The Intergovernmental Panel on Climate Change (IPCC) in their report has confidently confirmed that climate change is happening (IPCC 2007). If there is no effort to limit CO2 emissions, Melillo et al. (1990) predicts that CO2 concentration will reach 500 ppmv by the year 2040 and 800 ppmv by the year 2100. Continuous addition of the greenhouse gases such as CO2 and other greenhouse gases to the atmosphere may exacerbate the climate change in the future. A simulation using General Circulation Models (GCM) for doubled levels of atmospheric CO2 predict a mean annual global temperature increase in a range of between 1.5 and 4.5oC (Mitchell et al. 1990; McGuire & Joice 2005). The increase in global temperature is predicted to influence global hydrological cycle and leads to the possibility of more intense droughts in some places and conversely more floods in other places (Kattenberg et al. 1996).

Plants have adapted to climatic conditions such as temperature, CO2 concentration and precipitation over long periods of time to become better suited to its ecosystem (Streck et al. 2008). Changes in those climatic factors therefore have the potential effect to the plant, including plants productivity (McGuire and Joice 2005; Hawkins et al. 2008). Plant productivity and the carbon cycle are likely to be altered by these climatic changes. Furthermore, although there are uncertainties, various studies on the possible effect of climate change on plant productivity reveals that too high temperature and severe drought may decrease plant productivity while elevated CO2 may increase plant productivity if there is no limiting factor.

In this paper, I attempt to explain about how climate change will influence plant productivity and the carbon cycle. The first part of this paper describes correlation between climate, plant productivity and the carbon cycle. The second part explains the effect of climate change to plant productivity which is divided into the effect of temperature changes, elevated CO2 and changes in rainfall pattern. The last part discusses uncertainties and limitations in predicting the future impact of climate change to the plant productivity.

Linkage between Climate, Plant Productivity and the Carbon Cycle

Climate is long-term weather pattern at a particular place resulted from the interaction of the atmosphere, the hydrosphere, the cryosphere, the biosphere and the earth surface, which are the elements that influence the earth’s climate system dynamics (Birdsley et al. 1995; Pittock 2009; World Meteorological Organization, no date). The status of the climate system is often changes from year to year, or decade to decade. Changes in the weather pattern over long time period, such as one century to another, are usually referred to as ‘climate change’(Pittock, 2009).

Climate, plant and the carbon cycle are interrelated in a number of ways. First, plants are sensitive to climatic change. Plants have adapted to climatic, atmospheric and soil conditions such as temperature and precipitation over long periods of time as an adaptation to its ecosystem. It means that dramatic change of these variables may affect many aspects of the plants including its productivity, development, distribution and increase its vulnerability to pests and fires (Streck et al. 2008).

Second, plant productivity is interrelated with the carbon cycle. As part of the carbon cycle, plants take part in the transfer of carbon through converting carbon dioxide (CO2) from the atmosphere and water (H2O) into Carbohydrate (C6H12O6) using solar energy. This is part of the process called photosynthesis. Conversely, plants release carbon to the atmosphere when they respire (Ajtay et al. 1979; Garnaut 2008). Photosynthesis by terrestrial vegetation accounts for about half of the carbon that annually cycles between Earth and the atmosphere (Hawkins et al 2008). The ability of plants to photosynthesize is one of the explanations that terrestrial plants have been influencing the carbon cycle and become increasingly greater carbon sinks over the past 50 years (Houghton 2007).

The exchange of carbon between plant and the atmosphere may be explained by four related terms, namely Gross Primary Productivity (GPP), Net Primary Productivity (NPP), Net Ecosystem Productivity (NEP) and Net Biome Productivity (NBP). The definitions among those terms are sometimes different in the literatures. Grace and Zhang (2006) define GPP as the rate at which the vegetation capture and store the carbon from atmosphere in the process of photosynthesis. NPP is a result of GPP minus autotrophic respiration (Ra). NEP or sometimes called as Net Ecosystem Exchange (NEE) is the the result of GPP minus autotrophic and heterotrophic respiration (Rh). NBP is the term used to measure the flux at a broader spatial and time scale (1 km2 upwards and 1 year upwards), which also take the disturbances into account.

These terms are interconnected as follows (Grace and Zhang 2006):


NPP = P Ra

NEP = P Ra – Rh

NBP = P Ra – Rh – D

The Effect of Climate Change on Plant Productivity and the Carbon Cycle

The main variables of climate change are elevated carbon dioxide (CO2), changes in precipitation and changes in temperature (Hawkins 2008). In this part, I will explain the effect of climate change on plant productivity which mainly focuses on these main variables and divided into three parts: the effect of the increase in temperature, elevated CO2 and changes in rainfall pattern.


The Effect of Increases in Temperature

The existing studies shows that direct effect of temperature changes are expected to be more significant than any other climatic variables (Kehlenbeck & Schrader 2007). The increase of temperature may affect four major aspects related to plant growth, namely photosynthesis, respiration, soil nutrients and development (Lewis 2005).

