Sunday, July 31, 2011

Streams don't run from dry soils

Konza streamflow and precipitation as a function of soil moisture at 25 cm

The fraction of precipitation retained by soil is a major source of variation in soil water availability to plants. For a given site, much of the variation that we see in this is associated with the pattern of precipitation, external disturbances on vegetation not withstanding. Precipitation pattern is hard to quantify in an ecologically meaningful way, though. A large, intense rainfall event might be lost to the stream if soils are saturated, but if soils are dry might be retained completely. Yet, heavy rain on dry soils might also be associated with heavy runoff if the rain falls faster than the soil can absorb it. You can occasionally see it on your front lawn, but flow paths are pretty short there compared to an intact grassland. Then again, rivers do flood in deserts.

There has been a lot of uncertainty at Konza on this, so I dug into the data to test it. I used the biweekly soil moisture data and matched it up with precipitation and streamflow during April-July (day 105-214 from our critical precipitation periods). 27 years of data here.

First cut analysis shows high precipitation falling a range of soils, but high flows in the major stream draining Konza only when soils are wet. Really no cases of high flows off dry soils.

The data aren't perfect. Soil moisture is only taken biweekly, and I used the interpolated soil moisture for each day rather than actual or the previous soil moisture. We really need daily data on this and that doesn't exist. 

Upshot? Plants get access to all the precipitation that falls when soils are dry, but can lose a significant amount when soils are wet. Losing water from wet soils might not impact plants immediately, but likely does later as the soils dry out.

Turns out we have pretty good evidence of this. More on that later.

Heat waves and drought: it's all in the timing

Distribution from 1984-2010 of (a) mean daily maximum temperatures averaged over 15-d intervals and (b) soil moisture at 25 cm taken approximately every 15 days. Also shown (c) is the sensitivity to grass aboveground net primary productivity (ANPPG) to variation in drought and heat waves assessed every 15 d in 5-d increments. The critical climate period for drought (day of year 105-214) is shown in blue and for heat waves (day of year 190-214) is shown in red.
July in 2011 has been hot. And dry. Supposedly it's suppose to be like this more often in the future as future climates are likely to include more frequent droughts and heat waves. 

It's generally assumed that in most grasslands these events reduce grass production, yet their effects have been viewed somewhat monolithically. When it comes to forecasting the consequences of future climate variability, droughts and heat waves in early-, mid-, or late-summer are not viewed very differently. Absence of evidence is not necessarily evidence of absence though. 

The Konza LTER has built up datasets over the past 25 years that can really test this, though.

27 years of annual productivity
27 years of daily weather
27 years of daily stream discharge
27 years of biweekly soil moisture
17 years of biweekly productivity
11 years of remotely-sensed NDVI

I'll write about some of the datasets another time, but if one examines the annual productivity data and the climate data together with the critical climate period approach, it is clear that the timing of climate variability is just as important--if not more--than the magnitude.

First, grass productivity only responds to drought (or the converse precipitation) during part of the growing season (Apr 10-Aug 2). Drought in August doesn't reduce primary productivity. 

And heat waves? They only reduce productivity during a 25-d window. Jul 10 - Aug 2. Heat waves in August, no less June, just have no impact on productivity. 

We can use these data to come up with new relationships between productivity and climate variability.

A couple of lessons can be learned here, but the most striking is that droughts and heat waves in August just don't affect grass production. It's not that grasses aren't growing then. About 10% of the production happens then and in some years it can be as high as a third of the mean annual productivity. Yet, growth during that time is not tied to climate then.

It's hard to explain why this is so, but the practical consequences are clear. If droughts or heat waves are more likely to happen in August, it doesn't matter for the amount of grass we have. We've shown elsewhere it still impacts the bison, most likely because they cue in on grass quality than quantity. But ANPP is insensitive. If we  want to predict future productivity well, they we better know timing as well as magnitude.

**On a side note, the results are really the highest expression of what the LTER approach can accomplish. I think long-term datasets have fallen out of fashion in the ecological community. When was the last time Science or Nature published a paper that centered on a long time-series from an LTER site. Compared to experiments, models, and cross-site synthesis, long time series seems like a short leg of the table these days. No one has ever set up an experiment to test what natural variability has shown us about the timing of variability.

