Hart et al. begin by differentiating between two types of studies of AMF effects on plant coexistence.  Coarse-scale studies are defined as those that compare the outcome of plant competition with AMF presence or absence, which they say should be "relevant to the outcome of plant interactions mainly in early successional ecosystems," where AMF "are either absent or are in low abundance and patchily distributed."  Fine-scale studies, on the other hand, are defined as those that compare the outcome of plant competition when all experimental treatments contain AMF and the manipulations "involve the composition and diversity of AMF, and the ways in which they interact with plants and their soil environment."  These experiments, they say, "are more relevant to later-successional situations, in which AMF are more abundant and less patchy, and the roots of most plants come into contact with them."

Concentrating primarily on the latter category of experiments, Hart et al. note that "higher AMF diversity could lead to higher plant coexistence simply by increasing the probability of individual plant species associating with a compatible and effective AMF partner."  In addition, they point out that shared mycelial networks "might promote plant species coexistence by equalizing the distribution of soil resources among competitively dominant and subdominant host species," noting that "soil nutrients and plant-derived carbon might flow through the network from dominant to subordinate host plants, because of a concentration gradient created initially when the dominant plant takes up more nutrients than does a subordinate plant."  The former of these phenomena - the plant-to-plant transfer of nutrients - has been observed by Malcova et al. (1999) under laboratory conditions and by Walter et al. (1996) in the field.  Likewise, the second phenomenon - the plant-to-plant transfer of carbon - has been observed by Grime et al. (1987), Graves et al. (1997) and Robinson and Fitter (1999).

In concluding their review, Hart et al. say "our understanding of fine-scale factors is just starting to develop," and in this regard we note that many experiments conducted in recent years have added elevated levels of atmospheric CO2 to the mix of experimental variables considered within this context.

Several such studies are described in our Subject Index under the heading Biodiversity (Fungi), where it may be seen, as noted in the review of Hart et al., that (1) the presence of soil fungi helps to maintain, and sometimes even increase, the biodiversity of various ecosystems, and (2) elevated levels of atmospheric CO2 help these fungi to better perform this important function.  Also, under the Subject Index heading of Fungi, one can read how these principles operate in Grasses, Herbaceous Plants and Woody Plants, and how they help to enhance Carbon Sequestration.

It is also interesting, in this regard, to go back to the book of Idso (1989) -- Carbon Dioxide and Global Change: Earth in Transition -- and read what he wrote about the subject nearly a decade and a half ago:

"Considerable evidence may be mustered to support [an] optimistic view of the effects of atmospheric CO2 enrichment on species diversity.  Looking to the past, for example, several studies of Tertiary floras have demonstrated that many montane taxa of that period regularly grew among mixed conifers and broadleaf shclerophylls (Axelrod, 1944a, 1944b, 1956, 1976, 1987), whereas today these forest zones are separated from each other by fully 1,000 m in elevation and 10-20 km or more in distance (Axelrod, 1988).  Indeed, during this many-million-year period, when the CO2 content of the atmosphere was generally much greater than it is today (Volk, 1987), all three forest zones merged to form a 'super' ecosystem, which, in the words of Axelrod (1988), "was much richer than any that exists today."  Even under current conditions, in fact, modern forestry experiments have demonstrated that trees planted in mixtures sometimes grow better than they do in single-species plantings (Brown and Harrison, 1983; Carlyle and Malcolm, 1986a; Carlyle and Malcolm, 1986b; Chapman et al., 1988).

"One mechanism by which this type of mutualism may be fostered has to do with the stimulation of vesicular-arbuscular mycorrhizal fungi, which are ubiquitous in most terrestrial ecosystems (Gerdemann, 1968; Read et al., 1976) and the most prevalent of all soil fungi (Gerdemann and Nicolson, 1963).  These unseen inhabitants of the soil provide a number of benefits to the plants they 'service.'  They increase the absorption of water and nutrients by the plant, protect the plant from soil-borne diseases, and reduce the incidence of nematode infection of roots (Ingham, 1988).  And as Johnson and McGraw (1988) have noted, 'their vigor may be expected to reflect the vigor of their hosts,' which with CO2 enrichment would be expected to increase.  Consequently, whereas community ecology paradigms of the past, based largely on data pertaining to above-ground interactions, have tended to stress relatively short food chains with competition and antagonism as major organizing forces in community development, Edwards and Stinner (1988) note that, today, 'ecologists studying biotic interactions in soil systems generally have observed complex food webs, a great diversity of organisms, and a wide range of symbiotic interactions.'  Indeed, many endomycorrhizae (Chiariello et al., 1982), as well as certain ectomycorrhizae (Reid and Woods, 1969; Read et al., 1985), have even been demonstrated to actively transfer nutrients between individual plants of both the same and different species.  In fact, seedlings of some plants will not grow at all unless they interact with the mycorrhizae of an adjacent host plant (Warcup, 1988), while in other situations, both seed germination and initial plant growth rates are greatly stimulated by the presence of such fungi (Clements and Ellyyard, 1979; Masuhara and Katsuya, 1989).  As a result, Moore (1988) contends that mutualism is common below ground and that it 'can have profound effects on the structure and activity of soil microbial communities, the decomposition of organic matter, and ultimately plant growth'."

Yes, as the saying goes, "everything old is new again."  It's been shown over and over that arbuscular mycorrhizal fungi and atmospheric CO2 enrichment make a truly dynamic duo when it comes to getting plants to "cooperate" in both maintaining and enhancing ecosystem biodiversity.

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