Variation in biotic linkages with slope aspect in a temperate hardwood forest

Background/Question/Methods

Plant ecologists have long been interested in aspect-related contrasts of montane forests, with most studies focusing on north- versus south-facing slopes and on differences in overstory and herbaceous layer communities; far less emphasis has been placed on soil microbial communities.  The purpose of this study was to assess quantitative differences in plant and soil microbial communities between northeast (NE) - and southwest (SW) -facing slopes in a second-growth hardwood forest in western West Virginia.  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?  Field data were based on eight intensively-sampled 400-m²sample plots on each of NE- and SW-facing aspects of a forested ridge, a total of 16 plots for the study.

Results/Conclusions

As expected, there were notable differences in both soil characteristics and biotic community composition between aspects.  Moisture, organic matter, pH, extractable NO₃^-, and net nitrification were all significantly higher in NE soils, whereas extractable NH₄⁺ was significantly higher in SW soils; net N mineralization was virtually identical between aspects.  Overall, 29 tree and 118 herbaceous species were encountered in the study, 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 predominantly forbs, whereas graminoids dominated SW slopes.  Important microbial groups for NE soils were Gram + and Gram – bacteria; SW soils were dominated by fungal groups.  Using correlations of ordination axes of each biotic community, 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 resulted in more weathered soil conditions, selecting for microbial groups both adapted to—and maintaining—low N availability, with dominance of NO₃^- in more mesic NE soils and NH₄⁺ in more weathered SW soils.  Another important difference appears related to contrasts in humus formation, maintained with soil moisture conditions.   The higher moisture of NE soils is more conductive to humus formation and the support of different microbial communities than the SW slopes with drier and more exposed mineral soil.  These results demonstrate that plants and microbes can form biotic linkages in forest ecosystems which can vary with vegetation strata and slope aspect.