Evidence for large positive effects of species richness on biomass in U.S. forests

Background/Question/Methods

High rates of species loss have motivated intensive research into how the numbers of species in ecosystems (species richness) influences their functioning.  To date, most species richness-ecosystem function research has focused on grasslands, partly because of their global importance, but also because grassland species’ relatively rapid growth rates make them easy to study over short time frames.  Forest ecosystems are on the other end of the spectrum in terms of ease of study.  Trees take many decades or even centuries to reach mature sizes, and niche complementarity, a key mechanism believed to cause positive richness-function relationships, may be unimportant until trees grow large.  To date, richness-function relationships have been assessed experimentally only when synthetic forest stands were relatively young, generally 1-30 years of age.  Consequently, despite the overwhelming importance of forests for carbon sequestration, wood products and forest-obligate taxa, relatively little is known about how tree species losses affect functioning of mature forests.  We developed a multilevel linear model with over 170,000 parameters to jointly estimate relationships between species richness and live tree biomass at 57,575 U.S. sites occurring in 27 forest types.  The dataset we analyzed has replicate plots within sites, which allowed us to control for inter-site biomass variation.  We also controlled for tree density differences among plots within sites to avoid spurious correlations owing to spatial sampling variability. 

Results/Conclusions

Probabilities that relationships between richness and biomass were positive exceeded 0.975 for 25 forest types.  Point estimates for several forest types suggested biomass was reduced ~35% when species numbers were halved (e.g. 2 to 1, 4 to 2).  This fixed biomass reduction with a halving of species richness constitutes a form of saturating relationship.  Saturating relationships have been commonly observed in richness-function experiments, and they are thought to arise because redundancies in functional attributes cause niche complementarity to become increasingly less important as richness increases.  Saturating relationships may also arise through a biodiversity mechanism sometimes termed a selection effect.  So the saturating relationships we observed provide some evidence biodiversity mechanisms were responsible for the positive species richness-biomass relationships we observed.  However, mechanisms unrelated to biodiversity may also have influenced our estimates to some extent.  The large richness effect estimates we observed should encourage research designed to identify causal mechanisms driving correlations between species richness and tree biomass in forests.