Biodiversity, ecosystem function and plant traits in mature and immature plant communities

Several studies of synthesized herbaceous plant communities have reported a positive relationship between vascular plant diversity and biomass production (Naeem et al. 1994; Tilman et al. 2001), and the European BIODEPTH experiment has been one of the most influential in this regard (Hector et al. 1999). Much, though not all, of the evidence for this relationship has come from experimental communities established from seed and harvested after only a year or two. It is not clear that such immature plant communities would be expected to provide reliable evidence of the relationship between diversity and ecosystem properties. Here we compare the relationship between diversity and biomass in BIODEPTH with that at the long-term monitoring site at Bibury in Gloucestershire, UK (Dunnett et al. 1998), a natural plant community that has much in common with BIODEPTH. The Bibury site offers a rare opportunity to address questions similar to those tackled by BIODEPTH, while avoiding some of the limitations of short-term experiments. If a positive relationship between vascular plant diversity and biomass production is an important phenomenon, one could argue that it should be detectable in natural vegetation. Previous attempts to explore the relationship between biodiversity and productivity in natural plant communities have been criticized on the grounds that both variables are frequently affected by additional (usually abiotic) variables (Lawton et al. 1998). For example, over a wide range of moderate-to-high biomass values, there is a well known negative relationship between biomass and species richness, driven by increasing competitive exclusion at higher productivity (Grime 1973; Grace 1999; Stevens et al. 2004). In experiments such as BIODEPTH, one aim is to exclude such natural sources of variation and create synthesized plant assemblages that vary only in artificially imposed levels of species richness. Of course, random environmental heterogeneity cannot be completely excluded, and Huston & McBride (2002) have shown how such heterogeneity can give rise to spurious diversity-function relationships. However, synthesized communities may also exhibit systematic variation in productivity equalling or exceeding that occurring in natural habitats, arising from the higher nitrogen availability in communities sown with N-fixing legumes. In BIODEPTH, addition of TriJolium pratense (nomenclature follows Stace 1997) increased biomass, on average, by 360 g m (Hector et al. 1999). As this increase was much larger than the effect of any other species, it is reasonable to attribute this effect to a direct result of the extra N fixed by Trifolium. The increase was also much larger than the effect of a change in diversity itself; doubling species richness increased biomass, on average, by only 80 g m-. The Bibury road verge lies on the north side of Akeman Street, originally a Roman road, in Gloucestershire, UK (National Grid reference SP 119048). The 700 m length of experimental verge was originally chosen