Monday, July 18, 2011

Advancing plant functional trait science

I struggle at times to understand why we haven’t made much progress in understanding plant functional traits over the past ten years.

As has been well chronicled for over a century, plant functional traits are keys to understanding the evolution of plants, predicting ecosystem response to global change, and interpreting the distribution of species. None of the importance of plant functional traits has changed any time recently.

I would never argue that there has been no progress. For example, Wright et al.’s 2004 Nature paper on the worldwide leaf economic spectrum certainly is a landmark synthesis, but it was largely confirmatory from Reich’s work in the late 90’s. Baraloto’s recent work on the decoupling of leaf and stem economics is a good study that has the potential to be important. At the very least, advances have been sporadic and incremental.

Still, it just doesn’t feel like we’ve learned much in the past ten years about traits.

Regardless of how much advance there has (or hasn’t) been over the past 10 years, why hasn’t there been more?

In some ways I feel like there are a number of Catch-22 chicken-eggs involved in trait research. This isn’t an exhaustive list, but ones that seem to stand out.
  • Funding agencies do not fund trait work. Major screening projects just are not funded. Most of the funding has been into syntheses of extant data while new data has largely come from side projects and student work.
  • Pot-phobia. I’ve talked about the “pot effect” before and no one would deny that plants can sense their environment. Yet, proposing to grow plants in pots inevitably generates criticism. For example, one proposal review recently included the criticism “The plan to compare plants grown in small pots in growth chambers is questionable, since small pot effects will likely cause artifacts”. The word ‘artifact’ is a dismissively loaded term here. Besides, there is never criticism for the “field effect”—the artifact of growing small plants in an unconstrained soil volume with excess resources. Put briefly, intraspecific variation and plasticity is a bugaboo that can kill broader questions.
  • No new conceptual advances to test. There are still exciting untested hypotheses, but there is the perception that the discipline is a bit dead intellectually. CSR has never been modernized while LHS is just three orthogonal traits. Not much new seems to have taken its place. The lack of a exciting toe-hold here hurts.
  • Phylogenies are incomplete. It’s hard to describe what a drag on progress this is. Without a robust phylogeny, species and traits cannot be compared evolutionarily. For example, it’s hard to do congeneric comparisons when it can’t be agreed that species belong in the same genera. And to require ecologists to generate phylogenies is an unnecessary requirement that doesn’t promote long-term growth.
  • Post-SLA research in a SLA paradigm. The leaf economic spectrum is a great advance, but the common perception to many is that SLA is the central trait of plants. Here’s one comment we received recently on a proposal: “It was odd not to see conventional traits like SLA… included in this work”, even though we had proposed to measure the components of SLA: leaf thickness and tissue density. Put briefly, there is too much uncertainty on whether SLA is a central trait that structures plant evolution and ecology, or whether SLA should R.I.P. for a new generation of metrics.
  • The decline of traditional physiological ecology. This one is unquantifiable, but the central importance of physiological ecology to ecology has been diluted. The rise of model organisms and ecosystem ecology/global change biology has at the very least diluted the middle ground that was plant physiological ecology, instead of strengthened it as a discipline. I think there’s been a loss of the perspective that comes from being at the nexus of plant evolution, biogeography, and ecosystem function.

So what to do? I don’t have clear answers here. This post is just a scratchpad for me.  A couple thoughts come to mind.

  • We can’t always wait on phylogenies. If a phylogeny doesn’t exist, it is not helpful to insist that ecologists create one or that we wait. Some aspects of the work might later get revised, but functional trait work can be done in advance of the phylogenies. Especially considering how often phylogenies get revised.
  • We need model species sets. The rise of model organisms—and the resources devoted to their study—has  been quite amazing. Model organisms in and of themselves do not help us understand plant functional traits too much. They aren’t inherently comparative. I think we need model species sets to complement model organisms. For a broad clade, we need to identify a reasonable number of species that represents a broad set of evolutionary and ecological contrasts. This set then needs to be examined by multiple researchers, just like with model species.
  • The pot effect needs to become irrelevant. We will always need to grow plants in containers. We need clear understanding of how containers affect genetic expression and plant functional traits. Direct research is required here.
  • We need better theories. All disciplines need buzz. Plant functional trait research doesn’t have it. It needs it.
  • We need to measure old traits more, but we also need to develop new traits. SLA and rooting depth isn’t enough to help us understand how plants have been shaped and respond to everything from drought to herbivores.
  • We need to encourage breadth in our science. Plants have been shaped and respond to a multitude of environmental factors. You can’t describe an elephant by grabbing its tail, nor by having a number of scientists grabbing individual parts. Some need to try and feel the whole and understand how the parts come together.

