|How small is too small?|
It seems inviolable that species must be compared after having been grown under common conditions. This means growing plants either in field plots or pots. I've written a bit about autogenic plasticity involved with field plots and how feedbacks to resource availability affect expressed traits. One of the most enduring criticisms of plants grown in pots is the size of the pots. No pot is ever too big, and almost any pot can be too small.
The bias in results that comes from restriction of roots has been termed "the pot effect". I trace it back to Curtis and Wang's first ecological meta-analysis on elevated CO2. The authors wrote in their abstract "We found no consistent evidence for photosynthetic acclimation to CO2 enrichment except in trees grown in pots <0.5 l (-36%)". The take-home from the paper was that little pots bias the results.
My guess is that would be the wrong conclusion. For that study, photosynthesis acclimates when plants are nutrient-limited. Plants in small pots become nutrient-limited faster than those in the field or in big pots. It's not the pot size at all. But which is more representative of the real world?
Quite likely the pot. As I've shown at Cedar Creek, small plants in big plots at low density are not very nutrient limited and have the traits associated with the high-nutrient strategy. Plants of the same species in more mature plots at higher density resemble more what we seen in small pots--the low-water or low-nutrient strategy.
One could argue pretty easily that the pot effect is essential to mimicking the real world where resources limit plant growth. One could argue too that small pots cause artifacts, but it would take some careful logic chains for the argument to be convincing.
In the mean time, try small plots. They generate water or nutrient limitation faster. And increase your replication if you're tight on space.