Management Options for Exceedance of Empirical CLs for Nutrient N
This section focuses on the progressive negative changes in vegetation community structure and composition resulting from excess nitrogen (e.g., native forbs and other plant species being replaced by invasive grasses and other species that are more productive under elevated N deposition). These shifts in community composition and species richness can lead to losses in biodiversity, impact some threatened and endangered species, and affect fire dynamics (e.g., exotic annual grass invasion increases fine fuels and fire risk). Management options for mitigating exceedance of nutrient N CLs are based on reducing N emissions (and N deposition) and reducing the nitrogen available for plant use through biomass removal, prescribed fire, or control of invasive grasses. Ultimately, N deposition decreases will be needed for long-term ecosystem protection and sustainability, and this is the only strategy that will protect the most sensitive communities, such as lichens and native forbs.
1. Reduce nitrogen emissions at the source.
The most effective way to reduce the effects of nitrogen excess is to improve air quality, by decreasing emissions at the source. The Forest Service only has direct control of emissions produced from activities conducted or permitted by the forests, and these emissions generally contribute a small amount to overall N-deposition. For all other sources of pollution, the forest must work with state, tribal and federal air quality partners who have the authority and responsibility to regulate emissions of pollution into the atmosphere.
Since the mid-1980s, the Forest Service has reviewed and commented on proposed new or modified major point sources of air pollution to air regulators. This is an important approach, but the Forest Service role in the regulatory process only extends to sources that may negatively impact federally designated Class 1 wilderness areas, which is a limited number of sources. Participating in the development of state, tribal, or federal air implementation plans can be an effective tool for the increased protection of resources affected by air pollution. Forests can also support state and tribal partners who implement stricter health and mobile source standards, because the resulting emission reductions are likely to have positive effects on natural resources. Finally, the forests can be advocates for secondary air quality standards designed specifically to protect the environment. In all of these cases, CLs and TLs help the forests communicate the negative effects of air pollution on ecosystems, highlighting current and predicted resource conditions and identifying concerns.
2. Mitigate nitrogen effects through resource management.
The most extensive discussion of resource management options for nitrogen impacted ecosystems in the US is the Fenn et al. (2010) paper focused on California. Options for mitigating the effects of excess N vary widely among vegetation types due to the differing resource impacts caused by N deposition. All mitigation options require effective concomitant nitrogen reductions. Different combinations of the following management strategies should be considered, however many of these strategies may be counter-productive in areas where acidification is also a concern:
- Prescribed fire:
- Prescribed fire applied at regular intervals will reduce N pools in the aboveground ecosystem through volatilization. Fire has a limited capacity to reduce N pools in the mineral soil. The benefits of prescribed fire for N reduction are variable by vegetation type and should be weighed carefully prior to applying this management option.
- There is also an increase in N uptake through the vigorous regrowth of vegetation following burning.
- For areas where invasive exotic grasses have replaced native species, spring burning prior to seeds dropping can significantly reduce the seedbank of exotics. This provides a window of opportunity for re-seeding by residual native plants, or for planting native species if necessary. Because some invasives increase in abundance after burning, this approach should be used selectively and targeted to specific species.
- Prescribed fire can have a liming effect on soils, increasing soil pH as well as the Ca/Al ratio (Boerner et al. 2004).
- Mechanical stand thinning:
- Used in forested situations, this option can lead to increased N uptake by the vigorous regrowth of vegetation that occurs in openings and increased growth by residual trees. Similar to prescribed burning, this technique has a limited capacity to liberate N stored in the mineral soil.
- Mechanical control of invasive grasses through grazing, mowing, or manual weeding in areas where fire and herbicides cannot be used.
- Selective herbivory on N rich grasses is an essential process maintaining local diversity in grasslands affected by N deposition. For example, moderate grazing by cattle in the South San Francisco Bay was effective because the animals selectively fed on the invasive annual grasses, reduced biomass accumulation, and mechanically broke down litter leaving an environment conducive to the growth and propagation of native short annual forbs. Because invasives vary in lifecycle timing and palatability (e.g., cheatgrass is more palatable to livestock before it sets seed), this approach should be targeted to specific species and timed accordingly.
- Mowing is an effective tool for treatment areas too small for grazing. If possible, time the mowing so that it occurs when native plants have set seed but the exotic seeds have not fully ripened. Consider the impact of this technique on wildlife management concerns, like those for neo tropical birds and other species.
- Manual weeding is a labor-intensive technique best reserved for situations where other methods are not feasible, or for high value recreation areas like campgrounds or administrative sites. This method is recommended in desert areas where the establishment of shrub seedlings is desired.
- Grass-specific herbicides can be effective in controlling annual grasses, but some exotic forbs in California have been shown to increase more than native plants after herbicide application. Some herbicides are acidifying and should not be used in areas with acidification concerns.
- Mulch (applying wood chips to the soil in harvested areas):
- This technique provides some level of N immobilization and reduces nitrate leaching from the soil in northern hardwood forests with elevated N deposition (Homyak et al. 2008). The technique has been used in coastal sage scrub but the impacts were short-lived. Mulching is impractical for large-scale restoration projects.