Primary Productivity

Primary productivity is the rate of conversion of solar energy into plant matter. The total rate of solar energy conversion into carbohydrates (total photosynthesis) is gross primary productivity (GPP). However, a portion of GPP must be expended by the plant through metabolic processes necessary for maintenance, growth, and reproduction and is lost as heat through respiration. The net rate at which energy is stored as plant matter is net primary productivity. The energy stored in net primary production (NPP) becomes available to heterotrophs.

Primary productivity, turnover, and standing crop biomass are governed by a number of factors that differ among successional stages and between terrestrial and aquatic ecosystems. NPP is correlated with foliar standing crop biomass (Fig. 11.3). Hence, reduction of foliar standing crop biomass by herbivores can affect NPP. Often, only above-ground NPP is measured, although below-ground production usually exceeds above-ground production in grassland and desert ecosystems (W.Webb et al. 1983). Among major terrestrial biomes, total (above-ground + below-ground) NPP ranges from 2000 g m-2 year-1 in tropical forests, swamps and marshes, and estuaries to <200 g m-2 year-1 in tundra and deserts (Fig. 11.4) (S. Brown and Lugo 1982, Waide et al. 1999, W. Webb et al. 1983, Whittaker 1970).

Photosynthetic rates and NPP are sensitive to environmental conditions. Photosynthetic rate and NPP increase with precipitation up to a point, after which they decline as a result of low light associated with cloudiness and reduced nutrient availability associated with saturated soils (Schuur et al. 2001). These

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Npp And Photosynthesis

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I Relationship between above-ground net primary production (ANPP) and peak foliar standing crop (FSC) for forest, grassland, and desert ecosystems. From W. Webb et al. (1983) with permission from the Ecological Society of America.

rates also increase with temperature, up to a point at which water loss causes stomatal closure (Whittaker 1970).

Photosynthetically active radiation occurs within the range of 400-700 nm. The energy content of NPP divided by the supply of short-wave radiation, on an annual basis, provides a measure of photosynthetic efficiency (W. Webb et al. 1983). Photosynthetic efficiency generally is low, ranging from 0.065% to 1.4% for ecosystems with low to high productivities, respectively (Sims and Singh 1978, Whittaker 1970).

Photosynthetically active radiation can be limited as a result of latitude, topography, cloud cover, or dense vegetation, which restrict penetration of short-wave radiation.Terborgh (1985) discussed the significance of differences in tree geometries among forest biomes. Boreal tree crowns are tall and narrow to maximize interception of lateral exposure to sunlight filtered through a greater thickness of atmosphere, whereas tropical tree crowns are umbrella shaped to maximize interception of sunlight filtered through the thinner layer of atmosphere overhead. Solar penetration through tropical tree canopies, but not boreal tree canopies, is sufficient for development of multiple layers of understory plants.

The relationship between precipitation and potential evapotranspiration (PET) is an important factor affecting photosynthesis. Water limitation can result

Net Primary Productivity Biomes

Net primary production (NPP), total area, and contribution to global net primary production of the major biomes (top, data from Whittaker 1970); global calculation of total NPP using the light use efficiency model and biweekly time-integrated normalized difference vegetation index (NDVI) values for 1987 (from R. Waring and Running 1998).

Net primary production (NPP), total area, and contribution to global net primary production of the major biomes (top, data from Whittaker 1970); global calculation of total NPP using the light use efficiency model and biweekly time-integrated normalized difference vegetation index (NDVI) values for 1987 (from R. Waring and Running 1998).

from insufficient precipitation, relative to evapotranspiration. Plants respond to water deficits by closing stomata, thereby reducing O2 and CO2 exchange with the atmosphere. Plants subject to frequent water deficits must solve the problem of acquiring CO2, when stomatal opening facilitates water loss. Many desert and tropical epiphyte species are able to take up and store CO2 as malate at night (when water loss is minimal) through crassulacean acid metabolism (CAM), then carboxylate the malate (to pyruvate) and refix the CO2 through normal photosynthesis during the day (Winter and Smith 1996, Woolhouse 1981). Although

CAM plants require high light levels to provide the energy for fixing CO2 twice (Woolhouse 1981), desert plants often have high photosynthetic efficiencies relative to foliage biomass (W. Webb et al. 1983).

Air circulation is necessary to replenish CO2 within the uptake zone neighboring the leaf surface. Although atmospheric concentrations of CO2 may appear adequate, high rates of photosynthesis, especially in still air, can deplete CO2 in the boundary area around the leaf, reducing photosynthetic efficiency.

Ruderal plants in terrestrial ecosystems and phytoplankton in aquatic ecosystems usually have high turnover rates (short life spans) and high rates of net primary production per gram biomass because resources are relatively nonlimit-ing and the plants are composed primarily of photosynthetic tissues. Net primary production by all vegetation is low, however, because of the small biomass available for photosynthesis. By contrast, later successional plant species have low turnover rates (long life spans) and lower rates of net primary production per gram because shading reduces photosynthetic efficiency and large portions of biomass necessary for support and access to sunlight are nonphotosynthetic but still respire (e.g., wood and roots).

Usually, the NPP that is consumed by herbivores on an annual basis is low, an observation that prompted Hairston et al. (1960) to conclude that herbivores are not resource limited and must be controlled by predators. However, early studies of energy content of plant material involved measurement of change in enthalpy (heat of combustion) rather than free energy (Wiegert 1968). We now know that the energy initially stored as carbohydrates is incorporated, through a number of metabolic pathways, into a variety of compounds varying widely in their digestibility by herbivores. The energy stored in plant compounds often costs more to digest than the free energy it provides (see Chapters 3 and 4). Many of these herbivore-deterring compounds require energy expenditure by the plant, reducing the free energy available for growth and reproduction (e.g., Coley 1986). The methods used to measure herbivory often overestimate consumption but underestimate the turnover of NPP (Risley and Crossley 1993, Schowalter and Lowman 1999; see Chapter 12).

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