Environmental Effects on Photosynthesis and Primary Productivity

On land, four environmental parameters stand out as the major modifiers of photosynthetic activity - light, water availability, temperature, and nutrient supply. In aquatic systems, light, nutrients, and temperature are the leading environmental controls. The relationship between these factors and NPP are often directly proportional, as shown for water availability and accumulated intercepted radiation in tropical grasslands (Figure 5). In some cases, notably with temperature, NPP exhibits a strong response

Table 2 Ranges of NPP reported for selected ecosystems of the world

Primary productivity

Biome

(kgDMm2yr1)

C4-dominated

systemsa

Tropical wetlands

5-14

Sugarcane

6-11

plantation

Tropical grassland

1-4

(high rainfall)

Tropical grassland

0.2-1

(low rainfall)

C3-dominated

systemsb

Rice plantations

2-5

Wheat fields

1-3

Tropical rainforest

1-3.5

Deciduous

0.4-2.5

temperate forest

Evergreen forest

1-2.5

Boreal forests

0.2-1.5

Dry scrub

0.3-1.5

Arctic tundra

0.01-0.4

Deserts

0-0.3

Algae-dominated

systemsb

Reefs and tidal

0.5-4

zones

Coastal zones

0.2-0.6

Upwelling zones

0.4-1

Open ocean

0.0002-0.4

aC4 NPP values from Long SP, Jones MB, and Roberts MJ (eds.) (1992) Primary Productivity of Grass Ecosystems of the Tropics and Subtropics. London: Chapman and Hall.

bC3 and algal productivity values from Larcher W (2003) Physiological Plant Ecology, 4th edn. Berlin: Springer.

aC4 NPP values from Long SP, Jones MB, and Roberts MJ (eds.) (1992) Primary Productivity of Grass Ecosystems of the Tropics and Subtropics. London: Chapman and Hall.

bC3 and algal productivity values from Larcher W (2003) Physiological Plant Ecology, 4th edn. Berlin: Springer.

at low values, followed by a decline at elevated values. In addition to environmental controls, there are a number of physiological factors that influence NPP, notably photo-synthetic enzyme content, the rate of photorespiration, and the area of photosynthetic tissue. Of these, the best predictor of NPP is photosynthetic surface area (often measured as leaf area index, the ratio of total leaf area to ground surface area). Photosynthetic capacity, which reflects enzyme content in the cells, is not well correlated with NPP because high resource investment in photosyn-thetic enzymes reduces growth of new photosynthetic area.

Ultimately, the primary productivity in a system is limited by the light availability and the efficiency at which the vegetation converts light energy into biomass. Light input follows seasonal cycles and weather patterns, increasing during the long days of summer and the termination of wet seasons. High light is often associated with low rainfall or excessive heat, however, such that the light response of NPP has to be interpreted in the context of the overall resource availability and species present in a habitat. Where resources are abundant and it is not too hot, as in

3000

CL CL

2000

3000

2000

8 000 e

6 000 DL

4 000

2 000

100 200 300 Days per year without drought stress

0 1000 2000 3000 4000 Accumulated intercepted radiation (MJ m-2)

Figure 5 (a) The relationship between net primary productivity (NPP) and annual water availability for tropical grasslands and savannas. (b) The relationship between NPP and light availability for a tropical grassland in Thailand and a tropical wetland dominated by the C4 grass Echinochloa polystachya in Brazil. In panel (a), the relationship was compiled by House JI and Hall DO (2001) Net primary production of savannas and tropical grasslands. In: Roy J, Saugier B, and Mooney HA (eds.) Terrestrial Global Productivity, pp. 363-400. San Diego: Academic Press. In panel (b), the relationships are from Jones MB, Long SP, and Roberts MJ (1992) In: Long SP, Jones MB, and Roberts MJ (eds.) Primary Productivity of Grass Ecosystems of the Tropics and Sub-Tropics, pp. 212-255. London: Chapman and Hall.

