Stress-resistance traits disrupt the plant economics - decomposition relationship across environmental gradients in salt marshes

Understanding how plants respond to environmental gradients and influence ecosystem functions remains a core challenge in ecology. Across species and ecosystems, plants have been shown to coordinate leaf, stem, and root traits along a gradient with optimal resource acquisition or conservation strategies at its extremes, termed the plant economic spectrum (PES), in turn driving ecosystem functioning. PES theory has been successfully applied in coastal wetlands to disentangle how the strong abiotic gradients affect ecosystem functions, such as litter decomposition. Yet, wetlands can be dominated by monospecific stands, and it remains unclear whether the PES applies within species. Here, focusing on a globally widespread salt marsh plant, Spartina anglica, we investigated: a) if the PES holds at the intraspecific level along critical abiotic stress gradients (redox potential and soil salinity); b) how intraspecific changes in plant traits along the PES affect litter decomposition; and c) whether these changes in plant strategies influence the abundance of key macro-detritivores, mediating litter decomposition. We found remarkable variation in S. anglica leaf and stem traits, coordinating along the PES and adopting a conservative strategy under stronger abiotic stress. Unexpectedly, leaves with a conservative strategy (higher leaf dry matter content and C/N ratio) attracted more macro-detritivores and decomposed faster. Other facets of litter quality beyond C/N seem to drive these counter-intuitive effects: leaves with a conservative strategy were likely more palatable because of higher protein content and lower toughness (low total carbohydrate content). Our study highlights that intraspecific trait variability can strongly drive litter decomposition, potentially impacting on the carbon storage capacity of salt marshes; and that specific stress-resistance traits can disrupt the PES - ecosystem function relationship.