Important Definitions

      The following definitions are provided for potentially confusing terms and concepts employed in describing the succession-disturbance transition models.

Stand condition

A stand condition represents a discrete stage of stand development and is typically characterized by distinctive vegetation composition, structure, and function. Following a stand-replacing disturbance, a stand typically undergoes a sequence of somewhat predictable changes over time (through the process of succession) in the absence of another stand-replacing disturbance. Fine-scale disturbance processes typically play an important role in facilitating or inducing these developmental changes. Although the changes in stand composition, structure, and function are continuous and typically occur gradually over long periods of time, it is often convenient (and in our case necessary) to distinguish important discrete “stages” of development. We refer to these discrete “stages”, sometimes also referred to as seral or successional stages as stand conditions. We prefer the term stand condition over seral or successional stage because it has broader connotation; i.e., it implies that discrete conditions can be created through processes (such as coarse-scale disturbances) not typically associated with “succession.”

Succession

Succession involves the slow, gradual development of the stand and the corresponding transition among stand conditions (i.e., seral stages). Note, succession is defined as the progression of the stand over time; i.e., the transition from “early” to “later” stages of development. It is recognized that disturbance processes can and often do induce the progression of the stand. In RMLANDS succession occurs at the beginning of each time step. Thus, even if a particular succession transition is induced by a low-mortality disturbance (e.g., low-mortality fire inducing the transition to a fire-maintained open canopy condition in Ponderosa pine forest cover type), the transition doesn’t occur until the beginning of the next time step (after the disturbance).

Disturbance

Disturbance is any relatively discrete event (natural or anthropogenic) in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment, including both destructive, catastrophic events as well as less notable, natural environmental fluctuations. Typically, a disturbance causes a significant change in the system under consideration (Pickett and White 1985). In RMLANDS, the “disturbance” processes have been defined explicitly.

Retrogression

Retrogression involves the transition of a stand from a “later” to an “earlier” stage of stand development (i.e., stand condition). In RMLANDS retrogression is always induced by a disturbance process and occurs immediately following the disturbance; i.e., the retrogressive transition occurs immediately after the disturbance within the current time step (in contrast to succession, which always occur at the beginning of the next time step).

Recycle vs Maintain

In the transition models, we make a distinction between processes that "recycle" a stand in its current condition and those that "maintain" a stand in its current condition. We use "recycle" to imply that a disturbance has caused the condition-age (#years in current condition class) to be reset to zero, while we use "maintain" to imply that a disturbance has occurred but has not reset the condition-age. This is an important distinction as it will have an impact on how stands will be affected by successional probabilities following a disturbance. Generally, stands that are "maintained" will be older and therefore succeed quicker than stands that are "recycled".