What is Plant Productivity, and How to Measure It

• Plant productivity provides society with ecosystem services and is the basis of all heterotrophic levels.
• Environmental factors like light, temperature, and precipitation, determined by latitudes, influence plant productivity patterns, which are most in the tropics and decrease towards the poles.
• The combined negative and positive interactions are essential biotic factors influencing plant productivity.
• Plant productivity is measured by estimating the photosynthetic rate and biomass accumulation through harvest analysis or indirectly through associated parameters.

Plant productivity is essential as it is the yield of crops and biomass in ecosystems. It is crucial in limiting climate change, as terrestrial plants absorb and fix 30% of the CO2 produced through anthropogenic activities annually. Plant productivity determines the structure and functioning of terrestrial habitats and provides people with many ecosystem services. Therefore, it is necessary to understand what it is, how it is affected, and the means to measure plant productivity.

What is Plant Productivity?

Plant productivity is the rate at which plants gain new organic matter. That is, it is the amount of biomass that plants and photosynthetic algae add over time due to photosynthesis. Two types of plant productivity are usually considered- Gross primary productivity and Net primary productivity.

Gross Primary Productivity (GPP)

GPP is the rate at which plants fix carbon by capturing radiant solar energy to produce carbohydrates.

Net Primary Productivity (NPP)

NPP refers to the organic matter that the plants store after some photosynthates are used for respiration during the measurement period. Usually, around 33% of the carbon fixed by photosynthesis is used and released through respiration. The photosynthetic production remaining after respiration is available to build stems, roots, leaves, and other functions. Part of the organic matter is also stored for future use.

NPP is the energy available for other ecosystems’ components.

NPP can become negative when respiration rates are higher than photosynthetic rates. It can happen anywhere in the world. During dry seasons in the tropics or winter in the temperate regions

Plant productivity is different from plant production. Crops and standing crops refer to production and biomass, respectively, while yield refers to productivity.

Abiotic Factors Affecting Plant Productivity

Plant productivity is affected by abiotic and biotic factors and their interaction. The main abiotic factors affecting plant productivity are solar radiation, temperature, water, nutrients, and carbon dioxide (CO₂).

Light

As productivity is the measure of photosynthesis, light availability is crucial for plants. Light can vary in duration, intensity, quality, and angle of incidence. Any change can slow the photosynthetic rate, for example, through shading. However, since plants continue to respire, NPP can become negative in these cases.

The amount of solar radiation captured by plants is affected by plant features like leaf size, age, display angle, pubescence, and physiological condition. A higher Leaf Area Index (LAI) increases productivity and growth. A leaf area enough to capture 95% of incident light is considered optimum. As the LAI increases, productivity rates will decline due to canopy structure changes affecting light infiltration.

Water

Water is the most critical limiting factor for plant productivity and survival. Too much or too little can lead to stress for the plant.

Water deficits are more crucial. As water availability reduces, leaf water potential fall and causes a reduction in transpiration and stomatal conductance, eventually decreasing photosynthesis. The first effects are reduction in root and leaf growth. The consequent decrease in leaf area further reduces photosynthesis and plant productivity. Soil water deficit conditions that affect water absorption will also lead to nutrient deficiency, further decreasing plant productivity.

On the other hand, few plants can withstand excessive water since it creates anaerobic soil conditions. Since roots also respire, lack or reduction in soil oxygen can cause plant decay and a fall in productivity. Flood tolerance in species and cultivars varies widely and is associated with physiological and morphological adaptations.

Temperature

Plants have an optimum temperature range of 20-25^oC when growth is maximum. At high temperatures, 30-35^oC, photosynthesis is severely affected due to damage to the photosystems and changes in the chemical processes of photosynthesis.

At low temperatures below 10^oC, the enzyme activity necessary for photosynthesis falls, reducing plant productivity. Low temperatures can minimize plant productivity by reducing stomatal opening that hinders photosynthesis and restricting water movement. Extremely low temperatures can freeze water in tissues causing freezing injury, crown kills, and death of buds or the entire plant.

Low temperatures are the most critical factor limiting plant distribution around the globe.

Nutrients

Nutrient availability will influence plant productivity by influencing morphology and physiology.

• Nitrogen is the most essential nutrient that is limiting. It is necessary to synthesize chloroplasts, the site of photosynthesis, for light capture. Moreover, it also influences plant parameters like leaf area, canopy structure, and above-and below-ground development. For example, in boreal forests, nitrogen addition increased GPP by improving LAI by 7% and light use efficiency by 17%.
• Phosphorus is directly involved in photosynthesis. Plants convert solar energy to chemical energy during photosynthesis and store it as Adenosine triphosphate (ATP). Energy is also transferred to other plant parts and used in respiration as ATP. So any phosphorus deficiency reduces plant productivity.
• Potassium is also involved in energy storage and movement and is needed during photosynthesis to produce ATP. Moreover, potassium in guard cells is also responsible for stomatal opening that lets in CO₂ for photosynthesis.

Having monocultures or fewer species in farms, grasslands, or plantations will deplete nutrient availability in the required soil zone and lead to less productivity, yield, and biomass.

Atmosphere

The increase in CO₂ in the atmosphere due to anthropogenic causes has been shown to increase photosynthesis in many parts of the world. Under experimental conditions, where  CO₂ levels in the atmosphere were doubled, C3 plants increased their photosynthetic rate by 25 to 75%. There was no effect of CO₂ fertilization on C4 plants.

Leaf age also influences CO₂ fertilization. As the leaves age, the increase in area is slower and stomatal conductance that allows carbon dioxide in reduces.

CO₂ fertilization benefits are expected to be highest in temperate regions, followed by tropical forests and C3 plants. Colder areas, like the boreal forests and tundra, show little improvement in plant productivity.

