From 2005 to 2011, we studied the early vegetation succession of a six ha area in the artificial catchment Chicken Creek (German: Hühnerwasser) within the partly decarburised lignite mine Welzow Süd in NE Germany (details in Gerwin et al. 2009) (supplementary material Appendices A and B, Fig. B1). For the present study we used annual data on quantitative plant surveys from 107 single plots of 25 m2 (excluding a number of still empty plots in 2005 and 2006) (Fig. A1, details in Zaplata et al. 2013). For each plot, at the beginning of the study initial topsoil parameters (pH, organic carbon (C), and the percentage of sand in the soil) were determined. The complete set of raw data and the Pearson correlation coefficients between the soil variables are contained in the supplementary material Table A1 of Appendix A and Table B1 of Appendix B.
For each species, we estimated the cover degree (abundance) according to a modified Londo scale (Londo 1976; 0.1: ≤0.1%; 0.5: >0.1–0.5%; 1: >0.5–1%; 2: >1–2%, in 1% steps up to 10; 15: >10–15%, in 5% steps up to 30; 40: >30–40%, in 10% steps up to 100). Bryophyta and Marchantiophyta were not identified to lower taxonomic levels. In total, we found 168 morphospecies during the seven study years. The complete data of species identities and abundances of all study years used in this study are contained in Table A1 and in Ulrich et al. (2014, 2016).
We used the Leda (Kleyer et al., 2008) and BioFlor (Klotz et al., 2002) databases and compiled one plant morphological trait (specific leaf area, SLA), one reproductive trait (seed number), and one dispersal trait (seed weight). Additionally, we compiled data of three environmental demand traits assessed by Ellenberg indicator values (Ellenberg et al., 1992) (light, soil moisture, soil fertility). The traits served as proxies to the changing environmental conditions during succession. Raw data and pairwise correlations are contained in Tables A3 and B1.
For the present analysis we constructed three data matrices, a 168 × 4 species × traits matrix T containing for all 168 species the raw values of the four traits, a 168 × 642 species × plot/year matrix M containing the abundance data for all plot × consecutive study year combinations, and a 6 × 642 soil variable × plot/year matrix V, containing the soil parameters. These three matrices are provided in Appendix A.
The inner product TTM provides the total trait expressions at each of the 642 plot/year combinations. We calculated the FD for each plot from the functional attribute metric (FAD) of Walker et al. (1999) being the normalized sum of all functional pairwise distances between the species of a community. FAD values strongly depend on species richness and abundances (Walker et al. (1999). Therefore, we used two approaches for subsequent data analyses. First, we compared observed FAD values with those obtained from a statistical null standard. This was obtained from a reshuffling of species names across trait values breaking of the plant cover - trait combinations. The associated null distributions were obtained from 200 randomizations. For comparisons we used standardised effect sizes \({SES}_{j}= \frac{{y}_{j}-\mu }{\sigma }\) where yj denotes the observed parameter and µ and σ are the respective mean and standard deviation of the null model distribution.
Second, Ulrich et al. (2022) used the Price equation of evolutionary biology (Fox 2006) and partitioned the difference in the expression of FAD (ΔFAD) between two communities A and B into five parts:
$$\varDelta FAD=E\left({c}_{A}\right)\varDelta S+{N}_{B}\text{E}(\varDelta p\varDelta c)+{N}_{B}\text{E}(\varDelta c)+{N}_{B}\text{E}(\varDelta p)+f\left(\varDelta N\right)E\left({t}_{A}\right)$$
1
containing terms quantifying
1) the difference in species richness (ΔS)
2) the average difference in species trait expression (Δc)
3) the difference in relative abundance (Δp)
4) the combined effect of differences in relative abundance and trait expression (ΔpΔc)
5) the effect of differences in abundance (ΔN).
In Eq. 1 E denotes the statistical expectation (mean) in community A of 1) average trait expression cA, the combined and pure changes in c (Dc) and relative abundance p (Dp), and the average species trait value tA. NB denotes the total abundance (here plant cover) in community B (details in Ulrich et al. 2022). We note that these partitions, although being additive, are not linearly independent. Particularly the present richness partitions DS for seed number and seed mass were moderately to strongly correlated with the relative (Dp) and absolute (DN) abundance partitions (Table B2).
Here, we used FAD as the trait of interest and consequently ∆FAD refers to the total change in FAD in a given plot between two consecutive years. We calculated ∆FAD and its partitions for all 643 plot × consecutive year combinations. Eq. 1 allows for a comparison of the drivers that directly influence the changes in FAD and for separate analyses of the underlying covariates.
To answer to our first and second starting question, we used general linear mixed modelling with robust error estimation as implemented in SPSS 28 to relate the average annual changes in FAD and its Price partitions across the 107 plots to species richness (SA), total abundances (NA), total trait expression (TA), and environmental data (initial pH, concentrations of soil organic carbon, and proportion of soil sand), as well as the Ellenberg scores of plant light, soil humidity, and soil nitrogen demands. Study year served as random qualitative predictor. All variables were Z-transformed prior analysis. The environmental predictor variables used in the models were at most moderately corrected and multicollinearity should not influence parameter estimates (appendix B, Tab. B1). To reduce variability in bivariate comparisons we used average values and respective variance - mean ratios (VMR = σ2/µ; where σ and µ are the respective standard deviations and mean values) calculated from all 107 single plots. VMR = 1 indicates a Poisson random distribution. We note that the plots were temporarily not independent and significance levels might be inflated. However, this non-independence, expressed by the changes in trait space within each plot, is exactly what we wanted to study with this approach. Consequently, we focus on general trends and effect sizes.