Co-Occurrence of Oak Subgenera: Implications for Niche Differentiation

Abstract

MOHLER, C. L. (Section of Ecology & Systematics, Division of Biological Sciences, Cornell University, Ithaca, NY 14853). Co-occurrence of oak subgenera: implications for niche differentiation. Bull. Torrey Bot. Club 117: 247-255. 1990.-It is generally very difficult to determine whether the coexistence of two tree species in a forest is due to a difference in the species' niches or due to disturbance factors which prevent one species from driving the other to extinction. Thus, few studies have demonstrated that niche differences among tree species affect the composition of forest stands. Here an indirect approach is used in which the presence of niche interactions are deduced from the pattern of co-occurrence of species groups. Data from 14 regional vegetation studies were examined to determine whether oak subgenera co-occur more frequently than would be expected if species were randomly distributed among stands. Twelve of the 14 studies showed statistically significant departures from the expected frequency of same subgeneric pairs. The tendency for co-occurring oak species to be in different subgenera is apparently not due to the formation of particular coevolved pairs since in most regions each species is commonly found with several species in other subgenera. These results imply that niche differences are greater between species in different subgenera than between species in the same subgenus, and thus that niche relations play a role in the organization of forests with oaks. The pattern of co-occurrence is apparently not caused by reduced competition for light, water, or mineral nutrients between adults of different subgenera. The high frequency of cross-subgenera pairs may result from differences in the regeneration niches (sensu Grubb 1977) of species in contrasting subgenera. In particular, mast fruiting tends to be well synchronized for species within a subgenus but poorly synchronized between subgenera. Such asynchrony could facilitate coexistence of species in different subgenera. Seed predator behavior may also contribute to the observed pattern of co-occurrence. Differences in water relations, stature, and degree of deciduousness also appear to affect the co-occurrence of oak species.