Although each plant has different characteristic response to temperature, generally, the increase of temperature will have positive impact on plant growth and development especially in low temperature regions when limitations from other factors are absent (e.g. water). Warmer temperature will help plants in very cold regions to grow because most biological activities of plants are hard to occur when temperature is below 0-5oC (Melillo 1990; Bisgrove & Hadley 2002). Furthermore, higher temperature will make growth season longer which increases plant growth in polar region (Kehlenbeck & Schrader 2007).

However, although the response of photosynthesis is initially positive, it will slow or even decline after reaching the optimum range which varies from plant to plant. This decline occurs because too high temperature may increase the rate of respiration which may exceeds the optimum level and may caused death of the plant and plants becoming a CO2 source (Mellilo 1990; Bisgrove & Hadley 2002; Hawkins et al 2008).

Furthermore, Melillo (1990) stated that the optimum temperature range for photosynthesis is between 20-35oC. Figure 1 shows that the rate of respiration may increase when temperature continuously increases. When the rate of respiration exceeds the rate of photosynthesis, it may cause net carbon assimilation become negative. These differences between photosynthesis and respiration are used as the argument that the rising temperature may caused the decrease of net carbon uptake by plants.

Figure 1: the illustration of plant responses to temperature. (a) general responses of plant growth, (b) general responses of photosynthesis and respiration (Mellilo 1990).

In addition, Grace and Zhang (2006) find that the increase of temperature has more significant impact on respiration than its impact on GPP. This is because the increase of respiration is sharper than the increases of GPP. As a result, the NEP continuously declines. Furthermore, when annual temperature reaches 10-14oC, NEP will be negative, under both normal and doubled CO2 concentration. (Figure 2). This finding is based on data from the boreal forest which is analyzed using soil–plant–atmosphere (SPA) model. Grace and Zhang also claimed that the same result will be found if this modeling is applied with other biomes, as long as there is an assumption that the temperature can increase heterotrophic respiration.

Figure 2. Prediction of the effect of temperature on plant productivity in boreal forest; (a) is under normal CO2 and (b) doubled CO2 concentration (Grace & Zhang 2006).

The Effect of Elevated CO2

Lloyd & Farquhar (1996) suggest that there are axioms in various literatures related to plant responses to elevated CO2: first, correlation between plant growth and CO2 can be explained by the concept of photosynthesis dependence to CO2. Second, plant response to CO2 is limited by the availability of nutrients. This is because photosynthesis needs nutrients besides CO2. In other words, plant will respond less to rising CO2 if it grows in poor nutrient condition.

Experiments find that elevated CO2 may increase rates of photosynthesis, increase productivity and increase biomass in most C3 Plants (Houghton 2007). One of the explanations for this increase is because elevated CO2 may increase carboxylation rates and decrease oxygenation rates of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in C3 Plants, which leads to a higher net rate of photosynthesis and increased synthesis of carbohydrate (Stitt & Krapp 1999). Furthermore, Terrestrial Ecosystem Model (TEM) also predicts that doubled CO2 without temperature change will increase 16.3% of global NPP (Melillo et al. 1993).

However, TEM also predicts that different vegetation types express different responses to elevated CO2. For example, it is predicted that in tropical ecosystem, the NPP will increase 22.2% of the global increase while in many northern and temperate ecosystems, elevated CO2 may not much affect plant productivity. This is because in northern and temperate ecosystem type, NPP is limited by Nitrogen availability in the soil. Under low nutrient conditions, TEM predicts that this ecosystem will have difficulties to transform elevated CO2 into production (Melillo et al. 1993). Moreover, CO2 is just one of many organic substrates that are needed by plants, so the long-term response of photosynthesis and growth to elevated CO2 will depend on the availability of mineral nutrients (Stitt & Krapp 1999).

Besides nutrient availability, there are other limitation of the impact of CO2 to plant productivity, such as plant acclimatization and stomatal response to water availability. The increase of NPP is constrained by the acclimatization of plant to elevated CO2 because after the acclimatization, the photosynthetic response is decreased. This is because in the long term, elevated CO2 condition may cause the accumulation of carbohydrates in the plant tissues which may reduce the photosynthetic rates (Bisgrove & Hadley 2002).

The increase of CO2 may influence the response of stomata which in turn may affect the plant’s NPP. The exchange of carbon from the atmosphere to the plant is through the stomata. If the soil is poor of water, the stomata will close more often to restrict water loss. On one hand, it helps plant to save the water supply but on the other hand, this may prevent the movement of carbon into the plant which may reduce the GPP and NPP. Conversely, if water is abundant, it will increase plant productivity. So, the impact of rising CO2 to plant productivity may depend on the balance of water in the soil (McGuire & Joyce 2005).

The Effect of Changes in Rainfall Pattern

Water is a crucial element needed for photosynthesis and the main chemical component of plants (Boisvenue 2006). It is predicted that future changes in precipitation may impact water availability and substantially impact plant productivity. One of the explanations for such changes is that when water availability in the soil declines, it can reduce water uptake by plants and also restrict nutrient absorption (e.g. Nitrogen) by roots and its transportation to the plant cells (Hanson 2000).

