FIGURE 1. Multiple resource limitation of plant productivity.
From the following article:
Grassland species loss resulting from reduced niche dimension
W. Stanley Harpole & David Tilman
Nature 446, 791-793(12 April 2007)
doi:10.1038/nature05684
The greatest effect of factorial addition of resources occurred with the addition of all four resources: N, P, C and H represent the addition of nitrogen, phosphorus, cations and water. Vertical dotted lines group treatment combinations by the number of added resources, shown at the bottom of each group. Results are shown as means 
s.e.m.