Invasion by Ammophila arenaria alters soil chemistry, leaving lasting legacy effects on restored coastal dunes in California

Abstract Many restoration projects rely on invasive plant removal to restore ecosystems. However, success of restoration efforts relying on invasive removal can be jeopardized, because in addition to displacing native plants, invasives can also dramatically impact soils. Many studies have documented invasives' effects on soil chemistry and microbiota. While European beachgrass [Ammophila arenaria (L.) Link] is a worldwide invasive problem in coastal dunes outside northern Europe, little attention has been paid to effects of this species on soil chemistry following invasion, even though it establishes persistent, dense monocultures. In our study, we evaluated effects of A. arenaria invasion on soil chemistry of coastal dunes at Point Reyes National Seashore (PRNS); persistence of effects following removal by mechanical or herbicide treatment (legacy effects); and effects of treatment independent of invasion. Dune restoration efforts at PRNS have met with mixed success, especially in herbicide-treated backdunes, where decomposition of dead A. arenaria has been greatly delayed. Based on results, invasion impacted 74% of 19 variables assessed, although there was a significant interaction in many cases with successional status (earlier vs. later). Almost 60% of invasion effects persisted after restoration, with legacy effects prevalent in herbicide-treated backdunes where sand deposition from adjacent beaches could not mitigate effects as it could in herbicide-treated foredunes. Mechanical removal—or inversion of invaded surface soils with less-contaminated subsoils—resulted in fewer legacy effects, but more treatment effects, primarily in backdunes. Soil chemistry may decelerate decomposition of A. arenaria due to the limited nitrogen (N) available to enable microbial breakdown of the high carbon(C):N (70.8:1) material, but microbial factors probably play a more important role. Success of restoration at PRNS may not be fully realized until legacy effects are resolved through additional actions such as inoculation with healthy microbiomes or necromass reduction through controlled burning.