Resource availability mediates the importance of priority effects in plant community assembly and ecosystem function

Assembly history, including the order in which species arrive into a community, can influence long-term community structure; however we know less about how timing of species arrival may alter assembly especially under varying resource conditions. To explore how the timing of species arrival interacts with resource availability to alter community assembly, we constructed experimental plant communities and manipulated the interval between plantings of groups of seedlings (0, 5, 10, 15 or 20 days) at low and high levels of soil nutrient supply. To see if community changes influenced ecosystem-scale processes, we measured parameters across the plant -soil continuum (e.g. plant biomass and net ecosystem carbon dioxide exchange). We found that the timing of species arrival had a large impact on community assembly, but the size of the effect depended on soil fertility. As planting interval increased, plant communities diverged further from the control, but the divergence was stronger at high than at low nutrient supply. Our data suggest that at high nutrient supply, early-planted species preempted light resources more quickly, thus preventing the successful establishment of later arriving species even at short planting intervals. Finally, we found that assembly related divergence in plant communities scaled to impact ecosystem-level characteristics such as green leaf chemistry, but had little effect on total community biomass and net ecosystem exchange of CO2 and water vapor. Our data indicate that the effect of a stochastic factor, here the timing of species arrival on community composition, depends on the resource level under which the community assembles.