Friday, November 30, 2012

The timing of precipitation

Traces of cumulative precipitation (standardized across years) for Konza.  Date range is 145-250 (May 25 - Sep 7), which is the precipitation timing critical climate period.

When rain falls might not seem to be important, but in many ecosystems it is often more important than how much rain falls.

In annual grasslands, the germination of different species if dependent on when rains begin. And the end of the rains set the end of the growing season. Same could be said for many tropical grasslands.

Many deserts are structured by when rain falls as shrubs with deep roots benefit from winter rains and grasses summer rains.

But what about temperate grasslands where the timing growth is dependent more on temperature than rainfall? How important is timing?

Put another way, if climate change causes rains to fall later (or earlier), will it matter.

There has been little work to directly test the importance of timing of rainfall.

I recently dug through the Konza climate data to see how much the timing of precipitation varied among years. Quite a lot. For May-September rainfall, the timing of when the average unit of precipitation fell varied by over six weeks over a 25-y period. In some years, half the rain had already fallen by early June. In other years, it took to mid August.

Does it matter? If plants aren't running out of water and it all goes into the same bucket, how much impact can there be of the bucket filling early or late?

Turns out a lot.

Variation in the timing of rainfall explained as much variation in grass productivity as temperatures and about half as much as the total amount of rain.

From these calculations, shifting precipitation by just 1 week later in the season can reduce productivity by 5-10%. 

For reference, a decrease in productivity of this amount is enough to put most ranchers out of business, all other things equal. 

Thursday, November 29, 2012

How to say "It's going to be a hot one"

If there is anywhere where it would be hard to predict the weather, it would have to be here in Kansas. We are just too far from the sea for anything to have strong influence on our weather.

Yet, it seems like it must be a great temptation to try to predict the weather. Because many people have said to me in casual conversation some variation of, "It's going to be a hot summer." Whatever the weather is today (or the past few days) people seem to project forward a couple of months.

Is there any basis for this?

If it's hot this month, is it likely to be hot next month? Or dry?

Here's how you test that.

First you generate climate anomalies. In a seasonal climate like ours, it's simple to know what the climate will be like in a month or 6 months or a year.

If it's June, in the grand scheme of things, it's likely to be hot next month cold in 6 months and hot again in 12 months.

But, what we're interested here is in the climate anomaly and how they are correlated over time.

So we aren't saying it's going to be hot next month, but hotter than average next month.

To evaluate this, you look at past records of data and run autocorrelation analyses. This analysis looks at how different temperature or precipitation over a certain length of time and then examines the correlation between the current period's anomaly (hotter/colder, wetter/drier) vs. next month's.

Here are some data from May weather over the past 50 years for Manhattan KS.

I went to and accessed their climate explorer. The site runs autocorrelations for any weather station in the world.

Here's what the autocorrelation analysis for daily maximum temperatures look like for 30-d periods throughout May and June (start dates of May 1 to May 30).

What is important in this graph is the correlation after 30 d. Essentially it says that the correlation coefficient between the 30-d starting in May and the following 30-d is less than 0.2.

How helpful is that?

Here's data for Konza (close to Manhattan) for 1984-2010, which I've been using for analyses lately.

X-axis is mean maximum temperatures for day of year 120-149 (roughly May) vs. same temperatures in the next 30 d.

Correlation coefficient of this is about 0.3 (similar to autocorrelation analysis).

You can see that there is about 9 °C variation among years in May temperature. If it's the hottest May on record, June is likely to be just 1°C hotter that average. That's not much predictive capacity.

And the 30-d after that? r = 0.2 and no significant predictive capacity. It could be equally likely to be the hottest month on record as the coldest.

For precipitation, it gets even worse.

here's the same autocorrelation analysis for precipitation during that period.

Essentially for the 30-d following May, there is no significant predictive capacity (maybe slightly drier).

 Precipitation between the two 30-d periods at Konza: flat.

As a caveat, i should say that these patterns have nothing to do with general trends over long time periods associated with global warming. Only the structure of seasonal variability.

That said, it's probably more certain to say that the future is going to be hotter than it is to say that the next month is hotter (or colder or wetter or drier).

At least here in Kansas.

Friday, November 23, 2012

Where to send your paper after it's been rejected

Ever wonder where to send your paper after it's been rejected from a journal? 

The authors of a recent Science paper mapped the submission history of over 80,000 scientific articles published from 2006-2008 .

This monumental task allows tracking of the most likely flows of papers through journals. 

The network map provides more than a guide to where to send your next rejected paper.

A few interesting results came out of this. 

First, papers that had been rejected from another journal were cited more. Is this because papers were improved with further revisions? Or is it because papers that are likely to be the most interesting don't often fit into the neat model of a given journal's papers? 

Second, resubmissions often went to journals with lower impact factors. No surprise there, but good to quantify. 

Third, the proportion of papers published that had never been submitted elsewhere was similar across a range of impact factors of journals. The range was really narrow in the grand scheme of things.  

