Stevens' research has contributed to understanding how changes in the levels of nitrogen compounds in the soil, deposited from the atmosphere, have had significant effects on the composition of the UK flora. This has shown that the number of different species of plants present is reduced as soils receive more inorganic nitrogen compounds from the atmosphere. Stevens has been involved with a long-term project at Tadmore Moor that started in 1986 following the effects of nitrogen fertiliser on this wetland. No fertiliser was added after 1990 but she could still find effects in 2005.
Stevens is part of the Nutrient Network, an international collaboration investigating how grasslands are affected by global climate change, specifically how anthropogenic increases in nitrogen and phosphorus levels affect plant productivity and diversity and the interaction of the plants with grazing animals. This research involves the partners setting up the same experiment in their location so that global comparisons can readily be made. It started in 2005 and had grown to 130 sites by 2021.
She is a trustee of the Ecological Continuity Trust that maintains long-term ecological field experiments and their data in the UK.