Effects of productivity on species-area curves in herbaceous vegetation: evidence from experimental and observational data

The effects of productivity on the parameters of the species-area curve were investigated in this paper using two data sets on terrestrial plant communities: (1) one including 48 plots in 12 experimental sites on ploughed, formerly cultivated fields in the Siena region, Italy, and (2) one including 40 plots in hay meadows in the Bremen region, Germany. In both regions, species presence of vascular plants was recorded in nested plots ranging in size from 0.004 to 256 m(2) and 0.001 to 1000 m(2), respectively. Productivity was estimated as dry standing biomass. In the Siena data set, species richness showed a humped-back relation to biomass in the plot sizes up to 1 m(2). For the larger plot sizes, no significant correlations were found. In the Bremen data set, positive relation between species number and biomass was observed at the smallest spatial scale (0.001 m(2)), whereas the relation disappeared or tended to be negative for the larger plot sizes. In general, the slopes z of the log species-log area curves (SAC) were negatively related to biomass in both data sets, while the intercept c increased with biomass in the Siena data set and was unrelated to biomass in the Bremen data set. The relationship between c and z was negative in the Siena data set and positive in the Bremen data set. The above results differed somewhat depending on which plot sizes were considered for the calculation of the SAC. Literature data confirmed that there are no clear patterns in the inter-correlations between productivity, small scale and large scale species richness. Sites differing in productivity and in the slopes and intercepts of SAC may thus give rise to different species richness-productivity relationships. There can be one possible relation between species richness and biomass at one spatial scale (e.g. humped-back) and another type of relation, even opposite, at another spatial scale. This suggests that the properties of species-area curves do not respond in a uniform way to the changes in productivity, but depend on the type of habitat or plant community and its particular properties. The parameter of the SAC can then hardly be used as scale-independent parameter to investigate the effects of ecological factors, such as productivity, on species richness. The lack of clear patterns in the relations between small scale and large scale species richness implies that the predictions of the species-pool hypothesis may fail when applied to plot sizes as dealt with in this study.