Below-ground plant species richness: new insights from DNA-based methods

1.Ideas about mechanisms controlling plant species richness are founded on empirical studies of above-ground vegetation. In many ecosystems, however, the majority of vegetation (e.g. 5090% in temperate grasslands) occurs below-ground as roots, rhizomes and shoot bases. Whether the richness patterns described for above-ground vegetation also hold for the large below-ground component is still unknown. 2.Here, we provide a concise overview how the use of DNA-based techniques might alter our perception of richness patterns in plant communities. We focus mainly on temperate grasslands, but new patterns should also arise in other community types (except epiphyte-rich ecosystems). 3.We hypothesise that DNA-based measurements of below-ground plant richness will reveal that richness is greater below- than above-ground because many perennial plants persist below-ground even in the temporary absence of above-ground shoots, and because the roots and rhizomes of plant individuals occupy larger areas than do shoots. Consequently, the speciesarea relationship may show steeper slopes for below-ground than above-ground richness. Further, above-ground richness may not be a constant proportion of below-ground richness, so the ratio of below/above-ground richness may vary along environmental gradients of productivity, disturbance and heterogeneity. 4.We also hypothesize that the often-observed decrease in above-ground richness with increasing productivity may not occur for below-ground richness, partly for the reasons noted above, and partly because of differences between above- and below-ground resources. Light is supplied largely in one dimension and does not persist in the environment for later uptake, whereas nutrients and water are supplied in three dimensions and can persist in the soil. Together, these differences should allow more niche differentiation and greater species richness below-ground. 5.Current DNA-based methods that allow measurement of below-ground richness in the field are likely to reveal patterns different from those well-documented for above-ground richness and may also produce new insights about plant community structure and function.