Untangling Invasion to Address Restoration: Lower Nitrogen Levels Reduce Competitive Success of Arundo donax

Riparian ecosystems in Mediterranean climates are pressured by nutrient enrichment and opportunistic plants, such as the large perennial grass Arundo donax. Arundo grows more with nitrogen (N) amendments and can outcompete native woody species in high-N conditions characteristic of intensive agricultural runoff, but its competitive dynamics at N levels characteristic of less intensive agriculture have not been assessed. Restoration efforts typically do not consider minimising conditions that make ‘invaders’ so successful in a given area, such as the range of upland N inputs; however, such distinctions may be important in identifying areas with higher chances of successful native plant restoration. Here, we addressed how different levels of N addition influenced competition between Arundo and a dominant native California woody species, Salix lasiolepis (Arroyo Willow), by simulating a post-flooding scenario: Arundo rhizomes are deposited and establish within Arroyo Willow stands and receive nitrate (NO₃⁻) from agricultural runoff. We added Arundo rhizomes to established Arroyo Willows in individual pots of riparian soil (in the greenhouse), and added NO₃⁻ weekly for 4 months at the following levels: no addition, low (24 g/m² year.⁻¹ N), or high (120 g/m² year.⁻¹ N). Arundo inhibited Arroyo Willow growth by 57% in high-N conditions, but did not inhibit Arroyo Willow in low- or no-N treatments. Across all treatments, Arundo had greater leaf N%, higher potential photosynthetic N use efficiency, but produced less root biomass than Arroyo Willow, suggesting that it dominated access to NO₃⁻ without needing to invest in root infrastructure. Arundo had similar net assimilation of CO₂ (A_net) as Arroyo Willow, but greater leaf biomass, increasing its photosynthetic potential. These findings suggest that upstream agricultural NO₃⁻ management can either enhance or constrain Arundo success. Plant restoration practices should prioritise whole watershed management of Arundo removal areas, integrating consideration of upstream N dynamics and runoff so as to limit NO₃⁻ inputs.