Chronic nutrient enrichment and herbivores modulate salt marsh recovery after wrack disturbance

Understanding how top-down and bottom-up forces shape post-disturbance succession across environmental stress gradients remains a central challenge in ecology. Salt marshes are dynamic coastal ecosystems where disturbances, such as wrack deposition, play a key role in regulating plant community structure and diversity, yet the combined effects of herbivory and nutrient availability on post-disturbance recovery in these systems remain poorly understood. We conducted a two-year field experiment nested in a long-term fertilization framework in a Southwestern Atlantic salt marsh to evaluate how herbivore pressure and chronic nitrogen enrichment modulate successional trajectories after wrack disturbance in low- and high-marsh zones. Recovery trajectories differed sharply between marsh elevations. In the low marsh, vegetation recovery was extremely limited despite herbivore exclusion and chronic nitrogen enrichment. In contrast, recovery in the high marsh was rapid and strongly shaped by herbivory and chronic nutrient enrichment. In the high marsh, herbivore exclusion combined with nutrient enrichment promoted rapid convergence towards reference undisturbed communities. Conversely, herbivory or nutrient limitation allowed for more variable community composition. Our results indicate that abiotic stress sets fundamental limits on recovery at low elevations, whereas in more benign environments, the interplay of top-down and bottom-up forces governs both the rate and the predictability of successional trajectories. Abiotic conditions may thus determine whether these forces shape succession at all, rather than only how strongly they do so, with implications for recovery under global change.