Ecological Applications, 19(8), 2009, pp. 2093–2107 Ó 2009 by the Ecological Society of America Trajectories of vegetation-based indicators used to assess wetland restoration progress JEFFREY W. MATTHEWS, 1 GREG SPYREAS, AND ANTON G. ENDRESS Illinois Natural History Survey, Champaign, Illinois 61820 USA, and Department of Natural Resources and Environmental Sciences, University of Illinois, Urbana, Illinois 61801 USA Abstract. Temporal trends in attributes of restored ecosystems have been described conceptually as restoration trajectories. Measures describing the maturity or ecological integrity of a restoration site are often assumed to follow monotonically increasing trajectories over time and to eventually reach an asymptote representative of a reference ecosystem. This assumption of simple, predictable restoration trajectories underpins federal and state policies in the United States that mandate wetland restoration as compensation for wetlands damaged during development. We evaluated the validity of this assumption by tracking changes in 11 indicators of floristic integrity, often used to determine legal compliance, in 29 mitigation wetlands. Each indicator was expressed as a percentile relative to the distribution of that indicator among .100 naturally occurring reference wetlands. Nonlinear regression was used to fit two alternative restoration trajectories to data from each site: an asymptotic (negative exponential) increase in the indicator over time and a peaked (double exponential) relationship. Depending on the particular indicator, between 48% and 76% of sites displayed trends that were at least moderately well described (R 2 . 0.5) by one of the two models. Floristic indicators based on species richness, including native richness, number of native genera, and the floristic quality index, rapidly increased to asymptotes exceeding levels in a majority of reference wetlands. In contrast, indicators based on species composition, including mean coefficient of conservatism and relative importance of perennial species, increased very slowly. Thus, some indicators of restoration progress followed increasing trajectories and achieved or surpassed levels equivalent to high-quality reference sites within five years, whereas others appeared destined to either not reach equivalency or to take much longer than mitigation wetlands are typically monitored. Finally, some indicators of restoration progress, such as relative importance of native species, often increased over the first five to 10 years and then declined, which would result in a misleading assessment of progress if based on typical time scales of monitoring. Therefore, the assumption of simple, rapid, and predictable restoration trajectories that underlies wetland mitigation policy is unrealistic. Key words: bioindicators; Carex; emergent wetland; floodplain forest; floristic quality; Illinois, USA; invasive species; reference sites; species richness; succession; wetland mitigation. INTRODUCTION A major goal of restoration ecology is to identify commonalities in the temporal dynamics of restored ecosystems in order to make restoration practice more predictable. Restoration trajectories, which plot mea- sures of ecosystem attributes over time, are often used to monitor restoration progress. These trajectories are often assumed to follow smooth increases over time, followed by an eventual arrival at a stable level that is characteristic of a natural reference ecosystem (Brad- shaw 1984, Kentula et al. 1992, Aronson and Le Floc’h 1996). Although some conceptual models include alternative possible endpoints and exceptions to a deterministic increase (e.g., Hobbs and Norton 1996), both restoration practitioners and theorists often assume that restoration activities guide a site along a simple trajectory of self-repair or self-design by facilitating deterministic succession (e.g., Mitsch et al. 1998, Shuwen et al. 2001, Weinstein et al. 2001). This notion is manifested in the U.S. federal policy of compensatory wetland mitigation, which permits losses of natural wetlands under the assumption that equivalent, replace- ment wetlands can be constructed over a short time frame. In contrast, a more complex view of restoration trajectories would acknowledge uncertainty in restora- tion planning and monitoring. The assumption of predictable temporal trajectories in restored wetlands has been criticized based on empirical evidence (Race 1985, Zedler 1996, Zedler and Callaway 1999, 2000). For example, studies of restored saline and brackish marshes indicate that not all structural and functional properties of restored wetlands follow ex- pected, simple trajectories (Simenstad and Thom 1996, Streever 2000, Whigham et al. 2002), and even when Manuscript received 21 July 2008; revised 12 January 2009; accepted 2 April 2009. Corresponding Editor: J. C. Callaway. 1 E-mail: matthews@inhs.uiuc.edu 2093