Sunday, July 6, 2014

Update on seasonal weight gain of bison

More bison data are rolling in off the field scale. At Konza, we set up a scale for bison to walk over and weigh themselves. So far, they seem to be obliging.

As to the data, first, are examples of a 3-year-old cow (red) and a 2-year-old cow (blue). These are the individuals that we've had the most data for. You can see that over ~60 d they've put on almost 100 kg. As far as we know, neither of these animals have had a calf this year. Although the sample size is small here, it looks like weight gain has started to level off here. 

Bison mass (kg) vs. day of year for 2 cows at Konza.

With 90 calves tagged last year, we have a lot more data for this year's yearlings. If I standardize for individual variation in weight among animals (for those animals that have enough data to look at trajectories), this is the general trajectory. More or less linear increases in weight over the past 60 d. About 80kg of weight gain.

Residual weight vs. day of year for yearlings at Konza.
We still have some kinks to work out of the system. For example, we are just getting it to the point where we can remotely harvest the data. Also, we need to tag all the animals out there, which should happen this fall.

For everything we know about seasonal patterns of dietary quality, weight gain should be leveling off right about now.

This technique is going to be pretty important in order to understand seasonal weight dynamics and eventually how climate change will affect grazers in grasslands. For example, we see that bison in the north reach greater size than those in the south. Is this because they have a longer period of weight gain, or gain more weight at a given time? As it gets warmer, what mechanism will climate affect our grazers?

How much weight different animals will gain in the next month is going to be super interesting.

[Note: the average yearling mass as of July 12 is approximately 245 kg. If you look at the 2013 Oikos paper, average yearling mass in the October roundups ranged among years from 234.1–281.6 kg. That means, if this was a typical year the animals would gain just another 20 kg, but they might put on another 35 kg.]

Friday, July 4, 2014

The trajectory of nitrogen in grasslands

N concentrations for grasses from Konza's 1D watershed from 1983-2010.

Plant N concentrations might seem like another esoteric ratio, but they are the key to a number of ecosystem services. In grasslands, they determine the nutritional quality of grass for grazers, how much C plants take up, as well as how fast dead grass material decomposes. And grass that doesn't decompose fast is more likely to fuel burns later.

Whether plant N concentrations have been increasing or decreasing in grasslands is one of the greatest unknowns for modern ecosystem ecologists.

For example, Kendra's previous work had shown that N concentrations had declined by 25% in Kansas grasslands over the past 75 years. That study relied on plants collected for herbaria in Kansas over the past century.

Based on the timing of declines and what we know for other supporting evidence, the most likely explanation for the declines in plant N concentrations was that increasing CO2 concentrations have been driving down plant N concentrations.

Despite this single line of evidence, could grass N concentrations have actually been increasing?

N deposition rates have been increasing, for example. When N deposition is high enough, it's enough to eutrophy the grasslands with a cascade of effects.

To answer this question, we examined the N concentrations of grasses collected over 25 years at Konza Prairie. The grasses come from a single watershed under the same burn regime (annual burning) with no grazing during this time.

Now what trajectory the plants would take was uncertain. When grasses were first measured CO2 concentrations were 343 ppm. By 2010, they were 390 ppm. 14% higher.

Was that enough added CO2 to pick up a signal?

The quick answer was no. There were no significant declines in N concentrations (or 15N:14N ratios for that matter).

Could there be another driving factor that was offsetting the decline? We checked a lot of things. No trends in climate. No trends in species composition. No trends in productivity. No trends in water availability.

The grasslands was really similar to what it was like in 1982**.

**The Nature Conservancy's Joe Fargione's response to this was "Conservation works!" Essentially, we could hold a grassland pretty similar to what it was before.

Given the differences in results, now the rectification begins. The herbarium data suggest declines in N availability and plant N concentrations. The Konza data suggests no significant declines in either.

Is the difference time scale? Local conditions? Collection protocols?

Unknown at this point.

One thing is clear, though. Neither study supports eutrophication of the Kansas grasslands. Despite elevated N deposition, there is no indication of greater N availability.

Looking forward, the future of the grasslands is still uncertain for so many reasons.

For those grasslands that are preserved, whether N availability, plant N concentrations, and forage quality will decline is a question that only further monitoring and testing will be able to answer.

McLauchlan, K. K., J. M. Craine, J. B. Nippert, and T. W. Ocheltree. 2014. Lack of eutrophication in a tallgrass prairie ecosystem over 27 years. Ecology 95:1225-1235.

Wednesday, June 25, 2014

Bison calf weights

A few videos of bison calves walking over the scales and weighing themselves.

Once we get EID's on all the mothers, we should be able to ID the calves, since they stay together. 

The top one weighed in at 50 kg and the bottom at 59 kg. 

With weights about 50-60 kg right now, the animals are gaining about 1 kg a day (give or take)...

By fall, they'll weigh in at about 130 kg, which is a bit less than 1 kg a day every day until late October...

Monday, June 23, 2014

New videos for pressure bomb

Uploaded 2 new videos of us measuring water potentials of grasses with a pressure bomb and microscope camera. Higher resolution videos than the previous one...

Wednesday, May 28, 2014

Improvements on measuring drought tolerance

Just a quick note here on measuring drought tolerance.

In the past, we were using a pressure bomb to determine a plant's water potential at which stomatal conductance falls below a threshold (5 mmol m-2 s-1).

Using a pressure bomb on grasses can be tough. They are often just a few mm across and don't have that much water in them. Magnifying glasses just don't magnify enough.

I dug around a bit and bought a digital microscope. Turns out these work great.

First, we lock in the grass blade at a pretty high height. Note we spray painted everything black to reduce glare.

Then we place the microscope camera near the blade. The scope camera is 5 MP which gives fairly good resolution. A Macbook Pro with a Retina display is high enough resolution to show things well (most laptop monitors are only 2 MP).

The scope camera is under $100 at Amazon.

After that, we pressurize and wait for the water.

Holding things still can be problematic, but it's working pretty well to be honest. Also, what we see on the screen is a bit higher resolution than this video for some reason.

Much better than magnifying glasses.

Saturday, May 24, 2014

Bison weight trajectories

Weight trajectories of 16 bison at Konza Prairie. Colors don't mean anything here.

More and more animals are walking over the bison scale. I think because of what watersheds were burned, the bison have just been in other parts of Konza. Only about 80 of the 120 animals that have a tag have walked over it since we installed it about a month ago. We have a lot of other weights, but they most likely are for other animals that didn't get tagged (or ones with a tag that didn't get an EID reading).

Still, we're just now getting enough data to start to see some trajectories of weight gain.

Over the past month, there are the 16 animals that we have enough data for to start to look at weight trajectories...I've scrubbed out the animals with too few data. Data points that are obviously too low/high have also been removed.

The general trajectory since we installed the EID reader on May 1 has been for animals to be gaining weight.

Yearlings are gaining 1.3 kg a day.

2-year olds are gaining 1.6 kg a day.

The two older animals 1.9 kg a day (take with a grain of salt).

2-3 lbs per day is a pretty reasonable number.

Friday, May 16, 2014

Note to myself on standard errors

1.96 is well known, but not the others...

Found this here.

Confidence levelStandard Errors from mean