Current research analyzing satellite and global climate data in period of 2000 to 2009 shows that NPP declines of 0.55 petagram Carbon globally. This decline is attributed to widespread and intense droughts in some location in the last decade (Zhao & Running 2010). Many drought events are identified in period of 2000-2009. For example, high temperature driven drought has caused 30% reduction in GPP over Europe in 2003 (Ciais et al. 2003). Using terrestrial biosphere simulation model, Ciais et al. find a mean reduction in NPP of 16 gCm-2 month-1 in the summer of 2003 compared to 1998–2002, corresponding to a GPP reduction of 28gCm-2month-1.If the drought events continuously increase in the future, it is estimated that it will make temperate ecosystems as carbon sources and affect the pattern of the future carbon cycle (Ciais et al., 2003). In addition, the 100-milimeter increase in water decline has caused the Amazon forest lost 5.3 megagrams of carbon per hectare of aboveground biomass. This makes the Amazon more vulnerable to water stress in the future (Philips et al. 2009).

Those current data is contradictory to the previous studies by Nemani et al. (2003) who also use climatic and satellite data and suggest that global climate change increased NPP by 3.4 petagrams of carbon over 1982-1999 period (18 years). This increase is attributed to the increase of temperature and solar radiation which reduce several climatic restrictions to plant growth (Nemani et al. 2003)


In the previous section, I have reviewed some predictions of the effect of climate change to plant productivity. Even though the prediction of the effect of climate change has been the subject of many literatures and researches in the last few years, however, we have not reach a 100% confidence to accurately predict what and how the future climate change effect will be. Some tools, such as modeling has been used by researchers for developing climate scenarios that can be used to predict the effects that climate change may have on ecosystems. However, there are still weaknesses and uncertainties in the prediction (Massman 1995).

The weaknesses of a model which caused uncertainties may come from the lack of understanding on the complex biological process involved which may lead to the ‘weak’ hypothesis, difficulties in mathematical formulations and our inability to cover all the components of the natural variations (Tian et al. 2009).  Cramer et al. (1999) said that much of the remaining uncertainty comes from the role of human activities to alter terrestrial vegetation, such as land use change and deforestation which may increase CO2 in the atmosphere.

The uncertainties may also come from a range of possible response from different or specific species. For example, whilst many plant species may adapt to the rising CO2 quickly, many others maybe not. Plants with certain photosynthetic pathways or growth strategies maybe able to take advantage of changing conditions in any given habitat while the other maybe not. Hence, there are many potential effects of the climate change at plant community level (Hawkins et al. 2008)


This paper attempts to explain about the effect of climate change to plant productivity and the carbon cycle by focusing on three climate change variables; the increase of temperature, CO2 concentration and rainfall pattern. Productivity and carbon cycle are interrelated and may influence each other. In a simple way, it can be explained by the process of photosynthesis and respiration where the exchange of carbon between the atmosphere and plant occur.

The effect of temperature change is generally positive to increase the productivity by enhancing the photosynthesis as long as the temperature is in a range of optimum level. When temperature exceeds the optimum level, it will increase the rate of respiration causing the NPP continuously declined. Furthermore, Grace and Zhang (2006) find that increase of respiration is sharper than the increases of GPP. As a result, the NEP continuously declines. Furthermore, when annual temperature reaches 10-14oC, NEP will be negative under both normal and doubled CO2 concentration.

Experiments find that elevated CO2 may increase plant productivity by increase carboxylation rates of RUBISCO in C3 Plants. Terrestrial Ecosystem Model (TEM) predicts that doubled CO2 will increase 16.3% of the global NPP (Melillo et al. 1993). However, different response is occur in many northern and temperate ecosystems where elevated CO2 may not much affect plant productivity because of lack of Nitrogen in the soil.

The effect of CO2 to plant productivity may be limited by some factors, such as Nitrogen availability, plant acclimatization and water availability. Under low Nitrogen conditions, plants will have difficulties to transform elevated CO2 into production. Moreover, in the long term, elevated CO2 condition may cause the accumulation of carbohydrates in the plant tissues which may reduce the photosynthetic rates or decrease photosynthetic response to elevated CO2.

In addition, current study on the effect of drought on productivity shows that drought in the period of 2000-2009 has reduced NPP by 0.55 petagram Carbon globally. This is contradictory to the previous studies by Nemani et al. (2003) who suggest that global climate change increased NPP by 3.4 petagrams of carbon over 1982-1999 period.

Finally, this paper also identifies some limitations in predicting the effect of climate change to plant productivity, especially in the use of simulation model in the prediction. Regardless of their advancement and sophistication, many researchers (Massman 1995; Cramer et al. 1999; Tian et al. 2009) argue that climate model, as it is only simplified version of the complex climate system, has many weaknesses due to the lack of understanding on the complex biological process and our inability to cover all the components of the natural variations. Furthermore, there are also suggestions to cover the role of human activities in altering terrestrial vegetation, such as deforestation into climate modeling, because this activity may increase the concentration of CO2 in the atmosphere. It seems that climate modeling will continue to improve as understandings of the complexity of the earth system improve.



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