When plants absorb sunlight, part of the energy is inevitably lost due to breathing. As a result, net primary productivity (NPP) is computed by deducting the quantity of energy respired from gross primary productivity. NPP varies between species and across the globe. It is estimated to be 3-5% of absorbed light energy on average, but can reach up to 20% in bright areas with lots of photosynthesizing plants.
In practice, NPP is usually measured over time scales of years to decades. On shorter timescales (days to months), it is difficult to distinguish direct from indirect effects of plants on atmospheric chemistry. Direct effects include changes in water uptake or transpiration rates or growth that alter the surface area exposed to radiation, while indirect effects include changes in plant chemical defenses against pathogens or herbivores or nitrogen recycling rates.
Over longer periods (decades to hundreds of years), NPP decreases because old plants die and new ones don't take their place. This is called "old-growth depletion." The amount of carbon stored in old-growth forests is about 5% of global production, so they play an important role in climate regulation. Forests also store large amounts of other nutrients in their woody tissues which are released when they decay. These elements are then available for new growth.
Primary productivity is the process that results from the photosynthetic activity of plants and affects the buildup of biomass in forests. The difference between gross photosynthesis of plant leaves and energy losses due to respiration and plant tissue death is represented by net primary productivity (NPP). Biomass is the accumulated product of photosynthates over time, which is either stored as wood or fiber in the case of trees or returned to soil via livestock or decomposition in the case of plants. Biomass can be used as a source of fuel or chemical feedstocks such as cellulose or hemi-cellulose for paper and other products.
The amount of carbon fixed by photosynthesis is called the biospheric carbon pool. Carbon fixation by photosynthesis exceeds carbon loss by decay and combustion, so the total mass of carbon in the biosphere has increased over time. However some carbon is lost through oxidation and fossilization, so the total mass of carbon in the biosphere is not constant.
About 30 billion tons of carbon are absorbed by ocean waters each year, primarily by algae but also by certain types of bacteria. This is known as the oceanic carbon balance. If all of this carbon were to be released into the atmosphere, it would lead to global warming.
Forests play an important role in climate change because they act as carbon sinks.
Net Primary Productivity (NPP), or plant biomass output, is equal to all of the carbon taken up by plants through photosynthesis (also known as Gross Primary Production, or GPP) minus the carbon lost to respiration. GPP-respiration = NPP. Carbon dioxide molecules are removed from the atmosphere and used to make carbohydrates within plants via photosynthesis. Some of these carbohydrates are stored in fibrous roots or converted into oil or sugar for storage in seeds or fruit. The rest is released back into the atmosphere through a process called decomposition. As plants decompose, they can release more carbon into the atmosphere or store it again as soil organic matter. Changes in forest density or species composition can also affect NPP.
Scientists estimate NPP by using measurements of incoming light energy and other factors such as air temperature to calculate GPP and then subtracting estimated rates of loss due to decomposition and other processes.
The amount of carbon fixed by plants each year is about 2 x 10^9 metric tons (2 billion metric tons). That's more than enough to be worth mentioning. Most of it stays in the form of trees and other vegetation, some is stored in the soil, but some is released back into the atmosphere. If all of it were released back into the atmosphere, it would lead to positive feedback and increase global temperatures beyond what we see today.
The pace at which all plants in an ecosystem create net usable chemical energy is referred to as net primary productivity (NPP). The pace at which primary producers preserve and gather biomass for energy conservation is referred to as gross primary productivity (GPP). The difference between GPP and cellular respiration is represented by NPP. Net primary production equals GPP minus carbon loss through respiratory processes and other means.
Primary producers are the first organisms to gain energy from the sun's radiation and they make up most of the biomass on Earth. They convert the solar energy into organic material using photosynthesis. During this process, some organisms use the water-soluble pigment chlorophyll to capture light energy and use it to fix carbon dioxide and release oxygen in the process. Other organisms such as algae and some bacteria do not use chlorophyll but instead use a similar mechanism called "phytoplankton" for the same purpose. Plants provide the majority of the food for animals who cannot make their own nutrients. Some plant species can also generate small amounts of nitrogen and phosphorus that are taken up by others for use in growth. These are known as secondary consumers because they eat others after which they themselves become consumed.
Net primary production is the amount of new biomass produced per unit area over time. It is calculated by measuring the increase in mass above ground of living plants plus the loss of soil moisture minus the loss of biomass through mortality. Mortality includes death by fire, wind, ice, or disease.