Factors controlling the productivity of tropical Andean forests: climate and soil are more important than tree diversity

Jürgen Homeier,Christoph Leuschner

doi:10.5194/bg-18-1525-2021

Abstract

Abstract. Theory predicts positive effects of species richness on the productivity of plant communities through complementary resource use and facilitative interactions between species. Results from manipulative experiments with tropical tree species indicate a positive diversity–productivity relationship (DPR), but the existing evidence from natural forests is scarce and contradictory. We studied forest aboveground productivity in more than 80 humid tropical montane old-growth forests in two highly diverse Andean regions with large geological and topographic heterogeneity and related productivity to tree diversity and climatic, edaphic and stand structural factors with a likely influence on productivity. Main determinants of wood production in the perhumid study regions were elevation (as a proxy for temperature), soil nutrient (N, P and base cation) availability and forest structural parameters (wood specific gravity, aboveground biomass). Tree diversity had only a small positive influence on productivity, even though tree species numbers varied largely (6–27 species per 0.04 ha). We conclude that the productivity of highly diverse Neotropical montane forests is primarily controlled by thermal and edaphic factors and stand structural properties, while tree diversity is of minor importance.

Highlights

Research into the diversity–productivity relationship (DPR) has recently shifted its focus on forests because of their importance for the global carbon cycle and the high biodiversity, especially of tropical forests (Mori et al, 2017; Fei et al, 2018)
Results from mixed species plantations with tropical trees indicate a positive DPR in the majority of cases (Sapijanskas et al, 2014; Huang et al, 2018; Schnabel et al, 2019), suggesting complementary resource use and facilitative interactions between different tree species (McIntire and Fajardo, 2014; Chen et al, 2016) The results of these experiments may provide valuable insights into the mechanisms of diversity effects on productivity (Tuck et al, 2016; Schnabel et al, 2019), but they are difficult to extrapolate to tropical old-growth forests due to the young age and cohort structure of these tree plantations
Based on the existing knowledge about the dependency of productivity and aboveground biomass (AGB) on environmental and stand properties and assumed diversity effects on productivity, we developed an initial model of interaction paths between the environmental parameters, WSG and tree diversity, and the target variables (AGB and wood production (WP) or NPPa) (Fig. 1)

Summary

Introduction

Research into the diversity–productivity relationship (DPR) has recently shifted its focus on forests because of their importance for the global carbon cycle and the high biodiversity, especially of tropical forests (Mori et al, 2017; Fei et al, 2018). Observational studies on the linkage between tree diversity and productivity in natural or nearnatural tropical forests are usually burdened with additional confounding factors and are more difficult to interpret than experiments, but they have the advantage of considering natural systems. The few existing observational studies on the DPR in tropical forests have produced contradictory evidence. Chisholm et al (2013) analyzed the tree diversity–productivity relation in 25 permanent forest plots from temperate to tropical regions and found a positive diversity effect when plot size was small (0.04 ha), but the effect disappeared at larger spatial scales (0.25 and 1 ha). In a global analysis of forest productivity data, Liang et al (2016) found that positive DPRs predominate, but negative relations do exist, though they are restricted to a few of the tested data sets

Methods

Results

Discussion

Conclusion