Herbaceous Vegetation in Southwestern Pine Forests

Herbaceous forage in southwestern pine forests can be few and far between.  Especially in unnaturally-overgrown thickets, there is simply not enough light and too much acidic leaf needle litter to grow robust grasses and forbs.  The high carbon content and resistant nature of conifer bark and needles means that few nutrients are available to growing plants.  

Even when forest thinning restores natural stand densities, the recalcitrant accumulated biomass often seems to limit production.  There is simply too much carbon and it is clogging up microbial turnover of nutrients.  Sure, you'll find plenty of aboveground mushrooms and mycelial hypahe at work in the soil, but I have to wonder if there isn't a more productive alternate-stable state.

What is the limiting resource?  I've theorized that light may still be limiting when forests aren't thinned enough to create light gaps in the canopy.  But it also seems that the overabundance of carbon may be soaking up nitrogen.  In these nutrient-poor systems we typically find 'tolerator' species like sedges, that can eke out a frugal living in acidic conditions.

What about blueberries?  In some parts of the world, conifers and blueberries go together like apples and pie, but in many of the southwestern mountain ranges we have no native Vaccinium.  I'm still not sure why, except that they seem to prefer colder (moister?) climes than New Mexico can provide....

There are two species of Vaccinium in the southwest, with Vaccinium myrtillus much more common.  However, south of Santa Fe even this species only occurs at scattered locations in mixed conifer and subalpine forests.  These high-elevation forests are being rapidly lost to stand-replacing forest fires.  Even with current climate change, it is unlikely that extensive tracts of cooler forests will be able to regrow.

What about legumes?  Fabaceae are often able to supply their own nitrogen requirements and eventually supplement total ecosystem N.  But I'm surprised by how rarely I find good legumes in the forest.  Thermopsis is surprisingly rare, as are Lotus and I almost never find clovers growing in conifer needle duff.  

Robinia neomexicana seems to be one legume shrub that has found its way into a diversity of habitats, growing almost like a weedy in mountainous areas throughout the southwest.  

Seeding mixtures in forests typically use annual grasses, but I wonder if there isn't a Fabaceae that could dramatically enhance production to increase ground-cover, forage, pollinator, and wildlife habitat?  Should standard thin-and-chip treatments be supplemented with seeding efforts?

What about increasing disturbance to disrupt pine duff accumulation?  These thick, undecomposed layers and inhibit germination of many forbs...

Terrestrial Fertilization to Sequester CO2?

One of the main uncertainties in the global carbon cycle is measuring the amount of carbon bound up in ecosystems such as forests and grasslands. This Net Ecosystem Production (NEP) converts CO2 to plant material, detritus, and some animals. Most escapes back to the atmosphere as respired CO2, but some is sequestered in soil organic matter and trees.

Most ecosystems are Nitrogen limited because fertilization with nitrogen increases NEP. Interestingly, many ecosystems are already fertilized by Nitrogen deposition from drifting clouds of various nitrogen compounds emitted by urban areas, industry, and agriculture.

Nitrogen deposition increases productivity and decreases respiratory losses from decomposition (Hogberg). But how much? And how much is too much? Natural vegetation may be satiated/saturated with a low quantity of nitrogen, and any more would begin acidifying the soil, killing plants and washing away to pollute the watersheds.

Magnani et al attempt to answer some of these questions and measure the size of the "The human footprint in the carbon cycle of temperate and boreal forests" of Europe. They find that the amount of nitrogen deposited in European forests confers a huge increase in fertility; they find no sign of a decrease due to Nitrogen saturation.

However, their findings rested on a number of unpublished studies, and a flurry of correspondence questioned their main conclusions. De Shrivjer et al point out that just because NEP continues to increase with increasing Nitrogen deposition, this doesn't mean that the forest ecosystems aren't loosing nitrogen as runoff. Indeed, it makes sense that at very high applications of fertilizer an increasing fraction would be wasted. Many farmers have to contend with the problem that, beyond a certain point, a doubling of Nitrogen fertilizer may confer only an incremental increase in crop productivity, while vastly increasing the amount of Nitrogen that washes off.

de Vries present a more central problem in Magnani et al's results: according to Magnani's data correlation, for every unit of Nitrogen applied to European forests, 470 units of carbon are sequestered. Yet the only plant material with a C:N ratio that high is pure xylem wood, and it seems unlikely that all of the deposited nitrogen is being used to grow tree stems. Furthermore, de Vries et al find that Magnani et al failed to control for a range of other variables that could affect forest NEP. de Vries reanalyze that portion of Magnani's data that is publicly available and find a more plausible -- and vastly reduced -- C:N ratio of 20 to 40 (20-40 units carbon for every unit nitrogen).

It doesn't end there, though. Magnani et al respond that they agree with De Shrivjer, but refute de Vries. Magnani point out differences between wet and dry deposition, to argue that their stoichiometric ratio is really closer to 175-225. They claim that this ratio is not implausible, even though it is much higher than actual forest fertilization experiments (Nadelhoffer). They explain this difference by suggesting that up to 70% of the actual nitrogen deposited is absorbed by leaves, whereas the forest fertilization experiments applied nitrogen to the soil. (Nadelhoffer).

