- Igneous rock
- Rocks formed when high temperature, molten mineral matter cooled and solidified.
- Indicator species
- A plant or animal species related to a particular kind of environment. Its presence indicates that specific habitat conditions are also present. The spotted owl is an indicator species of certain ecosystems (old-growth forests).
- Individual tree selection
- The removal of individual trees from certain size and age classes over an entire stand area. Regeneration is mainly natural, and an uneven aged stand is maintained.
- Instream flow
- The quantity of water necessary to meet seasonal stream flow requirements to accomplish the purposes of the National Forests, including, but not limited to fisheries, visual quality, and recreational opportunities.
- Integrated pest management
- IPM evaluates alternatives for managing forest pest populations, based on consideration of pest- host relationships.
- Interdisciplinary team - (IDT, like FEMAT)
- A team of individuals with skills from different disciplines that focuses on the same task or project.
- Interior forest
- A forest area that is not influenced by edge effects.
- Intermittent stream
- A stream that flows only at certain times of the year when it receives water from streams or from some surface source, such as melting snow.
- Invasive species
- An alien species whose introduction does, or is likely to, cause economic or environmental harm or harm to human health. Example: tui chub.
- One of the categories of impacts mentioned in the National Environmental Policy Act to be included in statements of environmental impacts. An irretrievable effect applies to losses of production or commitment of renewable natural resources.
- A category of impacts mentioned in statements of environmental impacts that applies to non-renewable resources, such as minerals and archaeological sites. Irreversible effects can also refer to effects of actions that can be renewed only after a very long period of time, such as the loss of soil productivity due to road building.