Initial Floristics Composition

relay floristics model

There are two concepts posed as alternative views of vegetation development (Egler 1954). Relay floristics presents a very familiar pattern of development, one that many people would associate with Clements (1916). Well-defined seral stages, beginning with pioneer species, successively occupy a site, culminating in a relatively stable community of climax species. We also usually tend to think of pioneer species as shade intolerant and of climax species as shade tolerant (Loftis, 1993a).

In using this concept to predict the outcome of the application of various regeneration methods, we have associated species we classify as pioneer and shade intolerant with clearcutting, climax and shade tolerant species with selection methods, and species that are intermediate in their successional status and shade tolerance with shelterwood. And one frequently hears that "cutting sets succession back to an earlier stage." (Loftis, 1993a)

However, reconciling this neat scheme with observed outcomes of regeneration methods is problematic, particularly in hardwood forests. For example, clearcutting a stand of oaks on a relatively xeric site frequently results in a new stand dominated by oaks, but oaks are not usually considered pioneer species. Regeneration cuts of any kind in some hardwood forests result in excellent representation of species we regard as climax and shade tolerant in the new stand. Further, the application of different regeneration methods in some forest types frequently results in the same species composition. In short, categorization of species by presumed successional status lends little to our ability to predict species composition resulting from regeneration cutting. (Loftis, 1993a)

Initial floristics model

As an alternative to relay floristics, Egler proposed a concept he called initial floristic composition.

This concept suggests that species composition of vegetation following disturbance is determined by the propagules that exist on the site at the time of the disturbance and those which arrive early in the process of stand development. Changing species dominance over time results from differential growth and development of species present. Interestingly, it was Clements (1916) who first made this observation. According to Clements, logging does not initiate a successional sequence because propagules of a variety of species are left after logging. Rather, it creates a dysclimax. (Loftis, 1993a)

For forest trees, then, species composition following disturbance is determined by the initial load of propagules: stump- and root-sprouts and advance reproduction of the various species that occur in the stand, and by new seedlings that become established soon after the disturbance from buried seed or from seed blown or carried into the stand. The essential feature of the initial floristic composition concept that makes it useful for prediction modeling is that the individuals of species that form the dominant tree canopy 50 to 100 years after disturbance come from the initial load of propagules. A corollary to this concept is that variations in species composition among stands 50 to 100 years after similar disturbances results from variations among stands in the initial loads of propagules. (Loftis, 1993a)

A number of investigators have recognized the general applicability of the initial floristic composition concept to hardwood stand development following disturbance (e.g. Oliver 1981, Leopold and others 1985, Shugart 1984, Drury and Nisbet 1973). From the standpoint of developing prediction models, we can model the post-disturbance development of existing advance growth both for advance reproduction and stump or root-sprout potential and we can deal in a probabilistic sense with input and development of new seedlings. (Loftis, 1993a)