COS 132-6
Tree diversity enhances stand productivity in a large-scale field experiment in subtropical China
The increase in plant productivity with species diversity has been well documented in temperate grasslands. However, little is known about these effects in forests and (sub)tropical ecosystems, despite their major role in the global cycling of carbon.
To address this gap in knowledge, a large-scale forest biodiversity experiment was established in subtropical China in 2009-2010 (www.bef-china.de). Over 200,000 tree saplings, selected from a pool of 40 native tree species, were planted in 492 plots, creating communities with 1, 2, 4, 8, 16 or 24 species. In the range of 1 to 16 species, tree species richness manipulations reflected two extinction scenarios (random vs. trait-oriented), which were applied to ten partly overlapping species pools. The experimental site is located on a hilly terrain with steep slopes, near Xingangshan, Jiangxi province.
From 2009 to 2012, we monitored basal diameter (BD) and height of nearly 8,000 trees. Using mixed-effects models, we tested for diversity effects on stand-level basal area (BA) and on a proxy for stem volume (BA·height). In addition, we tested for transgressive and non-transgressive overyielding, and used additive partitioning to split the observed biodiversity effects into “complementarity” and “selection” components.
Results/Conclusions
From 2009 to 2012 the average tree BD and height increased four-fold. In 2012 the smallest species was Machilus grijsii (height<20 cm); the largest, Choerospondias axillaris(height>350 cm).
Diversity had a significant effect on both stand-level basal area and volume (p<0.04). Surprisingly, the extinction scenario had no effect (p>0.05). To control for local environmental heterogeneity, we included topographic covariates (altitude, slope and aspect) of each plot into the statistical model. These additional terms did not change the effects of diversity.
Transgressive overyielding was not observed: the most productive monocultures, C. axilaris and Elaeocarpus glabripetalus, had higher BA and volume than any high-diversity mixture. Non-transgressive overyielding occurred in ca. 30-35% of the plots. In contrast to selection, complementarity effect was affected by extinction scenario.
Further analyses indicated that the diversity-productivity relationship was strongly driven by positive effects of the presence of two fast growing species (C. axilaris and E.glabripetalus).
Overall, our results suggest an effect of diversity on stand basal area of subtropical tree communities already during the initial stages of stand development. In this phase, most tree individuals will interact only weakly by crown interference and diversity effects seem largely driven by the successful establishment of fast-growing species.