Direction of evenness-productivity relationship changes over a decade of stand development in a temperate forest

Tanvir Ahmed Shovon, Renewable Resources, University of Alberta, Edmonton, AB, Canada, Florian Hartig, Theoretical Ecology, University of Regensburg, 93053 Regensburg, Germany, Stephanie Kang, Thünen-Institut für Ländliche Räume, Braunschweig, Germany, Michael Scherer-Lorenzen, Geobotanik, Albert-Ludwigs Universität, Freiburg, Germany and Charles A. Nock, Faculty of Agricultural, Life and Environmental Science, University of Alberta, Edmonton, AB, Canada

Background/Question/Methods
Long term studies from both forests and grasslands experiments suggest that the positive diversity-productivity relationship (DPR) tends to strengthen during succession and attribute this pattern to an increase in niche complementarity. Most studies to date, however, did not control for species dominance and density, which also changes during forest succession. Further, how fine scale interactions between neighboring tree species affect the DPR is not well understood. We present data from simplex experiment established as a part of the BIOTREE, where tree diversity is manipulated to examine forest biodiversity-ecosystem function relationships. Our experiment consists of 36 experimental plots each planted with four important European tree species to create a gradient of tree species evenness at two levels of tree density (high density: 6667 trees ha-1 and low density: 3556 trees ha-1). We test whether: i) the effect of evenness on total above-ground biomass production increase with stand development, ii) the effect of evenness on productivity interacts with tree density; iii) neighboring conspecifics negatively affect the growth of dominant species more strongly compared to co-dominant species and whether this negative effect is stronger in denser plots.
Results/Conclusions
The relationship between evenness and total above-ground biomass was negative in both high- and low-density plots. However, the relationship between evenness and plot biomass increment shifted from negative to positive with time. In high density plots, the slope between evenness and increment rate between 2011-2014 was -3.34 Mgha-1 y-1 (95% credible interval: -10.43 to 2.87) and the slope turned into 6.40 Mgha-1 y-1 (1.15 to 11.39) in 2015- 2019 period. In low density plot, the slope between 2011 -2014 was -7.08 Mgha-1 y-1 (-11.43 to -2.81) and it turned into 3.79 Mgha-1 y-1 (-1.22 to 8.83) in 2015 - 2019 period. For both dominant species, Pseudotsuga menziesii and Picea abies, the effects of conspecifics on the individual growth rate were negative across density levels and years. The relationship between evenness and biomass increment turned into positive from negative in high-density plots. Strong negative conspecific effects limited the increment of the abundant species in uneven plots and a persistent increase in the abundance of shade tolerant trees in even plots led to a higher biomass increment in the even plots, which led to a positive DPR. Even species mixture and high-density planting can result in higher biomass increment compared to less diverse forests.