{[It is not clear to me what the consensus is on how much nitrogen can be absorbed by the canopy, or why wet versus dry deposition matters. }

News and Views: Hogberg P. Environmental science: Nitrogen impacts on forest carbon. Nature. 2007 June 14;447(7146):781-782.

Original Paper: Magnani F, Mencuccini M, Borghetti M, Berbigier P, Berninger F, Delzon S, Grelle A, Hari P, Jarvis PG, Kolari P, et al. The human footprint in the carbon cycle of temperate and boreal forests. Nature. 2007 June 14;447(7146):849-851.

Questions: De Schrijver A, Verheyen K, Mertens J, Staelens J, Wuyts K, Muys B. Nitrogen saturation and net ecosystem production. Nature. 2008 February 14;451(7180):E1.

More Questions: de Vries W, Solberg S, Dobbertin M, Sterba H, Laubhahn D, Reinds GJ, Nabuurs G-J, Gundersen P, Sutton MA. Ecologically implausible carbon response? Nature. 2008 February 14;451(7180):E1-E3.

Response:Magnani F, Mencuccini M, Borghetti M, Berninger F, Delzon S, Grelle A, Hari P, Jarvis PG, Kolari P, Kowalski AS, et al. Magnani et al. reply. Nature. 2008 February 14;451(7180):E3-E4.

More Information: Nadelhoffer, K. J. et al. Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests. Nature 398, 145–148 (1999)

Follow Up: SUTTON MA, SIMPSON D, LEVY PE, SMITH RI, REIS S, Van OIJEN M, De VRIES WIM. Uncertainties in the relationship between atmospheric nitrogen deposition and forest carbon sequestration. Global Change Biology. 2008 September 1;14(9):2057-2063.

Nutrient Pollution Management in 2011: New Regulations

Dr. Larry Antosch, Senior Director, Program Innovation and Environmental Policy, Ohio Farm Bureau Federation presented "Water Quality and Agricultural Nutrient Management – Many Forces Converge" to finish up the seminar series theme of Water Quality here in the School of Environment and Natural Resources. Dr. Antosch is well-placed to provide a comprehensive and up-to-date view of nutrient management and water quality, having worked in the field for 30 years, first for the EPA and now for the FBF, which recently sued the EPA over pending nutrient restrictions in the Chesapeake Bay (CB) watershed. Dr. Antosh pointed out that since Ohio is in the CB "airshed" (some of our air pollution is deposited in the CB watershed) these regulations could impact us directly. They could also impact Ohio indirectly as EPA gears up for an even bigger watershed nutrient reduction program for the entire Mississippi.

In Dr. Antosh's opinion, some of the regulatory impetus is misplaced, because science does not have all the answers yet. For example, the increased of dissolved Phosphorous (rather than particulate phosphorous) entering Lake Erie and causing harmful algal blooms, is a mystery. Farmers or cities? Which techniques could best eliminate excess nutrients? Some have even pointed to no-till as a possible source, because nutrients are applied to the surface rather than mixed in.

Although Dr. Antosch emphesized the uncertain nature of water quality science, I believe there is enough information to act today. Several simple examples come to mind: (1) Regulations against applying manure and fertilizer in the winter, when plants are not growing and hence don't absorb any of the nutrients; (2) Existing steps to eliminate phosphorous fertilization of residential lawns; (3) Creating new wetlands to filter the runoff that does occur.

Adaptive Nitrogen Management

Harold M. van Es, Cornell University Department of Crop and Soil Sciences, Soil Health

Dr. van Es introduced his team's effort at a comprehensive, quick, and cheap soil test that goes beyond the standard chemical measurements to include physical and biological properties as well. I would like to see a paper showing how much variation in yield (b/c that is the output variable of interest to farmers) this test can account for, compared to other more comprehensive tests or even expert in-field evaluation.

Dr. van Es than discussed his work creating an adaptive nitrogen management tool that would completely bypass soil tests. His on-line tool uses rainfall patterns to estimate loss of nitrogen from corn fields and than recommends how much "booster" N to add. The benefits include less overall use of N. Interestingly, the loss of N in wet weather is exacerbated in soils with high organic carbon because of increased decomposition rates, according to Dr. van Es. Unfortunately, he did not discuss the dynamics of N under alternative farming (no synthetic fertilizer, no-till with cover crops, etc) practices that could obviate the need to even add N. I realized that one thing that's nice about monoculture corn across most of the mid-West is that the standardization makes it easier for science research to be relevant to a lot of people. It would be harder for Dr. van Es to research all the different alternative management techniques and apply recommendations for adaptive management to each.

The reason for internet-based adaptive management is a lack of real-time on-the-ground data on, say, soil moisture levels, N content, etc. But this is changing and Dr. van Es did show a few slides about what may be the future, with combines and irrigation equipment festooned with hi-tech spectrometers to gauge how much N plants have. The technique has been shown to be successful with wheat, but is still being developed on other crops.

Some links:

Sustainable Agriculture Research and Education

Alternative Farming Systems Information Center