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Comparative study plots

In 2009 and 2010, the joint Chinese-German-Swiss research project “BEF China” has established a large forest Biodiversity and Ecosystem Functioning (BEF) experiment in subtropical forests at Xingangshan (Jiangxi Province, China). In total, 566 plots were established at two sites, using different pools of a total of 42 native tree species and 10 shrub species with more than 300 000 planted saplings, covering about 50 ha. In a parallel observational approach, 27 Comparative Study Plots (CSPs) were set up in existing forests in an adjacent National Nature Reserve (Gutianshan, Zhejiang Province).


Figure 1: Location of the experimental sites with Comparative study plots showing in green area and main experimetal sites in red area.



Figure 2: Map of a CSPs, indicating plot usage and measurements for different subprojects (SP).

Experimental design:

Twenty-seven plots of 30 × 30 m area were deliberately selected to span factorial gradients in both tree species richness and successional age resulting from timber cutting by local communities. Average distance between plot pairs was ~3km. The closest pair was 40 m apart, followed by 165 m and 243 m for the next-closest pairs. For each plot, we determined tree species richness from the inventory data we recorded. Successional age was assigned to five age classes (<20, 20–40, 40–60, 60–80, or >80 years old) based on the age of the fifth-largest tree of each plot (determined from a stem core), because the precise date of the last logging event could generally not be determined. Our goal was to evenly cover the range in tree diversity and successional ages present at the site, although it was not possible to keep these two fixed, independent variables fully orthogonal to each other. In the further course of the study, two plots were lost due to (illegal) timber cutting. All analysis presented are therefore based on data from the remaining twenty-five plots.
We did not select plots randomly, because such a “sample survey” design would have resulted in a concentration of plots around mean tree species richness values, with a typically bell-shaped distribution. In sample surveys (and meta-analyses based on sample surveys), correlations between species richness and productivity are bi-directional relationships between two dependent variables. This problem can be alleviated by fixing one variable as independent variable at different levels that are similarly replicated, and then measuring the other variable as dependent variable. This approach is recommended e.g. in the classical statistical textbook by Snedecor & Cochran [15] who refer to this type of study as comparative study and rank it between sample surveys and designed experiments (with randomized treatments) with regard to the power to detect causal relationships between variables (Baruffol, et al. 2013).


Figure 3 Comparative study plots in Gutianshan natural nature reserve. Different colors represent different successional stages.

There are many papers showing the tree diversity effect on ecosystem function from this comparative study design, such as:

Bruelheide H, Bohnke M, Both S, et al. (2011). Community assembly during secondary forest succession in a chinese subtropical forest. Ecol Monogr 81:25-41.

Baruffol M, Schmid B, Bruelheide H, et al. (2013). Biodiversity promotes tree growth during succession in subtropical forest. Plos One 8.

Schuldt A, Baruffol M, Bohnke M, et al. (2010). Tree diversity promotes insect herbivory in subtropical forests of south-east china. J Ecol 98:917-926.

Schuldt A, Assmann T, Bruelheide H, et al. (2014). Functional and phylogenetic diversity of woody plants drive herbivory in a highly diverse forest. New Phytol 202:864-873.

Schuldt A, Bruelheide H, Durka W, et al. (2014). Tree diversity promotes functional dissimilarity and maintains functional richness despite species loss in predator assemblages. Oecologia 174:533-543.

Schuldt A, Bruelheide H, Härdtle W, et al. (2015). Early positive effects of tree species richness on herbivory in a large-scale forest biodiversity experiment influence tree growth. J Ecol 103:563-571.


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Pilot experiment

Can niche plasticity promote biodiversity–productivity relationships through increased complementarity?

Pascal A. Niklaus published a paper from the results of pilot experiment in Ecology, showing that richness-productivity relationships are promoted by interspecific
niche complementarity at early stages of stand development, and that this effect is enhanced by architectural plasticity.

In the first stage of the BEF-China project, the BEF-China consortium agreed on an additional short-term Pilot Experiment. This small-scale tree biodiversity experiment was designed in order to study early growth patterns and species interactions of young trees. The establishment of the Pilot Experiment gave the opportunity to investigate mechanisms of competition between species with varied functional traits and under different environmental treatments. This knowledge can be considered and used as baseline data for the interpretation of results gained in the Main Experiment.

The Pilot Experiment has been established on a former agricultural field near the experimental sites in Xingangshan on a gross area of 7900 m2. A total of 1598 plots (size of 1 m × 1 m) were established. Except for certain density treatments, each plot was planted with 16 tree individuals, reflecting a diversity gradient from 1, 2 or 4 species. 21 tree species were chosen according to the species composition in nearby Gutianshan National Nature Reserve. Tree species were assigned to 8, partly overlapping, species pools containing 4 species each. The species assignments were chosen to reflect certain characteristics in natural forests, i.e. early-successional, commercial versus native and late-successional species. In addition, three species pools were comprised with species collected by seed families in order to control for genetic diversity.

