Definition
Temporal trajectory represents the rate and magnitude of changes in structure, function or services over time. It includes three main elements relevant to mitigation equivalency: historical condition, restoration timing, and future change or potential for future change.
Alternatively, the US Army Corps of Engineers definition reads “We have added a definition of temporal loss which clarifies that temporal loss is the time lag between the loss of aquatic resource functions caused by the permitted impacts and the replacement of aquatic resource functions at the compensatory mitigation site. Temporal loss is one factor that must be considered in determining compensation ratios. The definition also provides that the district engineer may determine that compensation for temporal loss is not necessary when a mitigation project is initiated prior to or concurrent with the permitted impacts, except for resources with long development times (e.g., forested wetlands)”. (after the USACE 2008 Mitigation Rule)
Representative Components
- Accrual time
- Historic structure and composition
- Inherent annual variability
- Range of responses (measure of uncertainty)
- Organism traits (growth rates, reproductive cycle)
- Triggers in the process that might lead to adaptive management
Utility
First, the historical aspect might be related to the assessment of the impact and the information available about historic conditions in the region. One element for the working group to consider is the messaging to the public around historic conditions. Next, restoration of degraded ecosystems often experience hydrological and biogeochemical time lags between the restoration implementation and ecosystem recovery and functional improvements. This lag will vary among habitat types based on the natural history characteristics of the species in the ecosystem. In addition, delays in a species’ response to habitat modification can occur after restoration, when species or entire associated communities take longer to reoccupy or recolonize. The USACE 2008 mitigation rule does encourage higher amounts of compensatory mitigation to account for this concept of temporal loss. The challenge around this temporal lag as it relates to mitigation is that immediate loss on the impact site may not result in replacement of function on the restoration/mitigation site. Finally, ideal mitigation projects would continue to function under a range of potential future conditions associated with land use change and predicted by climate change models.
Relevance
Impacts are often instantaneous and permanent, but compensation often occurs over time. Moreover, there is uncertainty in the recovery trajectory and it is difficult to discern inherent and expected variability from deviations in expected response (especially early in the recovery process). Accounting for the time lag between losses and compensation and the uncertainty associated with this process is critical to ensure adequate compensation occurs. This may need to be revisited over the course of the recovery process as the ultimate disposition becomes clearer
An understanding of past condition, function, and services can be informative in determining expectations for the future ecosystem. The goal is not to recreate historic conditions, but to use the understanding of the past to inform actions and performance targets.
Research Needs & Hurdles
Conceptually, restoring fundamental processes should impart ecosystem resiliency under changing conditions. Research needs are include:
- Historical condition (potential as a guiding principle)
- Restored decisions are informed by an understanding of historic conditions
- Go back as far as you can go since data was available and understand the trajectory
- This does not necessitate making returning to those conditions a goal.
- Time to recover function or structure (long lead time)
- Uncertainty
- Higher mitigation ratios needed in planning?
- Future potential
- Temporal modeling
- Potential for succession
- Migration potential, transgression
- Adaptability, resilience, persistence, stability
- Role of size (maybe move to landscape context, was also discussed in off site conversation in the context of combining several smaller mitigation projects into off site larger project)
- Large areas provide room to accommodate landscape-scale processes and large, diverse populations. Larger wetlands correlate with greater species richness (Keddy et al. 2009), and are more resilient to disturbances (Moreno-Mateos et al. 2012). (From WRP 2018)
Example/Representative Metrics
Key spatial elements that should be considered include:
- Rarity/historic loss
- Connectivity/isolatedness
- Ecotones
- Proximity to disturbance/urbanization
- Current and near future migration potential
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