Definitions

C3 Plants

The label for this group is the most basic photosynthetic mechanism. The majority of species globally especially in cooler and wetter environments are of this class. Typical examples include most trees and agricultural crops such as wheat, rice, barley, potatoes, soybeans, and cassava.

C4 Plants

These plants have a special mechanism within their leaves by which they are able to increase CO₂ concentration several times higher than ambient levels. These plants tend to be found in warmer and water-limited environments. Typical examples include many tropical grasses and agricultural crops such as maize (corn), sugarcane, and sorghum.

CAM Plants

This is a variant of C4. These plants take up CO₂ at night for normal photosynthetic processes the next day. Plants in this class grow in deserts such as cacti, but also includes pineapple.

GPP

Gross primary production (GPP) refers to the amount of carbon taken from the atmosphere by plants in the photosynthesis process.

AR

Autotrophic respiration (AR) is the amount of carbon that is respired back to the atmosphere as CO₂ by plants during the photosynthesis process.

NPP

Net primary production (NPP) is the carbon that remains stored in the plant after it has taken up CO₂ from the atmosphere in GPP and respired some back by AR: NPP = GPP - AR

HR

Heterotrophic respiration (HR) refers to the loss of carbon due to death of plants or shedding of plant parts to the soil where soil animals, fungi, or bacteria cause decomposition back to atmospheric CO₂.

NEP

Net ecosystem production (NEP) is the amount of carbon that remains in an ecosystem due to NPP but after HR is accounted for: NEP = NPP - HR

ET

Evapotranspiration (ET) describes the combined effect of evaporation of water from soil and standing water and transpiration of water vapor from plants seeking to lower the temperature of their leaves.

WUE

Water-use efficiency (WUE) is the ratio of difference in CO₂ concentration within the stomata to ambient values to the difference between water-vapor concentration at the same locations. Plants having high WUE are efficient (as far as using water is concerned) at converting atmospheric CO₂ to plant carbon for a given humidity condition. WUE = (C_CO2A - C_CO2S)/(C_H2OS - C_H2OA) where
C_CO2S is the concentration of CO₂ in the stomata
C_CO2A is the ambient concentration of CO₂
C_H2OS is the concentration of H₂O in the stomata
C_H2OA is the ambient concentration of H₂O

Biome

A biome is a life zone or biogeoclimatic region that shares a common climate, soil and collection of plant communities. Typical biomes include desert, scrubland, tundra, bog, forest, rainforest, woodland, and grassland (Figure 1).

Carrying Capacity

The carrying capacity of the planet is the maximum population of humans that the planet can accommodate and still supply essential food and other ecosystem services to sustain the population.

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