10 000

8 000 e

6 000 DL

4 000

2 000

100 200 300 Days per year without drought stress

0 1000 2000 3000 4000 Accumulated intercepted radiation (MJ m-2)

Figure 5 (a) The relationship between net primary productivity (NPP) and annual water availability for tropical grasslands and savannas. (b) The relationship between NPP and light availability for a tropical grassland in Thailand and a tropical wetland dominated by the C4 grass Echinochloa polystachya in Brazil. In panel (a), the relationship was compiled by House JI and Hall DO (2001) Net primary production of savannas and tropical grasslands. In: Roy J, Saugier B, and Mooney HA (eds.) Terrestrial Global Productivity, pp. 363-400. San Diego: Academic Press. In panel (b), the relationships are from Jones MB, Long SP, and Roberts MJ (1992) In: Long SP, Jones MB, and Roberts MJ (eds.) Primary Productivity of Grass Ecosystems of the Tropics and Sub-Tropics, pp. 212-255. London: Chapman and Hall.

tropical wetland systems with high nutrients, the light response of NPP is steep compared to systems where drought and soil nutrient deficiency limit leaf growth and photosynthetic activity (Figure 5b).

Temperature strongly alters light-use efficiency through effects on photorespiration, enzyme kinetics, and the rate of new tissue production. In cool conditions, C3 biomass has greater light-use efficiency than C4 biomass, while the reverse is true at warm temperatures because of the rise in photorespiration. When photorespiration is high, the conversion efficiency of radiation into biomass is reduced. In C3 plants, the rate of photosynthesis rises steeply with increasing temperature to an optimum between 20 and 30 °C (Figure 3). Increases in temperature above the optimum reduce photosynthesis because of accelerating photorespiration and a heat-induced reduction in enzyme activity. C4 leaves also show a high initial response to temperature increase above 5 ° C, but they have a higher thermal optimum of photosynthesis than C3 species because photorespiration is suppressed by the C4 pathway (Table 1).

In most C3 leaves, photosynthesis rises with increasing light levels to 30-75% of full sunlight intensity, above which further enhancements in light do not stimulate photosynthesis. C4 plants generally require more light to obtain maximum photosynthesis rates. Because plants cannot use every photon incident on a single leaf, they compensate by producing multiple layers of leaves. If the environment is stable enough to support a permanent leaf canopy, then the size of the canopy and leaf area index directly determine NPP by enhancing absorbed radiation. In seasonal environments, the rate of canopy formation and the length of time a canopy can be maintained are critical components of NPP. Extreme cold and drought force a plant to enter dormancy and potentially shed its canopy, during which time the radiation use drops to zero. Low temperatures and dry conditions also slow canopy growth at the beginning of the growing season, thus contributing to losses in potential NPP. Plants adapted to drought also produce fewer leaves, which reduces light harvesting in drought-prone landscapes. Drought also induces stomatal closure, causing a reduction in photo-synthetic capacity per unit leaf area.

With this understanding, it is relatively easy to explain the patterns of NPP found in the major biomes of the world (Table 2). The biomes with the greatest levels of NPP occur where conditions are warm but not too hot, water is abundant year round, and biomass is rapidly recycled so nutrients deficiencies are not excessive. In such systems, the plants can maintain a dense leaf canopy throughout the year, absorbing nearly all of the incoming radiation. For this reason, tropical rainforests lead all forested biomes in annual NPP, even though none of the plants in a rainforest are known for having high photosynthesis rates on a leaf area basis. Species in the arctic tundra have photosynthesis capacities per unit leaf area that are similar to rainforest species, yet the tundra is one of the least productive biomes due to the long, harsh winters, slow nutrient turnover, and a low total leaf area. The highest photosynthesis rates per leaf area actually occur in desert ecosystems, where rapidly growing, ephemeral species build a small number of highly photo-synthetic leaves after episodic rains. These species complete their lifecycles in the lush but brief periods of high soil moisture that the rains provide. After the rains pass and soils dry, the plants set seed and survive extended dry periods in a dormant state. For much of the year, NPP and radiation use is nil, and the landscape is devoid of productive vegetation.

Humans minimize resource limitations in agricultural settings through irrigation and fertilization, and thus realize high NPP levels, particularly in C4 crops. C4 maize and sugarcane, for example, have peak yields that are about double that of C3 wheat and rice. In nature, the highest NPP is found along floodplains of tropical lakes and rivers where nutrients are high, water is abundant, temperatures are optimal for C4 photosynthesis, and light is plentiful. Most photons incident on the canopy are used for carbon assimilation in such conditions, and NPP ofthe C4 grasses and sedges can exceed 10 kg dry matter m~ yr~ . Despite such high NPP in these systems, NEP is low because the vegetation is rapidly degraded upon death by the same conditions that promote high NPP.

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