Interpreting forest diversity-productivity relationships: volume values, disturbance histories and alternative inferences

Abstract

Understanding the relationship between stand-level tree diversity and productivity has the potential to inform the science and management of forests. History shows that plant diversity-productivity relationships are challenging to interpret-and this remains true for the study of forests using non-experimental field data. Here we highlight pitfalls regarding the analyses and interpretation of such studies. We examine three themes: 1) the nature and measurement of ecological productivity and related values; 2) the role of stand history and disturbance in explaining forest characteristics; and 3) the interpretation of any relationship. We show that volume production and true productivity are distinct, and neither is a demonstrated proxy for economic values. Many stand characteristics, including diversity, volume growth and productivity, vary intrinsically with succession and stand history. We should be characterising these relationships rather than ignoring or eliminating them. Failure to do so may lead to misleading conclusions. To illustrate, we examine the study which prompted our concerns -Liang et al. (Science 354:aaf8957, 2016)-which developed a sophisticated global analysis to infer a worldwide positive effect of biodiversity (tree species richness) on "forest productivity" (stand level wood volume production). Existing data should be able to address many of our concerns. Critical evaluations will improve understanding.

Plant species richness or soil fertility: which affects more the productivity of Scots pine in Central Europe?

“…Despite the fact that the clear relations between soil properties and forest productivity have been known for many decades, and that in recent studies the essential effect of species richness for the productivity has been additionally revealed, the forest productivity controlling agents remain still not fully described and understood (Ammer 2018, Sheil & Bongers 2020. Additionally, to date it has not been arbitrated if the importance of soil fertility or of species richness is more significant for the productivity.…”

Trade‐off between standing biomass and productivity in species‐rich tropical forest: Evidence, explanations and implications

“…To generalize, we see that large-stature, highbiomass species show slower turnover-that is lower recruitment, relative growth and mortality-than small-stature species. We note that the greater range of species sizes that can occur in taller forests, and the associated diversity of size-dependent species behaviours this permits, explains why taller forests tend to be both more productive, and richer in species, than otherwise comparable forests of smaller stature (Duivenvoorden, 1996;Huston, 1994;Sheil & Bongers, 2020).…”

The Impact of the Neophyte Tree Fraxinus pennsylvanica [Marshall] on Beetle Diversity under Climate Change

“…Assessing the functional relevance of canopy arthropods requires intrinsic knowledge of the overall diversity and the interactions among functional groups-knowledge which is still very limited [2,15,39]. In addition, causal biodiversityfunctioning relationships cannot simply be inferred from monitoring data alone but require manipulative experiments [40,41]. Such experiments are difficult to conduct in the field, particularly when canopy arthropods are involved.…”