COS 191-4 - How General is Constant Final Yield?

Abstract: Total biomass production of plant stands grown over a very wide range of densities increases with density at low densities and then levels off at higher densities, a pattern called Constant Final Yield (CFY). This pattern is considered to be one of the most general in plant ecology, but there has been no review of literature to support this claim. Support for its generality is actually very limited because most density series experiments do not include high enough densities to test it, or report data only on biomass components such as reproductive output, and very few studies have sufficient replication to test the hypothesis. We reviewed and evaluated graphically and analytically data from 110 studies on 65 annual plant species or cultivars from 25 families, many with several treatments in addition to density, giving a total of 787 density series in which total above-ground biomass was measured, and we asked if there is evidence that biomass production is lower at the highest density than at some lower density. We used 3 methods to do so: (1) Statistical analysis of published biomass data from the original report were used when possible. (2) For density series with 5 or more densities, we asked if a model consistent with CFY fit the data better than a model with decreasing biomass at higher densities. (3) To glean some information from the many experiments that did not meet the criteria for (1) and (2), we considered a decrease in biomass of 10% or more from a lower density to the highest density as weak evidence against CFY. Based on published statistical analyses (1) 61 density series were consistent with CFY and 20 were not. For those experiments with 5 or more densities (2), 58 density series fit the model consistent with CFY better than the alternative model and 7 fit the alternative model better. Based on our 10% criterion, (3) 377 density series were consistent with CFY and 72 were not. The remaining density series showed erratic behavior or were equivocal. Our review shows that CFY is a very general phenomenon, but there are numerous exceptions. CFY has important implications for plant population and community ecology because it sets a clear limit on plant abundance, and many, if not most, plant populations and communities in the field are at this limit.