Ecological Archives E093-073-A1

J.C. Douma, B. Shipley, J. P. M. Witte, R. Aerts, and P. M. van Bodegom. 2012. Disturbance and resource availability act differently on the same suite of plant traits: revisiting assembly hypotheses. Ecology 93:825–835.

Appendix A. Justification for the omission of moisture supply in the SEM.

To show that moisture availability is less important than nutrient availability and disturbance, we first calculated as a proxy for soil moisture conditions the process based root respiration stress (RS; Bartholomeus et al. 2008). Respiration stress occurs when oxygen availability decreases and it limits the metabolic activity of plants (Bartholomeus et al. 2008). RS was calculated using fortnightly series of groundwater levels measured with piezometers closely located to the plots (http://www.dinoloket.nl). Groundwater levels were interpolated with Menyanthes (Von Asmuth et al. 2002) to daily values and extrapolated to 30 years time series ({Bartholomeus, 2008 #582}), which were subsequently used in a detailed soil water atmosphere plant model to calculate gas filled porosity, which was used to calculate RS (kg O2/m2d; Bartholomeus et al. 2008). For a detailed description of this procedure we refer to Bartholomeus (submitted to JoH).

Secondly we submitted RS together with with TSD, soil C/P, soil C/N to a RDA to constrain 10 traits. The two drivers that are most related to the first two axes will be selected for the SEM. From the results it appears that Soil C/P ratios and TSD were more strongly related to the first two RDA-axes than RS, which justifies the use of soil C/P and TSD over RS as important drivers of species assembly in the Netherlands (Table A1).

TABLE A1. Results of the RDA with the relevé-traits (156 sites × 10 traits) constrained by 4 environmental factors (TSD, RS, Soil C/N and Soil C/P). The explained variance of the constrained axis, and the scores of the environmental constraints are shown. Abbreviations of traits: see Table 1. Abbreviations of environmental factors: time since disturbance (TSD), Respiration stress (RS), Soil CN ratio (Soil C/N), Soil CP ratio (Soil C/P).

  RDA1 RDA2 RDA3 RDA4 RDA5
Explained
variance
0.36 0.43 0.44 0.45 0.45
LogCN 0.17 -0.53 0.79 -0.24 -0.07
LogCP 0.35 -0.84 -0.05 0.18 0.38
Ox-stress 0.58 0.37 0.16 0.68 -0.21
TSD -0.95 -0.24 0.14 0.17 0.04

LITERATURE CITED

Bartholomeus, R. P., J. P. M. Witte, P. M. van Bodegom, and R. Aerts. (2008a). The need of data harmonization to derive robust empirical relationships between soil conditions and vegetation. J. Veg. Sci., 19, 799–808.

Bartholomeus, R. P., J. P. M. Witte, P. M. van Bodegom, J. C. van Dam, and R. Aerts. (2008b). Critical soil conditions for oxygen stress to plant roots: Substituting the Feddes-function by a process-based model. Journal of Hydrology, 360, 147–165.

Von Asmuth, J. R., M. F. P. Bierkens, and C. Maas. (2002). Transer function noise modeling in continuous time using predefined impulse response functions. Water Resources Research, 38, 2301–2312.


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