Tree invasion in secondary grasslands diminishes herbaceous biomass and diversity: A study of mechanisms behind the process

Abstract

Questions: How is plant biomass, composition and diversity in secondary grasslands affected by tree invasion? How do light conditions generated by invasive trees and soil legacy affect grassland's structure and functioning? Which mechanism is more important?. Location: Inland Pampa, Buenos Aires, Argentina. Methods: We established a long-term manipulative reciprocal transplant experiment by transplanting soil monoliths coming from two origins: grasslands (G) and invaded woody patches (W) into three different habitats: open grasslands (OG), woody patches (WP), and artificially shaded grasslands (SG). Results: Seven years after the beginning of the experiment, litter, above- and below-ground herbaceous biomass decreased from OG to WP habitats, with SG habitats showing intermediate values. However, differences between OG and WP habitats disappeared when tree (coarse) roots, and tree litter biomass were considered. No significant differences between the origin of the soil monoliths (G or W) were detected (p > 0.10), except for herbaceous roots biomass that, contrary to expectations, was higher in all habitats when the transplanted soil monoliths came from a W origin. Additionally, biomass of C3 grasses was higher in WP habitats and biomass of C4 grasses was higher in OG habitats, while biomass of forbs had the highest biomass in SG habitats. The inverse Simpson's index decreased from OG (2.08), through SG (1.73) to WP habitats (1.14), suggesting higher dominance in WP. Conclusions: Our study helps to glimpse the main mechanism driving community changes after tree invasions in temperate grassland communities. We demonstrated that light reduction beneath the tree canopies has a more important role than soil legacy in determining structure and functioning of grasslands, even though invasion of Gleditsia triacanthos also affected soil properties such as water content and organic carbon.