Scale‐dependent relationships between tree species richness and ecosystem function in forests
Authors: Chisholm Ryan A. , Muller‐Landau Helene C. , Abdul Rahman Kassim , Bebber Daniel P. , Bin Yue , Bohlman Stephanie A. , Bourg Norman A. , Brinks Joshua , Bunyavejchewin Sarayudh , Butt Nathalie , Cao Honglin , Cao Min , Cárdenas Dairon , Chang Li‐Wan , Chiang Jyh‐Min , Chuyong George , Condit Richard , Dattaraja Handanakere S. , Davies Stuart , Duque Alvaro , Fletcher Christine , Gunatilleke Nimal , Gunatilleke Savitri , Hao Zhanqing , Harrison Rhett D. , Howe Robert , Hsieh Chang‐Fu , Hubbell Stephen P. , Itoh Akira , Kenfack David , Kiratiprayoon Somboon , Larson Andrew J. , Lian Juyu , Lin Dunmei , Liu Haifeng , Lutz James A. , Ma Keping , Malhi Yadvinder , McMahon Sean , McShea William , Meegaskumbura Madhava , Mohd. Razman Salim , Morecroft Michael D. , Nytch Christopher J. , Oliveira Alexandre , Parker Geoffrey G. , Pulla Sandeep , Punchi‐Manage Ruwan , Romero‐Saltos Hugo , Sang Weiguo , Schurman Jon , Su Sheng‐Hsin , Sukumar Raman , Sun I‐Fang , Suresh Hebbalalu S. , Tan Sylvester , Thomas Duncan , Thomas Sean , Thompson Jill , Valencia Renato , Wolf Amy , Yap Sandra , Ye Wanhui , Yuan Zuoqiang , Zimmerman Jess K. 
Published: August 2013

The relationship between species richness and ecosystem function, as measured by productivity or biomass, is of long‐standing theoretical and practical interest in ecology. This is especially true for forests, which represent a majority of global biomass, productivity and biodiversity.

Here, we conduct an analysis of relationships between tree species richness, biomass and productivity in 25 forest plots of area 8–50 ha from across the world. The data were collected using standardized protocols, obviating the need to correct for methodological differences that plague many studies on this topic.

We found that at very small spatial grains (0.04 ha) species richness was generally positively related to productivity and biomass within plots, with a doubling of species richness corresponding to an average 48% increase in productivity and 53% increase in biomass. At larger spatial grains (0.25 ha, 1 ha), results were mixed, with negative relationships becoming more common. The results were qualitatively similar but much weaker when we controlled for stem density: at the 0.04 ha spatial grain, a doubling of species richness corresponded to a 5% increase in productivity and 7% increase in biomass. Productivity and biomass were themselves almost always positively related at all spatial grains.

Synthesis. This is the first cross‐site study of the effect of tree species richness on forest biomass and productivity that systematically varies spatial grain within a controlled methodology. The scale‐dependent results are consistent with theoretical models in which sampling effects and niche complementarity dominate at small scales, while environmental gradients drive patterns at large scales. Our study shows that the relationship of tree species richness with biomass and productivity changes qualitatively when moving from scales typical of forest surveys (0.04 ha) to slightly larger scales (0.25 and 1 ha). This needs to be recognized in forest conservation policy and management.