Untangling Invasion to Address Restoration: Lower Nitrogen Levels Reduce Competitive Success of <scp><i>Arundo donax</i></scp>

ABSTRACT 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 (NO 3 − ) from agricultural runoff. We added Arundo rhizomes to established Arroyo Willows in individual pots of riparian soil (in the greenhouse), and added NO 3 − weekly for 4 months at the following levels: no addition, low (24 g/m 2 year. −1 N), or high (120 g/m 2 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 NO 3 − without needing to invest in root infrastructure. Arundo had similar net assimilation of CO 2 (A net ) as Arroyo Willow, but greater leaf biomass, increasing its photosynthetic potential. These findings suggest that upstream agricultural NO 3 − 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 NO 3 − inputs.