The diversity of within-community plant species combinations: A new tool for assessing changes in forests and guiding protection actions
Stefano Chelli, James L. Tsakalos, Zhengxue Zhu, Luciano Ludovico Maria De Benedictis, Sándor Bartha, Roberto Canullo, Liubov Borsukevych, Marco Cervellini, Giandiego Campetella
Abstract
• Alpha diversity measures do not capture crucial information about plant interactions. • We propose an index called Compositional Diversity which takes into account species combinations. • Old-growth forests have unique understorey species assemblages. • Restoring forest plant assemblages requires a longer time scale than restoring alpha diversity. • Species combinations may be used to assess whether a given forest has reached old-growth features. Biodiversity is changing rapidly, and ecologists use various measures to monitor and conserve it, but not all are equally effective. In the European temperate forests, ecologists are tasked with assessing the impact of global changes on plant species richness; however, this fails at capturing vital information about plant interactions. Using a chronosequence of beech forest stands, spanning 600 years of growth, we demonstrate the application of a different measure of diversity compared to classical species richness in the understorey. This measure, called compositional diversity (CD), considers the number of species combinations and their relative frequency within a community. The response of both classical species richness and CD along with succession, corresponded with our expectations based on ecological theory’s U-shape prediction of diversity along the successional gradient. However, after 300 years, there was a significant decoupling between the two measures’ responses. While species richness remained low and constant across old-growth and primeval forests, CD peaked in primeval forests, implying that the same number of late-successional species generated more diverse assemblages. This new information emphasises the need to protect old-growth and primeval forests not only to conserve species richness but also to preserve their unique network of species co-occurrence patterns – a factor not well represented by the classical species richness measure.