Evaluating Monoculture Versus Polyculture Planting Regimes in a Newly-Restored Southern California Salt Marsh

Abstract

Salt marsh plants are a key source of primary productivity, ameliorate harsh abiotic conditions, and provide habitat structure to many organisms. As a consequence, rapid re-establishment of plant cover following restoration can speed the recovery of degraded ecosystems. Despite demonstrated positive relationships between plant biodiversity and ecosystem functions, many salt marsh restoration plans still incorporate single-species plantings under the belief that this approach will lead to faster increases in plant cover (a typical management goal). In this study, we evaluated post-restoration recovery of a non-vegetated high marsh berm in Brookhurst Marsh, Huntington Beach, CA, with two active planting strategies: monoculture plots of the competitive dominant Sarcocornia pacifica (pickleweed) versus polyculture plots of pickleweed and eight other common salt marsh plant species. Although monocultures did increase in total percent plant cover faster than polycultures, both treatments had reached 80-100% cover after one year, easily exceeding the permit-mandated goal of 20–40% cover in that time. The effects of increasing plant cover on abiotic parameters (e.g., % light reaching the ground, soil temperature, and soil salinity) were comparable between the two treatments and provided physical conditions sufficient to support similar macroinvertebrate communities. In contrast, plant species richness and canopy complexity were significantly higher in polyculture versus monoculture plots by the end of the experiment. Mean plant height was lower in polyculture plots, but maximum plant height (which can influence habitat use by perching birds) did not differ by treatment. Our data suggest that polyculture plots performed as well as, or better than, S. pacifica alone with respect to multiple indicators of ecosystem function. Active planting of high-diversity plots should therefore be seriously considered as a restoration tool to achieve common management goals in southern California salt marshes.