Mind the gap among patches in arid plant communities: Rapid root proliferation in response to N addition

Abstract

Aims: In arid communities, it has been proposed that individual plants can extend their roots beyond their canopy exploring neighbouring bare ground areas. This becomes relevant in systems where the vegetation is distributed in patches surrounded by bare soil. However, whether roots of different species may be overlapping under bare ground areas is still controversial. The factors controlling root responses when no plants appear to be directly influencing the gap among patches are still unclear. The aim of our study was to detect perennial grasses responses to an N enrichment pulse. Methods: In a semi-arid steppe (Patagonia, Argentina), we buried root traps filled with sieved soil with and without N addition, under bare soil patches. Traps were harvested after 4 and 6 months. Trap neighbourhoods (30 cm in diameter) included at least three of the dominant tussock species. After harvests, we identified species in the traps by root traits, and quantified diversity, biomass and specific relative growth rates. Important Findings: Bare ground areas show simultaneous root growth of different species. Diversity of perennial grass roots was higher with N addition than without it in the first harvest (4 months), but this difference disappeared in the second harvest (6 months). Root biomass was maximal after 6 months in N addition traps. Species preferred by herbivores (Bromus pictus and Poa ligularis) showed rapid growth and responses to N addition. Differences between harvests may be an indicative that N pulses interact with rising temperatures and soil water content as growing season progress.