Fine root is a short lived component of a plant but play important functional role in nutrient acquisition from distributed soil nutrients. If nutrients are limited mainly nitrogen and phosphorus then plant tend to allocate more energy (photosynthetic product-carbohydrate) for nutrient foraging as well as exudation of carbon (unused carbon) for attracting more micro-organisms especially mycorrhizae for P-acquisition.
Fine roots, as mentioned below your question, are most involved in nutrient acquisition, therefore as environmental stressors exist, plants will allocate C to fine root biomass, rather than say stem growth or leaves, in order to obtain the most limiting factor of their overall growth. Mycorrhizal exudates can be an effective strategy in this process, but come at a high C cost, and some plants may instead create finer, more highly branched roots that can more efficiently acquire N and P broken down by free living (saprotrophic) soil bacteria.
This issue is well discussed as 'Functional equilibrium theory': plant will allocate relatively more biomass (or carbon) to roots (or shoots) if the limiting factor is belowground (or aboveground). See the discussion in
H. Poorter, K.J. Niklas, P.B. Reich, J. Oleksyn, P. Poot, L. Mommer. Biomass allocation to leaves, stems and roots: meta-analyses of interspecific variation and environmental control. New Phytol., 193 (2012), pp. 30–50
Regards!
Xiaoying
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