Latitudes characterized by different temperatures, day lengths (and light availability), and precipitation determine plant productivity. Plants and, by extension, the ecosystem in the tropics with more temperature, rainfall, and light availability have higher productivity and biomass than higher latitudes, like temperate regions and polar regions with lower temperatures and solar radiation, see Figure 1.

Figure 1: “Net primary production (NPP):  The colors on these maps indicate how fast carbon was taken in for every square meter of land in 2015. Values range from -1.0 grams of carbon per square meter per day (tan) to 6.5 grams per square meter per day (dark green). A negative value means decomposition or respiration was more than carbon absorption; more carbon was released to the atmosphere than the plants took in,” Stockli, R for NASA, using data provided by the MODIS Land Science Team. (Image credits: https://wad.jrc.ec.europa.eu/primaryproduction)

Biotic Factors Affecting Plant Productivity

Intrinsic factors and interactions with other species also determine plants’ productivity. These interactions do not occur in isolation, and positive and negative interactions happen simultaneously. The net effect of these interactions is ultimately important for plant productivity, and because of the interaction combinations, relationships between plants can be many and complex.

Positive Interactions

There are more positive interactions than previously acknowledged. Some standard ones are between plants and microbes that help improve resource acquisition, like water and nutrients, to increase photosynthesis and plant productivity. Mycorrhizal associations are the most common to improve soil structure, and nutrient acquisition, especially phosphorus and water absorption.

Evidence shows that being part of a species-rich community can improve plant productivity. For example, every 1% reduction in plant diversity in Alaskan boreal forests reduced individual plant productivity by 0.23%.

Negative Interactions

The negative interactions that can impact plant productivity are herbivory and competition.

Large and small mammals or insects’ herbivory reduces the leaf tissues available for photosynthesis. Moreover, loss of culms in grasses can affect water and nutrient translocation from roots. The time of the season and plant life cycle will determine the effect that herbivory has on plant productivity. In some cases, moderate herbivory reduces canopy cover to let in light and minimize evapotranspiration to improve plant productivity.

Competition for water, nutrients, and light occurs in natural ecosystems and croplands. Changing environmental conditions or defoliation through herbivory can shift the advantage in a community to species whose growing conditions improve comparatively.

Management practices to optimize growing conditions will vary depending on the plant and expected interactions. In cropland, the efforts are to minimize competition from weeds and other crop plants or herbivory from pests. Agroforestry or plantations for coffee (and many spices) choose appropriate species and shade management to manipulate conditions to increase productivity. In forests, the effort is to increase species diversity.

How to Measure Plant Productivity

At the plant level, the following approaches can determine productivity in terrestrial habitats.

1. Measuring the Rate of Photosynthesis and Respiration

One method is to measure the amount of CO₂ assimilated or oxygen (O₂) produced during photosynthesis. The portable CI-340 Handheld Photosynthesis System, manufactured by CID Bio-Science Inc, is a non-destructive precision Infrared gas exchange analysis tool that can measure photosynthesis in real time. The amount of CO₂ in the air before and after entering the leaf chamber gives the amount of CO₂ assimilated. It is the difference between the CO₂ fixed by photosynthesis and the part released due to respiration. Similarly, the amount of O₂ before and after air enters the leaf chamber gives the net amount of O₂ after release during photosynthesis and use during respiration. Some modules allow for the control of light, temperature, CO₂, and water levels in the leaf chamber.

When the measurements are made in light, the gas analysis estimates NPP. CO₂ analysis conducted in the dark estimates only respiration. Adding respiration to NPP gives GPP.

Chlorophyll fluorescence is part of the unused solar radiation not used in photosynthesis that leaves reflect. It is used to measure photosynthesis efficiency. The CI-510CF Chlorophyll Fluorescence can be used with the CI-340 Handheld Photosynthesis System to measure chlorophyll fluorescence simultaneously with photosynthesis.

2. Estimating Biomass Production in a Specified Area Over Time

Plant productivity is measured as the rate at which biomass/energy is produced per unit area/ unit time.

The harvest analysis is used for crops and annuals where the biomass is zero initially. Vegetation is removed and dried at intervals to estimate accumulated plant biomass. The difference in biomass between readings is expressed in kcal/m2/unit time. This estimation is a measure of NPP. The harvest method is usually confined to only aboveground biomass since root mass estimations at periodic intervals are difficult.

3. Estimating the Standing Crop by Correlating Biomass to a Measured Variable

In forests, where the above harvesting methods cannot be used, biomass addition is measured in many ways. These can be destructive or non-destructive.

• The destructive way is to cut down trees of a species and weigh all the parts. However, this is not feasible in all cases.
• Indirect methods include using allometry and satellite-based models.
• The Chlorophyll Concentration index is also used to estimate aboveground biomass. The amount of greenness in leaves can be assessed through non-destructive methods. It can be done in the field or through remote imagery through spectroscopy. In the field, devices like the CI-710s SpectraVue Leaf Spectrometer can quantify the amount of chlorophyll in a leaf and estimate plant productivity.
• Radiation use efficiency is another way of estimating aboveground biomass accumulation. It is the amount of biomass formed per mol of incident photons. It is used for crops to measure daily growth rates.

Crop and Ecosystem Productivity

NPP is crucial to understanding ecosystem vegetation dynamics, agricultural yield, and limiting climate change. Plant productivity of single crop species gives crop productivity. However, to estimate ecosystem productivity, it is necessary to calculate the productivity of a community made of several species. For large-scale estimation, satellite imagery using indirect parameters is used. Since all organisms depend on plant productivity for their energy and as building blocks, we must keep updating our knowledge of this vital plant function.

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