Aggregation of conspecifics alters productivity in a grassland experiment

Background/Question/Methods

The spatial arrangement (pattern) of plant species in a plot determines the frequency of an individual’s intra- and interspecific interactions, and an individual’s performance may change with these frequencies.  Some species may be positively influenced by a planting pattern where the frequency of intraspecific interactions is high (aggregation), while this arrangement may be detrimental to others.   Because richness and evenness are confounded with species pattern, it is unclear if their effects on productivity arise due to direct effects or via indirect effects of species pattern.  To address the effect of pattern on grassland productivity, the plant Species Pattern and Community Ecology (SPaCE) field experiment was established in 2012 with transplants (64 individuals m2-; planted in 1 x 1 m plots) from a pool of 16 tallgrass prairie species.  Plots were planted with varying levels of plant species richness (2, 4, or 8 species and monocultures), evenness (0.64, 0.8, or 1.0), and pattern (planted randomly or in groups of four individuals).  Treatment diversity was maintained by weeding, and at the end of each growing season, aboveground plant biomass was clipped, dried, and weighed.

Results/Conclusions

In both years of the experiment, aggregation of conspecifics (clumping) reduced biomass production.  Differences in biomass production among treatments were driven by selection, but not complementarity effects.  In year one, clumping decreased selection, while in year two, selection effects did not differ between pattern treatments.  No indirect effect via self-association on biomass production or selection was found in either year 1 or year 2 using structural equation modeling, but significant total effects and inconsistencies between random and clumped model effects were found in both years.  These findings, along with significant treatment effects, suggest that pattern is contributing to community structure effects on grassland productivity.  We are currently investigating intraspecific competition and biota mediated effects as possible mechanisms of the observed responses.