One of the reasons why plants are able to respond to atmospheric CO2 enrichment in the face of significant shortages of light, water and nutrients is that CO2-enriched plants generally have more extensive and active root systems, which allows them to more thoroughly explore larger volumes of soil in search of the things they need. In the case of legumes, which are found in almost all of Earth's ecosystems, atmospheric CO2 enrichment additionally helps the nitrogen-fixing bacteria in the nodules of their roots to remove more nitrogen from the atmosphere and make it available to their hosts.
Symbiotic soil fungi attached to plant roots also function better when their hosts supply them with more photosynthetically-produced substances, as nearly all plants typically do in CO2-enriched air. These fungi then repay their hosts by supplying them with extra water and nutrients. They also produce hormones that further stimulate root growth; and they help to protect their hosts from toxic materials in the soil and from soil-borne plant pathogens.