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?

Comparing nutrient availability with traitscapes

An overlay of foliar N concentrations and nitrogen isotop ratios from the Konza flora (black) to a global dataset (gray).

One of the key questions for understanding plant community assembly is to understand the environments that species inhabit--not just the dominant species, but the hundreds of species that are only occasionally or rarely seen to the casual observer. Do the rare species mirror the more abundant species in their traits? Or are they rare because they are built for different environments?

At Konza Prairie, there are over 500 herbaceous species. Over last 2 years we measured the leaves of over 400 species at Konza. One of the interesting patterns was examining the relationships between the leaf N concentrations and the foliar N isotopes. Together these two best reflect the N availability of the environment the plant inhabits. High N concentrations and high del15N generally mean that the plant is growing in an environment with high N availability.

Konza is considered a strongly N-limited ecosystem. The responses of aboveground productivity to N addition are some of the highest in North American grasslands--ANPP triples with N addition. Given this, at Konza, one thing that was surprising was how many species had really high N concentrations in their leaves. A fair number of species that didn't fix nitrogen had N concentrations over 40 mg g-1, or 4%. That's really high.

When you look at the flora as a whole, there were a lot of species that were found in high N availability sites. Edges of roads. Bison wallows. Places with high dung inputs. A lot of the diversity of Konza is likely maintained because of these high N availability sites.

When you look at Konza species by species the picture changes from one dominated by severe N limitation to one with a broad spectrum of N availability.  In fact, we could compare Konza to the rest of the world with a global dataset on foliar N and N isotopes. Not only do many of the species occupy high N availability sites at Konza, but the typical species at Konza actually occupies areas of higher N availability than the "rest of the world".

The analysis of traits across a broad portion of a flora--the community's traitscape--is not novel, but definitely an undersubscribed approach. As we build more global datasets and measure more and more species, a lot more insight to how communities are constructed and florae assembled will come into new light.