Does functional trait diversity predict above‐ground biomass and productivity of tropical forests? Testing three alternative hypotheses

Abstract

Summary1. Tropical forests are globally important, but it is not clear whether biodiversity enhances carbon storage and sequestration in them. We tested this relationship focusing on components of functional trait biodiversity as predictors. 2. Data are presented for three rain forests in Bolivia, Brazil and Costa Rica. Initial above-ground biomass and biomass increments of survivors, recruits and survivors + recruits (total) were estimated for trees ≥10 cm d.b.h. in 62 and 21 1.0-ha plots, respectively. We determined relationships of biomass increments to initial standing biomass (AGB i ), biomass-weighted community mean values (CWM) of eight functional traits and four functional trait variety indices (functional richness, functional evenness, functional diversity and functional dispersion). 3. The forest continuum sampled ranged from 'slow' stands dominated by trees with tough tissues and high AGB i , to 'fast' stands dominated by trees with soft, nutrient-rich leaves, lighter woods and lower AGB i . 4. We tested whether AGB i and biomass increments were related to the CWM trait values of the dominant species in the system (the biomass ratio hypothesis), to the variety of functional trait values (the niche complementarity hypothesis), or in the case of biomass increments, simply to initial standing biomass (the green soup hypothesis). 5. CWMs were reasonable bivariate predictors of AGB i and biomass increments, with CWM specific leaf area SLA, CWM leaf nitrogen content, CWM force to tear the leaf, CWM maximum adult height H max and CWM wood specific gravity the most important. AGB i was also a reasonable predictor of the three measures of biomass increment. In best-fit multiple regression models, CWM H max was the most important predictor of initial standing biomass AGB i . Only leaf traits were selected in the best models for biomass increment; CWM SLA was the most important predictor, with the expected positive relationship. There were no relationships of functional variety indices to biomass increments, and AGB i was the only predictor for biomass increments from recruits. 6. Synthesis. We found no support for the niche complementarity hypothesis and support for the green soup hypothesis only for biomass increments of recruits. We have strong support for the biomass ratio hypothesis. CWM H max is a strong driver of ecosystem biomass and carbon storage and CWM SLA, and other CWM leaf traits are especially important for biomass increments and carbon sequestration.

A Review of Strong Evidence for the Effect of Functional Dominance on Carbon Stocks in Natural Forest Ecosystems

“…These above results provide support for the general hypothesis that plant economics may not only be reflected in leaves, but also in stems at species (Freschet et al, 2010) and at community level (Perez-Ramos et al, 2012). However, most of the studies focusing on the relationship between FTD and C stocks did not consider the whole plant coordination and root traits are particularly ignored due to technical difficulties and are often substituted by measurement of leaf functional traits (Conti and Diaz, 2013;Cavanaugh et al, 2014;Finegan et al, 2015). It is recommended to include root traits, in addition to leaf and stem traits, in further studies concerning FTD and C stocks in natural forest ecosystems (Orwin et al, 2010;Grigulis et al, 2013).…”

“…However, Cavanaugh et al (2014) conducted a global scale study in natural tropical forests and found a positive relationship between CWM of maximum diameter and C stocks, suggests that characteristics of the dominant trees (specifically their maximum potential diameter) results greatest aboveground C stock in mature forests, strongly supported the mass ratio hypothesis. Cavanaugh et al (2014) and Finegan et al (2015) did not find a significant relationship between FD and C stock which is in disagreement with the niche complementarity hypothesis (Tilman et al, 1997). Although, Cavanaugh et al (2014) found a positive association between taxonomic diversity and C stocks, supports partly the niche complementarity hypothesis.…”

“…However, some non-significant and negative FD effects were also retained in the final models of C stocks (Conti and Diaz, 2013). These negative FD and positive CWM effects reported for initial production suggest that strong dominance by tall species, rather than a set of coexisting species with diverse heights, results in greatest C stocks in natural forest ecosystems (Conti and Diaz, 2013;Finegan et al, 2015). Thus, these tall species may also have denser wood and greater below ground production and hence make a greater contribution to C stocks.…”

“…J. For., 9 (3): 65-70, 2015Finegan et al (2015 observed the strong relationship of leaf economics spectrum (CWM of leaf traits particularly SLA) to productivity including, initial aboveground biomass (thus, above ground C stock), biomass of survivors and recruits in tropical forest ecosystems. This implies that communities dominated by exploitative plants are associated with overall faster nutrient turnoverand the opposite are true for communities dominated by conservative plants (Lavorel and Grigulis, 2012).…”

“…Further, many studies have been independently tested the effects of different components of FTD, particularly CWM and FD effects, in natural forests and also assessed the trait-specific relationships with C stocks in natural forest ecosystems (Caspersen and Pacala, 2001;Delagrange et al, 2008;Jonsson and Wardle, 2010;Ruiz-Jaen and Potvin, 2011;Wardle et al, 2012). However, empirical studies focusing on the comparison of CWM (the mass ratio hypothesis) and FD (the niche complementarity) effects on C stocks in the naturally established forest ecosystems are still very scarce (Conti and Diaz, 2013;Cavanaugh et al, 2014;Finegan et al, 2015). Thus, our understanding about how functional dominance versus divergence affects the C stocks in natural forest ecosystems remains poorly understood.…”