Trajectories of vegetation‐based indicators used to assess wetland restoration progress

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 (R2 > 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.

Fighting carbon loss of degraded peatlands by jump-starting ecosystem functioning with ecological restoration

“…To be able to judge whether the goal of restoration is likely to be achieved, we need to understand the trajectories, i.e. the development of the community composition over time towards the target (Matthews et al 2009). …”

“…The time needed for judging the efficiency of restoration measures depends on studied ecosystem attributes, but in many cases it may be much longer than practitioners have anticipated (e.g. Matthews et al 2009;. The observed relatively slow recovery suggests that there, indeed, is a risk of drawing premature conclusions about the effectiveness of restoration in achieving the recent global expectations laid on restoration (Convention on Biological Diversity 2010; Halme et al 2013;Wortley et al 2013).…”

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“…Recovery trajectories can be at least partly understood in terms of the process of succession, which can often result in an accumulation of species over time that tends to saturate as a stable equilibrium is approached (Matthews et al 2009;Suding 2011). The curvilinear recovery trajectory documented here may therefore be explicable in terms of successional processes.…”

Impacts of introduced mammalian predators on New Zealand’s alpine fauna

“…3). Although trajectories for these targets are unknown, conceptually they encompass increasing the condition, richness, and/or abundance of sensitive indicator species until they eventually reach a yet-to be-defined asymptote representative of a reference ecosystem (Matthews et al 2009). The targets reflect thinking that is now common among many restoration ecologists in New Zealand.…”