What is the role of perennial plants in semi-steppe rangelands? Direct and indirect effects of perennial on annual plant species

Abstract

Biodiversity–productivity relationships have widely been studied in natural herbaceous communities but remain debated. Here, we hypothesized that species richness of perennial plants would increase species richness of annual plants but aboveground biomass of perennial plants would restrict the species richness and aboveground biomass of annual plants across plant functional types (i.e. shrubs, forbs and graminoids) in natural rangelands. We quantified the species richness of annual and perennial plants across plant functional types, and then measured aboveground biomass destructively in 735 quadrats of semi-steppe rangelands. We used linear path analysis model to test for the multivariate effects of perennial on annual plant species using species richness and aboveground biomass datasets, in addition to the influences of grazing disturbances and soil textural properties. Our results showed the species richness of perennial plants (forbs and graminoids) generally promoted the species richness of annual plants (forbs and graminoids). However, the aboveground biomass of perennial species (shrubs, forbs and graminoids) imposed negative or negligible (i.e. nonsignificant negative) influences on the species richness and aboveground biomass of annual species (forbs and graminoids). Hence, perennial species have probably restricted the aboveground biomass of annual plant species due to the resource acquisition, but protected annual plants from grazing and other anthropogenic disturbances. This study suggests that the relationship between species richness and aboveground biomass is positive in general, regardless of the confounding effects of perennial on annual species across plant functional types. This study highlights that simply by integrative modelling of perennial and annual plant species across plant functional types for the associations between species richness and aboveground biomass, better insights can be gained into ecological mechanisms playing a role in the biodiversity conservation of natural rangelands and other natural ecosystems in general.