Grasses of the world III--grasses can be incredibly drought tolerant

I've posted that I've been growing up 500 grass species from around the world to look at the geographic and phylogenetic distribution of physiological drought tolerance. Before we learned we were a bit constrained in quantifying the drought tolerance of grasses because we had grasses that could withstand pressures in excess of our previous pressure bomb, which maxed out at 10 MPa (~1450 psi). Jeff Hamel at PMS Instruments sent us one that goes to 14 MPa (~2000 psi). With that, we've rerun some grasses and measured some new ones, while we wait for some others to grow from seed again.

Still, the first results show that there are grasses that are incredibly drought tolerant.

We've now measured physiological drought tolerance (psi-crit) on 398 species. 13 of those were able to conduct water at pressures in excess of -14 MPa. That's more than 3% of the grasses we surveyed.

3% does not seem like a large number, but that number will only go up as we measure the most drought-tolerant species which are regrowing. 5% might not seem that high, but that'd be 500 species of grasses in the world if you extrapolate out. 

How drought tolerant could some of these species be? If you extrapolate out the lower bound of the width-psicrit relationship, we should have some that hit -17 MPa (~2500 psi).

Extreme weather summary

Just saw this today:

http://www.wunderground.com/blog/JeffMasters/article.html

It's an interesting summary of extreme weather events globally over the past year.

Another thing I hadn't know about was NOAA's Climate Extreme Index.

http://www.ncdc.noaa.gov/extremes/cei/index.html

For example, here's a graph of extreme summer precipitation events over the past 100 years. 2010 wasn't the most extreme, but pretty close.

Heat waves

I've been learning a bit about heat waves.

The WMO defines heat waves as a sequence of five straight days when the daily maximum temperature exceeds the average maximum temperature by 5 °C. Heat waves can extend much longer than 5 days and can exceed average maximums by much more than 5°C. Although heat waves are a categorical classification, they are part of continuous variation in climate.

Here at Konza, over the past 25 years, there has been a lot of variation in climate. We haven't had a major drought since 1980 (http://en.wikipedia.org/wiki/1980_United_States_heat_wave). One thing I didn't have in my head is when the heat waves come and how hot they can be. With the standard definition of heat waves, heat waves should be equally likely across the year. Yet, if you look at the climate data for Konza, the strongest heat waves happen a bit later in the year than one might think.

I took the climate data for Konza and averaged the maximum temperature for each day in 15-day periods from early July to early September. It turns out that the hottest mean daily temperatures are in late July (not shown), but the strongest heat waves are in late August.

Here's a graph of the min and max mean daily maximum temperatures for 15-day periods from 1984-2010.

Although the hottest mean temperatures are generally at the end of July (not shown), the greatest heat waves come at the end of August. In 2003, high temps averaged 40°C for 2 weeks in the last two weeks of August.

Ecologically, the interesting questions about heat waves become how plants (and animals) respond to heat waves at different times of year. Most C4 grasses are shutting down in early September. One would think that heat waves at that time of year would have much of an effect compared to ones in July or early August.

At some point, we'll have another summer like 1980. Preparedness is an important topic, but also for ecologists. I wonder if we're measuring the right things so that when it comes around we can understand our ecosystems better.

Drought tolerance: grasses of the world II

After 200 species, there still is a boundary between width and drought tolerance, just with a missing corner.

I haven't seen too many reviews of drought tolerance/cavitation resistance of grasses, but it is generally thought that permanent wilting points for most grasses is about -3MPa. It isn't considered that too many plants can resist below -10 MPa. Why -10 MPa? Because the machines to measure water potential don't go below there.

In the past few weeks, I've continued to measure the drought tolerance of grasses from across the world.
As the species have accumulated, the same tradeoff boundary observed earlier appears to hold. Yet, over 10% of the species appear to be able to conduct water at pressures below -10 MPa. For reference, if you could graft any of these species on the top of the tallest redwood, they could still conduct water.

We still need to nail down the actual psi-crits for the species. The problem is that our pressure bomb stops at -10MPa (100 bar).



The good news is that Jeff Hamel at PMS has been kind enough to build one that goes to -14MPa, which should allow us to determine the drought tolerance of the most drought-tolerant grasses. We'll grow up these species again and let them dry down. Hopefully, Jeff won't have to build one that goes below -14 MPa.

Drought tolerance: grasses of the world

One of the 500 species that are part of the Poa500 project to examine drought tolerance in grasses of the world.

I'm not sure this one is going to work. I believe that if you measure something interesting and have strong contrasts, you should learn something interesting. I've had pretty crazy schemes work out in the past. Hopped on a plane and measured roots on three continents. Had people send me soil from across the US to look at nutrient limitation. Measured foliar 15N for a couple hundred species at Konza. Even looked at spectroscopic assays of 20,000 cow poop samples to infer continental scale patterns of forage quality.

