Variation in vegetation and microbial linkages with slope aspect in a montane temperate hardwood forest

Plant ecologists have long been interested in aspect-related contrasts of montane forests. Few studies have assessed correlation (linkage) among vegetation strata; fewer have included soil microbial communities. This study assessed contrasts in overstory, spring herbaceous, and soil microbial communities between northeast (NE) - and southwest (SW) -facing slopes in a second-growth West Virginia hardwood forest. We addressed three questions: (1) how do soil microbial, herbaceous layer, and overstory communities vary with slope aspect? (2) do forest vegetation strata and soil microbial communities exhibit linkage? (3) do biotic relationships and linkage vary with slope aspect? Moisture, organic matter, pH, soil NO3-, and net nitrification were significantly higher in NE soils; soil NH4+ was significantly higher in SW soils, and net N mineralization was virtually identical between aspects. Vegetation communities markedly differed between the slopes. Overall, 29 tree and 118 herbaceous species were encountered, with overstory dominated by sugar maple and sweet buckeye on NE slopes and white oak on SW slopes. The herb layer of the NE aspect was characterized by forbs with fewer graminoids, whereas higher diversity and importance of graminoids were typical for SW slopes. Important microbial groups for NE soils were Gram-positive and Gram-negative bacteria; SW soils were dominated by fungal groups and Gram-negative bacteria associated with environmental stress. Linkage was detected between soil microbes and overstory on the NE slope and between soil microbes and herb layer on the SW slope. We suggest that higher net radiation to the SW slope has created more weathered soil conditions, selecting for microbial groups both adapted to, and maintaining, low N availability (bottom-up control). This has resulted in dominance of NO3[1] in less weathered NE soils and NH4þ in more weathered SW soils. Other differences arose from contrasts in humus formation, driven by overstory species composition and maintained by soil moisture (top-down control). Higher moisture of NE soils is more conducive to humus formation, supporting different microbial communities than the SW slopes with drier and often exposed mineral soil. Thus, plants and microbes can form biotic linkages in forest ecosystems which can vary among vegetation strata and with slope aspect