by examining recent average retreat dates of the East Asian Monsoon. In 2010 the average retreat date was on the 260th Julian day, which corresponds to the East Asian Monsoon ending in mid-September. According to the results in Model 1, a 2- to 3-day lengthening of the monsoon corresponds to a 0.481% reduction in yields over the historical period. In 2010, the average yield across the 15 provinces in our study was 1000 kg/ha, and thus a 0.481% reduction would cause yield to decline to 952 kg/ha on average. This is a notable reduction in yields resulting from such a small shift in the monsoon retreat.
The mechanism for yield loss with a delayed monsoon retreat can be explained by plant responses to changes in the distribution of rainfall over a season and, as a consequence, harvest timing decisions. Both plant phenological and physiological processes are impacted by extreme weather patterns, including precipitation, caused by climate change [44
]. It can also be explained through changes in management practices. To our knowledge tea farmers in some parts of China wait until the end of the monsoon to harvest tea and a delayed retreat would reduce the number of picking days available to farmers in the post-monsoon period. Additionally, excess water in soils after a longer or rainier monsoon season may leach nutrients from the soil thereby reducing crop growth or quality or both. These are merely hypotheses and further work is warranted. Some recent research has confirmed the monsoon-yield association with tea farmers in Yunnan Province, China who harvest premium tea especially during the spring, but also in the autumn once the monsoon has ended [18
]. Tea harvested outside of the monsoon garners higher prices in tea markets in China because of sensory characteristics preferred by tea buyers [18
]. As was illustrated in Figure 7
, the East Asian Monsoon retreat has experienced significant variability since 1980 to the present time across our sample even though the average retreat date is the same in 1980 and 2010. However, the IPCC expects that the East Asian Monsoon retreat will occur later in the year across China over the next half century [27
]. This could be detrimental to the success of tea production across China and adaptive management strategies may need to be identified to adjust to a longer monsoon season or a delayed monsoon retreat.
In our analysis, we also consistently find that an increase in average daily monsoon precipitation is negatively associated with tea yields. While we know that tea requires a relatively large amount of rainfall annually for adequate growth [38
], this finding suggests that an increase in daily precipitation, on average over the monsoon season, may be detrimental to tea yields. Two factors might explain this negative correlation. First, cloud cover associated with increasing daily precipitation limits plant and tree growth in tropical regions [60
]. From a farm management perspective less tea may be harvested during the monsoon season or during growing seasons with an increased in the frequency of heavy one-day rain events because this results in lower quality tea. Ahmed et al.
(2014) found that farmers in Yunnan harvest much less tea during the monsoon and some even note that the quality of tea is inversely related to the amount of precipitation that falls during the season which supports this hypothesis [18
]. We also postulate that tea farmers will not want to harvest plants in water-logged fields after a heavy rain event because it takes more effort to harvest and process wet leaves, which are already of lower quality.
Ultimately, new planting and soil management practices may need to be developed to help Chinese tea farmers cope with changes in precipitation and monsoon dynamics. These might include more robust soil management techniques that help to build and maintain soil organic matter which can increase water holding capacity so that more water can infiltrate and percolate the soil when extreme rain events occur [61
]. Soil with high levels of organic matter can also retain more water during drier conditions or in droughts. These management strategies are likely to be of increasing importance in light of the fact that the frequency of extreme rain events is expected to increase as a result of future effects of climate change [27
]. Farmers in Yunnan province report some other strategies to address changes in precipitation patterns. These include planting tea from seed instead of using clonal propugules which have less dense and deep root systems; mixed cropping of tea gardens in forests; and enhancing the drainage of tea gardens [19
We also find that the previous year’s monsoon retreat date and average daily monsoon precipitation are negatively associated with tea yields. In general, this is plausible because tea plants are perennial and so previous year’s weather patterns and seasonal dynamics could feasibly be associated with contemporaneous yields. To our knowledge the impact of previous year’s weather conditions on tea yields has not been established in the agronomic research. This is certainly an important area for further exploration, the results of which could validate our findings. It is unclear, however, why average daily solar radiation in the previous year was negatively associated with yields since tea generally prefers tropical temperatures in the range of 20 °C to 30 °C. Since surface radiation can be interpreted as a proxy measure of average daily maximum temperatures, we could interpret this finding to mean that average daily maximum temperatures (only included in the model through average daily solar radiation) during the monsoon from the previous year have a delayed, yet detrimental effect on tea yields contemporaneously. While this finding makes sense if the previous year experienced hotter than average daily maximum temperatures, there could also be a non-linear relationship between solar radiation and tea yields. In addition to being related to temperature, solar radiation is also generally found to be negatively associated with precipitation—hence, increased solar radiation may decrease tea yields because it represents a reduction in precipitation as well as an increase in temperature. Understanding which is more detrimental to tea growth the following year is a question for further research. As noted above, we assessed the potential for a non-linear relationship between solar radiation and yields by regressing the natural logarithm of yield on the natural logarithm of surface radiation and its quadratic term. We did not, however, find that this non-linear relationship was significant at our aforementioned statistical thresholds.
