A vast body of evidence supports the idea that biodiversity, in particular species richness, supports a wide range of ecosystem functions including primary production. To date, biodiversity-ecosystem functioning research has focused primarily on grassland, with forest studies underrepresented. Also, most studies have been carried out in the temperate zone, with only few in subtropical or tropical areas. Forests differ fundamentally from grasslands in a number of way that could result in different biodiversity effects. First, tree individuals are larger in size and longer-lived than the average herbaceous species. Second, niche complementarity may be lower in forests, at least in tropical systems, because of strong environmental filtering.
Here, we investigated biodiversity effects on leaf area index (LAI) and canopy structure in the world's largest forest biodiversity experiment "BEF China". We were interested in whether leaf area index increased with species richness, and whether leaf area and stem area responded in similar ways. In the present study, we used 144 (of a total of over 500) plots that were planted with 400 tree individuals each, forming communities with 1, 2, 4, 8 or 16 tree species. Leaf area index was determined three times at five locations per plot by digital hemispheric photography.
Results/Conclusions
LAI reached peak values of 6.6 m2 m-2 and increased significantly with tree species richness in the fifth year of stand establishment, paralleling increases in stem basal area. Similar but weaker and statistically non-significant trends were observed in the year before, suggesting that this effect developed progressively. The present study adopted a “broken stick” design with lower diversity communities formed by repeatedly splitting separate 16-species pools until monocultures were obtained. LAI in mixtures exceeded the average of the two component communities with half the species number by 0.48±0.20 m2 m-2. However, no transgressive leaf area overyielding was found, i.e. mixtures did not outperform the more productive component community. The presence of particularly productive and unproductive species explained large amounts of variation in leaf area, suggesting that selection-type effects contributed substantially to the biodiversity effects we found in this early phase of stand establishment. Effects sizes were moderate to large, and comparable in magnitude to the ones reported for grassland ecosystems. Subtropical (and tropical) forests harbor substantial parts of global net primary production and are important for the Earth’s carbon and hydrological cycle, and our results suggest that tree diversity critically supports these ecosystem services.