A Nitrogen fixation: N critical loads for California

Article Reviewed: Nitrogen critical loads and management alternatives for N-impacted ecosystems in California

M.E. Fenn , E.B. Allen, S.B. Weiss , S. Jovan, L.H. Geiser , G.S. Tonnesen, R.F. Johnson, L.E. Rao, B.S. Gimeno, F. Yuan, T. Meixner, and A. Bytnerowicz

Journal of Environmental Management 91 (2010) 2404-2423 doi:10.1016/j.jenvman.2010.07.034

The Plot Line

In this paper, Fenn and collaborators review the nitrogen input “critical loads” for a broad selection of California’s vegetation types and discuss management possibilities. Included are mixed conifer forest, oak woodlands, pinyon-juniper woodland, chaparral, desert scrub, coastal sage scrub, and annual grassland. This summary of the article focuses on the mixed conifer forest sections, but it is important to remember that vegetation types intermix and share nutrients throughout watersheds and ecosystems.

A critical load is “a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” (UBA 2004). Empirical critical loads are expressed as quantities per area over time, and are based on the responses of observed biological and chemical variables. High productivity long-lived systems like forests are slow to exhibit changes in species composition in response to nitrogen deposition, but may exhibit detectable changes in soil and stream water chemistry. Under extreme deposition rates, ponderosa pine trees show a measurable decrease in fine root biomass, as well as increased susceptibility to bark beetles and modified soil fungal communities. With even a minor addition to baseline N levels, acid-sensitive epiphytic lichens begin to disappear. These environmental thresholds were used to examine forest response to N loading. Lichens are the most sensitive, with shifts in tissue N concentration and species composition at 3.1 kg N ha^-1yr^-1; the most sensitive lichens are no longer present at 10.2 kgN ha^-1 yr^-1. Incipient N leaching to surface runoff and a 26% reduction root biomass reductions are expected to occur at 17 kgN ha^-1 yr^-1.

The authors used the Community Multiscale Air Quality model to create maps of N deposition statewide and predict where deposition is predicted to be in excess of the described critical load values. In some areas, nitrogen deposition was ground-truthed with canopy throughfall measurements. Deposition currently ranges from 15-20 kg ha^-1yr^-1 in the Central Valley and Sierra, and 30-70 kg ha^-1 yr^-1 in southern California. Natural background forest deposition is estimated to be much lower, around 1-4 kgN ha^-1 yr^-1. This means that an estimated 28.7% of California’s mixed conifer forest (30,596 km² of 106,663 km²) is already in exceedance, based on the extreme sensitivity of lichens. The potentially affected areas are primarily in the central Sierra at all elevations, and at decreasing elevations southward along their western flank. Portions of the San Bernardino and Santa Cruz mountains are also above the critical load for lichens. For nitrate leaching and root loss, the area is considerably lower, only 4.5% (4754 km²). This smaller area of elevated potential impact is primarily in the San Bernardino Mountains, low elevation central Sierra, and extreme southwest Sierra.

Relevant quote

“In the most polluted forests (e.g. estimated N deposition > 25-35 kgN ha^-1 yr^-1) N deposition in conjunction with ozone threatens forest sustainability by contributing to multiple stress complexes, thus increasing forest mortality and fire risk.”

Relevance to landowners and stakeholders

Many characteristics of ecosystem function, including plant species composition, are dependent upon the nutrients available. Systems are often limited by the amount of nitrogen available, and undergo major changes when released from this limitation. Impacts of excess N deposition to forests include: soil acidification and depletion of base cation pools, acidification or eutrophication of alpine lakes, leaching that makes nutrients inaccessible to plants, lower root: shoot ratios, decreased mycorrhizal diversity, and increased tree susceptibility to pests. Such impacts can lead to altered plant physiology, vegetation composition, and forest structure. Big enough impacts on their own, these effects can also act in concert with other pollutants and conditions to further alter forest function. This paper’s focus on critical loads for California adds to similar work from throughout the U.S. and Europe. Describing the ecosystem response to atmospheric deposition sheds light on this important ecological connection that can differ from region to region.

Nitrogen accumulation and flux may also be influenced by more severe droughts and increased extreme precipitation events as predicted by climate change scenarios. In general, a forest critical load would be expected to decrease somewhat due to litter accumulation over the course of repeated dry years, but would be accompanied by greater susceptibility to leaching in heavy rains. Lichen-based critical loads are also dependent upon climate: increases are expected in wetter regions where rainfall can leach accumulated N from lichen tissues, but critical loads may decrease in dry climates.

Relevance to managers

As described by the authors, in many cases there are no feasible management options available to amend the impacts of N deposition. Most likely, mitigation will only be applied in local high-value situations or locations where many interests combine to carry out a specific management practice (for example, a prescribed fire). That said, there are some ways to manipulate forests experiencing excess N input. One option is to relocate some of the N in mineral soil by the use of fire, which volatilizes N and deposits it elsewhere. Because mineral soil holds 65-80% of the N in a California mixed conifer forest, focusing on the subsurface N makes sense. However, fires typically release about 3% of total soil N, so they must be implemented repeatedly to effectively reduce N impacts; such a pattern would only be suitable in certain sites and forest types. Thinning or harvesting affected forests can also temporarily remove aboveground N accumulation. However, tree removal does little to decrease the primary, belowground, nutrient pool, which is largely inaccessible to managers.

Above all, the most direct route to avoid and reduce the impacts of N deposition to California ecosystems is to decrease atmospheric inputs. Nitrogen oxide emissions are currently declining somewhat, but ammonia emissions are on the rise and likely underestimated. Although statewide emissions reductions are far beyond the influence of the average forest manager, long term management planning should incorporate trends in air pollution and consider how N deposition and emissions policies could impact their stands in the future.

Critique

The model-based maps are very helpful in visualizing the areas where estimated N deposition is at or above the critical load. The authors clearly describe that the maps don’t necessarily indicate that ecosystem impacts have occurred in these locations, but one can’t help but wonder what such a map would look like. A display of the observations used to establish these critical loads might show the extent (or at the very least, location) of ecosystem effects thus far and illustrate some of the complexity behind the calculations.

The options for management section of the paper offers very few tools for N removal, focusing on biomass removal and fire. Even if the management options are indeed few, it would be useful to include a discussion of the merits of other approaches here. A broader discussion would provide some context for those seeking solutions to N deposition through management. Finally, there are two critical loads described for the mixed conifer type: one for the lichens, which are very N-sensitive, and one for streams and roots impacts, which are less so. It isn’t always clear which standard is an appropriate measure of ecosystem impacts for a given situation. Providing basic information on interpreting these disparate values (do I use 3.1 or 17 kgN ha^-1 yr^-1?) might help this interesting work reach a wider audience.

Other notes

A useful resource for understanding critical loads and how they are calculated:

http://nrs.fs.fed.us/clean_air_water/clean_water/critical_loads/faq/

UBA, (Ed.), 2004. Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels, and Air Pollution Effects, Risks and Trends. German Federal Environmental Agency, Berlin, Germany, 190 pp. Available from: www.icpmapping.org.