The effect of multi-nutrient fertilization on understory plant diversity

This study examined multi-nutrient fertilization effects on understory vege tation diversity at eight forested locations in the inland Northwest. Perce nt canopy covered by understory plant species of three growth forms (shrubs , forbs, grasses) and total understory were determined over a two year peri od following treatment. Two diversity indices (Shannon-Wiener index and Sim pson's index) were used to quantify multi-nutrient fertilization effects on understory plant diversity. Multi-nutrient fertilization prescribed to inc rease overstory tree growth did not typically reduce understory vegetation diversity, rather diversity increased following fertilization on some sites . Understory composition at the time of treatment greatly determined fertil ization effects on diversity as follows: 1) if most species in the plant co mmunity responded relatively the same after treatment, then diversity was u nchanged even though total biomass increase may have been large; 2) if a hi ghly responsive species was abundant Drier to treatment, then diversity dec reased following fertilization; and 3) if a highly responsive species was r elatively rare prior to treatment, then diversity increased following ferti lization. Multi nutrient forest fertilization generally did not affect unde rstory plant diversity, but where changes did occur, diversity increases we re more common than decreases.