Terrestrial ecosystems rely almost exclusively on the sun's energy to support the growth and metabolism of their resident organisms. Plants are quite literally biomass factories powered by sunlight, supplying organisms higher up the food chain with energy and the structural building blocks of lifeChennai Investment. Land plants, or autotrophs, are terrestrial primary producers: organisms that manufacture, through photosynthesis, new organic molecules such as carbohydrates and lipids from raw inorganic materials (CO2, water, mineral nutrients). These newly minted organic compounds lock up the sun's energy in chemical bonds, providing an energy currency accessible to heterotrophs, organisms that consume rather than produce organic moleculesJaipur Investment. In this way, primary producers are an essential vehicle for energy transfer from the sun to consumers, securing energy that can be passed from one consumer to another. The energetic and carbon-rich products of primary production supply consumers, including humans, with fuel to drive their metabolism while providing essential carbon-containing compounds that form the bricks and mortar of living cells.

Ecosystem ecologists have long been interested in two related metrics of terrestrial primary production. Gross primary production (GPP) is the total amount of carbon dioxide "fixed" by land plants per unit time through the photosynthetic reduction of CO2 into organic compounds. A substantial fraction of GPP supports plant autotrophic respiration (Ra), with the remainder allocated to the net primary production (NPP) of plant structural biomass in stems, leaves, and fruit, labile carbohydrates such as sugars and starch, and, to a much lesser extent, volatile organic compounds used in plant defense and signaling. Terrestrial GPP, therefore, relates to NPP as follows:

NPP = GPP - Ra

Figure 1: Net primary production (NPP) and standing biomass allocation for a 90-year-old Michigan forest estimated from inventory-based methods in which biomass growth is quantified over time (Gough et alIndore Investment. 2008)Both GPP and NPP are expressed as rates, usually in terms of their carbon currency (e.g., g C m-2 hr-1, tonnes C ha-1 yr-1). Because volatile organic compounds represent only a small fraction of NPP, the rate of total plant growth (or yield) in a terrestrial ecosystem is virtually synonymous with NPP, since biomass production is already discounted for respiratory expenditures that support plant growth and maintenance. The ratio of NPP to GPP, or carbon use efficiency, is the fraction of carbon absorbed by an ecosystem that is allocated to plant biomass production. Interestingly, carbon use efficiency is often remarkably similar across ecosystems located in different biomes, suggesting that ecosystems organize in a way that maximizes carbon allocation to growth.

Where do plants invest organic compounds designated for net primary production? Consider, as an example, a mature forest. The stems, leaves, flowers, and fruit are all visible displays of aboveground NPP (i.e., growth) that accrued over time — but what about belowground (root) NPP? Most of the NPP readily observed aboveground is matched in magnitude belowground by the less visible, but equally important, production of roots. For example, root growth comprised almost half of total ecosystem NPP in a ninety-year-old Michigan forest, indicating that belowground investments in biomass by plants are substantial (Figure 1). The total standing biomass of an ecosystem is a function of cumulative NPP over time minus biomass losses from senescence (i.e., death). In the same forest, stems (including trunks and branches) are the largest fraction of standing biomass, but roots comprise a quarter of the total biomass present in the ecosystem.