Plant Coexistence in the Interactive Environment: Arbuscular Mycorrhiza Should Not Be out of Mind

Abstract

Mechanisms responsible for species diversity in communities constitute central topic in ecology. Most of the theories explaining the coexistence of plant species in community share two basic assumptions. First, they recognize the importance of ecological phenomena, i.e. of contemporary forces which at least in theory can be experimentally studied. Accordingly, historic and phylogenetic explanations of species diversity have only superficially been taken into account (Ricklefs 1987, Zobel 1992, Eriksson 1993). Second, among the biotic interactions influencing species diversity, plantplant interactions (mainly competition) have received most attention (see, for example, reviews of Goldberg and Barton 1992, Gurevich et al. 1992). The importance of herbivory for plant coexistence and species diversity has also been recognized for long time (Whittaker 1972, Huntly 1991), although the true importance of less visible below-ground herbivory has only recently been fully recognized (Brown and Gange 1989a, b). Among biotic interactions the role of micro-organisms in particular has recently gained recognition: a plant out in the field is not simply plant, but rather merger of fungal cells with plant tissues (Wilson 1993: 379). Besides pathogenic fungi and leaf endophytes, mycorrhizal fungi may also have an important influence on plants. Several reviews concerning mycorrhiza-symbiotic relationships between plant roots and fungi also include chapters considering plant competition, community structure, and succession (Finlay and S6derstr6m 1989, Allen 1991, Brundrett 1991, Chanway et al. 1991, Ingham and Molina 1991, Goodwin 1992, Sanders et al. 1995, Schdnbeck and Raschen 1995). This topic has even been included in the new edition of textbook (Silvertown and Lovett Doust 1993: 131). Thus, the possible importance of mycorrhiza in plant coexistence is recognized in principle at least. Some experimental studies have also claimed that the presence of arbuscular mycorrhiza (AM) increases plant species diversity in microcosm experiments (Grime et al. 1987) or in early successional communities (Gange et al. 1990, 1993). Different results have been obtained from lichen-rich community, where benomyl treatment resulted in an increase in vascular plant species richness (Newsham et al. 1995a). Consequently, we can ask how important is the presence of mycorrhiza for plant coexistence and does it contribute to the variation of plant species diversity in time and space? Approximately two thirds or more of vascular plant species form symbiotic relationships with AM fungi (Trappe 1987, Gianinazzi 1991). Since the number of AM forming fungal species (ca 150) is relatively low and the number of AM forming plant species high (ca 225 000) (Sanders et al. 1995), one may conclude that the host specificity of AM fungi is low. In the case of ectomycorrhiza (EC), the situation is almost the opposite (Read 1991). Since we are mostly interested in herbaceous communities, where very high species richness but also considerable variation in richness are encountered (e.g. Grime 1979), we have focused on the role of AM.