Each time, we learned something interesting by having strong contrasts and measuring something interesting. But to start to understand global patterns of drought tolerance by growing 500 species of grass in the growth chamber in relatively tiny tubes? I'm just not sure this one is going to work. 

Granted, what we're doing right now is just a pilot project and would be easier with the NSF Dimensions of Biodiversity grant funded. But, questions about the evolution and geographic distribution of drought tolerance are just too important not too try. At the heart of it, we just don't understand the traits that are associated with drought tolerance--what does a drought-tolerant plant consistently look like. In what climates are they most likely to be found. Are some lineages more likely to have evolved drought tolerance than others?

To begin to answer the question, USDA sent me seeds for 500 grass species from their seedbanks and I've serially germinated them over the past 2 months. After about a month, we measure a couple of gas exchange and morphological metrics on the leaves and then stop watering. When they stop conducting water (shut their stomata), we measure their water potential, which we call psi-crit.

A little over 100 species have hit their psi-crit so far. Here's probably the most interesting graph so far--drought tolerance (psi-crit) vs. the maximum width of the largest leaf on the plant. 

It seems like you can have narrow leaves on plants that aren't drought tolerant--species like Enneapogon oblongus. You can also have narrow leaves on plants that are drought tolerant--species like Bouteloua repens. You can also have wide leaves on plants that are not drought tolerant--species like Dichanthelium scoparium. But you can't have wide leaves on plants that are drought tolerant. Doesn't exist.

Of course, it doesn't take more than one species to prove something not impossible.

We still have a few species left to measure, of course.

What controls rooting depth?

What controls the maximum height of plants is relatively well understood. Plants can grow no taller than they can support themselves and than they can move water. Roughly every 10m of addition height requires xylem to resist an addition -1MPa of pressure. There are many other factors that could lead to selection for shorter plants, but tall plants need to be able to resist high negative pressures in their xylem.

Though we understand relatively well what controls the maximum height of trees, what controls maximum rooting depth is not well understood. If roots are moving water from their tips to the shoots, then similar constraints should apply to roots as stems. A root cannot go deeper than its xylem can resist the negative pressures of moving water that height. Even if it is belowground, the same physics apply. 

But is there another constraint besides this? Is maximum rooting depth determined largely by hydraulics? If moving water was not a limitation, e.g. in wet places, what would constrain rooting depth? Or lateral extent for that matter.

I suspect that phloem and sucrose transport could be just as big a constraint on rooting depth/extent as hydraulics are aboveground. I'm not sure I understand the details on this, though. If a typical plant tried to produce a root 10m long, could it move enough sugar through its phloem to sustain the growth of the root tip as well as intermediate tissue? What about 100m? Some plants can apparently go this deep, but could any plant? 

I have to admit, I'm not even sure what to begin measuring here. This summer, we're suppose to start doing root cross-sections on long grass roots, e.g. 2 m long, and look at the anatomical characteristics of xylem and phloem. Maybe this'll start to shed some light on what's hidden belowground.

Modification of Classic State Factors

Modified State Factors and Interactive Controls Diagram to account for external supplies of resources.

The general framework to understand ecosystem properties has been the State Factor framework. Created by Hans Jenny to understand differences in soils, it has been applied to ecosystem properties such as plant species composition and stand structure. Essentially, with Jenny's approach, soils were determined by the climate, the organisms present, the landscape relief or topography, parent material and time since a major disturbance.

Terry Chapin and others modified the state factors approach to understanding ecosystems to include interactive controls of ecosystem properties. Interactive factors are not independent of the ecosystem properties, but sit somewhere in between. For example, the macroclimate is independent of what species are present at a site, but the microclimate can be influenced by species composition. These interactive controls include disturbances, microclimate, resource availability, and species composition.

With these state factors and interactive controls, one can have a better framework for understanding how ecosystems are structured and function. For example, the amount of biomass in a forest is not just a function of the state factors, but is influenced by interactive controls such as disturbance and the species that are present in a stand, which is influenced but not determined solely by state factors.

The state factor/interactive control approach is a big improvement in our conceptual framework, but it's generally left out external supplies of resources. Atmospheric CO2 concentrations, dust inputs, and nitrogen deposition can have profound influences on ecosystem properties, but are not adequately included in the current conceptual framework. These factors are somewhat influenced by ecosystem properties. The amount of dry deposition is influenced by canopy structure, for example. Yet, it's probably better to consider these state factors. N deposition plumes are wide and more similar to climate than disturbance regimes.

It's a minor tweak in many respects, but a likely necessary improvement to a long-tenured conceptual framework.