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Natural habitats have a "critical load" threshold in regards to deposition of nitrogen compounds (both wet and dry deposition). The critical load is calculated in kg N / hectare and is specific to local ecoregion type. I've heard that critical loads are based on lichens or other simple vegetation, but I'm not sure. How are critical loads calculated? What are the factors that are considered?

An example of critical loads are shown below for the western USA. In general the ecoregions are pretty coarse and in my mind over-generalized. I'm curious about how these ecoregion distinctions are made. How many types of ecoregions are there and what are they based on?Is it based on the vegetation that currently exists in that area (which can sometimes be distinctly different than indigenous vegetation)? Or is it based on some historical vegetation type? Furthermore, what is the expected result if deposition surpasses the critical load and what is the time horizon involved? Critical Load by Eco-Region!

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  • $\begingroup$ Peculiar how the critical load 10 or 5 seems to follow exactly the boundary between Missouri/Iowa/Minnesota and Kansas/Nebraska/Dakotas. $\endgroup$ – gerrit Feb 5 '15 at 16:48
  • $\begingroup$ I think that's the Missouri river boundary. $\endgroup$ – farrenthorpe Feb 5 '15 at 17:22
  • $\begingroup$ I can see how watershed boundaries are natural boundaries, but why would a river be? $\endgroup$ – gerrit Feb 5 '15 at 17:35
  • $\begingroup$ It's the boundary of the "Great Plains" ecoregion. This is part of the reason I posted the original question... I can't figure out what all factors they are considering in the calculation that would produce a map like this. Seems quite over-generalized. $\endgroup$ – farrenthorpe Feb 5 '15 at 20:40
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These two links were the sources I used when researching this question:

"Mapping critical loads of nitrogen deposition for aquatic ecosystems in the Rocky Mountains"

http://co.water.usgs.gov/publications/non-usgs/Nanus_2012.pdf

"Approaches for estimating critical loads of nitrogen and sulfur deposition for Forest Ecosystems of U.S. Federal Lands"

http://www.nrs.fs.fed.us/pubs/gtr/gtr_nrs71.pdf

It looks like there are actually several different approaches which can be used when calculating the Critical loads, and depending on the approach the factors considered change:

  1. Empirical (Regression) modeling: These are generally based on observations of ecosystem response (this is tracked off of Foliage, lichens, or soils) and tracked to a given deposition level. This is generally calculated in one area, and applied elsewhere.

  2. Simple mass-balance models: estimating a net loss or accumulation of nutrients based in the in/out of a system for that nutrient. These are basically steady state models and are measured generally on base cations or nitrogen.

  3. Dynamic models using a mass balance approach which incorporates internal feedbacks over time. This is a combination of the two and is generally used where there is an abundance of existing data.

The links contain further information which may be more helpful.

According to "Wolfe, A.P., Baron, J.S., Cornett, R., 2001. Anthropogenic nitrogen deposition induces rapid change in alpine lakes of the Colorado Front Range (USA). Journal of Paleolimnology 25, 1e7." as cited in the first article I linked, This excess N (caused by exceeding the critical load) can result in acidification and nutrient enrichment, increasing primary productivity in high-elevation lakes and streams, and altering diatom community structures that form the base of the food web –

The USFWS Ecoregion classification system was designed by Robert G. Bailey working out of the Rocky Mountain Research Station in Colorado. In his paper, "Identifying Ecoregion Boundaries" found Here: http://www.fs.fed.us/rm/ecoregions/docs/publications/identifying-ecoregion-boundaries.pdf he outlines the rational used for identifying the boundaries, and the 20 different principals he used. They are: 1. The series of ecoregions should express the changing nature of the climate over large areas. 2. Boundaries of ecoregions coincide with certain climatic parameters, 3. , 4. , 5 - 20

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  • $\begingroup$ Hi, thanks for your answer. Could you say more about what the consequences are of exceeding said "critical load"? $\endgroup$ – farrenthorpe Aug 14 '15 at 20:43
  • $\begingroup$ @farrenthorpe Sure. According to "Wolfe, A.P., Baron, J.S., Cornett, R., 2001. Anthropogenic nitrogen deposition induces rapid change in alpine lakes of the Colorado Front Range (USA). Journal of Paleolimnology 25, 1e7." as cited in the first article I linked, This excess N (caused by exceeding the critical load) can result in acidification and nutrient enrichment, increasing primary productivity in high-elevation lakes and streams, and altering diatom community structures that form the base of the food web $\endgroup$ – Patrick Meddaugh Aug 14 '15 at 20:49
  • $\begingroup$ If you would like the bounty, I think it would be necessary to discuss the determination of ecoregions, as discussed in the question. $\endgroup$ – farrenthorpe Aug 15 '15 at 6:27
  • $\begingroup$ @farrenthorpe Will Do. I should have something for you soon. It appears that the determinations of the Ecoregions by the USFS was mainly done by one scientist working out of colorado and he has some literature on his rationale you may find insightful. $\endgroup$ – Patrick Meddaugh Aug 17 '15 at 15:29
  • $\begingroup$ @farrenthorpe So, the USFWS Ecoregion classification system was designed by Robert G. Bailey working out of the Rocky Mountain Research Station in Colorado. In his paper, "Identifying Ecoregion Boundaries" found Here: fs.fed.us/rm/ecoregions/docs/publications/… he outlines the rational used for identifying the boundaries, and the 20 different principals he used. They are: 1. The series of ecoregions should express the changing nature of the climate over large areas. 2. Boundaries of ecoregions coincide with certain climatic parameters $\endgroup$ – Patrick Meddaugh Aug 20 '15 at 23:07

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