5. Conclusions and Limitations
Overall, our findings indicate that the monsoon retreat date and average daily precipitation during the monsoon are the strongest predictor of yields in major tea producing regions in China over the historical period. These findings suggest the need for adaptive management and harvesting strategies given the known negative effects of excess rainfall and delayed monsoon retreat have on tea [18
]. More research is needed to continue to understand the relationship between climate change, weather patterns and tea yields, not just in China but also in other major tea producing countries. The analytical limits of the present study also warrant further examination of this relationship.
Our study has three major limitations that warrant clear explanation. First, tea is grown in a variety of ecosystem types and under a fairly wide range of acceptable temperature and precipitation values as noted in the introduction. Thus, some of our general conclusions about the association with specific weather factors and yields may not hold when examined at a finer scale (i.e., at the county or prefecture level). A sub-provincial analysis would require significant data collection efforts in China that were beyond the scope of the current study. Because of the scale of our analysis we are also unable to determine the relationship between yields and extreme weather events on tea production. This is because the association between extreme weather events and yield are diluted by the geographic scale of our data.
Second, weather factors are not the only driver of tea production. There are likely intra- and inter-province differences in tea production including fertilizer and pesticide application rates, labor use, and water use. These non-weather input use differences would certainly affect the results of our analysis. To our knowledge, no data on these farm inputs was readily available at the time that this study was being conduced. Entity fixed-effects were used to capture the effect of some of these unobservable differences across provinces. However, there may still be some bias in our estimates for each climate indicator because of these data limitations. We also did not control for wind factors because data on prevailing winds was not available. This could potentially cause biased parameter estimates as noted by Auffhammer et al.
Third, at this level of analysis we cannot account for the fact that the production mix of Camellia sinensis
varies across provinces. Average yield does vary according to the tea type being produced and because of differences in post-harvest processing [53
]. Many varieties of tea can be produced from Camellia sinensis
including green, black, Oolong, and white. The only difference in production for these teas is the plucking standard. For example, only one leaf bud is typically harvested for oolong tea, whereas one to two leaves and one bud are plucked for premium green tea [53
]. Unfortunately, a specific measure of the production mix of tea in each province was not available to us but we did account for differences in yield across provinces using a fixed-effects term.
These limitations notwithstanding, we believe this research is an important contribution to the literature on how climate change impacts agriculture for two reasons. First, our analysis is the first to quantitatively assess China’s tea yield using a yield response model. This body of research tends to focus on calorie-rich crops like corn, wheat, and rice. However, tea is an important agricultural crop in terms of nutrition, economic value, and to the cultural heritage of many societies. Second, instead of quantifying the relationship between yield and historical average monsoon onset and retreat dates, we utilize a technique to estimate the monsoon empirically, shedding light on how the dynamics of seasons are correlated with tea yields. To our knowledge, this is the first study to examine the association between crop yields and empirically derived seasons. This is an important contribution to this area of research because it allows our quantitative model to better reflect what is happening in the real world. As noted before, Chinese tea farmers interviewed by Ahmed et al.
(2014) report the timing of the monsoon is as important as the amount of rain that falls in a particular growing season [18
Our study is the first of its kind to estimate the effect of climate change on Chinese tea production it by no means provides a complete picture of the relationship between climate, weather, and tea productivity. However, we do believe our approach can be coupled with case studies and smaller-scale analyses to create a detailed picture of the climate-tea yield-quality relationship in China. Continued research in this area is most certainly warranted given tea’s importance to human societies globally.