Experimental design

The basic design comprised pair-wise competition diallels and four-species mixtures within each species pool (Figure 1). In each pool, there were four monocultures, the six possible pair-wise mixtures and the four-species mixture, resulting in eleven communities. All experimental plots contained 16 individuals in an array of 4 x 4, and in mixed communities each species was represented by the same number of individuals at the edge and in the center (Figure 1). All treatment combinations were replicated four times, once in each of four blocks. The treatment combinations were randomly assigned to plots within blocks. This basic design was modified regarding the particular requirements of the different subprojects (see below). For example, the treatment using different soil nitrogen tracers to assess belowground complementarity used only monocultures and four-species mixtures yet a greater number of replicates to allow for sequential harvests.



Figure 1: Eleven different species compositions of one of the six pools in the Pilot Experiment. All possible monocultures and 2- and 4-species mixtures of the four species in the pool (here species A, B, C, D) were established.

Each experimental community consisting of 16 individuals was densely planted on an area of 1 m2 (= 1 plot). Planting distances between neighbors were 25 cm, except for additional density treatments of subproject 2 where neighbor distances were 15 cm and 25 cm (plots with 16 individuals) and 50 cm (= plots with only one individual). The dense planting at a distance of 25 cm was intended to induce competitive interactions already in the early stages of tree growth (i.e. within months to a few years). In the first block, individuals within the community were planted regularly (Figure 1), while in the three additional blocks the individuals were randomly distributed, randomizing first the four individuals of the inner square (= center), and second the twelve individuals in the outer square (= edge). Each random distribution was repeated within the block for the different treatments and for the different diversity levels.

The Pilot Experiment was established in March 2009 and finished in July 2011, depending on the subprojects involved, has been studied for 1, 2 or 3 growing seasons. The treatments included light vs. shade (Figure 2); shallow vs. deelp nitrogen labeling; high vs. low genetic diversity; density low vs. hight; fungicide vs. mycorrhizal inoculation; pesticide vs. control (with herbivory), etc.

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Figure 2 Light vs. shade plants in the plot.

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BEF modification experiment

Complementary effect is discussed for the explanation of diversity–ecosystem functioning (BEF) relationships in some forest ecosystems, however the importance of the resistance to the leaf pathogens and herbivores in higher diversity forest as one of the mechanisms for the BEF relationship is unknown. Here we conducted an experiment in a newly established large forest biodiversity–ecosystem functioning experiment in JiangXi Province, China (BEF-China). Using experimental manipulations of pathogens, herbivores and nutrient resources availability across 6 biodiversity levels (1, 2, 4, 8, 16, 24 species), we assessed the relative importance of herbivore and pathogen release versus resource competition as mechanisms driving diversity-productivity relationships in a subtropical forest.

Experiment treatments

Each plot has an area of 25.8 × 25.8 m (1 mu in Chinese unit), consisting of 400 trees planted in a grid of 20 × 20 individuals at a horizontal planting distance of 1.29 m. Because the main experiment is based in the plot center, we set up five additional subplots in the northern border area (rows 16 – 19). Each subplot had 4×4 trees and an area of 5.16 × 5.16 m. The subplots were randomly assigned to one of five experimental modification treatments: 1) fungicide for pathogen exclusion (F); 2) insecticide for herbivore exclusion (I); 3) no weeding, in order to increase resource competition by allowing weeds (NW); 4) phosphorus fertilizer to decrease resource competitions (P) and 5) a control with the standard manipulation of the overall experiment (C).

For the insecticides and fungicides, the chemicals were applied every 4 weeks (and every 2 weeks during the rainy season because chemicals were more rapidly leached) and only on days without (or with very little) wind. All C, P, and NW plots were sprayed with 4 liters of water every time when the F and I plots were sprayed with insecticides/fungicides.

For insecticides, we added 10 ml dimethoate and 10 ml deltamethrin to the water (a maximum of 4 liters) and mixed water and insecticides by stirring the solution for one minute. For fungicides, we added 8 g of mancozeb and 25ml myclobutanil to the water (previously dissolved in a small amount of water) and then mixed water and fungicides by stirring the solution for one minute.

For P fertilizer, the fertilizer was only applied to the soil once at the beginning of the experiment. We used Triple Superphosphate (Ca(H2PO4)2), with the application rate of 100 kg P per ha (i.e. 10 g/m2). Long groove (3-4 cm deep) were made 30 cm above and 30 cm below each tree (at each of the 16 planting positions, even when the tree is not there anymore). Half the fertilizer was added to the upper groove, half to the lower groove. Then fertilizer in the grooves was covered with soil.

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