Publishing is tricky. There is an immense amount of wasted effort trying to get papers published. Given that, journals and authors are reasonably efficient in many respects. 

Flows of Research Manuscripts Among Scientific Journals Reveal Hidden Submission Patterns
V. Calcagno et al. Science 338, 1065 (2012)
DOI: 10.1126/science.1227833

Thursday, November 15, 2012

Planetary boundary for nitrogen

A paper a few years ago (Rockström et al Nature 2009) laid out a conceptual framework for how humans are impacting the earth ecosystem. They identified nine ways that humans impact the Earth and  identified thresholds at the global scale (planetary boundaries) that once exceeded could have severe consequences for humans.

They stated that three boundaries have already been exceeded. The rate of climate change and biodiversity loss were the first two. The third was biogeochemical flows, specifically the N cycle.

Direct and indirect N fixation can have a host of unintended consequences from acidification to biodiversity loss to enhanced warming to hypoxia. Clearly, too much N is bad from local to global scales.

The authors set their limit as 35 MT N y-1. In the longform version, they state this value is " ~25% of the total amount of N2 fixed per annum naturally by terrestrial ecosystems" Estimates are that we've exceeded this by a factor of 4.

Basically, they suggest reducing N fixation by 75%.

The authors state that this value is "is a first guess only. Much more research and synthesis of information is required to enable a more informed boundary to be determined."

If you take the negative consequences on N fixation one by one, the key question to ask is what have the trends in these states been over time at the global scale.

Not easy to do.

One of the more tractable is to ask whether N availability to plants, a key component of terrestrial eutrophication and acidification, has actually been increasing or not.

One thing ignored in the paper is that CO2 concentrations in the atmosphere have also been rising. If N2 fixation were drastically reduced, could N availability actually decline as plants receive more C than N?

To answer those, we need better monitoring of N availability to plants at a global scale.

And there is nothing in place to try to answer those questions.

Friday, November 9, 2012

What did bison once eat in November in Kansas?

Most people think the greatest mystery with bison is "How many bison were there before Europeans arrived"?

That number to me seems trivial. If it was 5 million or 20 million is not unimportant, but it's a quantitative question, not a qualitative question.

Today I was out at Konza today. It's November. A warm day, but almost everything is brown. A few rosette forbs were green in the uplands. Some scattered grasses and sedges in other places. Almost nothing good for bison to eat.

Except by the roadsides and some of the firebreaks. Those areas are kept mowed throughout the year and tend to be dominated by the annual grass Bromus arvensis--japanese brome.

The thing about japanese brome is that it greens up early and stays green late. It's an order of magnitude more nutritious than almost anything else out there right now.

The other cool-season grasses just aren't that similar to japanese brome. They are mostly bunch grasses and aren't green or at least vigorous throughout the Kansas winter.

As you can see, the bison really work hard to get it and it must provide an important part of their current diet. As you can hear, they really rip at the turf. The grass there might have been 15 mm tall.

The only thing is that japanese brome didn't use to be in Kansas. When it arrived is uncertain but probably not until the early 1900's.

One thing I've wondered is that if japanese brome wasn't introduced, what would the bison be eating? What would be different about our bison if the grass wasn't there?

Those are just two of the the qualitative questions I think are pretty interesting about bison these days.

Revisiting foliar N isotopes

In 2009, I led a paper on foliar N isotopes. In it, we synthesized >10,000 samples to come up with some global patterns, but also looked at individual studies that related foliar del15N to indices of N availability. In general, we said, N availability and plant del15N scale positively, but there were exceptions. 

In my mind at least, I remember thinking it doesn't always work, but looking at it again, it didn't work sometimes when potential N mineralized was measured. It always seemed to work with actual measurements.

I replotted all the actual measurements of Nmineralization on to a common scale (0-1) and centered the foliar del15N across sites (gave them the same mean of zero).

When you do that, it looks like this:

That's a pretty good summary of almost 4‰ increase in del15N across N availability gradients within sites.

Actually, my first attempt at showing the N availability data was a 1-panel summary rather than 16 panels.

Looking back, reviewers didn't like the 16 symbols and I thought 16 panels were necessary. Reviewers always seem to ask for more data, more panels, more analyses. They rarely tell me (at least) to simplify. 

The 16 panel graph is pretty good. It's clear I should just not have shown the potential N mineralization data in the same graph. 2 out of 3 times they didn't work and should have been shifted out. 

Either way, the 1-panel graph is still a pretty good summary of the data....

Wednesday, November 7, 2012

Teaching Biogeochemistry and Ecosystem Ecology

I surveyed approximately 150 colleges and universities in the US for how they teach biogeochemistry and ecosystem ecology.

I'll submit the full report to the ESA Bulletin, but can attach a draft of the report here.

Here are a few highlights:

  • Less than half of the surveyed institutions teach a course that centers on BiogeoEcosystem science.
  • 40% of these were only offered to graduate students. 
  • Approximately 1300 students a year take these courses in the US. 40% are at just 10 institutions. 
  • 75% of the classes have no lab or field component.

