Exploring competition for water

From Lobet et al. 2014

It is true that drought kills plants. 

Drought lowers soil moisture. Low soil moisture kills plants. Therefore, drought kills plants. 

Yet, the rate at which drought lowers soil moisture is dependent on the plants that are present in soil. 

Plants lower soil moisture. Low soil moisture kills plants. Therefore, drought kills plants. 

Therefore, it is also true to say that plants kill plants. 

And when plants kill plants by using resources, that's resource competition. 

There has been a lot of great work over the past decade examining the mechanisms of how drought kills plants. 

But not so much on how plants kill plants when water is limiting. 

About a decade ago I was curious about some of the mechanisms of how plants compete for nutrients.

To explore this, I put together a model...actually I asked Trevor to put together a model...to simulate the movement and uptake of nutrients in soils.

This model was parameterized at a fine scale and could simulate the supply, movement, and uptake of nutrients in soils. 

It was able to show patterns of nutrient distribution in soils like this.. 

2 cm x 2 cm cross section of soil with all roots orthogonal to the plane. Red indicates high nutrient concentrations in soil solution. Blue is low.

With the model, I was able to show that plants acquired nutrients in proportion to the fraction of all the root length they had in a given volume of soil. Plants had a lot more roots than they needed to take up nutrients...if there was no competition. Once more than one plant had roots in a given volume of soil, a race set in.

As a result, plants can have 1000 times more roots that is optimal for maximizing growth.

That all pertained to nutrients though.

I'm curious about how competition for water works.

For example, does competition for water favor plants with high root length density? Would this also lead to a race like nutrients.

For competitive purposes, is there any benefit to being able to sustain a low minimum water potential? Under what conditions, if any, does drought tolerance affect competitive outcomes?

I'm also curious about the interactions between nutrients and water. Does increasing transpiration rate help with nutrient competition? Do dry soils exacerbate nutrient limitation?

But first, I need to adjust the model to handle water.

That means making soil moisture dynamic, parameterizing water fluxes between pixels and into roots.

The hardest part of all of this is figuring out how to parameterize water uptake by a  given root. There is no simple Michaelis-Menten equation here. Roots are a 1000 connected little straws

Still, I've been impressed by some of the developments in root modeling over the past few years.

I can expand on that later.