Predicting dietary phenology for bison

Here is some science in action today.

We have been measuring the quality of the diet of bison for 5 years now. Every 2 weeks or so, Gene Towne collects fecals from the bison at Konza. We then send them off to Texas AM to be analyzed for dietary quality. Because protein is most limiting to bison, we focus on % crude protein in the diet as an index of quality.

What you see across years is that when grass greens up in the spring, the quality of the diet rapidly goes up, peaks, and then declines. 

This happens every year in the spring, but the timing of the peak varies among years. These peaks can vary by over a month across years. Sometimes grass is best to eat in mid-April. Sometimes mid-May. 

One question is what is driving this? How do we predict this? **

**"Why do this?" is another question. Partly, we're curious. Partly, it could explain a lot about why animals gain more weight in some years and help us predict what happens if we get warmer springs. 

I adapted the critical climate period techniques to help out on this. Essentially I calculate average temperatures for different ranges of dates before the phenological event each year. Then see whether temperatures 10 d before the event predicts the timing of the event better than temperatures 20 d before the event, etc.

Here's what the technique reveals...

For Females, the best predictor is that peak crude protein occurs right after having 20 d of average maximum temperature of 23°C.

For Males, the best predictor is that peak CP occurs right after having 25 d of average maximum temperature of 22.6°C.

When each year, you calculate when you get these average temperatures, you explain 73% of the variance in the timing of peak CP.

So, in any year, take a look at maximum daily temperatures (I haven't tried means yet). When you get 20 or 25 d of temperatures hitting ~23°C (73°F), that's when you get peak dietary quality. After that, it's all downhill.

What is interesting is that 2012 is an outlier. We averaged 23°C a month earlier than when peak CP hit. 

Turns out March 2012 was the super heat wave. And 2012 peak should have happened 30 d earlier than it did. 

Daily maximum temperatures from 2009-2013. Line is a spline for each year. Red is 2012.

It seems that when those temperatures hit early in the year, the peak happens later than expected. 

That could some phenological constraint of our warm-season grasses...

I'll play around with this some more (and probably have to amend this), but not bad for a first pass.