Plants are largely constructed of carbon, almost 47 percent, and therefore have a large influence on the pattern in which carbon is cycled and stored at local, regional and global scales (Barnes et al 1998). The production of plant material not only influences the global carbon budget but represents important food, fuel, and fiber production for humanity. The carbon balance of plants is controlled by the quantity of CO2 fixed through photosynthesis and the rate of return to the atmosphere by respiration. If the amount of carbon fixed through photosynthesis is greater than the amount lost due to respiration a plant can grow.

The processes of photosynthesis and respiration occur in the leaves of all plants and control the net carbon gain of the leaves. Although these are the two processes which are controlling carbon gain they are influenced by light, temperature and the availability of water and nutrients (Barnes et al. 1998).

In many ecosystems plants will be the most important influence on the carbon balance; however we must consider other organisms to completely understand the transfer of carbon to and from an ecosystem. Soil microorganisms for example use dead plant material as an energy source, thus incorporating some of the carbon located in the material and returning some of it back to the atmosphere during respiration.

When considering the carbon balance of an ecosystem we must look at the biomass found with in the system. Biomass is the dry mass of living organisms and dead organic mater contained in a specific area. The net ecosystem production (NEP) can be calculated using the following equation (Barnes et al. 1998):

Where GPP equals the gross primary production, Ra is equal to the total plant respiration and Rh is equal to the hetorotrophic respiration. Gross primary production is the total amount of carbon fixed in an ecosystem during photosynthesis.

The primary building blocks of all plant tissue is essentially carbon, hydrogen and oxygen, however there are 14 other elements that are required to maintain existing tissue and build new biomass. These elements are termed nutrients.

Plants collect nutrients from the soil and combine them with carbon to build biomass and later return the nutrients to the soil in the form of dead leaves, needles, branches and stems (Mackenzie et al. 1998). The material then undergoes decomposition in the forest floor to complete the nutrient cycle. The nutrient cycle is termed a biogeochemical process since it is controlled by the activity of plants, soil microorganisms, and geochemical process of the soil.

Nutrients can enter an ecosystem through geological, hydrological and biological process. Specifically the processes of mineral weathering, atmospheric deposition and biological fixation are the primary mechanisms which allow nutrients to be introduced into an ecosystem.

Mineral weathering is the process by which nutrients are released from the chemical structure of rocks and soil minerals through abrasion, and chemical dissolution. This process is strongly influenced by the climate, temperature, and precipitation regimes. All plant nutrients except nitrogen can be supplied to an ecosystem through mineral weathering.

Atmospheric deposition is the process of nutrients entering an ecosystem from the atmosphere (Barnes et al. 1998). There are three processes which can contribute to atmospheric deposition: 1) wet deposition 2) dry deposition and 3) cloud deposition. Wet deposition occurs when nutrients are added to an ecosystem through rain or snow. Dry deposition occurs when nutrients enter an ecosystem directly from the atmosphere to the soil, vegetation or water surfaces within that ecosystem. Cloud deposition is the process of nutrients being deposited in an ecosystem from the non-precipitating water droplets found in clouds and fog. Although wet and dry deposition occurs in all ecosystems their importance in terms of nutrient additions varies from place to place. Cloud deposition typically occurs in coastal and mountainous regions where the vegetation is more likely to be exposed to low clouds or fog.

Inputs of nitrogen to forest ecosystems occur through the biological fixation caused by bacteria, lichens, and other free living organisms. Other plants such as legumes can also fix nitrogen within an ecosystem. Nitrogen fixing plants such as legumes can become abundant after disturbances such as fire. In many ecosystems increased levels of nitrogen can dramatically increase biomass production, especially in soils which are nitrogen limited.

A small portion of nutrients is lost annually through the hydrological cycle and biological export to the atmosphere (Barnes et al. 1998). Major pathways in which these nutrients are lost include: soil erosion, leaching and gaseous losses.

Leaching is a physical process where nutrients exit terrestrial ecosystems in the downward flow of water through the soil. It is important to note that nutrients are only subject to leaching when water is moving downward through the soil. Once nutrients are transported below the roots of plants they are lost from the terrestrial ecosystem (Barnes et al. 1998). However as they move into the ground water they become part of the aquatic ecosystem, therefore the hydrological process of leaching links the terrestrial ecosystem and aquatic ecosystem together.

Another loss of nutrients from ecosystems is due to denitrification. Denitrofication is the microbial reduction of NO3 to N2O, which results in the loss of nitrogen to the atmosphere (Barnes et al 1998). Denitrofication has been of great interest to the scientific community for two main reasons, first the loss of nitrogen could have implications for plant growth and second N2O is a greenhouse gas. However the requirements of denitrofication limit where this process can occur.

Other processes such as wildfires and forest harvesting also can effect both nutrient loss and gain in ecosystems.