Evaluating the relative importance of woody versus non-woody plants for alpha-diversity in a semiarid ecosystem in Brazil

Abstract

Background and aims – Floristic surveys in tropical ecosystems are strongly biased towards collecting woody plants, overlooking the non-woody component even in semiarid ecosystems, where they can be predominant. This has led to an undervaluation of plant biodiversity in semiarid ecosystems because, as we show here, these areas have a high diversity of herbaceous species, sometimes much higher than that of woody plants. The semiarid Caatinga, in Northeastern Brazil, is one of such areas. Previously thought to be a species poor biome, these ecosystems are now understood as having a diverse flora, well adapted to dry conditions, including many therophytic species. Methods – We performed an extensive survey on plant diversity in a semiarid Caatinga site paying special attention to the non-woody component and compared it with the previously available data on woody plants for the same site. We used rarefied and extrapolated sampling curves to evaluate to what extent woody species are well sampled, based on the published data for trees and shrubs. We also constructed habit spectra to compare the relative importance of woody versus non-woody species in our site and across different Caatinga sites. Key results – Although the asymptotic richness for woody species is low when compared to other areas within Caatinga, the total alpha-diversity in the region is high if non-woody plants are considered in the study. We present habit and life form spectra to show that in this semiarid area most of the local biodiversity is composed by short-lived, usually under sampled, therophytes. Conclusion – We discuss how the focus on woody plants has underestimated the total alpha diversity of semiarid ecosystems. We call the attention of ecologists and phytosociologists collecting in semiarid ecosystems to focus not only on large trees and shrubs, but also to look down and collect the usually neglected, but very rich non-woody component.