|Sensitivity of water uptake to changes in parameters for crops grown with weeds or weed-free. Parameters in the lower right are a lot more important when competing against weeds than when weed-free.|
Dunbabin 2007 modeled this by simulating the growth of plants with 3-d models of root systems in order to examine sensitivity of uptake of water, nitrogen, and phosphorus to variation in key root parameters.
Looking at what parameters are important when crops are competing against weeds, here's what Dunbabin says about that:
"the ability to quickly (growth rate) and effectively occupy (rooting density) the soil volume during crop establishment, may be important for denying weeds water and nutrients, thereby conferring competitive ability"
When there is no competition between crops and weeds, effective exploration is important:
"The ranking of P uptake efficiency as important for the acquisition of mobile nitrate and water
resources by weed-free crops...suggests that foraging for the least mobile, and often most limiting nutrient, may provide the best strategy for acquiring all soil resources (Robinson, 1996a)."
Here, the modeled plants are growing in relatively low-P soils, so to acquire the most water, you have to have a big plant. Having a strategy for effective acquisition of P becomes the most important parameters there.
One parameter not important regardless of whether there is competition or not? Potential transpiration rate.
Water is a mobile resource, but roots still have to wait for water to move to them.
Considering that nitrogen uptake kinetics aren't important for nitrogen competition, and mass flow is slower than diffusion, this makes sense.
It is important to note that the arena of competition is important here. The plants were started from seed (I think) and allowed to grow for 12 weeks, simulating a quick crop rotation.
The question here is what aspects of roots become important when competition is not a 12-week race? What becomes important when perennials occupy the same space for years? And if nutrients aren't limiting? Then what?
What does the optimal root system look like for plants growing in the absence of interspecific competition when water is limiting, but nutrients aren't?
A few thick roots?
Many thin roots?