The Vegetation of Wisconsin: An Ordination of Plant Communities

24. Interrelations of communities

CHAPTER 24 Interrelations of communities THE STRUCTURE OF A STAND The accounts of Wisconsin plant communities given in the previous chapters were designed largely to convey information about the distribution , structure, and floristic composition of the several types. The interrelations of the communities were discussed only to a limited extent, except in those vegetation types which had been shown to form a vegetational continuum. It is the intention here to explore the interrelationships of all types more fully, but before this is done, it may be advantageous to review and summarize some of the findings that have been described earlier with respect to the structure of stands, communities, vegetation types, and especially to their variability. In considering first a typical stand of any community, it has been shown that the individuals of the component species differ in their spatial arrangement on the ground. These individuals may be disCopyrighted Material INTERRELATIONS OF COMM U NITIES 477 tributed at random or they may approach a regular or uniform disper &al, but more frequently they are aggregated into clusters or clumps with relatively large intervening spaces which are devoid of the species. These clusters may be large or small, dense or diffuse, numerous or rare. Furthermore, the kind of distribution may be uniform throughout the stand or it may vary from strongly aggregated in one microsite to nearly random at another place in the same stand. Similar variation in spatial patterning is shown to be typical of many vegetations by Greig-Smith (1957). Some species may be closely associated with others, since individuals of the two sorts are found together within small lateral distances to a greater extent than would be expected by chance. On the other hand, other species pairs or groups may be significantly disassociated; an individual of one species is rarely found in close proximity to an individual of another species. These differences in local distribution and local association may be attributed to local micro-environments in some cases. Such micro-sites as tip-up mounds, animal burrows, ridge tops, drainage ways, and spot openings in the canopy of forests may favor certain species or groups of species and be detrimental to others. Frequently, the micro-sites show no present-day differences from the surrounding area but are relics of historical gaps created by past disturbances such as lightning-fire or wind throw. Direct interspecific competition in the form of antibiotic or probiotic compounds may be responsible for some association, while indirect competition due to the dense shade of scattered dominants may bring about a non-random dispersion of the groundlayer species. An additional factor influencing the spatial patterns is the difference in immigration rates of species just entering the community from adjacent assemblages. The first individuals to enter usually are dispersed at random, while later stages tend toward aggregation (Whitford, 1949). Some of the immigrants , which are chance adventives from non-related communities. may lack the necessary adaptations to establish themselves permanently ; their period of tenure is thus fleeting and highly unpredictable. From this, we must conclude that stands of any major community are highly variable, differing widely from place to place within the stand and, to a considerable extent, from year to year at the same place. THE STRUCTURE OF A COMMUNITY In earlier chapters, communities were delimited by various methods as groups of stands with sufficient characters in common to produce Copyrighted Material 1·78 VEGETATION AS A WHOLE studiable assemblages. The major vegetation types of a floristic pro\'· ince were identified by gross physiognomy, as forests, savannas, or grasslands. Each of these types was split arbitrarily into communities by an objective division of a floristic compositional gradient. The lesser communities were characterized largely by physiography, as beaches, dunes, cliffs, and aquatic areas, although some were separated by the nature of their dominants, as shrub communities or sedge meadows. In spite of these diverse ways of delimiting the communities , it is believed that most of them are comparable to each other in terms of degree of homogeneity. This contention is supported by the data in Table IV-2, which show the index of homogeneity for all communities. Most of these indices range between 50 and 65 (24 out of 34) although the extremes vary from 34.1 to 70.3. Similar indices were reported by Raabe (1952) fOLa series of communities in Germany. Many of the communities reported above for \Visconsin are comparable to the "alliances...