When researchers are measuring plant productivity, they usually harvest and measure above-ground biomass. But with grassland plants, it is common to find that most of an individual's biomass is in its root system. Why don't researchers measure below-ground biomass as well?

Because of the fibrous root network of grasses.

Explanation:

Measuring below ground biomass of grass plants is technically impossible and difficult because of the fibrous root network different plants form with one another and within the soil. The network virtually makes it impossible to individually extract all the underground biomass of each grass.