Water and fertilizer have opposite effects on plant species richness in a mesic early successional habitat

Herbaceous plant species richness typically declines with increasing productivity, but differences in the resources underlying these gradients are often ignored. This study adds to the small number of studies examining the effects of water and mineral nutrients on biomass and richness in oldfield communities. We established 60 4 m2 plots in a goldenrod-dominated oldfield to test the differential effects of water and mineral nutrients on community properties. Species richness declined with added nutrients, but increased with added water. Aboveground biomass increased only when both nutrients and water were added. Leaf area index increased with added nutrients alone, although the increase was greater when water was also added. Understory light levels decreased with added nutrients, but not with added water; however, the per-gram effect of biomass on understory light levels did not vary significantly among nutrient and water treatments. Our results suggest that water tends to enhance productivity, but does not cause the common decline in species richness that typically accompanies nitrogen additions. Rather, water increased richness through positivie effects on either germination and establishment, or increased survival. These results are consistent with either increased light limitation or increased water limitation causing loss of species from nitrogen-rich habitats.