Processes Causing the Formation of Recalcitrant Understory Layers

This section discusses how natural processes, including such stressors as: (1) overstory disturbance, (2) elevated herbivore regimes, and (3) altered fire regimes may be treated either as threats or benefits to forest communities. Overstory disturbances reinitiate stand development (Oliver and Larson 1996) and characteristic fire and herbivore regimes often promote species coexistence (reviewed in Bond and Keeley 2005, Huntly 1991). What constitutes a threat or a risk often is intrinsically linked (and, thus, often critiqued) to a subjective value of what constitutes a loss in biological or ecological diversity, function, or service (see Power and Adams 1997 for a vigorous debate). In order to be workable, an uncharacteristic disturbance regime is narrowly categorized as a threat or risk if its occurrence results in a persistent negative impact on the ability of the disturbed stand to regenerate its predisturbance tree species composition. Utilizing that definition, it is clear that alterations to disturbance regimes can constitute a threat to forest regeneration if they result in a degraded understory plant community composition monopolized by a select species that interferes with tree regeneration.

It was found that major anthropogenic changes to disturbance and browsing regimes underlie the development of most recalcitrant understory layers (see Hobbs and Huenneke 1992 for their similar conclusion regarding exotic invasives). Indeed, overbrowsing, altered fire regimes, and increased overstory disturbance were implicated in 18, 34, and 82 percent, respectively, of the cases in (Table: Interfering Species Examples). More importantly, this review suggests that the formation of a dense understory canopy layer arises approximately 53 percent of the time in the cases when overstory disturbances and altered understory fire and browsing regimes occur in tandem (Table: Interfering Species Examples). Additionally, these understory layers are depauperate because repeated canopy disturbances combined with other processes, (i.e., fire and browsing) strongly favor a small subset of species.