37513.2.2 Transitions
The other fundamental component of a conceptual STM are transitions representing
either succession between phases or disturbances that alter the structure or composi-
tion of phases and, eventually, states. Transitions can be natural (e.g., fire, flooding)
or managed (e.g., prescribed burning). Furthermore, natural disturbances can repre-
sent pre-settlement (e.g., surface fire) and European post-settlement (e.g., Bromus
invasion) events. Most transitions are reversible given succession, natural distur-
bances, or management actions; however, some transitions can result in crossing of
biotic or abiotic thresholds that irreversibly change either the diagnostic species
Fig. 13.1 (continued) Boxes within each state represent vegetation phases. Arrows represent either
transitions between phases or states (i.e., 1.2a is a replacement event from late successional shrubs
to grass forbs, while T2c represents invasion by annual grasses or invasive forbs). This conceptual
state-and-transition diagram is accompanied by a detailed description of each state, phase, and
transition in the NRCS source documentation. (b) Screen capture of STSM software showing a
model developed for the same ecological site. The box-and-arrow “pathway diagram” shows
phases for the current potential state (#2), and the three undesirable states (#3–5). Arrows in the
diagram represent probabilistic or deterministic transitions driven by different process such as fire,
grazing, or succession. The graphs on the right show model outputs for three different example
management scenarios representing the amount of land in modeled states or phases. Blue repre-
sents ongoing management, green is no management, and red is delayed management. Lines rep-
resent the mean across multiple iterations and shaded areas represent model uncertainty
13 State-and-Transition Models: Conceptual Versus Simulation Perspectives...