Sugar maple, an economically and ecologically important tree in the northern hardwood forest, has experienced regeneration failure that in the Northeast portion of the range has been variously attributed to soil acidification and resultant changes in soil chemistry, impacts of climate change, and effects of species composition. In a 5-year study spanning a latitudinal gradient in the state of New Hampshire, we examined evidence for these three hypotheses to explain sugar maple regeneration patterns. Overall, sugar maple seedling survival was highest in the two sites with lower sugar maple abundance. Alternatively, the two other sites with greater than 50% sugar maple relative dominance shared the following outcomes: higher seed production per area, greater foliar pest damage, lower seedling survival, lower sapling density, and higher canopy maple mortality, while the sites with lower dominance of maple had opposite outcomes. Based on field data and a common garden experiment, conspecific impacts on seedling survival were related to foliar pests and fungal pathogens rather than through soil feedbacks. These results lend support to other studies encouraging promotion of stand tree diversity and avoidance of monocultures.

Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils are expected to become colder and experience more winter soil freeze-thaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root-microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snow- and/or soil frost-mediated) vs. indirect (e.g., root-mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2mm pore size) or excluded root ingrowth (50m pore size) for up to 29months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P<0.05; R-2=0.25-0.46) than to root biomass, snow or soil frost, or winter soil temperature (R-2<0.10). Root ingrowth was positively related to soil frost (P<0.01; R-2=0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re-allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30% (P<0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plant-microbe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.