A controlled experiment to assess relationships between plant diversity, ecosystem function and planting treatment over a nine year period in constructed freshwater wetlands

Two fundamental questions for wetland restoration science are: how do early management practices affect long term plant species diversity, and how does diversity affect ecological function? Here, we report on a nine year study conducted in six replicate experimental wetland ecosystems designed to shed light on the impact of different planting treatments and nutrient enrichment on plant species diversity and on a variety of indicators of structure and function. Pairs of high-and low-intensity planting treatments were initiated with native seeds and vegetative propagules. In the high-intensity treatment, vegetative propagules that did not establish were replanted in years three and four. An unplanted treatment was seeded through natural recruitment alone. Annual inventories of the presence and percent cover of plant species were used to derive measures of native Shannon-Weaver diversity and the floristic quality assessment index (FQAI). We found that higher plant species diversity was achieved early and was sustained in planted versus unplanted wetlands. However, no differences were evident between high-and low-intensity planting treatments. Although the prevalence of dominant species shifted over time, overall measures of plant diversity did not change between early and later succession in either planted or unplanted treatments. We found no relationships between plant diversity and a variety of direct and indirect measures of ecosystem structure and function, including aquatic community metabolism, aboveground productivity, soil organic matter accumulation and nutrient concentration. Fertilizer was applied in years eight and nine to simulate nutrient loading from agricultural runoff. Plant species diversity had no detectable effects on nutrient uptake rates in response to fertilization. Conversely, fertilization did not measurably alter species diversity in the two years following addition. We conclude that early management efforts were important in achieving a sustained and resilient enhancement in plant species diversity, but that elevated biodiversity resulting from planting did not measurably alter function. We speculate that control of invasive species in all treatment groups during early years of management resulted in high species diversity levels in unplanted as well as planted groups and that species diversity levels in all wetlands were therefore well above levels at which effects of plant species diversity on ecosystem function saturate. (C) 2015 Elsevier B.V. All rights reserved