I'll admit that almost half offering these courses is more than I thought. I had it pegged at 25%. Still what is being offered is in line with expectations.

One thing that is clear is that most US institutions do not offer undergraduates access to rich BiogeoEcosystem courses.

Whether the current level of access to BiogeoEcosystem subject material at the national scale is sufficient is a more difficult question. Clearly defined goals and benchmarks need to be defined for that.

One opinion though is that considering the fundamental importance of an understanding of BiogeoEcosystem science, it seems like access can be improved.

  • More institutions need to teach more students earlier. 
  • On-line courses are needed to supplement the curricula of institutions that cannot provide access to these courses. 
  • BiogeoEcosystem students need richer experiences that include lab and field experiences.

There aren't many levers to implement recommendations like these at the national level, though.

Hopefully, information like this will help.

Saturday, November 3, 2012

Top ten nitrogen papers

Someone the other day made a statement that some recent work was one of the most important papers on the N cycle published in the last 5 years. Statements like that are hard to refute, but I wondered what that list would like.

I had a short list in my head of papers I could think of. There has been a lot of work on N limitation, new developments in our understanding of microbial processes, and continued advances in the consequences of N deposition.

Most important?

Not sure

I did a search for “nitrogen” and “cycle” and sorted by number of times cited. Then did a search for “nitrogen” in Science, Nature, or PNAS and sorted by # times cited and then relevance. For each I looked at the top 100 papers. I then pulled papers relevant to terrestrial N cycling, but left out reviews and anything with my name on it. I didn’t try to judge 2012 papers.

Here are the 12 papers I came up with for the "Top Ten" list…

Effects of nitrogen deposition
Clark, C. M. and D. Tilman. 2008. Loss of plant species after chronic low-level nitrogen deposition to prairie grasslands. Nature 451:712-715.
Janssens, I. A., W. Dieleman, S. Luyssaert, J. A. Subke, M. Reichstein, R. Ceulemans, P. Ciais, A. J. Dolman, J. Grace, G. Matteucci, et al. 2010. Reduction of forest soil respiration in response to nitrogen deposition. Nature Geoscience 3:315-322.
Mulholland, P. J., A. M. Helton, G. C. Poole, R. O. Hall, S. K. Hamilton, B. J. Peterson, J. L. Tank, L. R. Ashkenas, L. W. Cooper, C. N. Dahm, et al. 2008. Stream denitrification across biomes and its response to anthropogenic nitrate loading. Nature 452:202-U246.
Thomas, R. Q., C. D. Canham, K. C. Weathers, and C. L. Goodale. 2010. Increased tree carbon storage in response to nitrogen deposition in the US. Nature Geoscience 3:13-17.

Global patterns of N cycling
Houlton, B. Z., Y.-P. Wang, P. M. Vitousek, and C. B. Field. 2008. A unifying framework for dinitrogen fixation in the terrestrial biosphere. Nature 454:327-330.
Davidson, E. A. 2009. The contribution of manure and fertilizer nitrogen to atmospheric nitrous oxide since 1860. Nature Geoscience 2:659-662.
Clarisse, L., C. Clerbaux, F. Dentener, D. Hurtmans, and P. F. Coheur. 2009. Global ammonia distribution derived from infrared satellite observations. Nature Geoscience 2:479-483.

Nitrogen limitation to productivity
Elser, J. J., M. E. S. Bracken, E. E. Cleland, D. S. Gruner, W. S. Harpole, H. Hillebrand, J. T. Ngai, E. W. Seabloom, J. B. Shurin, and J. E. Smith. 2007. Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems. Ecology Letters 10:1135-1142.
LeBauer, D. S. and K. K. Treseder. 2008. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology 89:371-379.

Soil N cycling advances
Di, H. J., K. C. Cameron, J. P. Shen, C. S. Winefield, M. O'Callaghan, S. Bowatte, and J. Z. He. 2009. Nitrification driven by bacteria and not archaea in nitrogen-rich grassland soils. Nature Geoscience 2:621-624.
Manzoni, S., R. B. Jackson, J. A. Trofymow, and A. Porporato. 2008. The global stoichiometry of litter nitrogen mineralization. Science 321:684-686.
Morford, S. L., B. Z. Houlton, and R. A. Dahlgren. 2011. Increased forest ecosystem carbon and nitrogen storage from nitrogen rich bedrock. Nature 477:78-81.

Summary of these papers:

N deposition causes loss of species even at low levels, it reduces soil CO2 production, increases stream denitrification, and increases C sequestration in trees.

Nitrogen isn’t fixed in cold places because of enzymatic limitations, nitrous oxide concentrations have been driven by anthropogenic N, and India produces a lot of ammonia.

N and P are limiting everywhere, N is limiting everywhere.

Archaea aren’t important in nitrification, microbes are less efficient with C in high C:N litter, plants can get N from rocks.

Which of these is the most important? Probably not necessary to try and answer.

Did I miss any?