We conducted a long-term field experiment in northeastern Kansas to investigate the interplay of local and regional processes affecting plant community diversity in a grassland ecosystem undergoing secondary succession. Our study asks the following questions to further understand the forces influencing plant species coexistence, diversity, and community development over the course of secondary succession: 1) To what extent are plant diversity and its dynamics of change over time governed by soil nutrient availability? 2) To what extent are plant diversity and its dynamics of change over time constrained by seed limitation and the availability of colonists from the regional pool? 3) How may propagule pools and dispersal limitations interact with soil nutrient availability to govern species diversity and its dynamics of change over time? To address these questions, we implemented a fully replicated, factorial experiment in 2001 on abandoned agricultural land. To examine effects of soil nutrient availability on plant diversity, we manipulated soil nitrogen content (0, 4, 8, or 16 g N m-2 yr-1 added to plots annually) and soil phosphorus treatment (0 or 8 g P m-2 yr-1 added to plots annually). To examine the effect of propagule pools and dispersal limitations, we manipulated seed availability be either adding fifty-three native and naturalized plant species to half of the plots at the start of the experiment. Plant species cover was measured every year from 2001 to 2010, and diversity indices were calculated for each year (i.e. richness, evenness, Shannon’s diversity index, and Simpson’s diversity index) before conducting factorial ANOVAs.
Seed addition increased plant colonization and species accumulation early on in succession and led to a significant enhancement of species diversity in sown plots that was sustained over the entire course of the study. Nitrogen additions increased plant production and generally reduced species diversity, an effect that became more evident over time and that was more pronounced in sown than non-sown plots. Effects of phosphorus on plant diversity were less consistent than the effects of nitrogen and were much more subtle in magnitude. Our findings illustrate strong interactions between regional constraints on seed availability and soil fertility in regulating the successional dynamics of plant diversity in this grassland system.