Elevation and micro environmental conditions directly and indirectly influence forests’ soil seed bank communities

The soil seed bank (SSB) plays an important role in determining future community composition and health of forests. The extent to which abiotic conditions alter the abundance and diversity of the future forest composition via modulating the regenerative capacity of the SSB and their relationship with each other is not fully understood. We studied how elevation and abiotic conditions (canopy openness, slope, aspect, soil pH, and a natural gradient of these soil nutrients: total nitrogen [N], total carbon [C], C:N ratio, available phosphorus [P], available potassium [K], calcium [Ca], iron [Fe], magnesium [Mg], manganese [Mn], and sulfur [S]) affect community characteristics, calculated as: (1) seedling abundance, (2) species richness, (3) Shannon-Weiner diversity index, (4) Simpson index, (5) Chao 2 richness estimator, (6) number of species represented by only one individual, and (7) number of species that occur only in one plot. We used soils sampled in 13 1-ha plots along an altitudinal gradient from 400 to 1850 m above sea level. We determined both direct and indirect causal relationships between abiotic factors and SSB community characteristics using linear mixed models and structural equation modeling. Increased elevation and canopy openness resulted in lower species richness and diversity as well as fewer rare species. However, greater canopy openness resulted in higher abundance. Abundance and richness of the SSB were lower in steeper slopes, but this negative effect appeared to be along an indirect pathway through soil pH and nutrients such as soil Mn and S. Soil P had a significant negative relationship with SSB characteristics for all indices evaluated here, except for SSB abundance, which was positively affected by increased soil pH. Our results highlight the role of abiotic factors in forest regeneration and resilience through their influence on the soil seed bank community. This study is one of the very first to provide data on the SSB along a wide elevational range in subtropical forests, and can be the basis to assess the regeneration potential and health of the study region.