Plant removals in perennial grassland: Vegetation dynamics, decomposers, soil biodiversity, and ecosystem properties

The consequences of permanent loss of species or species groups from plant communities are poorly understood, although there is increasing evidence th at individual species effects are important in modifying ecosystem properti es. We conducted a field experiment in a New Zealand perennial grassland ec osystem, creating artificial vegetation gaps and imposing manipulation trea tments on the reestablishing vegetation. Treatments consisted of continual removal of different subsets or "functional groups" of the flora. We monito red vegetation and soil biotic and chemical properties over a 3-yr period. Plant competitive effects were clear: removal of the C-3 grass Lolium peren ne L. enhanced vegetative cover, biomass, and species richness of both the C-4 grass and dicotyledonous weed functional groups and had either positive or negative effects on the legume Trifolium repens L., depending on season . Treatments significantly affected total plant cover and biomass; in parti cular, C-4 grass removal reduced total plant biomass in summer, because no other species had appropriate phenology. Removal of C-4 grasses reduced tot al root biomass and drastically enhanced overall shoot-to-root biomass rati os. Aboveground net primary productivity (NPP) was not strongly affected by any treatment, indicating strong compensatory effects between different fu nctional components of the flora. Removing all plants often negatively affected three further trophic levels of the decomposer functional food web: microflora, microbe-feeding nematode s, and predaceous nematodes. However, as long as plants were present, we di d not find strong effects of removal treatments, NPP, or plant biomass on t hese trophic groupings, which instead were most closely related to spatial variation in soil chemical properties across all trophic levels, soil N in particular. Larger decomposer organisms, i.e., Collembola and earthworms, w ere unresponsive to any factor other than removal of all plants, which redu ced their populations. We also considered five functional components of the soil biota at finer taxonomic levels: three decomposer components (microfl ora, microbe-feeding nematodes, predaceous nematodes) and two herbivore gro ups (nematodes and arthropods). Taxa within these five groups responded to removal treatments, indicating that plant community composition has multitr ophic effects at higher levels of taxonomic resolution. The principal ordin ation axes summarizing community-level data for different trophic groups in the soil food web were related to each other in several instances, but the plant ordination axes were only significantly related to those of the soil microfloral community. There were time lag effects, with ordination axes o f soil-associated herbivorous arthropods and microbial-feeding nematodes be ing related to ordination axes representing plant community structure at ea rlier measurement dates. Taxonomic diversity of some soil organism groups w as linked to plant removals or to plant diversity. For herbivorous arthropo ds, removal of C-4 grasses enhanced diversity; there were negative correlat ions between plant and arthropod diversity, presumably because of negative influences of C-4 species in the most diverse treatments. There was evidenc e of lag relationships between diversity of plants and that of the three de composer groups, indicating multitrophic effects of altering plant diversit y. Relatively small effects of plant removal on the decomposer food web were a lso apparent in soil processes regulated by this food web. Decomposition ra tes of substrates added to soils showed no relationship with treatment, and rates of CO2 evolution from the soil were only adversely affected when all plants were removed. Few plant functional-group effects on soil nutrient d ynamics were identified. Although some treatments affected temporal variabi lity (and thus stability) of soil biotic properties (particularly CO2 relea se) throughout the experiment, there was no evidence of destabilizing effec ts of plant removals. Our data provide evidence that permanent exclusion of plant species from th e species pool can have important consequences for overall vegetation compo sition in addition to the direct effects of vegetation removal, and various potential effects on both the above- and belowground subsystems. The natur e of many of these effects is driven by which plant species are lost from t he system, which depends on the various attributes or traits of these speci es.