Sunday, July 3, 2011

Grasses of the World IV--Taxonomic differences

Relationships between leaf width and physiological drought potential for six genera of grass.

No one know exactly what the ancestral grass looked like or the environments it inhabited. But one could imagine a bright, open wet environment with a narrow leaved bunch grass or weakly rhizomatous grass inhabiting it.  Some tens of millions of years later the BEP and PACMAD clades would have diverged and the major radiations of grasses still a long way off. 

But what were the forces that drove the radiations. Aridity is often cited as one. Fire another. Grazers still a third. But this might be somewhat of a skewed, biased perspective, since there has been little work to characterize the modern ecology of the whole of grasses.

When we look at the global traitscape of grasses, we saw clear patterns for leaf width and drought tolerance. One can imagine some selective force favoring wide-leaved grasses and drought narrow leaved grasses, until an ecological or physiological tradeoff was reached.

But what does the pattern of radiations for individual clades look like?

If I map the distribution of 6 genera in traitspace, clear unique patterns show up. The genus Panicum, for example, has species with wide and narrow leaves, but none that are very drought tolerant. In contrast, Festuca species all have leaves that are narrow, but span the full range of drought tolerance.

We still haven't mapped all this onto a phylogenetic tree. That's coming. But the value of screening programs like this are pretty clear for understanding the ecology and evolution of grasses. 

But why the separation among genera? Are individual genera constrained physiologically, or are they constrained evolutionarily by the presence of other species that lead to the apparent differentiations. 

Part of what we still need to do is understand the importance of traits such as leaf width and understand the benefits (and constraints) of narrow and wide leaves.

Traitscape of drought tolerance for Konza

One of the keys to understanding community assembly will be assembling traitscapes for communities and comparing them to global traitscapes. Earlier, I showed how we could assemble a nitrogen traitscape for Konza and compare that to the global distribution to show that the typical Konza species has higher foliar N concentrations and experiences higher N availability than the typical species at the global scale.

We're getting close to being able to do something similar for Konza, but for physiological drought tolerance. We're working to collect all the grass species of Konza and measure their psi-crit in order to compare them to the global distribution. We've only fully measured 28 of Konza's 86 species of grasses, but the patterns so far our interesting.

Part of the power of the traitscape is to understand inter- vs. intra-site importance of environmental variation. For drought, if we expect Konza to be more likely to experience frequent and severe drought than other grasslands of the world, you could expect to see the typical species be more drought tolerant than the global distribution. We can also look at the distribution of drought tolerance at a site and see how that compares to the global range. Means might be different, but if there is high spatial or temporal variability in water availability, a community could encompass a large part of the global range.

Expectations for Konza are a bit uncertain--it's a humid prairie (835 mm y-1 precip), but can experience severe droughts. Within site, there are dry habitats--south facing slopes with thin soils--and wet ones--seeps, riparian areas, and ditches.

The pattern?

So far the global mean psi-crit is -4.8 MPa. Konza? -4.5 MPa.

The global range is -1.4 to <-14 MPa. Konza? -1.8 to -13 MPa.

Here's the pattern of psicrit with leaf width (red = Konza species):

After 28 species, most of the global trait-space is covered. If anything, Konza might be underrepresented in fine-leaved, drought tolerant grasses. I haven't measured Agrostis hyemalis yet, but it's leaves are about 1mm across--we'll see how drought tolerant it is.

I think there's an amazing range of diversity in drought tolerance at a single site. Konza might be an exception, but the diversity in soil moisture availability at a site can be high.

One question that comes up is that if there can be such high diversity at a site, what are the differences in among sites? How important is drought tolerance in differentiating grasslands and contributing to gamma diversity?

Saturday, July 2, 2011

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).