Soil seed bank characteristics measurements
The density of SSB was expressed as the number of seeds per m2 and SSB richness was measured as the number of species for each soil sample.
SSB species diversity indices were calculated for each individual under-shrub using greenhouse data. The Shannon index is most frequently used to characterize the diversity of communities; it is sometimes referred to as the Shannon-Wiener index (Equation 1).
Equation 1 H’ =\(\sum_{i=1}^{s}\text{pi}\log\text{pi}\)
Where pi is the relative abundance of SSB of the ith species in a soil sample, and S is the number of detected species in SSB in that sample (Chernov et al., 2015). Another diversity index frequently used in ecology is the Simpson index, which is frequently determined as the probability of belonging to different taxa for two plant species randomly selected from an indefinitely large community. The Simpson index was calculated from the equation 2 (Chernov et al., 2015).
Equation 2 SI\(=\)\(\sum_{i=1}^{s}\frac{ni\ (ni-1)}{N(N-1)}\)
Where ni is the individual number of each plant species in the SSB in a sample, and N is the total number of all germinants of all plant species in SSB in that sample.
The diversity indices were calculated using the Past software.
In addition, during the growth season, we recorded the presence of all plant species within each of the patches sampled for SSB. Species abundance of the AGV was not estimated because it was not possible to place a sampling frame beneath the shrubs and we used presence-absence data for AGV in the analyses. Qualitative similarity between the species composition of the AGV and the SSB was assessed using the Jaccard similarity index (Kent and Coker 1994) in each patch using equation 3.
Equation 3\(\text{IS}_{j}=\left[\frac{C}{\left(C+A+B\right)}\right]\times 100\)
Where C is the number species common between AGV and SSB, A, the number of species found only in the AGV and B, the number of species found only in the SSB.
We also estimated the mean canopy surface of our shrubs on ground and the height of each individual shrub using metal tape measure.