Restoring Wetland Plant Diversity: A Comparison of Existing and Adaptive Approaches

Abstract

Most restoration projects are not designed or assessed in ways that identify cause–effect relationships.When plants die, even detailed postmortem examinations cannot pinpoint causes; e.g., mortalities of 7% vs. 90% in two salt marsh transplantation projects were attributable to differences in hypersalinity and sedimentation, but other effects could not be ruled out. Adaptive restoration (the experimental testing of alternative approaches in restoration sites), however, can clarify cause–effect relationships, while simultaneously restoring plant diversity and informing future restoration efforts. Projects in Tijuana Estuary (California) and Greene Prairie (Wisconsin) demonstrate the approach: (1) A large field experiment at Tijuana Estuary showed that species-rich plantings of halophytes accelerated the development of ecosystem structure and function (over single-species plantings) while simultaneously vegetating an intertidal plain. The six-species assemblages produced more complex canopies and accumulated more biomass and nitrogen than singlespecies and unplanted plots. (2) Also at Tijuana Estuary, an experiment is testing the ability of tidal creek networks to accelerate revegetation and increase food-web support (via increased growth of plants, invertebrates, and fish) in an 8-ha project that simultaneously restored tidal flushing. (3) In Greene Prairie, the ability to establish 33 native species is being tested as replacements for an invasive grass (Phalaris arundinacea). In each case, the adaptive approach informs both the science and practice of restoration. Without experimentation, restorationists are hard-pressed to explain past mortality and to suggest better methods for restoring structure and function. Adaptive restoration can provide the knowledge required, especially when large projects are implemented as sequential modules with experiments that sequentially provide essential information.