Invasive Earthworms Alter Forest Soil Microbiomes and Nitrogen Cycling

Northern hardwood forests in formerly glaciated areas had been free of earthworms until exotic European earthworms were introduced by human activities. The invasion of exotic earthworms is known to dramatically alter soil physical, geochemical, and biological properties, but its impacts on soil microbiomes are still unclear. Here we show that the invasive earthworms alter soil microbiomes and ecosystem functioning, especially for nitrogen cycling. We collected soil samples at different depths from three sites across an active earthworm invasion chronosequence in a hardwood forest in Minnesota, USA. We analyzed the structures and the functional potentials of the soil microbiomes by using amplicon sequencing, high-throughput nitrogen cycle gene quantification (NiCE chip), and shotgun metagenomics. Both the levels of earthworm invasion and soil depth influenced the microbiome structures. In the most recently and minimally invaded soils, Nitrososphaera and Nitrospira as well as the genes related to nitrification were more abundant than in the heavily invaded soils. By contrast, genes related to denitrification and nitrogen fixation were more abundant in the heavily invaded than the minimally invaded soils. Our results suggest that the N cycling in forest soils is mostly nitrification driven before earthworm invasion, whereas it becomes denitrification driven after earthworm invasion.

Preferences for Northern Hardwood Silviculture among Family Forest Owners in Michigan’s Upper Peninsula

Managing northern hardwood forests using high-frequency, low-intensity regimes, such as single-tree selection, favors shade-tolerant species and can reduce tree species diversity. Management decisions among family forest owners (FFO) can collectively affect species and structural diversity within northern hardwood forests at regional scales. We surveyed FFOs in the Western Upper Peninsula of Michigan to understand likely future use of three silvicultural treatments—single-tree selection, shelterwood, and clearcut. Our results indicate that FFOs were most likely to implement single-tree selection and least likely to implement clearcut within the next 10 years. According to logistic regression, prior use of a treatment and perceived financial benefits significantly increased the odds for likely use for all three treatments. Having received professional forestry assistance increased likely use of single-tree selection but decreased likely use of shelterwood. We discuss these results within the context of species diversity among northern hardwood forests throughout the region.

Long-Term Evolution of Composition and Structure after Repeated Group Selection Over Eight Decades

: In northeastern North America, group selection is frequently used in northern hardwood forests to maintain uneven-aged stand structure and promote regeneration of tree species spanning a range of shade tolerances. For this study, long-term application of group selection at the Bartlett Experimental Forest, New Hampshire, USA provided a unique opportunity to address cohort and stand level progression after 80-years of treatment. Cohort-level evolution reflected successional and developmental dynamics associated with even-aged forest systems, whereas aggregate, stand-level conditions were consistent with expectations for uneven-aged systems. As cohorts aged, diameter distributions progressed towards descending monotonic forms and species composition transitioned from shade-intolerant species to shade-tolerant species. Standing deadwood and downed woody material in cohorts followed trajectories of aging even-aged stands through time. Although American beech (Fagus grandifolia Ehrh.) was a primary species across cohorts and at the stand level, stand level regeneration included a mixture of ecologically and commercially valuable species. These long-term results offer important insights into emergent cohort and stand-level conditions and processes that may affect continued recruitment of desirable compositional and structural conditions in stands managed using group selection over numerous cutting cycles.

Tracing leaf litter-derived 15N to mineral soil organic matter in forests of different ages

<p>Forest soils are important for retaining nitrogen (N), especially in areas where anthropogenic activities have led to historically high inputs of N. As forests age and their N demands for biomass accumulation decline, the capacity for N retention of soils may change as well, although little work has been done to further our understanding of this process. We conducted a mineral soil reciprocal transplant study in three northern hardwood forests of different ages (young, recently mature, and old growth) in New Hampshire, USA to determine how the retention of isotopically labeled nitrogen from leaf litter would differ depending on characteristics of the incubated soil’s origin and destination. After 18 months of incubating the soil bags below the <sup>15</sup>N-labeled litter, we did not find retention of litter-derived N to be related to the age of the incubation site forest, but rather that it differed based on the origin of the incubated soil.  We found that the soil C content was the strongest predictor of how much of the tracer was recovered in the transplanted soil bags. Furthermore, the C content of soils changed during incubation and tended to change in the direction of equilibrating with the soil C concentration of the incubation site. This finding suggests that site characteristics are important in determining soil C concentrations and consequently N retention capacities.</p>